New Feature Implementation Guide

For understanding the underlying architecture (backend runtime, frontend rendering, WebSocket communication, element tree), see the understanding-streamlit-architecture skill.

Most features need implementation in three areas:

• Backend: lib/streamlit/
• Frontend: frontend/
• Protobufs: proto/

New features should include:

• Python unit tests in lib/tests
• Vitest unit tests
• E2E Playwright tests in e2e_playwright/

Order of Implementation

1. Protobuf changes in proto/ then run make protobuf

   • New elements: add to proto/streamlit/proto/Element.proto

2. Backend in lib/streamlit/

   • New elements: add to lib/streamlit/__init__.py
   • Public st.* commands: decorate with gather_metrics. Use it only for those public APIs, not internal helpers.

3. Python unit tests in lib/tests

   • Run: uv run pytest lib/tests/streamlit/the_test_name.py
   • New elements: add to lib/tests/streamlit/element_mocks.py

4. Frontend in frontend/

   • New elements: add to frontend/lib/src/components/core/Block/ElementNodeRenderer.tsx

5. Vitest tests in *.test.tsx

   • Run: cd frontend && yarn test lib/src/components/elements/NewElement/NewElement.test.tsx

6. E2E Playwright tests in e2e_playwright/

   • Run: make run-e2e-test e2e_playwright/name_of_the_test.py

7. Autofix formatting and linting: make autofix

8. Verify the implementation: make check

9. Bundled agent skills (user-facing features): update lib/streamlit/.agents/skills/ following lib/streamlit/.agents/skills/AGENTS.md