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:

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.

The code-related AI assistance was:

User:

I identified that ServerSentEvent(data=...) does not respect by_alias when
the data is a Pydantic model, and confirmed it in the discussion. Before opening
a PR, check whether the same path misses any other response model serialization
options.

Codex:

I will inspect the SSE serialization path and compare it with the regular
streamed response model serialization path.

In fastapi/routing.py, plain streamed items use:

But ServerSentEvent(data=...) with a Pydantic model calls:

So the same path is not only missing by_alias; it is also missing the route's
include, exclude, exclude_unset, exclude_defaults, and exclude_none
serialization options.

User:

Update the fix so it handles the missing response model serialization options,
and add regression tests for the changed behavior.

Codex:

I updated the model_dump_json() call to pass the route serialization options:

I also added focused SSE regression tests for:

• serialization aliases
• response_model_by_alias=False
• response_model_include
• response_model_exclude
• response_model_exclude_none
• response_model_exclude_unset
• response_model_exclude_defaults

User:

Run the focused checks.

Codex:

The focused checks passed:

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.