HX

A terminal coding agent. Python core, Textual TUI, OpenRouter for models.

HX runs in your project directory, reads and edits your code, runs commands in a sandboxed shell, and shows you what every turn costs.

Install

curl -fsSL https://raw.githubusercontent.com/aletisunil/hx/main/install.sh | sh

The script bootstraps uv if you don't have it, then installs HX as an isolated tool with a pinned Python. Re-running it upgrades in place.

If you'd rather not pipe a script into a shell:

uv tool install hx-cli      # or: pipx install hx-cli

hx-cli on PyPI, hx in your terminal

The package is published as hx-cli. The command it installs is hx, and that is what you type — the longer name never appears again after install.

The plain name hx on PyPI is registered by someone else and has no releases, so uv tool install hx fails with "no versions of hx". Install hx-cli.

If the command isn't found after installing, add uv's bin directory to your PATH:

export PATH="$(uv tool dir --bin):$PATH"

Requires Python 3.11+. macOS and Linux. Update with hx upgrade.

Credentials

HX reaches models over two routes. Which one serves a turn is decided by the model id alone, so "what paid for that" is always answerable by reading it:

Credentials live in ~/.hx/auth.json, mode 0600, one entry per route.

OpenRouter. On first run HX asks for a key. Get one at https://openrouter.ai/keys. Change it later with /configure in the TUI, or:

hx auth          # which routes have a credential, and where from
hx auth set      # paste a new OpenRouter key (hidden input)
hx auth clear    # remove the saved key

Resolution order is HX_OPENROUTER_API_KEY, then OPENROUTER_API_KEY, then the saved file. The environment wins, and both /configure and hx auth say so — otherwise saving a key while a variable is set looks like a no-op.

ChatGPT Plus/Pro. Sign in over OAuth and run Codex models against your subscription instead of paying per token (OpenAI endorses this for OSS harnesses):

hx auth login openai-codex      # or /login inside the TUI
hx --model openai-codex/gpt-5.3-codex
hx auth logout openai-codex

The browser opens to auth.openai.com and redirects to a loopback listener on port 1455. Over SSH that redirect cannot reach your machine, so paste the final URL into the prompt instead — or set HX_LOGIN_DEVICE_CODE=1 for the device-code flow. Access tokens are refreshed automatically; /model only offers routes you are actually signed in to.

Web search. Optional, and the one credential that is not a model route. It powers two tools:

Tavily bills per search rather than per token, so it works identically on both routes — including the ChatGPT subscription one, where a provider-native search tool is not on offer.

Get a key at https://app.tavily.com — the free tier is 1000 searches a month and takes no card — then save it:

hx auth set tavily      # paste the key, hidden input, saved to ~/.hx/auth.json
hx auth                 # confirm: `tavily  key tvly-a…f3d9  …/auth.json`
hx auth clear tavily    # remove it again

Or keep it in the environment instead, which wins only when nothing is saved:

export TAVILY_API_KEY=tvly-...     # HX_TAVILY_API_KEY is read first

Without a key, HX works exactly as before — WebSearch and WebFetch are simply not registered, and hx auth says so:

tavily         not configured      WebSearch and WebFetch are off

There is no nag, no failing tool and no startup warning. Leaving them out is deliberate: an advertised tool that always fails still costs cached-prefix tokens every turn, and a model told "no key" just calls it again. Adding a key turns both on from the next hx; no other setting changes.

Credits are reported per call and per session at the foot of every result — [Tavily: 1 credit, 4 this session]. A search costs 1 credit, search_depth: "advanced" costs 2, and fetching costs 1 per 5 URLs. Tavily's docs describe a usage field for this, but the live API does not send one, so HX prices every call from those published rates and marks the figure ~1 rather than passing an estimate off as measured. Check app.tavily.com for the authoritative number.

Credits are not in the status bar's cost, which prices tokens: /cost under-reports a session that searched.

Behind a TLS-inspecting proxy. HX verifies certificates against the operating system's trust store, so a corporate CA the machine already trusts needs no setup. When the CA lives only in a file, name it:

export HX_CA_BUNDLE=/path/to/corp-ca.pem   # SSL_CERT_FILE and REQUESTS_CA_BUNDLE also work

HX_SSL_NO_VERIFY=1 turns verification off entirely; it prints a warning at startup every session, because it is not a fix. A TLS failure is reported in one line - /model and the startup notice both name the certificate problem rather than reporting an empty catalogue with no cause.

There is no Claude Pro/Max route. Anthropic rejects OAuth tokens unless the request impersonates Claude Code, which HX will not do. Claude models stay available through OpenRouter.

Running it

hx                          # interactive TUI in the current directory
hx -p "explain this repo"   # headless: streams to stdout, tool activity to stderr
hx resume                   # resume the last session here
hx resume <session-id>      # resume a specific one
hx prompt                   # print the system prompt this directory would use
hx --model openai/gpt-5     # override the model for one run
hx --mode plan              # start read-only
hx --cwd ../other-project   # run against a different directory
hx --no-sandbox             # disable OS sandboxing (rules still apply)
hx --system-prompt @p.md    # replace the system prompt for one run
hx --append-system-prompt "Always run the tests"   # add to it; repeatable
hx mcp list|add|remove      # manage MCP servers
hx auth [set|clear]         # manage the OpenRouter key
hx auth login [provider]    # sign in (openrouter, openai-codex)
hx auth logout <provider>   # forget a stored credential
hx docs [section]           # the manual; no argument lists its sections
hx changelog [version]      # what shipped in each version
hx upgrade                  # update to the latest release

Print mode is the scriptable one: stdout carries only the assistant's text, so it pipes cleanly.

The docs ship with the binary

README.md and CHANGELOG.md are included in the wheel, so the installed tool can print the manual and the release record for the version you are actually running:

hx docs                     # the section titles
hx docs credentials         # one section, printed whole
hx docs --all               # the entire manual
hx changelog                # every release, newest first
hx changelog 0.1.5          # one release
hx changelog unreleased     # what has landed since the last tag

The system prompt points a session at these two commands, so "how do I set the API key?" or "what changed in the last release?" is answered from the shipped documentation rather than from the model's recollection of some other version. The call still goes through the permission engine like any other command; allowlist it if you would rather not be asked:

{ "permissions": { "allow": ["Bash(hx docs:*)", "Bash(hx changelog:*)"] } }

CHANGELOG.md lists what each version added, and a release cannot skip it: tests/test_docs.py fails when __version__ has no section (see Releasing).

Keys

Every key below is defined once, in src/hx/keys.py. /help, the hints line under the prompt and this table all read from that registry, and any of them can be rebound (see Keybindings).

! still goes through the permission engine and the sandbox — it skips the model, not the safety layers.

The prompt is a readline-style editor: ctrl+a/ctrl+e for line start and end, ctrl+b/ctrl+f by character, alt+b/alt+f by word, ctrl+w and alt+d to kill a word, ctrl+u and ctrl+k to kill to the start or end of a line, then ctrl+y to yank it back and alt+y to walk further down the kill ring. ctrl+z undoes, ctrl+shift+z redoes.

Typing / or @ opens a completion list above the prompt; tab cycles it, enter accepts, esc dismisses.

Copying

ctrl+x copies the message the cursor is on, and /copy copies the last reply. HX writes through the platform's own clipboard tool first (pbcopy, wl-copy, xclip, xsel) and falls back to OSC 52, which is also always sent over SSH so the text lands on the machine you are actually sitting at.

Because ctrl+c clears the prompt, drag-selecting in the transcript is your terminal's own selection rather than the TUI's — use your terminal's copy key for that. HX does not emit OSC 133 prompt markers yet, so shell-integration features that jump between prompts will not see HX's messages.

Keybindings

Any key can be rebound in ~/.hx/keybindings.json, keyed by the action ids in src/hx/keys.py:

{
  "app.tools.expand": "ctrl+r",
  "app.message.copy": ["ctrl+x", "alt+c"]
}

Conflicts and unknown action names are reported as a notice at startup rather than being silently resolved.

Commands

The status bar

Two lines: working directory and permission mode above; token counts, cache read/write with hit rate, spend, last-turn latency, context gauge and model below. The cache and cost fields are the point of it — a hit rate that collapses after an edit is the visible symptom of a broken prefix.

Configuration

Settings are JSON, merged lowest to highest:

defaults  <  ~/.hx/settings.json  <  ./.hx/settings.json  <  HX_* env  <  CLI flags

Permission rule lists and prompt.append are unioned across layers, so a project can add a deny rule — or a line to the system prompt — without discarding yours. Everything else is replaced.

{
  "theme": "dark",                    // dark | light | ansi, or a file in ~/.hx/themes
  "quiet_startup": false,             // skip the startup header
  "telemetry": false,

  "models": {
    "model": "anthropic/claude-sonnet-4.5",
    "subagent_model": null,           // defaults to "model"
    "title_model": null,              // model that names sessions; defaults to "model"
    "max_tokens": 8192,
    "temperature": null
  },

  "permissions": {
    "mode": "default",                // plan | default | acceptEdits | bypass
    "allow": ["Bash(git status:*)"],
    "ask":   [],
    "deny":  ["Read(**/.env)"],
    "sandbox": true,
    "allow_network": false            // outbound network for sandboxed commands
  },

  "context": {
    "compact_at": 0.80,               // fraction of the window that triggers compaction
    "keep_recent_turns": 6,           // turns kept verbatim across a compaction
    "tool_output_char_cap": 25000,
    "tool_output_line_cap": 2000,
    "git_notices": true               // late-inject the branch and outside file changes
  },

  "bash": {
    "timeout_seconds": 120,
    "max_timeout_seconds": 600,       // ceiling; caps what the model may ask for
    "shell": null                     // defaults to $SHELL
  },

  "prompt": {
    "system": null,                   // replaces the built-in system prompt
    "append": []                      // added after it; unioned across layers
  }
}

Environment overrides: HX_MODEL, HX_SUBAGENT_MODEL, HX_MAX_TOKENS, HX_PERMISSION_MODE, HX_SANDBOX, HX_COMPACT_AT, HX_GIT_NOTICES, HX_THEME, HX_QUIET_STARTUP. Also HX_HOME to relocate user state, HX_TAVILY_API_KEY for web search, and HX_CA_BUNDLE / HX_SSL_NO_VERIFY for TLS (see Credentials).

The system prompt

hx prompt prints the prompt this directory resolves to, with its source on stderr so stdout stays pipeable. /prompt shows the same thing inside a running session.

The built-in prompt is SYSTEM_PROMPT in src/hx/core/context.py. It is replaced by the first of these that exists:

Appended text is added after whichever prompt won, in this order: ~/.hx/system-prompt-append.md, ./.hx/system-prompt-append.md, then each --append-system-prompt value (and prompt.append) in the order given. Nothing appended is ever discarded by a later layer.

This is a different lever from HX.md: HX.md describes the project and is injected as its own context section, while these files change the agent's instructions. Both are read once at startup and frozen, because they sit above the first cache breakpoint — an override edited mid-session applies on the next run, and /prompt says so when it spots one.

Sessions are named

After the first exchange, HX asks the model for a short name for the session and writes it to ~/.hx/sessions/<id>/meta.json, so /resume lists work rather than timestamps. It is one small call — cap 32 output tokens, models.title_model if you want a cheaper model for it — and it is counted in /cost like any other. If the call fails the session is still named, from your first message. /title <text> renames it.

Themes

A theme is a JSON file: a vars block of raw colours, and a colors block mapping semantic roles onto them. Drop one in ~/.hx/themes/mine.json and /theme mine picks it up — no restart, no code change.

{
  "name": "mine",
  "dark": true,
  "vars": { "green": "#b5bd68", "gray": "#808080" },
  "colors": {
    "background": "#18181e",
    "success": "green",           // a vars key, or a literal
    "muted": "gray"
    // ...every role; a missing one is an error, not a silent black
  }
}

The role names are pi's, so pi theme files load here unchanged. src/hx/tui/themes/dark.json is the reference.

Where things live

HX.md is the place for things a newcomer would get wrong: how to run the tests, conventions, what not to touch. /init writes a first draft. It is loaded once per session and frozen, so it costs one prefix, not one per turn.

Safety

Two independent layers guard every tool call, and both must pass.

Permission rules are Tool(specifier) strings — Bash(git commit:*), Edit(src/**), Read(**/.ssh/**). Deny beats ask beats allow, and a deny holds even in bypass mode. Shell commands are decomposed into their real segments first, so an allow rule for git status does not carry && rm -rf / along with it; a command that cannot be decomposed with confidence prompts rather than passing.

An OS sandbox wraps command execution: Seatbelt on macOS, bubblewrap on Linux. The filesystem is readable, writes are confined to the project and the temp dir, credential paths (~/.ssh, ~/.aws, and HX's own auth.json) are unreadable, and outbound network is off. If neither backend is present the status bar says no-sandbox rather than implying protection that is not there.

Modes cycle with shift+tab: plan (read-only — mutating tools are not even offered to the model), default, acceptEdits, bypass.

Context engineering

Long sessions are the normal case, so the harness is built around keeping the provider's KV cache warm and the window from filling up.

A stable prefix. System prompt, tool schemas and project context are assembled in a fixed order and never mutated mid-session. Cache breakpoints sit at the end of that static block and at a rolling point before the recent turns, which only advances once enough tokens have accumulated behind it.

Late injection carries everything that changes per turn - the todo list, files that changed on disk since HX read them, the git branch and any file the working tree gained or lost outside the session - on the tail of the newest user message rather than in the prefix. Stale copies are stripped and regenerated each turn, so six todo updates leave one copy in context, not six.

Working-tree notices come from one git status --porcelain=v2 --branch -z per provider call, run without a shell and bounded by a two-second timeout. It runs in a worker thread, as every injector does: a turn makes one of these per tool round-trip, and on the event loop a slow repository would freeze the UI each time. A file HX has already read is left to the stale-file notice, so no path is reported twice; outside a repository the watcher disables itself for the session. Turn it off with "context": {"git_notices": false}.

Rewind (/rewind) picks an earlier prompt and takes the session back to it: the transcript is cut there and every file HX wrote after it is restored from a content-addressed snapshot taken before each Write and Edit. A file someone else changed in the meantime is reported and left alone, and shell commands are outside the scheme - the report says so rather than implying the tree was fully returned. The prompt that was cut comes back in the input box, because a rewind is usually the first half of saying it differently. Nothing is deleted from the session file: the rewind is one more append, and replaying the file reproduces the state - which is also how an undone compaction gives its messages back. The cost ledger is deliberately not rewound; those tokens were spent.

Compaction fires at 80% of the window, or on /compact [focus]. Older turns are replaced by a structured summary; the recent turns and the todo list survive verbatim, and the boundary snaps to a turn edge so a tool call is never severed from its results. Superseded messages are flagged, not deleted, so resume replays exactly what happened.

Output capping keeps the head and tail of a large tool result, spills the rest to the session directory, and hands the model that path to grep.

Extending it

Skills are directories containing SKILL.md with YAML frontmatter:

---
name: deploy
description: Tag, build and ship a release
allowed-tools: Read, Bash          # optional; narrows the toolset while active
---

1. Run the tests.
2. Tag the commit.

Drop them in .hx/skills/<name>/ or ~/.hx/skills/<name>/; a project skill shadows a user one of the same name. Only the name and description enter the context — the body loads when the model calls Skill(name), so a hundred installed skills cost a hundred lines, not a hundred documents.

Subagents run in their own context with their own transcript, tool allowlist and model. Only the final report returns to the parent, so a long search costs the caller one paragraph instead of every intermediate tool result. explore, plan and general ship built in; add your own as .hx/agents/<name>.md:

---
name: reviewer
description: Reviews a diff against the project's conventions
tools: Read, Grep
model: openai/gpt-5                 # optional
---

You review code. Be specific and cite file:line.

A subagent never gets the Task tool, so recursion is impossible by construction.

MCP servers go in .hx/mcp.json:

{
  "mcpServers": {
    "local": { "command": "python", "args": ["server.py"] },
    "remote": { "url": "https://example.com/mcp" }
  }
}

Or hx mcp add local python server.py. Tools arrive namespaced mcp__<server>__<tool> in a deterministic order. Servers connect concurrently with a per-server timeout; one that is broken or slow logs a warning and is dropped rather than taking the session with it.

Development

git clone https://github.com/aletisunil/hx && cd hx
uv sync --extra dev

uv run pytest                # the suite; live tests are deselected
uv run ruff check . && uv run ruff format --check .
uv run mypy                  # strict
uv run hx                    # run from the checkout

Tests marked live hit a real API. The OpenRouter ones cost money:

OPENROUTER_API_KEY=... uv run pytest -m live tests/test_live.py

# the Codex route draws on your ChatGPT subscription instead
hx auth login openai-codex
uv run pytest -m live tests/test_live_codex.py

They are the only place the wire formats, streaming, tool use and a genuine cache hit are proven against a real service; everything else runs against the scripted provider.

Tests marked sandbox exercise the real OS sandbox and are skipped where no backend exists.

The TUI has SVG layout snapshots. Read the diff before accepting a change to them - they exist to catch a frame that quietly lost a row:

uv run pytest tests/tui/test_snapshots.py --snapshot-update

Layout

src/hx/
  cli.py config.py paths.py frontmatter.py git.py
  core/         loop, context assembly, compaction, late injection, sessions, usage
  auth/         credential store, OAuth flows, per-route resolution
  providers/    OpenRouter, Codex, the model catalogue, a scripted provider for tests
  tools/        Bash, Read, Write, Edit, Glob, Grep, TodoWrite, Task, WebSearch,
                WebFetch, output capping
  permissions/  rule engine, shell decomposition, Seatbelt/bubblewrap
  skills/ agents/ mcp/
  keys.py       keybinding registry: ids, defaults, descriptions, user overrides
  tui/          Textual app, commands, per-tool renderers, widgets
    themes/     the shipped palettes, as JSON

The core is headless and emits events; the TUI and print mode are both just consumers. Nothing under core/, tools/, providers/ or permissions/ imports tui/.

Releasing

For maintainers. HX publishes to PyPI as hx-cli from CI, on a tag.

One-time setup:

1. Push the repo to github.com/<owner>/hx and update the URLs in pyproject.toml.
2. Create a GitHub environment named pypi.
3. On PyPI, add a trusted publisher for the project: owner, repo hx, workflow ci.yml, environment pypi. No API token is stored anywhere.

Each release:

# 1. move CHANGELOG.md's [Unreleased] entries under the new version, dated,
#    and leave a fresh empty [Unreleased] behind
# 2. bump __version__ in src/hx/__init__.py
# 3. uv run pytest tests/test_docs.py     # the version must have a section
git commit -am "release: X.Y.Z"
git tag v$(uv run hx --version | cut -d' ' -f2) && git push origin main --tags

Step 1 is not optional and is not a convention: tests/test_docs.py fails the whole suite when __version__ has no CHANGELOG.md section, so a release that skips it cannot get past CI to the publish job.

The publish job runs only on refs/tags/v* and only after lint, the test matrix, and an install-script run in a clean Debian container have passed. It builds with uv build and uploads via OIDC.

To check a build before tagging:

uv build && ls dist/

Status

Feature complete against the original plan: the agent loop, OpenRouter streaming with prefix caching and accurate cost accounting, session persistence and resume, the tool suite, the permission engine and OS sandbox, late injection, compaction, output capping, skills, subagents, MCP, and the TUI.

Two routes to a model: an OpenRouter API key, or a ChatGPT Plus/Pro subscription over OAuth. The model id decides which.

Published to PyPI as hx-cli, released from CI on a tag.

The one thing still unproven in CI is a live call on either route: the live tests exist and cover both wire formats, tool use, reasoning replay and a genuine cache hit, but they need a credential and are deselected by default.