oneharness

One CLI across many agentic coding harnesses. oneharness drives Claude Code, Codex, OpenCode, Goose, Qwen Code, Crush, GitHub Copilot CLI, and Cursor through a single non-interactive interface, runs them in parallel, and returns one stable JSON shape built for programmatic consumers.

It exists to make cross-harness automation boring: instead of hand-rolling a run_agent() for each tool — different flags, different output, different "don't prompt me" switch, different skip-if-not-installed dance — you call one command and read one JSON document.

$ oneharness run --all --prompt "Reply with the single word: pong" --model haiku

{
  "schema_version": "0.1",
  "oneharness_version": "0.1.0",
  "prompt": "Reply with the single word: pong",
  "model": "haiku",
  "resume": null,
  "fork": false,
  "bypass_permissions": true,
  "dry_run": false,
  "config_files": ["/home/me/.config/oneharness/config.toml"],
  "results": [
    {
      "harness": "claude-code",
      "bin": "claude",
      "available": true,
      "status": "ok",
      "exit_code": 0,
      "duration_ms": 1840,
      "command": ["claude", "-p", "Reply with…", "--permission-mode", "bypassPermissions", "--output-format", "json"],
      "output_format": "json",
      "text": "pong",
      "text_source": "json:result",
      "usage": { "input_tokens": 1234, "output_tokens": 8, "cache_read_tokens": 7, "cache_write_tokens": null, "cost_usd": 0.0095 },
      "usage_source": "json",
      "session_id": "0f3c…",
      "events": null,
      "events_source": null,
      "failure_kind": null,
      "failure_kind_source": null,
      "stdout": "{\"type\":\"result\",\"result\":\"pong\"…}",
      "stderr": "",
      "error": null
    },
    { "harness": "codex", "available": false, "status": "skipped", "error": "`codex` not found on PATH; harness skipped. Install it: npm install -g @openai/codex", "…": "…" }
  ]
}

Supported harnesses

The table doubles as the config support matrix: each column after the binary is a unified setting (CLI flag and/or oneharness.toml field) and shows how — or whether — it reaches that harness.

The --resume column shows each harness's headless continuation flag and whether it can fork (run --resume <id> --fork: branch a new session from the resumed one, leaving the original — and its cached prefix — untouched). Only Claude Code (--fork-session) and OpenCode (--fork) fork headlessly; the rest resume linearly (append in place), and --fork is a usage error for them, never a silent linear resume. ¹ Goose and Copilot emit no session id to stdout headlessly, so their continuation handle is caller-supplied (a --name, or a minted UUID respectively) and reused on the next run — session_id stays null for them (nothing to extract); every other harness reports an id oneharness captures.

Above --resume sits the higher-level --session <name> (see the Session handle section): a stable, caller-owned name oneharness maps to the harness's native session id in a small store, so a consumer threads one name across turns instead of extracting and re-passing the id itself. It is supported exactly for the harnesses that expose a session id headlessly — claude-code, opencode, codex, cursor, qwen (session_capable: true in oneharness list); for the rest (which have no id to bind a name to) --session is a loud usage error. A named session automatically selects that harness's session-id-bearing output format unless you explicitly set --output-format/config output_format; an explicit format remains authoritative only when it can emit the id, otherwise the run fails with a usage error before spawning.

• model — ✓ means the harness takes a model flag. Goose selects its model from its own provider config, so model is intentionally not mapped for it.
• system — "native flag" means the system prompt maps to a real flag (Claude Code's --append-system-prompt, Goose's --system); "prepended" means the harness has no such flag, so the text is prepended to the prompt — it always reaches the model, never silently dropped.
• reasoning — reasoning / thinking effort (--reasoning <effort> / ONEHARNESS_REASONING / config reasoning, and per harness next to its model), an opaque string forwarded verbatim in each harness's native shape: Claude Code's --effort (low/medium/high/max/auto), Codex's -c model_reasoning_effort= (minimal…xhigh), Copilot's --reasoning-effort (low/medium/high), and Cursor's model-id tier suffix (--model claude-opus-4-8
  • --reasoning high → --model claude-opus-4-8-high; cursor-agent bakes the effort into the model name and rejects a bracketed [effort=…], so Cursor needs a base model whose family accepts the tier suffix, and a loud usage error when no model is set). Effort is a provider/model capability with no shared spelling, so oneharness does not interpret the value — an effort the model rejects surfaces as that harness's own error, never a guess. The remaining harnesses set effort only in their own config file (OpenCode, Qwen, Crush) or expose no headless knob at all (Goose), so --reasoning is a loud usage error for those rather than a silent drop; scope the setting per harness when a selection mixes capable and incapable ones. (supports_reasoning in oneharness list; each capable harness's delivery is drift-alarmed live by oh_reasoning_enforce.)
• synced config file — the project-scoped file oneharness sync merges the unified settings into. Because the policy lands in each harness's own config, it also governs the tools when used directly — oneharness is not in the loop at run time. Codex and Goose read only user-global config, and Copilot takes permission rules only as flags (deliverable via [harness.copilot] args), so they have no sync target.
• allow / deny — whether allowed_tools / denied_tools lists have a place in that file, in each harness's own rule syntax: Claude Code, Qwen, and Cursor use permissions.allow / permissions.deny. Qwen's rules govern its interactive approval flow only — live testing showed its headless mode never auto-approves from settings (only the -y CLI flag executes approval-gated tools), so synced qwen rules protect regular usage, not headless runs. Crush uses permissions.allowed_tools, with deny mapped to options.disabled_tools (the tool is hidden entirely — its strongest deny). OpenCode's permission is a policy map, not a list, so the lists are rejected for it — express it with [harness.opencode.settings] instead. A rule aimed at a harness with no mapping is a parse error (per-harness fields) or reported unmapped (top level) — never silently dropped.
• hooks — Claude Code's hooks table in .claude/settings.json. Other harnesses keep hooks in places oneharness doesn't manage yet (Copilot's .github/hooks/, Cursor's hooks.json, OpenCode's JS plugins).
• output format — ✓ means the harness takes a format flag the output_format setting maps onto; a — harness emits plain text and the setting only changes how text is extracted. Codex defaults to its --json stream so plain runs capture thread_id; Qwen remains text by default and is upgraded to stream-json automatically when --session needs its id.
• --resume — the flag each adapter maps run --resume <session> onto (every harness supports headless continuation). The cell also shows whether the harness can fork (--fork): Claude Code and OpenCode branch a new session from the resumed one; the rest resume linearly (append in place), so --fork is a usage error for them, never a silent linear resume.

The remaining unified settings — timeout, env, bin, per-harness args, cwd, selection — are enforced by oneharness itself at run time, so they work for every harness — as does --schema (structured output, prompt-based where a harness has no native schema flag). oneharness list prints this registry as JSON, including each adapter's exact command, its sync_file, and supports_resume / supports_fork / supports_native_schema / supports_reasoning / supports_allowed_tools / supports_denied_tools / supports_hooks capability flags.

Install

# from PyPI (per-platform wheel wrapping the prebuilt binary — no Rust toolchain)
pip install oneharness-cli          # installs the `oneharness` command
# or from npm (per-platform package wrapping the prebuilt binary — same promise)
npm install -g oneharness-cli       # also installs the `oneharness` command
# typed Node API (includes the matching CLI package)
npm install @oneharness/sdk
# typed Python API (includes the exact matching CLI package)
pip install oneharness-sdk
# or the latest prebuilt release for your platform via the install script
curl -fsSL https://raw.githubusercontent.com/nickderobertis/oneharness/main/scripts/install.sh | sh
# or pin a release tag / install directory
curl -fsSL https://raw.githubusercontent.com/nickderobertis/oneharness/main/scripts/install.sh \
  | sh -s -- --version v0.1.0 --to ~/.local/bin
# or from crates.io / a published release tag
cargo install oneharness --locked
cargo install --git https://github.com/nickderobertis/oneharness --tag v0.1.0 --locked
# or from a clone
cargo install --path .
# or just build the release binary
just build-release            # -> target/release/oneharness

A tagged release ships five ways: PyPI wheels (pip install oneharness-cli, the distribution is oneharness-cli, the command is oneharness), npm per-platform packages (npm install -g oneharness-cli, same distribution name, same command), crates.io (cargo install oneharness), prebuilt checksummed binaries on its GitHub Releases page for Linux, macOS, and Windows, and cargo install --git. The PyPI and npm CLI packages both wrap the prebuilt binary — no Rust toolchain, no compile — carrying the platform-specific binary in a per-platform artifact (a wheel; an @oneharness/cli-<platform>-<arch> optional dependency) that the package manager selects for your OS and CPU. Building from source requires a stable Rust toolchain and just. The same release publishes matching @oneharness/sdk and oneharness-sdk language clients; each pins its packaged CLI dependency to that exact version.

Applications can use @oneharness/sdk (Node 20+) or oneharness-sdk / the oneharness_sdk import (Python 3.9+) for the same complete surface: run, streaming run, registry list/detect, and history lookup/list/watch. Both streaming methods are async iterators; every JSONL envelope is validated before it is yielded, and breaking or cancelling an iterator terminates the subprocess. Both SDK distributions are stamped from the root Cargo version and depend on the exact matching oneharness-cli package.

The SDK declarations, input contracts, and runtime validation schemas are generated from one Rust JSON Schema bundle and drift-checked by just check. Outputs preserve unknown fields for additive forward compatibility; inputs are strict, so unknown option names and misspellings fail before a subprocess starts. Missing history records, sessions, and watch cursors raise a typed HistoryNotFoundError. See the Node SDK guide and Python SDK guide. The install script honors ONEHARNESS_VERSION, ONEHARNESS_INSTALL_DIR, ONEHARNESS_RELEASE_BASE_URL/--base-url, ONEHARNESS_CHECKSUM_BASE_URL, and GITHUB_TOKEN (for higher GitHub API rate limits when resolving the latest release).

Supply-chain verification

The install script never trusts a mirror to attest its own download. It verifies every archive against a trust root independent of where it was downloaded, and aborts if nothing independent can vouch for it. Two roots, tried in order:

1. Sigstore build-provenance attestation (preferred). Each release ships a keyless Sigstore bundle beside the archive (oneharness-<tag>-<target>.sigstore.json), logged to the public Rekor transparency log and bound to this repo's release workflow's OIDC identity — no signing key or secret. When a verifier is present — cosign, sigstore (pip install sigstore), or gh — the installer verifies the archive against the bundle offline. The trusted digest comes from the signed attestation itself (no checksum file is consulted), so a mirror cannot forge it, and it works behind a mirror that can't reach github.com. Where github.com is unreachable a verifier is one registry install away (pip install sigstore, npm i -g @sigstore/cli, or go install …/cosign@latest).
2. SHA-256 checksum from canonical GitHub (fallback, only when no verifier is installed). The .sha256 is fetched from github.com, never from the mirror. A checksum that shares the mirror's origin is no trust root at all — the mirror would just serve a matching tampered checksum — so the installer refuses it and tells you to install a verifier, rather than trust the mirror to vouch for its own download.

Serve the archive from a mirror with ONEHARNESS_RELEASE_BASE_URL (or --base-url) — for a network that can reach a mirror but not github.com, ship the .sigstore.json bundle on the mirror too and install a verifier, and the whole flow works offline. ONEHARNESS_CHECKSUM_BASE_URL points the checksum fallback at a specific independent root. You can also verify any archive out of band:

cosign verify-blob-attestation --new-bundle-format \
  --bundle oneharness-v0.1.0-x86_64-unknown-linux-gnu.sigstore.json \
  --type https://slsa.dev/provenance/v1 \
  --certificate-oidc-issuer https://token.actions.githubusercontent.com \
  --certificate-identity-regexp '^https://github.com/nickderobertis/oneharness/\.github/workflows/release\.yml@' \
  oneharness-v0.1.0-x86_64-unknown-linux-gnu.tar.gz
# or, more simply:
gh attestation verify oneharness-v0.1.0-x86_64-unknown-linux-gnu.tar.gz \
  --repo nickderobertis/oneharness

Every release runs a verify-attestation CI job that installs real cosign and sigstore-python and runs these exact commands against the just-published bundle, so a drift in the signing identity or flags reddens the release instead of silently degrading installs to the checksum fallback.

Usage

list/detect/config/sync/run emit JSON to stdout (diagnostics go to stderr), gate speaks a harness's hook protocol on stdin/stdout, and init scaffolds a starter config with a plain confirmation line.

oneharness init                                   # scaffold a starter oneharness.toml (refuses to overwrite; --force to replace)
oneharness init oneharness.judge.toml             # scaffold under a specific name
oneharness list                                   # describe the registry
oneharness detect --all                           # which harnesses are installed (+ versions)
oneharness config                                 # effective layered config + where each value came from
oneharness sync                                   # merge the unified settings into each harness's own config file
oneharness sync --global                          # install [[hooks]] into the user-global config instead of the project
oneharness run --all --prompt "…"                 # run everywhere, in parallel
oneharness run --harness claude-code,codex --prompt-file task.md
oneharness run --harness claude-code --system "$(cat ctx.md)" \
  --prompt "Q1" --prompt "Q2" --prompt "Q3" --batch-strategy min-tokens  # batch: one harness, N prompts, shared cache prefix
oneharness run --all --print-command --prompt "…" # dry run: show commands, run nothing
oneharness gate claude-code --deny-if-contains X  # the pre-tool gate an installed hook invokes (reads stdin)

Useful run flags:

• --all / --harness <id,…> / --exclude <id,…> — selection.
• --prompt <text> or --prompt-file <path|-> — the prompt (file or stdin). Both are repeatable; passing more than one prompt switches to a batch run (one harness, N prompts). Each --prompt-file is read whole as one prompt (not split per line); - (stdin) may appear once. Combined order is every --prompt, then every --prompt-file. A large prompt is delivered to the harness off the argv (piped to its stdin) so it never trips the OS argument limit at the harness spawn either — see Large prompts.
• --batch-strategy <speed|min-tokens> — for a batch run, how the calls are scheduled to exploit the shared prefix cache (speed, the default, or min-tokens); see batch runs. No effect on a single-prompt run.
• --run-mode <parallel|fallback> — how the selected harnesses are run (parallel, the default, or fallback); also run_mode in config / ONEHARNESS_RUN_MODE. See Fallback mode.
• --model <m> — passed to each harness that supports a model flag. Repeatable: pass it more than once (or set config models / ONEHARNESS_MODELS) to fan out over several models — see Multiple models. A CLI value overrides config model/models.
• --system <text> or --system-file <path|-> — portable system prompt for every harness: mapped to a native flag where one exists (Claude Code's --append-system-prompt, Goose's --system), and prepended to the prompt otherwise, so the instructions always reach the model. The two are two spellings of one input and mutually exclusive; config system is the fallback for both. Use --system-file (the file-based counterpart to --prompt-file, - for stdin) for a system prompt too large to pass as an argv string — a big --system value can trip the OS single-argument limit and fail at spawn with Argument list too long (E2BIG) before any harness runs. Only one input may read stdin, so --system-file - cannot combine with --prompt-file -. However the system prompt reaches oneharness, a large one is also delivered to the harness off the argv (a temp file on Claude Code, folded into the stdin prompt elsewhere), so it clears the harness-spawn ceiling too — see Large prompts.
• --resume <session> — continue a prior session, sending the prompt as its next turn. Single-harness only (a session belongs to one harness); every harness supports it, but multi-harness selections are still a usage error. The continued session_id is surfaced on each result (see below). Harnesses that emit no id headlessly (Goose, Copilot) take a caller-supplied handle you reuse across runs (a --name, or a minted UUID).
• --fork — with --resume, branch a new session from the resumed one instead of appending to it, leaving the original (and its cached prefix) untouched — so one expensive initial prompt can seed many independent follow-ups that each reuse the cached prefix. Only Claude Code (--fork-session) and OpenCode (--fork) fork headlessly (supports_fork in oneharness list); requesting it for any other harness is a usage error, never a silent linear resume. Requires --resume.
• --session <name> — continue (or start) a named conversation by a stable, caller-owned handle: oneharness maps <name> to the harness's native session id in a small store, so you thread one name across turns instead of extracting and re-passing the id yourself. The first --session <name> run starts fresh and captures the id; later runs with the same name resume it. Single-harness in the default parallel mode; under --run-mode fallback it binds to the first session-capable harness in the chain. Only for harnesses that expose a session id headlessly (session_capable in oneharness list) — others are a loud usage error. Without an explicit format, oneharness selects the harness's session-id-bearing format automatically; an explicitly pinned incompatible --output-format/config output_format is a usage error instead of a silent empty store. The higher-level counterpart to --resume; mutually exclusive with --resume/--fork/--all and with a batch. See Session handle.
• --session-dir <dir> — directory the --session store lives in (default: <platform state dir>/oneharness/sessions). Like --resume/--fork, a per-invocation knob with no config/env layer; mainly for isolating the store in tests and scripts.
• --output-format <text|json|stream-json> — override the format requested from each harness (default: per-harness); affects the emitted flag and how text is extracted. With --session, the explicit choice must be one of that harness's session-id-bearing formats or the run is rejected before spawning.
• --schema <path> / --schema-max-retries <n> — structured output: constrain each harness's final answer to a JSON Schema, validate it, and re-prompt on failure. See Structured output below.
• --output-dir <dir> — also write each harness's raw stdout/stderr to <dir>/<harness>.stdout and <dir>/<harness>.stderr (read transcripts from files without a JSON parser).
• -- <args…> — extra arguments appended verbatim to each harness command (for single-harness runs, since flags differ per harness).
• --timeout <secs> — per-harness timeout (default 120); a hang becomes a timeout result, not a stuck process. oneharness owns the launcher's whole process tree, terminates descendants too, and bounds final pipe draining, so a native child cannot survive an npm/Node wrapper or hold the report open.
• --cwd <dir> / --env KEY=VALUE — run each harness in a directory / with extra env (useful for sandboxed e2e).
• --max-parallel <n> — cap concurrency (default: all selected at once).
• --mode <read-only|plan|default|edit|auto|bypass> — the approval mode requested from each harness (default default; see Approval modes below). A mode a selected harness can't express is a loud usage error before anything spawns; one that may block on a prompt headlessly is warned about and run, with --timeout as the backstop.
• --no-bypass / --bypass — shorthands for --mode default / --mode bypass; --bypass forces bypass on over a config's mode / bypass.
• --permit-prompts — silence the "may block on a prompt" warning for the chosen mode (use once allow-rules are synced so the prompt never fires).
• --require-available — treat a not-installed harness as a failure.
• --bin <id>=<path> — override a harness binary (also via ONEHARNESS_BIN_<ID>).
• --config <path> / --no-config — load exactly one config file / ignore all config files (see below).
• --compact — single-line JSON.

Configuration

Most run flags have a persistent counterpart in oneharness.toml, so a project (or a user) states its defaults once instead of repeating flags. oneharness init scaffolds a commented starter oneharness.toml (a fallback-mode chain) to edit from; it refuses to overwrite an existing file unless --force, and takes an explicit path (oneharness init oneharness.judge.toml) to scaffold a differently named config. Several sources layer per field, lowest precedence first:

1. Built-in defaults.
2. User-level — ~/.config/oneharness/config.toml (honoring $XDG_CONFIG_HOME; %APPDATA%\oneharness\config.toml on Windows), or the file named by $ONEHARNESS_CONFIG.
3. Project-level — the nearest oneharness.toml (or .oneharness.toml), discovered by walking up from the directory the harnesses run in (--cwd, else the current directory).
4. Environment overrides — ONEHARNESS_<FIELD> variables (see below); beat every config file.
5. CLI flags — always win.

Every top-level field with a run flag also has a standard ONEHARNESS_<FIELD> environment override, the field name upper-snake-cased so the env var, config key, and flag stay in sync (model → ONEHARNESS_MODEL, schema_max_retries → ONEHARNESS_SCHEMA_MAX_RETRIES). List fields are comma-separated like their repeatable flags (ONEHARNESS_HARNESSES=claude-code,codex, ONEHARNESS_MODELS=opus,sonnet for the model fan-out), booleans take true/false (or 1/0), and an empty value counts as unset. A malformed value (bad boolean/integer/format, unknown harness id) is the same loud usage error a file would raise. The sync-policy fields (allowed_tools, denied_tools, hooks, settings), the [env] table, and the [harness.<id>] overrides have no env form by design.

ONEHARNESS_MODEL=gpt-5 ONEHARNESS_TIMEOUT=300 oneharness run --harness codex --prompt hi
ONEHARNESS_HARNESSES=claude-code,codex oneharness run --prompt hi   # selection from the env

Within one file, a [harness.<id>] value beats the top-level value for that harness. Every field is optional, and an unknown field or harness id is a loud usage error (exit 2), never silently ignored. The run report's config_files array records exactly which files shaped a run.

# oneharness.toml — every field optional; shown with its CLI counterpart
harnesses = ["claude-code", "codex"]  # --harness (or `all = true` for --all)
exclude = ["cursor"]            # --exclude (applies to an `all` selection)
model = "gpt-5"                 # --model (single model, per harness)
models = ["opus", "sonnet"]     # repeated --model: fan out over the model axis
system = "Be terse."            # --system
bypass = true                   # legacy --bypass toggle (opt-in; default false)
mode = "default"                # --mode; beats `bypass` (default: "default")
timeout = 120                   # --timeout, in seconds
output_format = "json"          # --output-format
schema_file = "person.json"     # --schema (structured output; relative to project)
schema_max_retries = 2          # --schema-max-retries (default 2)
max_parallel = 4                # --max-parallel
run_mode = "parallel"           # --run-mode ("parallel" or "fallback")
require_available = false       # --require-available
history = false                 # --history / --no-history (opt-in run history)
history_dir = "~/logs/oh"       # --history-dir (default: platform state dir)
allowed_tools = ["Bash(git log:*)"]  # synced into each harness's config file
denied_tools = ["Bash(rm:*)"]        # (see `oneharness sync` below)

# A normalized pre-tool hook, fanned across every synced harness and rendered
# into each one's native shape (a shared config file, a dedicated hooks file,
# or a plugin). `{harness}` is replaced with the harness id. Unlike the
# verbatim `[harness.<id>.hooks]` table below, this reaches ALL harnesses.
[[hooks]]
command = "mygate hook {harness}"   # required; {harness} → claude-code, codex, …
matcher = "Bash"                    # optional tool-name matcher (harness dialect)
timeout = 10                        # optional; honored where the schema has one
# plugin_name = "mygate"            # optional identity for plugin/Copilot files
# harnesses = ["claude-code"]       # optional; default = every synced harness

[env]                           # --env, for every harness
RUST_LOG = "warn"

[harness.claude-code]           # per-harness: beats the top level for this id
model = "claude-sonnet-4-5"     # each harness can name its own model
bin = "/opt/claude"             # like --bin (the flag and ONEHARNESS_BIN_* win)
args = ["--max-turns", "6"]     # extra argv appended for this harness only
allowed_tools = ["Bash(git:*)", "Read"]  # this harness's rule syntax
env = { ANTHROPIC_LOG = "debug" }

# Lifecycle hooks, in the harness's own hooks schema, synced into its config
# file (Claude Code's .claude/settings.json `hooks` key) uninterpreted.
[harness.claude-code.hooks]
PreToolUse = [{ matcher = "Bash", hooks = [{ type = "command", command = "./validate.sh" }] }]

# Raw settings merged verbatim into a harness's config file — the escape
# hatch for shapes the unified fields don't model, like OpenCode's
# permission policy map.
[harness.opencode.settings.permission]
edit = "deny"
bash = { "git *" = "allow" }

Syncing harness configs

allowed_tools, denied_tools, hooks, settings, and the top-level [[hooks]] are sync settings: instead of being passed on each invocation, oneharness sync merges them into each harness's own project config file (the synced config file column in the matrix). That makes oneharness a config-sync dev tool: state the policy once in oneharness.toml, run sync, and it governs Claude Code, Cursor, Qwen, crush, and OpenCode even when they're used directly — oneharness is not needed at run time.

Hooks come in two forms. A [harness.<id>.hooks] table is written verbatim in that harness's own hooks schema, so it only reaches harnesses whose hooks live in the config file oneharness already syncs (Claude Code). A top-level [[hooks]] entry is normalized: oneharness renders it into each harness's native shape and delivers it the right way for that harness — merged into a shared file (Claude Code, Qwen, crush), written to a dedicated hooks file (Codex, Cursor, Copilot), or installed as a plugin (Goose's manifest + hooks.json, OpenCode's JS shim). One [[hooks]] entry therefore installs the same gate into all eight harnesses. The per-harness install appears under a hooks array in each entry of the sync JSON report.

oneharness sync                  # write/merge the harness config files in this project
oneharness sync --check          # CI mode: exit 1 (writing nothing) if out of sync
oneharness sync --harness claude-code --cwd ~/proj
oneharness sync --global         # install [[hooks]] into the user-global config (~ / $XDG_CONFIG_HOME)

By default sync writes the project config files. --global instead installs the normalized [[hooks]] into each harness's user-global location (~/.claude/settings.json, ~/.codex/hooks.json, ~/.copilot/hooks/…, $XDG_CONFIG_HOME/crush/crush.json, $XDG_CONFIG_HOME/opencode/plugin/…, etc.), so the gate applies to every project. Permission rules and raw settings are project-scoped only, so configuring them under --global is a loud usage error rather than a silent half-write.

The runtime gate (oneharness gate)

A normalized [[hooks]] entry's command is what each harness runs before a tool call. oneharness gate <id> is a ready-made such command: it reads the harness's pre-tool hook event on stdin, and — when the event matches --deny-if-contains <substr> — emits that harness's native deny verdict on stdout (otherwise nothing, so the call proceeds). It always exits 0, so a gate never blocks a call on its own error. The per-harness deny shapes are sourced from each CLI's hook protocol. The decision is a deliberately trivial substring match: gate exists to prove a synced hook is honored end to end (the live e2e drives a real harness through it), not to be a policy engine — that is allowlister's role, which consumes oneharness-core's installer as a library.

The mock/spy responder (oneharness mock)

oneharness mock <id> is the gate's read-write sibling, for behavioral test suites (the skilltest consumer — see docs/mock-spy-design.md): the same stdin/stdout hook loop, but driven by a --rules <file> JSON ruleset that can intercept tool calls, not just deny them. Every observed event is also appended to a --spy-file JSONL log (or $ONEHARNESS_SPY_FILE) — the spy channel, which records the original tool call even when a rewrite substituted its input (the transcript events can only show post-rewrite reality).

// rules.json — first matching rule wins; no match = allow through (spy-only)
{
  "rules": [
    {
      // all listed criteria must hold (AND). `tool_regex` spans a harness's
      // tool-name casing; `input` matches specific argument fields.
      "match": {
        "tool_regex": "^(?i)bash$",
        "input": { "command": { "regex": "git\\s+push" } }
      },
      "action": { "deny": { "message": "pushes are mocked in this test" } }
    },
    {
      // fake a shell result by declaring ONLY the output: oneharness generates
      // a safely-quoted printf stub itself, so no user-authored command — and
      // nothing real — executes; the model receives this text (+ trailing
      // newline) as the tool's genuine result. `exit_code` fakes a failure.
      "match": { "input": { "command": { "contains": "git status" } } },
      "action": { "stub": { "output": "nothing to commit, working tree clean" } }
    },
    {
      // the general rewrite: substitute any input fields — here redirecting a
      // file read to a fixture (shell stubs are better written with `stub`)
      "match": { "tool": "Read", "input": { "file_path": { "equals": "/etc/prod.yaml" } } },
      "action": { "rewrite": { "input": { "file_path": "/tmp/ws/fixtures/config.yaml" } } }
    }
  ]
}

Matching — a rule's match combines any of these criteria (all present ones must hold): tool (case-insensitive exact tool name) or tool_regex; event_contains (substring of the raw hook event — the portable, harness- agnostic option) or event_regex; and input, a map from an argument name (command, file_path, …) to a predicate — equals, contains, or regex — so you can match on the specific tool input rather than the whole event. An absent input field fails the rule (never fabricated); a non-string argument is compared against its compact JSON, so a predicate can still target an array or object. Regexes are RE2 (linear-time — a caller-supplied pattern can't hang the responder), unanchored (use ^…$ for exact); an invalid pattern, an empty needle, or a match with no criteria is a loud usage error before anything runs.

Actions: deny (the model reads the message as the tool's failure), stub (declare a shell call's output — compiled to a safe printf rewrite, so it needs the same mock_rewrite capability), and rewrite (substitute raw input fields — the primitive under stub, and the way to mock file reads). The model never perceives the substitution: its own tool call (the original) is already in its context when the hook fires, and what it receives back is just the result — keep canned output plausible for what was asked, since a self-inconsistent result (a fixture whose content names a different file) is the one thing a model has been observed to notice.

Per-harness capability — deny works wherever the gate does; rewrite (and therefore stub, which compiles to one) needs the harness's mock_rewrite shape (see supports_mock_deny / mock_rewrite in oneharness list; a rule using an action the harness can't express is a loud usage error, never a silent allow):

The single-flag path — run --mock-rules (and/or --spy-file) delivers the hook for one invocation, works in an original workspace whose existing config keeps applying (the mock is layered on top), and leaves no trace afterwards:

$ oneharness run --harness claude-code --cwd ~/proj \
    --mock-rules rules.json --spy-file spy.jsonl --prompt "…"

Per-harness delivery (all live-verified; see mock_delivery in the registry): Claude Code takes the hook on the argv via a per-run --settings temp file — zero workspace mutation, project/user settings untouched and still in effect. The others get a project-scope install through the non-destructive merge (existing hooks and unrelated keys preserved), with every touched file snapshotted before and restored byte-identically after the run — files the install created are deleted, directories it created are pruned. Codex's hook-engine opt-in flags (-c features.hooks=true --dangerously-bypass-hook-trust) are appended to its argv automatically. --spy-file alone (no ruleset) installs a pure observer. A selected harness whose hooks can't fire this way is refused loudly before anything is touched: qwen (user-scope-only hooks — use the sync --global + redirected-HOME pattern instead) and copilot (hooks never fire headlessly). The report records mock_rules and spy_file so a mocked run is distinguishable from a clean one. One caveat: the restore runs on the normal exit path, so a hard kill (SIGKILL) mid-run can leave the hook installed — re-running any oneharness mock/sync in that workspace, or git checkout, puts it back; prefer throwaway workspaces for suites.

The standing-policy path — a [[hooks]] entry synced into the harness's own config — remains available for policies that should persist (and is how qwen's user-scope delivery works):

[[hooks]]
command = "oneharness mock {harness} --rules /tmp/ws/rules.json --spy-file /tmp/ws/spy.jsonl"

The rewrite path AND the ephemeral delivery are drift-alarmed live per harness by the oh_mock_enforce e2e phases (driven through run --mock-rules: the substituted command must run, the original must not, the spy log must keep the original event, and the workspace must carry no trace of the hook afterwards).

The merge is deliberately conservative:

• Unrelated keys are never touched — objects merge per key, and only the keys oneharness manages are written.
• Lists union — existing entries keep their order and place; missing ones are appended. Re-syncing is therefore idempotent (sync adds and updates, it never removes — delete by hand or edit the harness file directly).
• Scalars oneharness manages take the config's value — the unified config is the source of truth for the keys you declared, and only those.
• Unparseable files are refused, untouched — a JSONC file with comments, say, fails loudly rather than being rewritten without them. Writes are atomic (temp file + rename), and an existing higher-precedence variant (crush's .crush.json) is merged into rather than shadowed.
• Nothing is dropped silently — a setting with no mapping for a harness is a parse error (per-harness fields) or surfaced as unmapped in the JSON report plus a stderr warning (top-level fields).

To opt out: --config <path> loads exactly that file and skips discovery (the ONEHARNESS_<FIELD> overrides still apply on top); --no-config (or ONEHARNESS_NO_CONFIG=1 for wrappers and hermetic test suites) ignores every config file and the env overrides, leaving only flags and defaults. detect honors the configured bins too, so it probes the same binaries run would invoke.

oneharness config is the debugging surface for the layering: it prints the effective configuration with every value's provenance — the config file path that supplied it, "environment" for an ONEHARNESS_* override, or "default" for a built-in — plus per-key attribution for [env] and per-field attribution for each [harness.<id>] section. It takes the same --cwd, --config, and --no-config as run, so it shows exactly what a run from that directory would load:

$ oneharness config --cwd ~/proj | jq '{config_files, model, timeout}'
{
  "config_files": ["/home/me/.config/oneharness/config.toml", "/home/me/proj/oneharness.toml"],
  "model": { "value": "gpt-5", "source": "/home/me/proj/oneharness.toml" },
  "timeout": { "value": 30, "source": "/home/me/.config/oneharness/config.toml" }
}

Which settings can reach which harness is the support table above: model, system, bypass, and output format are per-harness capabilities; timeout, env, bin, and args are enforced by oneharness and work everywhere.

Exit codes

• 0 — every selected harness was ok or skipped (or it was a dry run).
• 1 — at least one harness nonzero/timeout/spawn-errored (or, under --require-available, was missing; or, under --schema, never produced a schema-conforming answer).
• 2 — usage/configuration error (bad args, unknown harness, no prompt, an unreadable or invalid --schema file).

Under --run-mode fallback the rule is different: 0 when the harness that ran succeeded, 1 when it ran but failed or when no candidate could run at all — the fallen-through candidates never count against the run.

The result envelope vs. the normalized signals

The execution envelope — command, exit_code, duration_ms, status, stdout, stderr — is guaranteed and identical across harnesses.

Alongside it, oneharness lifts a few best-effort signals out of each harness's bespoke stdout so consumers don't have to parse it per harness. Each is null/empty when it can't be found, is never fabricated, and (where there's more than one possible method) records how it was found. This also applies to bytes captured before a timeout: the result stays timeout, while complete records can still populate text, usage, session_id, and events (a truncated final JSONL record is ignored rather than invalidating earlier ones):

• text / text_source — the final assistant message, normalized to one clean string across harnesses (json:result for Claude Code's terminal event, json:opencode-parts for OpenCode's JSONL text parts, stream-json:result for Cursor, raw for a plain-text harness, …). text is a convenience, not a guarantee: it is null whenever extraction isn't possible, and text_source is then null too. A consumer that needs certainty reads the guaranteed stdout — when text is null, stdout is the fallback that always carries the harness's real output.

• usage / usage_source — { input_tokens, output_tokens, cache_read_tokens, cache_write_tokens, cost_usd }, each field independently null when the harness doesn't report it (cost is commonly absent on subscription auth). The usage object is always present so the shape is stable for cross-harness cost/latency tables. usage_source records the method: json for a harness that reports a whole-run total in one event (Claude Code), json:summed-steps for one that reports per-step usage that oneharness sums (OpenCode). The two cache fields surface provider-side prompt-cache counts — cache_read_tokens is prefix tokens served cheaply from cache, cache_write_tokens is tokens written to it (a.k.a. cache creation) — so a consumer can confirm a repeated/forked run actually hit the cache. Cache-reporting support today (the rest leave both cache fields null — never 0 as a guess):

  harness	cache fields	source field(s)
  claude-code	✓ read + write	usage.cache_read_input_tokens / usage.cache_creation_input_tokens
  opencode	✓ read + write	summed part.tokens.cache.{read,write}
  all others	—	(no cache counts emitted; cursor emits no usage at all)

  Each supported harness has a live drift alarm (oh_cache_assert in its scripts/e2e-<id>.sh): a second run within the cache TTL must surface cache_read_tokens > 0, proving the extraction matches the real output shape.

• session_id — the handle a harness exposes for continuation, read from the snake_case session_id (Claude Code, Cursor, Qwen), camelCase sessionID (OpenCode), or Codex's thread_id; feed it back via run --resume <session> (single-harness) to drive a faithful multi-turn against the real agent, or add --fork (Claude Code / OpenCode) to branch independent follow-ups off one cached prefix. null for a harness that emits no id headlessly (Goose, Copilot) — their handle is caller-supplied, never scraped (see the support matrix).

• session — the uniform --session handle in play (else null): {name, phase (create|continue), token, store_file}. Lets a consumer thread one stable name across turns while oneharness maps it to the harness's session_id above. See Session handle.

• events / events_source — a normalized array of tool-call / action events the harness took, in order, so a consumer can assert on behavior (ran bash with a command matching /…/, edited exactly config.yaml, used ≤ 3 tool calls), not just the final text. Each entry is { kind, name, input, output, index }: kind is tool_call or tool_result, name is the normalized tool name (null for a result), input is the structured, tool-shaped arguments (so a consumer reads the command string / file path without re-parsing), output is the observation when exposed, and index is the position in the run. events is null (never []) when the harness's output carries no machine-readable trace — a plain-text harness (Goose, Qwen, Crush, Copilot), or Claude Code's single-document json result, which omits the intermediate transcript — with events_source then also null, so a consumer tells "harness doesn't expose it" from "no tools were used." Like text, it is best-effort and never fabricated; consumers needing certainty parse stdout.

  Events require the harness to run in a format that carries a tool transcript. Some emit one in the format oneharness already uses; the rest need a richer format, which --events (and --stream) selects automatically per harness (HarnessSpec.events_format) without you knowing the quirk — and without breaking text extraction. Every shape below was sourced from a real transcript captured from the live CLI (never guessed) and is drift-alarmed by a per-harness live e2e (oh_events_assert):

  harness	events via	events_source
  opencode	json (default)	json:opencode-parts
  cursor	stream-json (default)	stream-json:cursor-tool-calls
  claude-code	--events → stream-json (adds the required --verbose)	stream-json:content-blocks
  codex	exec --json (default; --events needs no upgrade)	json:codex-items
  qwen	--events → --output-format stream-json	stream-json:content-blocks
  goose, crush, copilot	no machine-readable transcript headlessly	— (null)

  Four recognizers cover these, each harness-agnostic: OpenCode tool parts, the Anthropic content-block shape (Claude Code + Qwen), Cursor's tool_call events, and Codex's command_execution items. goose, crush, and copilot emit only decorative TUI text headlessly (confirmed by probing the live CLIs), so events stays null for them — the honest answer, not a gap.

  Streaming (oneharness run --stream <one harness>) emits each event as an NDJSON {"type":"event","event":{…}} line the instant it is observed, then a terminal {"type":"result","report":{…}} line with the full envelope. A consumer can short-circuit the moment it sees a disallowed action by closing the stream — oneharness's next write fails (broken pipe) and it tears the harness down, so a bad turn is cut off instead of paid for in full. Stream runs a single harness (no batch, no --schema); --stream implies the --events format selection.

• failure_kind / failure_kind_source — on a non-zero run, a coarse reason (auth, rate_limit, model_not_found, quota) so a caller can tell a retryable condition from a broken request. This is distinct from status, which only records oneharness's relationship to the process. One kind, tool_deferred, is reported even on a status: ok run: the harness exited cleanly but only deferred a builtin tool call (Read, Bash, …) instead of executing it, so it produced no result. This happens in bridged/managed Claude Code deployments (where tengu_non_deferrable_builtins is empty and every builtin is deferred) — a tool-using run there silently dead-ends. When detected, oneharness sets failure_kind: "tool_deferred", writes an actionable error naming the tool, and fails the run (non-zero exit) instead of letting it look like an empty or schema-invalid answer. Agentic / tool-using runs against claude-code require a deployment that executes tools inline (a standalone environment or CI); a deferring deployment is unsupported for them, and there is no consumer-side flag to force inline execution.

Coverage is keyed off each harness's documented output shape — Claude Code's result JSON, OpenCode's JSONL (text parts for the answer, step_finish for usage), Cursor's stream-json — and widens as more shapes are sourced; an absent signal is the honest answer, not an error. Consumers that need certainty should parse stdout themselves.

Streaming events

The CLI already emits the normalized events incrementally with run --stream, using the Rust-owned RunStreamEnvelope contract described above. Non-streaming runs still return the same events at the end in RunReport.results[].events.

The Node and Python SDKs expose this contract as runStream / run_stream async iterators. A behavioral consumer such as skilltest can short-circuit the moment it observes bad behavior (a forbidden rm -rf, an out-of-scope network call), killing the run instead of paying for a full turn before judging it.

Structured output

run --schema <path> constrains each harness's final answer to a JSON Schema and validates it, so a programmatic consumer gets a checked JSON value instead of prose to parse. The schema is delivered two ways, chosen per harness:

• Native where the CLI supports it — Claude Code's --json-schema (with --output-format json), which returns the conforming value in its result document's structured_output field. supports_native_schema in oneharness list flags these.
• Prompt-based for every other harness — the schema is appended to the prompt as an instruction to emit only a conforming JSON value, which oneharness then recovers from the final text (unwrapping a ```json fence or an object embedded in prose).

Either way oneharness validates the result itself (with the jsonschema crate), so a native flag the harness ignores is still caught. On a validation failure it re-prompts the harness with the prior answer and the exact errors, up to --schema-max-retries times (default 2 — so at most 1 + N invocations per harness). The loop runs per harness, in parallel, so a --schema run across many harnesses is still concurrent.

Codex CLI also has a native --output-schema, but it takes a schema file and is reportedly ignored once the agent uses tools, so oneharness uses the more reliable prompt-based path for it today. The registry's native_schema hook makes adding more native deliveries a one-line, well-tested change.

Each result gains four fields (all null when no --schema was given):

• structured — the JSON value extracted from the answer and validated. Carries the last-attempted value even when it failed, so you can see what the harness produced; null only when no JSON could be extracted at all (never fabricated).
• schema_valid — true/false for the final attempt. A false here makes the run a failure (exit 1), so you can gate on "did I actually get conforming output".
• schema_attempts — how many times the harness was invoked under the loop (1 + retries).
• schema_error — the validation errors from the final attempt, joined for display; null when valid.

The top-level report echoes the applied schema and schema_max_retries. Both the schema path and the retry budget are also configurable (schema_file / schema_max_retries in oneharness.toml).

oneharness run --harness claude-code --prompt "extract the person from auth.py" \
  --schema person.json --compact | jq '.results[0].structured'

Windows note. A JSON Schema is quote-heavy, and a harness installed as an npm .cmd shim receives its arguments through cmd.exe's %* forwarding, which mangles quote-containing arguments. So on Windows the native --json-schema delivery (and a schema appended to the prompt) may not reach a .cmd-shim harness intact — structured output is most reliable on Linux/macOS, or on Windows against a real .exe harness. oneharness's own argv construction and validation are exercised on Windows by the hermetic test suite regardless.

Session handle

Driving a faithful multi-turn conversation against a real agent means continuing the same harness session each turn. The low-level way is --resume <id>: run once, read session_id from the report, pass it back next turn. That works, but the caller carries per-harness bookkeeping — extract the id, thread it, and know which harnesses even emit one.

--session <name> removes that. You pick a stable name and pass it every turn; oneharness maps the name to the harness's native session id in a small store (<state dir>/oneharness/sessions/<project-slug>/<name>.json, or --session-dir), so:

# Turn 1 — starts fresh, captures the harness's session id under the name "triage".
oneharness run --harness claude-code --session triage --prompt "Investigate the flaky test."
# Turn 2 — same name resumes the same session; you never touched the native id.
oneharness run --harness claude-code --session triage --prompt "Now propose a fix."

The report's session block echoes {name, phase, token, store_file} — phase is create on the first run and continue after, token is the bound native id. A named session is bound to one harness (reusing the name on another is a loud error) and cannot combine with --resume/--fork/--all or a batch, and is supported only for harnesses that expose a session id headlessly (session_capable in oneharness list: claude-code, opencode, codex, cursor, qwen) — for the rest, --session is a usage error rather than a silent fresh start. Session ids are format-dependent: with no explicit format oneharness selects the harness's preferred session-bearing format (notably Qwen stream-json; Codex now defaults to --json for every run). An explicit --output-format or config output_format still wins only when that format can emit the id; pairing --session with an incompatible format such as text is a usage error before the harness runs, never a warning after a lost capture. In the default parallel run mode it is single-harness; under --run-mode fallback it is allowed on a multi-harness chain and binds to the anchor — the first session-capable harness in the priority order, which fallback deterministically settles on given stable availability (the token is applied to, and captured from, that harness only, so a transient fall-through to a different harness never resumes it with a foreign id). This is the substrate a multi-turn driver (e.g. a simulated-user / skill-testing framework) builds on: thread one handle, get faithful state, read events for what the agent did.

Fallback mode (first that runs wins)

By default run drives every selected harness in parallel and reports them all. --run-mode fallback (or run_mode = "fallback" in config) instead runs them in priority order and stops at the first harness that actually runs the task — falling through only the candidates that cannot run at all. This is graceful degradation across a set of harnesses a repo declares it supports: list a few, and whichever one a given contributor (or CI runner) has installed and authenticated is the one that runs.

# Try claude-code first; if it isn't set up, fall through to codex, then opencode.
oneharness run --run-mode fallback --harness claude-code,codex,opencode \
  --prompt "Explain the failing test" --compact | jq '.fallback, .results[].status'

What falls through vs. what stops. The distinction is deliberate: a setup problem tries the next harness; a real run — success or failure — stops the chain, so a long, genuine run can never be mistaken for "try the next one".

¹ Exception — a model list. When the run is fanning out over several models (repeated --model / config models; see Multiple models), a per-model rejection means "try the next model", so model_not_found (fall through — model-not-found) and rate_limit (fall through — rate-limit) do fall through. With a single model both still stop the chain, as above.

The report gains a fallback block, { "ran", "fell_through": [{ "harness", "reason" }] }: ran is the harness that executed (or null when every candidate failed to start), and results holds only the harnesses attempted — the fallen-through ones in priority order, then the one that ran. Priority order is the --harness / config harnesses order (registry order under --all). Under --print-command nothing executes, so the block is null and every candidate's command is printed in priority order.

The command must be valid for the whole set. Every listed harness is validated up front, so a flag no candidate could honor (an approval --mode a listed harness can't express, an unsupported --mock-rules action, …) is a loud usage error before anything spawns — even for a harness that is never reached. This keeps people and agents writing commands that work for every harness the fallback config supports, not just the one that happens to run.

Fallback is single-outcome by nature, so it refuses a batch run, the low-level --resume / --fork continuations (each pins one specific harness's native id), and --stream as loud usage errors. The higher-level --session handle is allowed: it binds to the anchor (the first session-capable harness in the chain), so a named conversation degrades gracefully across the same priority set. Exit code: 0 when the harness that ran succeeded, 1 when it ran but failed or when no candidate could run at all.

Multiple models (fan out over the model axis)

By default a run uses one model per harness — a single --model (or config model, overridable per harness with [harness.<id>].model). Pass --model more than once (or set config models = [...] / ONEHARNESS_MODELS) and run fans out over the model axis, and it composes with the two run modes exactly as you would expect:

• parallel (the default) — the harness × model cross-product. Every selected harness runs once per model, all concurrently, and results holds one entry per (harness, model) pair (harness-major, then model-minor). One harness × three models is three runs; --all × two models is every harness twice.

  # Compare two models across two harnesses — 4 runs in parallel:
  oneharness run --harness claude-code,codex --model opus --model sonnet \
    --prompt "Explain this diff" --compact | jq '.results[] | {harness, model, status}'
  

• fallback — the (harness, model) priority chain. The same cross-product becomes the fallback order (harness-major, model-minor); the run stops at the first pair that actually runs. Here a per-model rejection falls through: an unavailable model (model_not_found → model-not-found) or an over-limit one (rate_limit → rate-limit) tries the next model, exactly as a missing harness tries the next harness — graceful degradation across models. (With a single model those still stop the chain — see the fallback table.)

  # Prefer opus; if it's unavailable or rate-limited, fall through to sonnet:
  oneharness run --run-mode fallback --harness claude-code --model opus --model sonnet \
    --prompt "Explain this diff" --compact | jq '.fallback'
  

Each result carries its own model (the value put on the harness's model flag, also visible in command), and the report gains a models list — the presence of which is the signal a consumer keys on to read each result's model. The top-level model is the first of the list. A one-element list is not a fan-out (it behaves like a single --model). Because a fan-out multiplies the run into several units, more than one model is a loud usage error with a batch (its cache prefix is per harness/model) and with the single-unit continuations --resume / --fork / --session / --stream.

Batch runs (same-prefix prompt caching)

A common workload is many prompts that share a prefix — the same --system context (a spec, a big reference doc, few-shot examples) with a different question each time. Pass more than one prompt and run switches to a batch: it drives one harness over each prompt and returns one report with a result per prompt (in order), each tagged with its own prompt. The top-level report gains a batch block ({ "strategy", "prompt_count", "forked" }); results[].prompt is authoritative, and the top-level prompt repeats the first for back-compat.

# 3 questions over one shared context, warming it once then forking:
oneharness run --harness claude-code --system "$(cat reference.md)" \
  --prompt "Summarize section 2" \
  --prompt "List the open questions" \
  --prompt "What changed since v1?" \
  --batch-strategy min-tokens --compact | jq '.batch, .results[].usage'

Two strategies:

• speed — the default — fire all prompts at once for minimum wall-clock. Every call is independent; this optimizes latency, not tokens. It is the default precisely because the token-saving alternative only helps one harness today (see the support matrix below) and never hurts — speed is the safe choice for any harness.
• min-tokens — minimize redundant token spend on the shared prefix. On a harness whose fork reuses the cache (today Claude Code only; see the matrix below) it runs the first prompt as a warm-up that establishes a session carrying the shared --system, then forks that session for the remaining prompts, so each fanned-out call reuses the warmed cached prefix instead of re-sending it. The report sets batch.forked: true, and the fanned-out results report usage.cache_read_tokens > 0 with a lower cache_write_tokens than the warm-up. oneharness never claims a saving it can't measure — read the counts. On every other harness min-tokens falls back to order-only (no saving) with a stderr warning, so it is never worse than speed.

Why fork rather than just repeating --system: provider prompt caching keys on the harness's byte-exact request prefix, but these CLIs inject per-invocation content (Claude Code, for instance, re-creates a user-supplied --append-system-prompt on every separate claude -p process — only its own global prefix gets cross-process cache reads). So a static --system repeated across processes is not reused; the reliable cross-call reuse is a warmed session, which is exactly what --fork branches from (see --fork). min-tokens operationalizes that.

Support matrix — where min-tokens reduces tokens. The saving needs a cache-reusing fork (fork_reuses_cache in oneharness list), which today is Claude Code only:

So exactly one harness is confirmed to save tokens; every other harness runs min-tokens as a plain scheduler (results are correct, just no token reduction) and oneharness prints a stderr warning rather than implying a saving. Two findings shape this (both measured live, not assumed):

• A static --system is not reused across separate harness processes. Even on Claude Code (a native --system harness) a repeated --append-system-prompt is re-created on every claude -p — only the harness's own global prefix gets cross-process cache reads. The other five non-Goose harnesses merely prepend --system (no cacheable breakpoint), and the six non-fork harnesses report no cache counts at all (so a saving couldn't even be observed). So a system-prompt approach saves nothing on them.
• Only a cache-reusing fork helps. Claude Code's --fork-session branches from the warmed session and reuses its cached prefix (the fan-out reads it and writes little). OpenCode's --fork instead re-sends the branched conversation cold (the fan-out reads no cache and re-writes the whole prefix — so forking it would raise tokens), so oneharness leaves OpenCode's min-tokens order-only.

On every order-only harness min-tokens just orders the calls, and oneharness says so on stderr rather than implying a saving.

Caveats. A batch is single-harness by nature (a session/cache prefix is per harness/model/tools) — selecting more than one harness (or --all), or combining with --resume/--fork, is a usage error. The token saving needs a harness with a cache-reusing fork (fork_reuses_cache in oneharness list — today Claude Code only); on any other harness min-tokens only orders the calls (no reuse) and oneharness says so on stderr. Note that where it does fork, this changes the fan-out's semantics: because the fan-out branches from the warm-up's turn, the later prompts share the first prompt's context (the fork model — "one initial prompt seeds independent follow-ups"), rather than being fully independent questions. Caching itself is best-effort and provider-side (a ~5-min TTL refreshed on hit, a minimum prefix length, a byte-identical prefix), so the reuse only lands when the warmed session's prefix clears the minimum and the fan-out runs within its TTL. Use speed when you want N strictly-independent answers with no shared context.

Large prompts (off-argv delivery)

Passing a prompt or system prompt as a command-line argument is bounded by the OS: Linux caps a single argv string at 128 KiB (MAX_ARG_STRLEN), and macOS / Windows cap the whole argv+env. A prompt past that limit fails the spawn with Argument list too long (E2BIG). --prompt-file / --system-file clear the caller → oneharness hop (the value arrives in a file, not on oneharness's own argv); oneharness then clears the oneharness → harness hop too, delivering a large prompt (or system prompt) to the harness off its argv rather than re-inlining it (issue #1115).

The switch is automatic and size-gated at 64 KiB: below it, the argv is byte-identical to before (so --print-command and small runs are unchanged); above it, oneharness routes the value off-argv where the harness's CLI supports it. The user/system text the model sees is identical either way. Delivery per harness (sourced from each CLI's headless docs, drift-alarmed by the live oh_long_prompt_enforce e2e phase):

¹ These CLIs have no system-prompt flag, so oneharness already prepends --system to the prompt; the combined text rides the same stdin stream. (Cursor's stdin-only-prompt behavior was verified live — see scripts/explore-cursor-stdin.sh.) ² Goose's --system takes inline text with no file/stdin route, so a >128 KiB system prompt for Goose still risks E2BIG — oneharness warns on stderr rather than failing silently. Its large user prompt is delivered via -i -. oneharness list exposes supports_prompt_stdin / supports_system_file per harness.

Run history

Every harness keeps its own session history in its own place and shape (Claude Code under ~/.claude/projects/, Codex under its sessions dir, …). run --history records a standardized, cross-harness history instead: one normalized record per harness run — the same signals the JSON report carries (harness, prompt, model, status, usage, session_id, events, text), and only those (no raw stdout/stderr) — streamed to disk as the run finalizes. If a run times out after emitting parseable records, their normalized signals are preserved here just as they are in the report; the record's status remains timeout.

It is off by default and opt-in three ways, layered like every other setting (CLI > env > project file > user file):

oneharness run --harness claude-code --prompt "…" --history          # this run

# ~/.config/oneharness/config.toml — the per-user opt-in: on for all your
# projects, without committing anything to any project's own config.
history = true
history_dir = "~/logs/oneharness"   # optional; default below

ONEHARNESS_HISTORY=1 ONEHARNESS_HISTORY_DIR=/data/oh oneharness run …  # env

Records can carry validated task-graph labels. Labels merge by key with the same precedence (CLI > environment > project file > user file), so a nearer layer can replace one key without discarding the others:

history_labels = { graph = "release", owner = "platform" }

ONEHARNESS_HISTORY_LABELS='graph=release,owner=ci' oneharness run … \
  --history-label owner=agent --history-label task=verify

Keys are 1–64 ASCII letters/digits/./_/- and must start alphanumeric; values are non-empty, at most 256 characters (Unicode code points, so a multibyte value is bounded by what you can read, not by its encoded size), and contain no control characters — every character Unicode calls Cc, which is C0, DEL, and C1. The CLI and the language SDKs enforce this one contract identically. Malformed config, environment, and CLI values are rejected before a run starts.

--no-history (or history = false in a nearer layer) turns it back off. Nothing is written under --print-command (nothing runs).

Layout. <history_dir>/<project-slug>/<session>.jsonl — one file per oneharness run invocation ("session"), partitioned by a slug of the project directory. history_dir defaults to <platform state dir>/oneharness/history ($XDG_STATE_HOME or ~/.local/state on Linux, ~/.local/state on macOS, %LOCALAPPDATA% on Windows); set it with --history-dir, history_dir, or ONEHARNESS_HISTORY_DIR. Relative project/cwd paths are canonicalized before the record and slug are written, so later list/show lookups resolve the same project even when the original run used ...

Each v0.2 record has a time-ordered UUIDv7 history_id, which is both an exact lookup key and a watch cursor; empty labels are omitted. Readers continue to accept v0.1 records, assigning deterministic UUIDv5 IDs and empty labels so the same legacy line always migrates to the same identity. A history_id is canonical hyphenated UUID text (8-4-4-4-12 hex, either case) carrying the RFC 4122 variant and a defined version; the unhyphenated, braced, and urn:uuid: spellings are not the contract and are refused, as they are by the SDKs' schema.

Session name. Each session has a human-meaningful name shown next to its id. Harnesses don't expose a readable title headlessly (only an opaque session_id, which oneharness already records per run), so the name is derived from the session's first prompt — or set explicitly with --history-name <NAME>.

Programmatic handoff. The run report echoes the session file as history_file (absolute), so a consumer captures it and reads the session back later. The oneharness history verb views and manages the store — JSON on stdout by default (the programmatic contract), --format text for a human view:

oneharness history list [--project <dir> | --all-projects]   # sessions, newest first
oneharness history show <session-id-or-name> [--last] [--all] # a session's records
oneharness history show <history-id>                          # one exact record
oneharness history watch [--label key=value] [--after <history-id>] --format jsonl
oneharness history clear [--all-projects] [--yes]            # dry-run unless --yes

show resolves its argument against a session id or name (name is non-unique — the newest match wins, or --all shows every match); a UUID history_id instead performs an exact record lookup across projects. watch first emits matching records after its optional cursor, then follows the locked, append-only .index.jsonl without rescanning the history tree. Reconciliation on startup adds missing session records, ignores removed sessions, and truncates a partial final index line left by an interrupted writer. Reusing the last emitted history_id with --after resumes without duplication; repeated --label filters are ANDed. clear reports what it would remove and deletes nothing until --yes, so it is safe to run non-interactively first.

Safety note: bypass by default

A headless agent run hangs waiting for a human to approve tool calls. run's default mode (default) maps each harness to its cleanest non-interactive variant — deny-and-continue, fail-closed, or auto-deny — so it neither hangs nor blanket-approves; an agent in default mode can read and answer but is denied the tools it would otherwise prompt for. To let it take real actions, pass --mode bypass (or --bypass) — the "allow everything" mode — ideally against a throwaway sandbox (see --cwd). --mode (below) selects any other point on the spectrum.

Approval modes

Every harness has its own approval vocabulary (Claude Code's --permission-mode, Codex's --sandbox, Qwen's --approval-mode, Goose's GOOSE_MODE, …). --mode <m> is oneharness's single spectrum across all of them, from least to most autonomy:

• read-only — no mutations; the agent may read but not edit files or run commands. No plan workflow — it just does whatever read-only work the task allows. Mapped to each harness's strongest per-run no-mutation enforcement.
• plan — like read-only, but additionally engages the harness's native plan workflow (research the task, write a plan, don't act).
• default — the harness's ask flow, mapped to its cleanest non-interactive variant.
• edit — auto-approve edits, gate commands.
• auto — auto-approve what the harness deems safe.
• bypass — approve everything (the default).

The default when nothing is passed is default. Each mode is mapped to the harness's own mechanism; oneharness list shows the per-harness modes (each tagged clean or hangs), and the report echoes permission_mode. A harness that can't express a requested mode is a loud usage error before anything spawns (there's no command to build). A mode that may block on a prompt headlessly (a hangs tag) is warned about on stderr but still run, with the --timeout as the backstop (a real hang becomes a timeout result, never an infinite stall); --permit-prompts silences that warning once allow-rules are synced so the prompt never fires.

✓ supported & clean headless · ⚠ supported but may block on a prompt headlessly (warns + runs; --timeout backstops, --permit-prompts silences the warning) · — unsupported (refused). read-only is enforced where marked — ˢ Codex's read-only sandbox (OS-enforced), ᵈ deny rules (Claude's --disallowedTools Bash Edit Write NotebookEdit, Copilot's --deny-tool shell/write — deny beats allow) — and ᵖ behavioral where its only mechanism is the plan agent (OpenCode --agent plan, Qwen --approval-mode plan, so read-only and plan coincide there). Cursor's read-only is native --mode ask. Codex has no native plan mode in exec, so plan (ⁱ) is synthesized — the read-only sandbox enforces no-mutation and a plan instruction is prepended to the prompt, reproducing Codex's own interactive Plan mode (= read-only sandbox + a plan template). Goose has no plan workflow and its only no-mutation option (chat) disables reads too, so it offers neither plan nor read-only (a plan instruction alone can't help — it has no read-only enforcement to stop the agent acting); Crush's run can't gate, so it supports only default/bypass (¹ it auto-approves the whole session, so the two are identical). Only Cursor's default can still block on a prompt (no fail-fast deny) — every other harness's default is clean: it maps to that harness's cleanest non-interactive variant — Claude Code's dontAsk (deny-and-continue), Codex's read-only exec, Goose's fail-closed approve, Copilot's auto-deny, and OpenCode/Qwen auto-reject gated tools and continue rather than hang. Modes ride the argv except: Goose carries the whole spectrum in GOOSE_MODE, and OpenCode's edit (ᵉ) rides the inline-config env var OPENCODE_CONFIG_CONTENT (its per-tool permission map has no argv flag). Copilot's edit is a composed --allow-tool write --allow-tool read list (shell omitted → auto-denied); edit/auto for Cursor remain a permission config concern (oneharness sync), not --mode.

Relatedly, a harness can carry a small default environment so headless runs stay clean — e.g. oneharness sets QWEN_CODE_SUPPRESS_YOLO_WARNING=1 for Qwen Code so its --yolo/no-sandbox startup warning doesn't litter stderr. These defaults are per-harness data in the registry, and an explicit --env KEY=VALUE always overrides them.

Why it exists

nickderobertis/allowlister verifies its policy engine against every real agent CLI. Each check had its own bash run_agent() — Claude wants -p … --permission-mode bypassPermissions --output-format stream-json, OpenCode wants run --dangerously-skip-permissions --format json, Codex wants exec --dangerously-bypass-approvals-and-sandbox, and so on — plus its own timeout, output capture, and skip-if-missing logic.

oneharness collapses that to one call per check:

# before: ~40 lines of harness-specific bash per agent
# after:
result="$(oneharness run --harness claude-code \
  --prompt "$prompt" --cwd "$proj" --timeout 150 --compact)"
status="$(jq -r '.results[0].status' <<<"$result")"

The same uniform interface is the intended driver for a future cross-harness skill-testing framework: set up a sandbox, fire one prompt at every harness via oneharness run --all, and assert on the JSON.

Development

just bootstrap   # toolchain components + fetch (works from a clean clone)
just check       # full gate: fmt-check, clippy -D warnings, shellcheck, tests, build, smoke
just test        # tests only
just smoke       # hermetic end-to-end smoke of the built binary
just run -- list # run the CLI through cargo

The gate uses just (pinned in .tool-versions for asdf/mise users) and shellcheck for the shell scripts; CI installs both, so install shellcheck (apt-get/brew install shellcheck) to run the full gate locally.

Tests are hermetic: the subprocess path is exercised against a mock harness fixture (no network, no real CLI), and every adapter's command construction is pinned with --print-command assertions. just check also runs scripts/smoke.sh, an end-to-end smoke of the built binary, including a local-release installer check that drives scripts/install.sh without network. To exercise the real harnesses you have installed, run just smoke-live — it makes real model calls, skips any harness that isn't installed, and is intentionally never part of the gate or CI. See AGENTS.md and tests/AGENTS.md.

Released CLI and SDK consumers can use that same deterministic provider seam; see Testing patterns for the stable MOCK_* contract and CLI, Python, and Node examples.

Live end-to-end testing

just smoke-live is the quick "does any installed harness work" check. The per-harness suite is the allowlister-style counterpart: each scripts/e2e-<harness>.sh drives one real harness through oneharness with that provider's model/auth and asserts the JSON contract end to end — it plants a high-entropy marker, asks the harness (via oneharness run) to echo exactly that marker, and asserts status == ok, exit_code == 0, and that the marker surfaced. So a pass means the model genuinely ran, not just that the process exited. A missing CLI or missing auth is a skip, never a failure.

For the sync-capable harnesses (Claude Code, OpenCode, Qwen, Crush, Cursor) the live check also proves sync enforcement end to end: it syncs an allow + deny policy into the harness's own config file, then drives the real CLI with --no-bypass — the allowed touch must execute (the positive control) and the denied one must not. This is the only tier that can prove a synced file is honored, not merely written; it doubles as the drift alarm for the encoded config formats.

The live check also proves hook enforcement the same way: it syncs a [[hooks]] entry whose command is oneharness gate <id> into the harness's own config, then drives the real CLI under bypass (so the hook is the sole decider) through a marked command (the gate must block it) and an unmarked one (the gate must let it run). For Qwen the gate is synced with --global — Qwen only fires user-scoped hooks headlessly — which also exercises sync --global live. Two harnesses are excluded by design: Codex (codex exec loads hooks only when the invocation opts in with -c features.hooks=true --dangerously-bypass-hook-trust — probe-verified; the oh_mock_enforce codex phase passes those flags and is the live proof its hooks load, so the plain gate phase stays omitted) and Copilot (its project hooks sit behind a trusted-folder + prompt-mode setup that belongs in allowlister's adapter e2e); both keep their hermetic install coverage.

Alongside the per-harness checks there is a per-feature one for structured output: scripts/e2e-schema.sh (just live-schema) drives the real Claude Code CLI through oneharness run --schema and asserts a schema-valid round-trip — it plants a marker, asks for a conforming JSON object carrying it, and checks schema_valid == true with the marker in .structured. claude-code is chosen because it is the one with native delivery (--json-schema → structured_output); this is the live drift alarm for that flag and field, which the hermetic suite can only mock. (The portable prompt-based path is harness- agnostic; any per-harness script can add a live leg by calling oh_schema_enforce <id>.)

just live-claude     # one harness (installs the release binary, runs the live check)
just live-schema     # the structured-output feature (drives claude-code via --schema)
just live-all        # every harness + feature in sequence; skips pass, only real failures fail

Each harness needs its CLI installed and that provider's auth in the environment:

Per-harness CI workflows (.github/workflows/e2e-*.yml) run the same checks, each gated to the canonical repo and non-fork PRs so secrets are never exposed. The structured-output feature has its own (e2e-schema.yml), reusing the Claude auth secret. Locally a missing CLI or auth is a clean skip, but those workflows set OH_E2E_NO_SKIP=1, which turns any skip into a hard failure: in CI the harness is installed and auth verified up front, so a skip there can only mean detection/install/spawn silently broke (classically an unresolved npm .cmd shim on Windows) and the job would otherwise go green having run zero model calls. A genuine per-platform gap is therefore expressed as a matrix exclude or an if-guarded phase, never a runtime skip. Every workflow runs a fail-fast: false matrix across Linux, macOS, and Windows (ubuntu-latest, macos-latest, windows-latest), so the adapter argv, JSON contract, and sync/hook enforcement are proven on each platform independently — the scripts run under bash everywhere (Git Bash on Windows), and the two curl | bash installers (cursor, goose) use their PowerShell equivalents on Windows. A per-harness model can be overridden with <HARNESS>_E2E_MODEL (e.g. CLAUDE_E2E_MODEL, OPENCODE_E2E_MODEL).

The one per-platform gap is cursor hook enforcement on Windows: cursor-agent builds its hook command as a PowerShell wrapper but executes it through bash (Git Bash on PATH), so the wrapper dies on a syntax error and cursor blocks every command. This is an acknowledged cursor-agent bug with no shell flag, config field, or env lever ($SHELL and $COMSPEC are ignored; the only workaround is WSL), so that single phase is skipped on windows-latest. Cursor's echo and sync enforcement still run on Windows, and hook enforcement is still proven on Linux and macOS. Every other harness's hook enforcement runs on all three platforms.

Secrets

The auth above is managed with gh-secrets: gh-secrets.json is a committed manifest that pulls each secret from Bitwarden (secure notes) and pushes it to two destinations — a local .env (for just live-*) and the repo's GitHub Actions secrets (for the workflows). COPILOT_GITHUB_TOKEN is sourced from the GH_TOKEN vault item.

just secrets-sync    # gh-secrets manifest sync: Bitwarden -> .env + GitHub Actions

The manifest names which secrets go where; the values never touch the repo. .env and the sync-state file are gitignored.

Releasing

Releases are automated from conventional commits by release-plz — do not hand-bump the version or CHANGELOG.md. Land commits on main (feat → minor, fix/perf → patch, !/BREAKING → major; docs/test/chore/ci do not release), and release-plz opens a release vX.Y.Z PR that bumps Cargo.toml/Cargo.lock and writes the changelog. That PR auto-merges once the gate is green, then:

1. release-plz tags vX.Y.Z and cuts the GitHub Release;
2. that Release fires .github/workflows/release.yml, which re-runs the complete gate, publishes both Cargo crates idempotently in dependency order (oneharness-core first, then the oneharness binary that depends on it), attaches archived, sha256-checksummed binaries for Linux, macOS, and Windows, signs each archive with a keyless Sigstore build-provenance attestation and publishes its .sigstore.json bundle (see Supply-chain verification), builds per-platform PyPI wheels with maturin and publishes them to PyPI via Trusted Publishing, and builds the per-platform npm packages and publishes them to npm.

So each release ships five ways: PyPI (pip install oneharness-cli), npm (npm install -g oneharness-cli), crates.io (cargo install oneharness), the GitHub Release binaries, and cargo install --git. Only the binary gets a vX.Y.Z tag and GitHub Release; oneharness-core is published to crates.io and tagged in its own oneharness-core-v* namespace (no GitHub Release) so its version never collides with the binary's vX.Y.Z tags.

PyPI publishing is keyless Trusted Publishing (OIDC — no token secret), and stays dormant until the PYPI_PUBLISH repo variable is set to true; the wheels still build on every release so a packaging break surfaces early. Activating it requires the PyPI project oneharness-cli to register this repo's release.yml as a Trusted Publisher (no GitHub Actions environment).

npm packaging mirrors the wheels: maturin's bindings = "bin" wraps the prebuilt binary in per-platform wheels; scripts/npm-build.mjs wraps the same binary in per-platform npm packages (@oneharness/cli-<platform>-<arch>), pulled in as optional dependencies of the oneharness-cli launcher so npm installs only the one matching the host. npm publishing stays dormant until the NPM_PUBLISH repo variable is true (the packages still build on every release, so a break surfaces early) and authenticates with an npm token in the NPM_TOKEN secret (an automation or granular-access token with publish rights to oneharness-cli and the @oneharness scope).

Two repo secrets gate the automation (the workflow no-ops until both are set): RELEASE_PLZ_TOKEN (a PAT with contents: write + pull-requests: write) and CARGO_REGISTRY_TOKEN (a crates.io API token). Creating a GitHub Release by hand (gh release create vX.Y.Z) is the supported fallback if the automation is wedged; it triggers the same idempotent registry publication and artifact jobs.

License

MIT — see LICENSE.