Pull Request

Discussion: N/A - docs-only clarification with tutorial test coverage.

Description

This PR documents how to add custom response headers to JSON Lines and SSE streaming endpoints.

FastAPI already documents the common pattern in Response Headers: use a Response parameter and mutate response.headers. This PR adds the streaming-specific caveat. For generator endpoints, setting a header inside the generator body is too late because FastAPI has already prepared the streaming response before the generator starts yielding data.

The examples use a dependency with a Response parameter to set the header before FastAPI creates the streaming response. Dependencies are resolved before the response is prepared, so the header is available at the correct point.

Changes:

• Add a JSON Lines tutorial section and runnable docs example for custom stream headers.
• Add an SSE tutorial section and runnable docs example for custom stream headers.
• Add tutorial tests that verify the custom header is present, the response media type is still correct, the stream body is produced, and OpenAPI keeps the expected streaming schema.

Validation

python -m pytest tests/test_tutorial/test_stream_json_lines tests/test_tutorial/test_server_sent_events tests/test_sse.py tests/test_stream_bare_type.py tests/test_stream_json_validation_error.py -q
52 passed

python -m ruff check docs_src/stream_json_lines/tutorial002_py310.py docs_src/server_sent_events/tutorial006_py310.py tests/test_tutorial/test_stream_json_lines/test_tutorial002.py tests/test_tutorial/test_server_sent_events/test_tutorial006.py
All checks passed

git diff --check

I did not run the full docs build locally because this venv does not have the full docs dependency group installed.