onejudge

A Rust library that drives a simulated interaction and evaluation loop on top of oneharness: take a skill or agent, drive it through a multi-turn conversation with a simulated user, and score the resulting transcript with natural-language (judge) verdicts and tool-event queries.

It is the engine extracted from skilltest (see nickderobertis/skilltest#31). The layering:

oneharness  →  one harness invocation, one JSON report   (pure substrate)
onejudge    →  simulated interaction + judging loop        (this crate)
skilltest   →  test-framework surface: cases, evals-as-assertions, SDKs

Reach for onejudge when you want to "drive a harness through a simulated conversation and score the transcript" without skilltest's YAML / case framing.

Install

cargo add onejudge

Minimum supported Rust version: 1.82.

The onejudge CLI

The same engine that tests a skill can drive real work. onejudge run points a harness at a task and lets an LLM-driven simulated user supervise it — pushing back, asking for verification, re-prompting — until a done_when condition holds or max_turns is hit. Configured by YAML; the library API is unchanged and CLI deps (clap, a YAML parser) are opt-in behind the non-default cli feature.

Spin up a run in three steps:

cargo install onejudge --features cli   # or: install.sh (prebuilt archives)
onejudge init                           # scaffold onejudge.yaml + oneharness configs
onejudge run                            # reads ./onejudge.yaml, drives to completion

init shells out to oneharness init (needs oneharness 0.11.0+) to scaffold oneharness.toml (the agent side) and oneharness.judge.toml (the judge side), then writes a fully-commented loop-only onejudge.yaml. The fields that make a run yours are task (what to do), the system framing — a skill (a SKILL.md directory) and/or a system_prompt, both optional — and the user block (persona / done_when / max_turns — omit it for a single-turn run). After each nonterminal agent turn, one unified supervisor call either completes with a reason or supplies the exact next user message. It sees compact normalized tool summaries by default, never raw dumps; when needed it may inspect the agent-side recording with oneharness history show <session>-skill --project <worktree> --format text. Agent and judge harnesses run in that worktree, but only agent runs are automatically history-recorded. Harness and model selection lives in those oneharness.toml files, not onejudge.yaml. onejudge schema prints the annotated config, the single source of truth for every field.

Flags override the file (flags > file > defaults), so one config serves many tasks: onejudge run --task - < task.txt, --max-turns 8, --format json -o result.json.

Config

A run is a YAML file carrying only the loop's own concerns. The fields that make it yours — task, the system framing (skill and/or system_prompt), and the user block. Everything else has a default; omit user for a single-turn run. A minimal config:

system_prompt: You are a senior engineer. Complete the task and keep tests green.
# skill: ./skills/my-skill    # optional: a SKILL.md dir; its body is appended

task: Add a --version flag to the CLI.

user:                         # the simulated supervisor that drives the loop
  persona: A demanding tech lead. Do not accept "done" until you have verified it.
  done_when: the task is complete and all tests pass
  max_turns: 8

evals:                        # optional: score the finished transcript
  - criterion: the change is well-scoped and readable
    kind: numeric
    scale: [1, 5]
assessment: Identify useful follow-up work left out of scope.

The harness and model come from oneharness's own config (oneharness.toml for the agent, oneharness.judge.toml for the judge side) — onejudge init scaffolds them. More keys — provider (oneharness / command / split, with the oneharness judge_config path), session, boolean evals. onejudge init writes a fully-commented starter and onejudge schema prints the annotated field reference (the single source of truth); it is validated strictly (deny_unknown_fields) so a typo is a loud error.

Human output is the conversation + tool actions + completion status + eval verdicts; --format json emits the versioned Report. The exit code is 0 only when the task completed and every boolean eval passed, 1 if it hit max_turns or a boolean eval failed, 2 on a bad config. Add --stream to publish tool events on stdout as they occur, ahead of that same report (docs/streaming.md). Full docs: docs/cli.md.

Concepts

• Provider is the boundary — onejudge never talks to a model directly. Every model call goes through oneharness, and harness/model selection lives in oneharness's config files, not onejudge.
  • OneharnessProvider (default) shells out to the oneharness CLI (v0.11.0+): the agent side uses the discovered oneharness.toml, and the judge side uses a separate --config file (default oneharness.judge.toml). With stream: true it consumes the agent turn as it happens — NDJSON tool events, then a terminal report — instead of only when it ends (docs/streaming.md).
  • CommandProvider speaks a small JSON-lines protocol, for a custom backend or a deterministic test double.
  • SplitProvider composes two providers — one that runs the skill, one that judges and role-plays the user (e.g. run the skill on one harness, judge on another).
• Engine runs a Conversation (a Skill, an initial input, and an optional SimulatedUser) into a Transcript, bounded by max_turns / done_when / the skill declaring itself done.
• Transcript carries each turn plus the normalized **ToolEvent**s the skill took, so the judge — and a ToolQuery — can reason over what the skill did, not just what it said.
• Report is onejudge's own versioned contract (SCHEMA_VERSION): a serializable bundle of the transcript, verdicts, optional free-text assessment, and usage that higher-level frameworks compose over and re-export. See docs/contract.md.
• SpawnHook is the seam for an in-process embedder: onejudge offers every process it is about to create so the embedder can place it in a group it owns — a POSIX process group, a Windows job object — and still reap the whole harness tree when a run is cancelled. The embedder owns the group; onejudge reports what a hook claimed on Report::processes. Install it on each provider, or on a Plan (Plan::with_spawn_hook) when you drive the CLI's run driver — the plan reaches both sides of a two-party worker + judge run. See docs/spawn-hook.md.

Two things it improves over the in-skilltest engine:

1. The judge sees tool events. Verdicts render the transcript with a compact, token-budget-aware summary of each turn's tool calls, so a criterion like "the change was committed" can be decided from the git commit the skill actually ran — not only from what it said. Transcript also exposes a ToolQuery primitive for events-backed assertions with no judge call.
2. One caller-owned session name. The engine always threads a single --session <name> across turns instead of extracting and re-passing a native id; if a harness cannot bind a session, the provider gracefully retries the call without it, re-prompting the inlined transcript.

Example

use onejudge::{Conversation, Engine, OneharnessProvider, Settings, SimulatedUser, Skill};

let provider = OneharnessProvider::new();
// Harness/model selection lives in oneharness's config files, not here; Settings
// carries only the loop's own concerns (turn cap, session name).
let settings = Settings::new();
let engine = Engine::new(&provider, settings);

let skill = Skill::new("greeter", "./skills/greeter", "Greet the user warmly.");
let user = SimulatedUser::new("A curious first-time visitor.")
    .done_when("the assistant has answered the visitor's question")
    .max_turns(6);

let outcome = engine.run(&Conversation::multi_turn(skill, "hi", user))?;

let verdict = engine.judge_boolean("the reply was welcoming", &outcome.transcript)?;
println!("{:?}: {}", verdict.value, verdict.reason);
# Ok::<(), onejudge::Error>(())

Drive a deterministic backend instead of a live harness by pointing a CommandProvider at any command that speaks the protocol.

Development

The command surface is a just recipe set; just --list is the index.

just bootstrap   # clean-clone setup: toolchain + cargo tools + fetch
just check       # the full gate: format, lint, doc, coverage-enforced tests, audit
just test        # fast unit + integration + e2e

The gate is deterministic and offline — the model is faked by real subprocess test doubles, never mocked. The one path that needs a real external service is proven in an opt-in tier, kept out of check:

• just test-live — the OneharnessProvider path against a real harness (see docs/live-tier.md).

See AGENTS.md for the durable contributor guide.

License

MIT.