Pull Request

🐛 Fix response model serialization options for SSE event data

Discussion: #15703

Description

This fixes the ServerSentEvent(data=...) serialization path for Pydantic models.

Previously, this path called model_dump_json() without forwarding the route's response model serialization options. Because of that, models used as SSE event data could ignore options such as response_model_by_alias, response_model_include, response_model_exclude, response_model_exclude_none, response_model_exclude_unset, and response_model_exclude_defaults.

This now passes those options to model_dump_json(), matching the behavior used by regular streamed response items.

I added regression tests for serialization aliases, response_model_by_alias=False, response_model_include, response_model_exclude, response_model_exclude_none, response_model_exclude_unset, and response_model_exclude_defaults.

I ran locally:

uv run pytest -q tests/test_sse.py
uv run pytest -q tests/test_sse.py tests/test_response_by_alias.py tests/test_response_model_include_exclude.py tests/test_serialize_response_model.py
uv run ruff check fastapi/routing.py tests/test_sse.py
git diff --check

AI Disclaimer

I used Codex as a coding assistant while preparing this PR.

After identifying the missing by_alias handling and confirming it in the discussion, I used Codex to inspect the same ServerSentEvent(data=...) serialization path for other response model serialization options.

Codex helped me check for similar missing options, draft the focused regression tests, run local checks, and prepare this PR description. I reviewed the code changes, the test coverage, and the command results before opening the PR.

stream_item_field.serialize_json(
    value,
    include=response_model_include,
    exclude=response_model_exclude,
    by_alias=response_model_by_alias,
    exclude_unset=response_model_exclude_unset,
    exclude_defaults=response_model_exclude_defaults,
    exclude_none=response_model_exclude_none,
)

item.data.model_dump_json()

item.data.model_dump_json(
    include=response_model_include,
    exclude=response_model_exclude,
    by_alias=response_model_by_alias,
    exclude_unset=response_model_exclude_unset,
    exclude_defaults=response_model_exclude_defaults,
    exclude_none=response_model_exclude_none,
)

uv run pytest -q tests/test_sse.py
uv run pytest -q tests/test_sse.py tests/test_response_by_alias.py tests/test_response_model_include_exclude.py tests/test_serialize_response_model.py
uv run ruff check fastapi/routing.py tests/test_sse.py
git diff --check

Checklist

• This PR is an obvious typo fix, or it links to a GitHub Discussion for the proposed code change.
• I added tests for the change.
• The new or updated tests fail on the main branch and pass on this PR.
• Coverage stays at 100%.
• The documentation explains the change if needed.