Ensure FastMCP handles nested SSE and SHTTP apps properly in ASGI frameworks #390
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Pull Request Overview
This PR ensures that FastMCP handles nested SSE and Streamable HTTP apps correctly when integrated into ASGI frameworks, while also patching low-level SSE transport issues and updating documentation accordingly.
- Introduces nested server configurations in tests for both HTTP and SSE transports.
- Refactors Starlette app creation in server code and patches the low-level SSE transport.
- Updates project timeout configuration and enhances ASGI integration documentation.
Reviewed Changes
Copilot reviewed 10 out of 10 changed files in this pull request and generated 3 comments.
Show a summary per file
| File | Description |
|---|---|
| tests/client/test_streamable_http.py | Adds nested server test and corresponding helper function. |
| tests/client/test_sse.py | Adds nested server test for SSE and corresponding helper function. |
| src/fastmcp/server/http.py | Refactors base app creation and updates import for SSE transport. |
| src/fastmcp/low_level/sse_server_transport.py | Introduces a patched low-level SSE transport implementation. |
| src/fastmcp/low_level/README.md | Updates README with a brief note on patched low-level objects. |
| pyproject.toml | Reduces the test timeout from 5 to 3. |
| docs/* | Adds documentation for ASGI integration, custom routes, and nested mounts. |
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
jordicore
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Jul 2, 2025
Ensure FastMCP handles nested SSE and SHTTP apps properly in ASGI frameworks
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Closes #365
Closes #370
Closes #382
Closes #234
Closes #225
Closes #189
Addresses the following low-level / FastMCP 1.0 issues: modelcontextprotocol/python-sdk#673, modelcontextprotocol/python-sdk#563, modelcontextprotocol/python-sdk#507, modelcontextprotocol/python-sdk#464, modelcontextprotocol/python-sdk#412
Since it is unclear if the low-level SDK will merge modelcontextprotocol/python-sdk#659, we are adding a patched low-level SSEServerTransport to FastMCP (and can remove it if/when the bug is fixed). This should resolve issues with nested ASGI applications.
However, note that you must pass the MCP lifespan to the outer-most app, due to the current design of the
StreamableHTTPSessionManager. We are looking at whether we can replace that implementation with a more self-contained one.This also includes complete documentation for ASGI framework integration and tests for nested SSE and SHTTP starlette apps.