camas 0.1.22

Parallel and sequential task-tree runner with matrix expansion and effects plugins.

Camas

A task runner with parallel execution, matrix expansion, MCP, and pluggable output effects.

• For developers: live tree view that updates in place as tasks stream
• For CI/CD: one definition drives both local runs and CI
• For LLMs: a closed edit→validate→run loop over structured MCP

 

Example

from camas import Parallel, Sequential

ci = Sequential(
  Parallel(
    "ruff format . --check",
    "npx prettier ."
  ),
  Parallel(
    "ruff check .",
    "mypy .",
    "npx eslint src/",
    "pytest",
    "npx tsc --noEmit"
  ),
)

The animated tree above is from a live test fixture — see the walkthrough.

Install

[!TIP] Add extras with PEP 621, e.g. camas[github_checks]

pipx:

pipx install camas

uv:

uv tool install camas

Nix:

nix run github:JPHutchins/camas                     # default
nix run github:JPHutchins/camas#with-github-checks  # adds httpx for the GitHubChecks effect
nix run github:JPHutchins/camas#with-check          # adds ty for `camas --check`
nix run github:JPHutchins/camas#all                 # both extras

Then scaffold a starter tasks.py in your project root:

camas --init

The starter demonstrates leaf tasks, Sequential/Parallel composition, a matrix, a Config default task, and the optional PEP 723 standalone block — cross-platform placeholder commands ready to be swapped for your real ones.

--init also creates a gitignored .camas/ directory beside tasks.py. Camas writes run logs and a per-leaf timing cache there, so camas --list can annotate tasks with an estimated duration; delete the directory to opt out. Rename or relocate it with Config(camas_dir=...).

Why camas?

camas is not a build system. camas is for the specific job of running structured trees of shell commands.

	Python task runners†	just	Task	Mage	camas
Project scope	Python projects only	Any	Any	Go projects only	Any
Definition language	pyproject.toml TOML or @task decorator	justfile DSL	YAML	Go	Python (typed AST)
Inline anonymous parallel groups	No	No	No (must be a named task)	No	Yes
Parallel execution	poe yes; Invoke / taskipy no	[parallel] attr	deps: (parallel)	mg.Deps(...)	Parallel(...)
Matrix expansion	No	No	for: + parallel:true	Go loops	matrix=
CLI matrix override (e.g. --PY 3.13)	No	No	No	No	Yes
Live tree output	No	No	prefixed / group modes	No	Termtree (default)
Pluggable output renderers	No	No	3 built-ins	No	--effects
Type checking on task definitions	Partial	No	Editor schema	Yes (Go)	mypy / pyright
First release / status	2013–2020, stable	2016, stable	2017, stable	2017, stable	2026, alpha
Ecosystem	Moderate (Python)	Large	Large	Moderate	None yet

†poethepoet, Invoke, taskipy — pyproject.toml-bound runners that assume a Python project.

If you need...	Reach for
Reproducible, hermetic builds	Nix
Incremental file-based builds (skip when inputs unchanged)	Task
Simple project command menu	just
Parameterized tasks (--env=staging, prompts, vars)	Task
Go project, build logic in Go	Mage
Inline parallel/sequential trees with a live view	camas
Pluggable output effects (live tree locally, summary in CI)	camas
Matrix runs across versions/platforms, overridable from the CLI	camas

CI integration

The renderer is swappable, not the tree. Run the same tasks.py locally with the live Termtree and in CI with Status — one flag changes the output, the pipeline definition is unchanged.

On GitHub Actions, no flag is needed. Camas detects GITHUB_ACTIONS=true and defaults --effects to (Status(output_mode="github"),) — collapsed workflow groups, ISO timestamps with millisecond precision, ANSI colors preserved.

Under an AI coding agent, no flag is needed either. Camas detects agents (the CLAUDECODE env var, or set CAMAS_AGENT=1) and defaults to the line-oriented Status renderer instead of the live Termtree, whose cursor-redraw frames bloat captured output. Prefer the camas mcp server over the CLI from an agent; a Config default_effects always overrides the detection.

- run: uv run camas check

On other CI providers (or to opt into a specific mode), spell it out:

- run: uv run camas check --effects='(Status(output_mode="errors"),)'

See the OutputMode literal and block_for doctests for the per-mode behavior. The status-modes-demo job renders one CI run per mode for visual comparison.

For per-leaf visibility in the PR Checks panel, add the GitHubChecks effect alongside Status (opt-in extra: camas[github_checks]). Each leaf task becomes its own check run, so reviewers see lint / mypy / pytest pass-or-fail individually instead of one monolithic log.

- run: |
    uv run --extra github_checks camas matrix --effects='(
      Status(output_mode="github"),
      GitHubChecks(sha="${{ github.event.pull_request.head.sha || github.sha }}"),
    )'
  env:
    GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}

The job needs permissions: checks: write. Defaults read GITHUB_TOKEN, GITHUB_REPOSITORY, and GITHUB_SHA from the Actions env. The pull_request.head.sha override attaches checks to the PR head rather than the synthetic merge commit. See the github-checks-demo job for a working example.

When to use it. Two things it gives you:

• SSOT between local dev and CI. Your camas matrix terminal view and the PR Checks panel show the same per-leaf shape (lint [PY=3.10], mypy [PY=3.14], …). The matrix definition lives once in tasks.py; CI doesn't re-encode it in YAML.
• One runner instead of N. Camas-side parallel matrix on a single runner produces the same per-(cell, leaf) granularity that a GHA matrix gets across N runners — at 1/N the minutes and camas gives you efficient task parallelization pushing that runner to 100% utilization as much as possible. Worth it on paid-runner budgets.

The downside is that fork PRs get a read-only GITHUB_TOKEN from GitHub and can't write checks — usually a non-issue for Enterprise teams where contributors have push to the org repo.

When not to bother. OSS gets free runners — just GHA-matrix them in parallel for faster wall-clock time. The cost is small: you give up either SSOT (matrix definition moves into YAML) or per-leaf-UI granularity (one PR check entry per runner instead of per leaf) — pick one. What is a deal-breaker for OSS is fork PRs: external-contributor PRs return 403 from the Checks API, so per-leaf entries silently don't appear. The github-checks-demo job is marked continue-on-error so it doesn't block CI when that happens, but GitHubChecks isn't a workable OSS solution.

Machine-readable report (Ctrf)

For a CI artifact or input to an AI code review, add the Ctrf effect (opt-in extra: camas[ctrf]). It writes the run as a CTRF JSON report — each leaf a test with status, duration, output, command, and exit code. path= writes a file; the default is stdout.

uv run --extra ctrf camas check --effects='(Status(output_mode="errors"), Ctrf(path="ctrf-report.json"))'

Config

Bind a Config in tasks.py and bare camas (no arguments) runs its default_task:

from camas import Config, Sequential, Task

lint = Task("ruff check .")
test = Task("pytest")
ci = Sequential(lint, test, name="ci")

_ = Config(default_task=ci)

Now camas runs ci, camas --dry-run previews it, and the default's matrix axes stay overridable (camas --PY 3.13). With no Config (or no default_task), bare camas prints the full help — task listing, effects, hints — and exits non-zero.

github_task is the CI counterpart, falling back to default_task when unset; it runs under GITHUB_ACTIONS=true. Paired with the automatic Status effect, one bare camas does the right thing in both places:

_ = Config(
  default_task=ci,                            # bare `camas` locally
  github_task=Sequential(ci, "pytest --cov"), # bare `camas` under GitHub Actions
)

Config is discovered by type, so the binding's name never matters — _ by convention. Defining two is an error.

Because github_task reproduces CI, running it before a push catches a CI failure locally. If it is a named task, a one-line git hook guards every push:

echo 'exec camas check' > .git/hooks/pre-push && chmod +x .git/hooks/pre-push

git push --no-verify still bypasses it deliberately. An LLM agent needs no hook — camas_list reports the CI task as github_default, so it runs camas_run with that name before pushing.

Time budget (--under)

Once camas has timed a task's leaves (the .camas/ cache, surfaced by camas --list), camas --under=<duration> runs only the leaves whose estimate fits a wall-clock budget — the fast inner-loop subset, picked for you instead of by hand.

camas --under=1s            # budget the Config default task
camas --under=500ms check   # budget a named task or expression

It runs the mutating leaves first, in sequence, then the read-only rest in parallel — so formatters never race a checker over the same files. Mark a leaf that writes the workspace with mutates=True:

fix = Task("ruff check --fix .", mutates=True)
fmt = Task("ruff format .", mutates=True)
lint = Task("ruff check .")

$ camas --under=1s --dry-run
Time budget 1.00s — running 6 leaf(s) (0 unmeasured), excluded 2 over budget.
  over budget: pyright ~4.57s, coverage ~20.98s
fix → fmt → (mypy | ty | zuban | pyrefly)

Durations are 1s, 500ms, 2m, 1h, or a bare number of seconds. Only leaves measured to exceed the budget are excluded; a leaf with no recorded timing yet runs anyway (and is thereby measured) — skipping it would keep it forever untimed. The budget is per-leaf: a measured leaf runs when its own estimate fits, so the parallel group's wall-clock stays near the budget.

For agents, camas_run exposes the same budget as its under argument (omit task to budget the project default), and the response's budget field reports what was selected and excluded — a tight, time-boxed validate loop over structured MCP.

Path scoping (--paths)

camas <task> --paths <path>… scopes a run to changed paths instead of the whole tree. A command opts in by writing the {paths} placeholder and declaring its scope with paths= — a directory prefix, or a (changed) -> paths callable. A Sequential/Parallel may carry paths= too: it's the default scope for descendant {paths} commands that set none (the same way env/cwd propagate into a group's leaves):

py = Task("ruff format {paths}", mutates=True, paths="src")
web = Task("prettier --write {paths}", mutates=True, paths="web")
# the group's paths="." is the default for both children (neither sets its own):
autofix = Parallel(Task("ruff format {paths}"), Task("ruff check --fix {paths}"), paths=".")
_ = Config(agent=Claude(fix=Sequential(py, web, autofix)))

--paths works on any task — camas check --paths src/a.py. Without it, every {paths} resolves to its full-run default (ruff format src); with it, each {paths} command runs only over the changed files it covers, and one covering none is dropped. A command without {paths} can't be narrowed, so its paths= is a no-op and it always runs — unless it declares when= (below), camas errs on correctness (a tool it can't narrow might be affected by the edit). --paths is repeatable, or comma-separated.

A command that can't take {paths} (cargo build, nix flake check, ctest) is scoped with when= instead — a directory-prefix string, a tuple of prefixes, or a (changed) -> bool callable. On a scoped run a leaf whose when= doesn't match the changed set is dropped; a full run never consults it. Like paths=, a group's when= is the default for descendant leaves that set none:

build = Task("cargo build", when="src")                # runs only when src/ changed
flake = Task("nix flake check", when=("flake.nix", "nix"))

A paths= callable is called with () on a full run — one that filters the changed set would return () and strip the command's arguments entirely (a formatter reading stdin on no args hangs). by_suffix(suffixes, default=...) is the safe factory: it filters the changed files by suffix on a scoped run and returns default on a full run:

tidy = Task("clang-tidy {paths}", paths=by_suffix((".c", ".h"), default=("src",)))

camas --check (and the MCP camas_check) flags both authoring mistakes as advisory warnings: a leaf whose own paths= can never apply (no {paths} token — use when= instead) and a {paths} callable that goes empty on a full run.

For the Claude Code plugin, you register the auto-fix node — whatever you named it — to Config.agent.fix; the PostToolBatch hook runs that node over the just-changed files, zero model tokens. Run camas mcp init --claude to write the full Claude Code setup in one command (.mcp.json + PostToolBatch autofix hook + camas-fixer subagent + gate skill), resolving the launcher for your project (uv run camas/uv run tasks.py, uvx, or a PATH camas) and pinning it — to tasks.py's PEP 723 block when present, else to the running camas release version (a dev/local build is left unpinned, since it isn't published to pin against). Pass --launcher uv|uvx|camas to force a strategy instead of auto-detecting — e.g. --launcher camas for a nix/flake-provided camas on PATH. For a bare .mcp.json for any MCP client, camas mcp init alone (same launcher/pin resolution, no Claude Code files). Or wire it by hand:

// .claude/settings.json
{ "hooks": { "PostToolBatch": [
  { "hooks": [{ "type": "command", "command": "camas mcp fix" }] } ] } }

camas mcp fix runs the registered Config.agent.fix node (not a task named fix — that's just camas fix, your own task); it reads the changed files from the PostToolBatch event on stdin (--paths still works for a manual run). With no fix registered it is a clean no-op, so the hook is harmless without it. The launcher runs in your project's environment — camas mcp init --claude resolves and pins it: to tasks.py's PEP 723 declaration (dependencies = ["camas>=X.Y"]) when present, else to the running camas release version; re-run camas mcp init --claude after bumping either to keep it current.

Effects plugins

Define an Effect in your tasks.py and it's discovered automatically — usable by name from --effects and listed under camas --effects. See examples/effect-plugin/ for a typed Tail effect that streams per-task output as it arrives.

Versioning

The public API is published through versioned namespaces — camas.v0 today, camas.v1 and beyond later. Import from a generation to pin the API shape: a name a generation exports is never removed or changed within that generation. The scheme tracks semver — v0 pairs with camas 0.x and is as loose as semver says 0.x is (the surface prefers to grow; breaking changes stay possible until 1.0, made deliberately and noted in releases). At 1.0 a generation freezes: a breaking change forces the next camas.vN, and published generations keep shipping, so a tasks.py or effect plugin pinned to one keeps working across upgrades.

The top-level camas namespace is the unversioned alias for the latest generation: from camas import Task, Sequential, Parallel, Effect, Config re-exports that generation's five headline definers and is kept 1:1 with its package surface. The rest of the public API for a generation — TaskNode, the TaskEvent stream, LeafState, Completion — lives in that generation's submodules, e.g. from camas.v0.task_event import TaskEvent. Everything under camas.core / camas.main is internal — it consumes whatever generations are installed and carries no stability promise.

To pin a minimum camas feature level, use your dependency declaration (camas>=0.x in pyproject.toml, or PEP 723 inline metadata) — the import path covers API shape; the package pin covers feature availability.

Standalone tasks.py (PEP 723)

For a non-Python project that wants a single-file task runner — no pyproject.toml, no venv to manage — give tasks.py a PEP 723 header and a run_cli(globals()) entry point, then run it with any PEP 723-aware tool:

# /// script
# requires-python = ">=3.11"
# dependencies = ["camas>=0.1.8"]
# ///
"""Build tasks for my project."""

from camas import Parallel, Task, run_cli

lint = Task("ruff check .")
test = Task("pytest")
check = Parallel(lint, test)

if __name__ == "__main__":
    run_cli(globals())

uv run tasks.py check     # uv reads the header, builds an ephemeral env, runs
uv run tasks.py --list    # every camas flag still works
pipx run tasks.py test

run_cli(globals()) introspects the module for Task / Sequential / Parallel and Effect bindings — exactly what camas does when it auto-discovers a tasks.py — so the standalone file behaves identically to a discovered one: camas <task> --help, --dry-run, matrix overrides, and --check all work and cite the file. run_cli is part of the stable surface (from camas import run_cli, or from camas.v0 import run_cli to pin the generation); it's imported lazily, so a plain from camas import Task doesn't pull it in.

The header owns version pinning (dependencies = ["camas>=0.1.8"]) and the interpreter floor (requires-python); it's inert to camas --check and to auto-discovery, which read the module the same way with or without it.

Reference

• examples/ — full project layouts under test coverage. The canonical reference for how to structure tasks.py, use [tool.camas.tasks] in pyproject.toml, drive a matrix from .python-version, or scope a 2-axis matrix from the CLI.
• src/camas/ — typed Python with thorough docstrings. camas --help and camas <task> --help link back here.
• camas with no args runs the Config default task (or prints the full help when none is defined); camas <task> --help shows the expanded tree, matrix axes, and override flags.

Walkthrough

The animated tree at the top is generated from this tasks.py:

examples/tauri-app/tasks.py — Rust + TypeScript + Python in one tree

from pathlib import Path

from camas import Config, Parallel, Sequential, Task

src_tauri = Path("src-tauri")
python_sdk = Path("python-sdk")
node = Path("node_modules/.bin")

frontend = Sequential(
    f"{node}/prettier --write .",
    Parallel(
        f"{node}/eslint src/",
        f"{node}/tsc --noEmit",
        f"{node}/vitest run",
    ),
)

backend = Sequential(
    Task("cargo fmt --all", cwd=src_tauri),
    Parallel(
        Task("cargo clippy --all-targets --locked -- -D warnings", cwd=src_tauri),
        Task("cargo test --all-targets --locked", cwd=src_tauri),
    ),
)

sdk = Sequential(
    Task("uv run ruff check --fix .", cwd=python_sdk),
    Task("uv run ruff format .", cwd=python_sdk),
    Parallel(
        Task("uv run mypy .", cwd=python_sdk),
        Task("uv run pytest", cwd=python_sdk),
    ),
)

all = Parallel(frontend, backend, sdk)

fix = Parallel(
    f"{node}/prettier --write .",
    Task("cargo fmt --all", cwd=src_tauri),
    Sequential(
        Task("uv run ruff check --fix .", cwd=python_sdk),
        Task("uv run ruff format .", cwd=python_sdk),
    ),
)

build = Parallel(
    Task("npm run tauri build {FLAG}"),
    matrix={"FLAG": ("-- --debug", "")},
    help="Debug and release builds (FLAG='-- --debug' debug, FLAG='' release)",
)

_ = Config(default_task=all)

$ cd examples/tauri-app

List the tasks —

$ camas --list

output

Available tasks from .../tauri-app/tasks.py:
  all       frontend | backend | sdk
  backend   cargo fmt --all, cargo clippy --all-targets --locked -- -D warnings | cargo test --all-targets --locked
  build     Debug and release builds (FLAG='-- --debug' debug, FLAG='' release)  [matrix: FLAG×2 (-- --debug..)]
  fix       node_modules/.bin/prettier --write . | cargo fmt --all | (uv run ruff check --fix ., uv run ruff format .)
  frontend  node_modules/.bin/prettier --write ., node_modules/.bin/eslint src/ | node_modules/.bin/tsc --noEmit | node_modules/.bin/vitest run
  sdk       uv run ruff check --fix ., uv run ruff format ., uv run mypy . | uv run pytest

Preview what all would run —

$ camas --dry-run all

output

all ∥
┃ frontend →
┃ ├─ node_modules/.bin/prettier --write .
┃ └─ node_modules/.bin/eslint src/ | node_modules/.bin/tsc --noEmit | node_modules/.bin/vitest run
┃   ┃ node_modules/.bin/eslint src/
┃   ┃ node_modules/.bin/tsc --noEmit
┃   ┃ node_modules/.bin/vitest run
┃ backend →
┃ ├─ cargo fmt --all  (cwd: src-tauri)
┃ └─ cargo clippy --all-targets --locked -- -D warnings | cargo test --all-targets --locked
┃   ┃ cargo clippy --all-targets --locked -- -D warnings  (cwd: src-tauri)
┃   ┃ cargo test --all-targets --locked  (cwd: src-tauri)
┃ sdk →
┃ ├─ uv run ruff check --fix .  (cwd: python-sdk)
┃ ├─ uv run ruff format .  (cwd: python-sdk)
┃ └─ uv run mypy . | uv run pytest
┃   ┃ uv run mypy .  (cwd: python-sdk)
┃   ┃ uv run pytest  (cwd: python-sdk)

Tree-symbol key: ∥ ends a Parallel header, → ends a Sequential. Children hang off ┃ (parallel siblings, run concurrently) or ├─ / └─ (sequential steps, short-circuit on first failure).

Discover a task's matrix axes and override flags —

$ camas build --help

output

usage: camas build [-h] [--dry-run] [--effects EFFECTS] [--FLAG VAL[,VAL...]]

Debug and release builds (FLAG='-- --debug' debug, FLAG='' release)

runs the 'build' task:
build ∥
┃ npm run tauri build -- --debug [FLAG=-- --debug]: npm run tauri build -- --debug  FLAG=-- --debug
┃ npm run tauri build  [FLAG=]: npm run tauri build   FLAG=

Matrix axes (override with --AXIS VAL[,VAL...]):
  --FLAG  -- --debug,

Pin the matrix from the CLI —

$ camas build --dry-run --FLAG '-- --debug'

output

build ∥
┃ npm run tauri build -- --debug [FLAG=-- --debug]: npm run tauri build -- --debug  FLAG=-- --debug