streamlit (Agent Skills)

# Streamlit Repo Overview

[Streamlit](https://github.com/streamlit/streamlit) is an open-source (Apache 2.0) Python library for creating interactive web applications and dashboards with focus on data apps and internal tools.

## Tech Stack

- **Backend (Server):** Python, Starlette/Uvicorn server, pytest
- **Frontend (Web UI):** TypeScript, React, Emotion (CSS-in-JS), Vite, Vitest
- **Communication:** Protocol Buffers (protobuf) over WebSocket.

## Folder Structure

- `lib/`: All backend code and assets.
  - `streamlit/`: The main Streamlit library package.
  - `streamlit/elements/`: Backend code of elements and widgets.
  - `streamlit/runtime/`: App runtime and execution logic.
  - `streamlit/web/`: Web server and CLI implementation
  - `tests`: Python unit tests (pytest).
- `frontend/`: All frontend code and assets.
  - `app/`: Streamlit application UI.
  - `lib/`: Shared TypeScript library that contains elements, widgets, and layouts.
  - `connection/`: WebSocket connection handling logic.
  - `utils/`: Shared utilities.
  - `component-lib/`: Library for building Streamlit custom components v1.
  - `component-v2-lib/`: Library for building Streamlit custom components v2.
- `proto/streamlit/proto/`: Protobuf definitions for client-server communication.
- `e2e_playwright/`: E2E tests using playwright (via pytest).
- `scripts/`: Utility scripts for development and CI/CD.
- `specs/`: Product and tech specs for Streamlit features.
- `.github/workflows/`: GitHub Actions workflows used for CI/CD.
- `wiki/`: Documentation relevant for development of Streamlit.

## Shell & Build Policy

- Prefer `make` targets for all dev tasks (tests, lint, format, builds).
- Always use `uv run` to run any Python command (e.g. `uv run streamlit`, `uv run pytest`, `uv run ruff`, `uv run mypy`, etc.).
- Always use `uv run` for git commands that trigger hooks (e.g. `uv run git commit`, `uv run git push`). Pre-commit hooks require the uv environment to run linters and formatters.
- The committed root `uv.lock` is the source of truth for normal development and CI. Do not hand-edit it.
- For Python dependency changes, edit the relevant `pyproject.toml`, run `uv lock`, and commit both files. Also run `uv lock` after changing the package name or version. Use `uv lock --upgrade-package <package>` for a targeted upgrade, `uv lock --upgrade` for a full compatible upgrade, and `uv lock --check` to verify consistency.
- The dev/CI `[dependency-groups]` in the root `pyproject.toml` use bare package names because `uv.lock` owns the versions: do not add lower-bound floors, reserve exact `==` pins for tools we deliberately hold back, and add upper caps (`<`) only for versions known to break us (mirrored by `ignore` entries in `.github/dependabot.yml`); a `!=` exclusion of a single known-broken release needs no `ignore` entry. This does NOT apply to `lib/pyproject.toml`, whose version ranges are the published package's contract with users. See the comment above `[dependency-groups]` in the root `pyproject.toml` for details.
- If `uv.lock` conflicts during a merge, restore it with `git checkout origin/develop -- uv.lock`, then run `uv lock`; never hand-merge it.
- For Python unit tests: `uv run pytest` commands are allowed and encouraged for running specific tests during development.
- For E2E tests: `uv run pytest` commands targeting `e2e_playwright/` files are blocked by policy.
  Use `make run-e2e-test <filename>` instead.

## `make` commands

Selection of `make` commands for development (run in the repo root):

- `help`: Show all available make commands. [~1s]
- `check`: Run all checks (format, lint, types, unit tests) on changed files only. Add `E2E_CHECK=true` to include E2E tests. [varies by changes]
- `protobuf`: Recompile Protobufs for Python and the frontend. [~5s]
- `autofix`: Autofix linting and formatting errors. [~30s]

**Backend Development (Python):**

- `python-init`: Install the locked Python environment selected by `PYTHON_DEPENDENCY_GROUP` (`dev` by default).
- `python-lint`: Lint and check formatting of Python files (ruff). [~1s]
- `python-tests`: Run all Python unit tests (pytest). [~3min]
- `python-types`: Run the Python type checker (mypy & ty). [~30s]
- `python-format`: Format Python files (ruff). [~1s]

**Frontend Development (TypeScript):**

- `frontend-fast`: Build the frontend (vite). [~40s]
- `frontend-dev`: Start the frontend development server (hot-reload). [until stopped]
- `frontend-lint`: Lint and check formatting of all frontend files (oxlint + eslint). [~45s]
- `frontend-knip`: Run Knip dependency analysis. [~5s]
- `frontend-types`: Run the TypeScript type checker on all files (tsc). [~15s]
- `frontend-format`: Format all frontend files (oxfmt). [~2s]
- `frontend-tests`: Run all frontend unit tests (vitest). [~5min]

**E2E Testing (Playwright):**

- `run-e2e-test`: Run e2e test, via: `make run-e2e-test st_command_test.py`. [varies by test]

**Debugging backend & frontend:**

- `debug`: Start Streamlit backend and Vite dev server together, via: `make debug my_app.py`. [until stopped]
  - Frontend hot-reload: Changes to frontend code (`frontend/`) are applied within seconds.
  - Backend hot-reload: Only changes to the **app script** trigger a rerun. Changes to the Streamlit library itself (`lib/streamlit/`) require restarting `make debug`.
  - Logs are written to a per-session directory under `work-tmp/debug/` (e.g. `work-tmp/debug/<session>/backend.log` and `work-tmp/debug/<session>/frontend.log`).
  - `work-tmp/debug/latest/` is a symlink to the most recent debug session (e.g. `work-tmp/debug/latest/backend.log`). If multiple sessions are running simultaneously, this symlink can move—prefer using the session directory printed by `make debug`.
  - Log files are cleared at the start of each session and persist after exit for post-mortem analysis.
  - Browser `console.log()` output appears in the session’s `frontend.log`.
  - See [.claude/skills/debugging-streamlit/SKILL.md](.claude/skills/debugging-streamlit/SKILL.md) for the full debugging guide.

### Development Tips

- **Follow existing patterns**: Check neighboring files for conventions.
- **Subagent model**: When launching subagents, use the same model as the parent session (`model: inherit` / omit any model override). Do not switch to a different or faster model unless the user explicitly requests it. Prefer the named custom agents in `.claude/agents/` when available.
- You can use the `work-tmp` directory to store temporary files, specs, and scripts.
- Use `agent-wiki/` (gitignored, cloned on first use) to share intermediate files (specs, plans, learnings) relevant for the current PR. This is a local checkout of [streamlit.wiki](https://github.com/streamlit/streamlit.wiki.git). Files are stored in `pull-requests/<pr-number>/` and accessible at `https://issues.streamlit.app/agent_wiki_explorer?file=<relative-path>`. See `sharing-pr-agent-artifacts` skill.
- If you fail to run a `make` command, remember to run it from the root / top-level directory.
- Use `make debug <script.py>` to start both backend and frontend with hot-reload for debugging. The app URL will be printed on startup (default `http://localhost:3001`; `3000` is reserved for manual `make frontend-dev`; it may use `3002+` if you have other sessions running). Avoid pinning `VITE_PORT` unless you have a specific hard requirement (last resort).
- Run `make check` after completing changes to run formatting, linting, type checking, and unit tests on all uncommitted files.
- The main branch of this repository is `develop`.
- For adding new elements, widgets, or features that span backend, frontend, and protobufs, see `wiki/new-feature-guide.md`.

## Testing Strategy

- Most new user-facing features should be covered by both unit tests and E2E tests.
- **Python Unit Tests**: Test internal behavior without frontend. Located at `lib/tests/streamlit/<package>/<module>_test.py` mirroring `lib/streamlit/<package>/<module>.py` (legacy tests may vary).
- **Frontend Unit Tests**: Test React components, hooks, and related functionality with Vitest and React Testing Library. Co-located as `<Component>.test.tsx` next to `<Component>.tsx`.
- **E2E Tests**: Test the entire app logic end-to-end with Playwright. Located at `e2e_playwright/<name>_test.py` with app code in `e2e_playwright/<name>.py`. User-facing features should be covered by E2E tests (e.g., parameters and commands in the public `st.` API).
- **(Python) Type Tests**: Verify public API typing with mypy `assert_type`. Located at `lib/tests/streamlit/typing/<command>_types.py`.
- Prefer running specific tests / test scripts for newly added tests instead the entire test suite.