fix(mcp): Handle all awaitable return types #5415

alexander-alderman-webb · 2026-02-03T09:50:43Z

Description

The handler registered with Server.call_tool is always awaited in the mcp internals. Registering a handler which is not a coroutine function or does not return a type implementing __await__ raises an exception. This exception is swallowed by mcp if raise_exceptions=False.

https://github.com/modelcontextprotocol/python-sdk/blob/050aeb64569ae2115f1946f721efd6d7c439d43e/src/mcp/server/lowlevel/server.py#L781

The mcp.tool.result.content attribute now holds the awaited return value of the registered handler, in the case that the return value is awaitable but not a coroutine function.

The test below previously fails.

class AwaitableDict:
    def __init__(self, value):
        self.value = value
    
    def __await__(self):
        yield None
        return self.value

async def test_awaitable(
    sentry_init, capture_events, stdio
):
    """Test that synchronous tool handlers create proper spans"""
    sentry_init(
        integrations=[MCPIntegration(include_prompts=True)],
        traces_sample_rate=1.0,
        send_default_pii=True,
    )
    events = capture_events()

    server = Server("test-server")

    @server.call_tool()
    def test_tool(tool_name, arguments):  # Note: synchronous function
        return AwaitableDict({"result": "success", "value": 42})

    with start_transaction(name="mcp tx"):
        result = await stdio(
            server,
            method="tools/call",
            params={
                "name": "calculate",
                "arguments": {"x": 10, "y": 5},
            },
            request_id="req-123",
        )

    (tx,) = events
    span = tx["spans"][0]

    assert span["data"][SPANDATA.MCP_TOOL_RESULT_CONTENT] == json.dumps(
            {
                "result": "success",
                "value": 42,
            }
        )

Issues

Reminders

Please add tests to validate your changes, and lint your code using tox -e linters.
Add GH Issue ID & Linear ID (if applicable)
PR title should use conventional commit style (feat:, fix:, ref:, meta:)
For external contributors: CONTRIBUTING.md, Sentry SDK development docs, Discord community

github-actions · 2026-02-03T09:51:07Z

Semver Impact of This PR

🟢 Patch (bug fixes)

📋 Changelog Preview

This is how your changes will appear in the changelog.
Entries from this PR are highlighted with a left border (blockquote style).

New Features ✨

Span Streaming

feat(span-streaming): Add spans to telemetry pipeline, add span name and attributes (3) by sentrivana in #5399
feat(span-streaming): Add span batcher (2) by sentrivana in #5398
feat(span-streaming): Add experimental trace_lifecycle switch (1) by sentrivana in #5397

Other

feat(integration): add gen_ai.conversation.id if available by constantinius in #5307

Bug Fixes 🐛

Google Genai

fix(google-genai): Token reporting by alexander-alderman-webb in #5404
fix(google-genai): deactivate google genai when langchain is used by shellmayr in #5389

Span Streaming

fix(span-streaming): Always preserialize attributes by sentrivana in #5407
fix(span-streaming): Warn about thread usage if any batcher is active by sentrivana in #5408

Other

fix(anthropic): Token reporting by alexander-alderman-webb in #5403
fix(arq): handle settings_cls passed as keyword argument by nc9 in #5393
fix(dramatiq): cleanup isolated scope and transaction when message is skipped by frankie567 in #5346

fix(mcp): Handle all awaitable return types by alexander-alderman-webb in #5415

fix(openai): Token reporting by alexander-alderman-webb in #5406
fix(openai-agents): Inject propagation headers for HostedMCPTool when streaming by alexander-alderman-webb in #5405
fix: Adapt to new packaging in toxgen by sentrivana in #5382

Internal Changes 🔧

Fastmcp

test(fastmcp): Wrap prompt in Message by alexander-alderman-webb in #5411
test(fastmcp): Remove test_fastmcp_without_request_context() by alexander-alderman-webb in #5412
test(fastmcp): Use AsyncClient for SSE by alexander-alderman-webb in #5400
test(fastmcp): Use TestClient for Streamable HTTP by alexander-alderman-webb in #5384
test(fastmcp): Simulate stdio transport with memory streams by alexander-alderman-webb in #5333

Mcp

test(mcp): Use AsyncClient for SSE by alexander-alderman-webb in #5396
test(mcp): Use TestClient for Streamable HTTP by alexander-alderman-webb in #5383
test(mcp): Remove unused stdio helpers by alexander-alderman-webb in #5409
test(mcp): Simulate stdio transport with memory streams by alexander-alderman-webb in #5329

Other

ci: 🤖 Update test matrix with new releases (02/02) by github-actions in #5413
ci: Update tox and pin packaging version for tox by alexander-alderman-webb in #5381
ci: migration to the new codecov action by MathurAditya724 in #5392

_{🤖 This preview updates automatically when you update the PR.}

github-actions · 2026-02-03T09:52:06Z

Codecov Results 📊

✅ 13 passed | Total: 13 | Pass Rate: 100% | Execution Time: 8.97s

All tests are passing successfully.

❌ Patch coverage is 0.00%. Project has 13557 uncovered lines.

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alexander-alderman-webb added 2 commits February 3, 2026 10:48

fix(mcp): Handle all awaitable return types

314375c

force await for fastmcp

e804de5

force_await=False

dfc2b98

alexander-alderman-webb marked this pull request as ready for review February 3, 2026 10:19

alexander-alderman-webb requested a review from a team as a code owner February 3, 2026 10:19

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fix(mcp): Handle all awaitable return types #5415

fix(mcp): Handle all awaitable return types #5415

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New Features ✨

Span Streaming

Other

Bug Fixes 🐛

Google Genai

Span Streaming

Other

Internal Changes 🔧

Fastmcp

Mcp

Other

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Description

Issues

Reminders

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Semver Impact of This PR

New Features ✨

Span Streaming

Other

Bug Fixes 🐛

Google Genai

Span Streaming

Other

Internal Changes 🔧

Fastmcp

Mcp

Other

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