Camas

A task runner with parallel execution, matrix expansion, 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: coming soon — structured JSON stream + 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

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(StatusOptions(output_mode="github")),) — collapsed workflow groups, ISO timestamps with millisecond precision, ANSI colors preserved.

- 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(StatusOptions(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(StatusOptions(output_mode="github")),
      GitHubChecks(GitHubChecksOptions(
        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.

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.

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 lists tasks; 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 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)",
)

$ 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