polylith-dependency-management

name	polylith-dependency-management
description	Add or manage third-party dependencies in a Polylith workspace. Use when the user wants to install a new library (e.g., requests, fastapi) for a brick or project.

Dependency Management Skill

💡 Terminology: this skill uses Polylith terms like brick, project, and development project. If they're unfamiliar, load polylith-concepts first.

Polylith manages third-party dependencies differently than standard Python projects.

Rules for adding dependencies

1. The Development Project (Root): Every third-party library used anywhere in the workspace must be added to the root pyproject.toml (the development project). This ensures local REPLs, tests, and the shared lockfile work correctly. Use your package manager (e.g., uv add <package>, poetry add <package>) at the workspace root.
2. Deployable Projects (projects/<name>): If a library is required for production by a specific deployable project, it must also be added to that project's pyproject.toml under projects/<name>/. Do this by manually editing the dependencies array in projects/<name>/pyproject.toml.

⚠ Never add dependencies to a brick's directory. Bricks do not have their own pyproject.toml files.

Example Workflow: Adding requests to an api project

1. Install it at the root so the lockfile updates and it's available for local development:

   uv add requests
   

2. Manually add it to the deployable project that needs it: Edit projects/api/pyproject.toml.

   Important Versioning Rule: Before adding the dependency to the project, check the root pyproject.toml. If it contains a [tool.uv.workspace] section (e.g., members = ["projects/*"]), then uv workspaces is enabled.

   If uv workspaces is enabled, do not specify the version in the project's pyproject.toml. Just use the package name, because versions are centrally pinned in the root. If workspaces are not enabled, then you should specify the version.

   # WITH uv/poetry workspaces enabled (Recommended):
   dependencies = [
       "requests"
   ]
   
   # WITHOUT workspaces enabled:
   dependencies = [
       "requests>=2.31.0"
   ]
   

3. Run poly check (load polylith-check) to verify the workspace is valid. poly check will catch if a project imports requests but forgot to declare it in its pyproject.toml.

Notes for the agent

• Use poly libs (load polylith-libs) to inspect current dependency usage across projects.