OpenTelemetry: FastMCP client does not propagate trace context on initialize and list methods

[strawgate]

Summary

FastMCP's client still behaves like an opaque MCP client on some protocol requests. initialize, tools/list, resources/list, resources/templates/list, and prompts/list create local client spans but do not inject trace context into MCP _meta.

Use case

Wrap a FastMCP client in an outer OpenTelemetry span, then connect and call initialize() or any of the list methods. The client emits a local span, but the server side does not receive traceparent / baggage through MCP _meta, so the handshake or discovery request is disconnected from the caller's trace.

Expected vs actual

Expected: these client requests should propagate _meta.traceparent and _meta.baggage the same way tools/call, resources/read, and prompts/get already do.

Actual: the client calls the MCP SDK methods directly, so those requests cross the protocol boundary without MCP trace context.

Related: #3887 tracks broader method coverage. This issue is specifically about propagation on methods the FastMCP client already wraps or exposes today.

[marvin-context-protocol]

Claude finished @strawgate's task in 8m 36s —— View job

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tl;dr: Confirmed — the four list methods call MCP SDK session methods directly without injecting trace context into _meta. The MCP SDK already supports _meta on PaginatedRequestParams, so the fix follows the same send_request pattern already used by read_resource_mcp and get_prompt_mcp. initialize is a separate, harder problem because the SDK's session.initialize() internally handles capability negotiation.

Recommendation: Add trace context propagation to list_tools_mcp, list_resources_mcp, list_resource_templates_mcp, and list_prompts_mcp using the conditional send_request pattern already established in the codebase. Defer initialize to #3887 since it requires reimplementing SDK initialization internals.

Findings

Root cause

The four list methods call SDK session methods directly without _meta:

Method	File	Line	Current call
list_tools_mcp	src/fastmcp/client/mixins/tools.py	L65	self.session.list_tools(cursor=cursor)
list_resources_mcp	src/fastmcp/client/mixins/resources.py	L60	self.session.list_resources(cursor=cursor)
list_resource_templates_mcp	src/fastmcp/client/mixins/resources.py	L136	self.session.list_resource_templates(cursor=cursor)
list_prompts_mcp	src/fastmcp/client/mixins/prompts.py	L61	self.session.list_prompts(cursor=cursor)

In contrast, the three methods that do propagate trace context all call inject_trace_context() from src/fastmcp/telemetry.py:50 and either pass meta=propagated_meta directly (for call_tool_mcp at L159) or construct a raw request with _meta=propagated_meta and use session.send_request() (for read_resource_mcp at L220-237 and get_prompt_mcp at L157-175).

MCP SDK support

The MCP SDK (v1.26.0, pinned in pyproject.toml) already supports _meta on all list requests:

• PaginatedRequestParams inherits _meta from RequestParams (line 79)
• The SDK's own higher-level client already passes meta through to PaginatedRequestParams._meta for all list methods

Why initialize is different

The SDK's session.initialize() takes no parameters — it internally constructs an InitializeRequest with the client's capabilities, protocol version, sampling/elicitation callbacks, etc. Injecting _meta would require either:

• Reimplementing the full initialization logic in FastMCP (fragile, breaks on SDK updates)
• Patching the request after construction (not possible with the current SDK API)

Since initialize is a one-time handshake per session, the trace gap is less impactful than for list methods. It makes sense to track it under the broader #3887 effort.

Existing tests

tests/client/telemetry/test_client_list_tracing.py verifies that client spans are created for list operations, but does not verify trace context propagation (i.e., that server spans are children of client spans). The test test_list_tools_creates_both_client_and_server_spans (L43-81) checks that both spans exist but not that they share a trace_id or have a parent-child relationship. New tests should follow the pattern in TestClientServerSpanHierarchy.test_trace_context_propagation (L288-339 in test_client_tracing.py).

Action Plan

1. Update list_tools_mcp in src/fastmcp/client/mixins/tools.py

Replace lines 64-65:

result = await self._await_with_session_monitoring(
    self.session.list_tools(cursor=cursor)
)

With the conditional send_request pattern:

propagated_meta = inject_trace_context()

if propagated_meta:
    request = mcp.types.ListToolsRequest(
        params=mcp.types.PaginatedRequestParams(
            cursor=cursor,
            _meta=propagated_meta,
        )
    )
    result = await self._await_with_session_monitoring(
        self.session.send_request(
            request=request,
            result_type=mcp.types.ListToolsResult,
        )
    )
else:
    result = await self._await_with_session_monitoring(
        self.session.list_tools(cursor=cursor)
    )

inject_trace_context is already imported (L21).

2. Update list_resources_mcp in src/fastmcp/client/mixins/resources.py

Same pattern at lines 59-61, using mcp.types.ListResourcesRequest, mcp.types.ListResourcesResult.

3. Update list_resource_templates_mcp in src/fastmcp/client/mixins/resources.py

Same pattern at lines 135-137, using mcp.types.ListResourceTemplatesRequest, mcp.types.ListResourceTemplatesResult.

4. Update list_prompts_mcp in src/fastmcp/client/mixins/prompts.py

Same pattern at lines 60-62, using mcp.types.ListPromptsRequest, mcp.types.ListPromptsResult.

5. Add propagation tests to tests/client/telemetry/test_client_list_tracing.py

For each list method, add a test verifying that the server span is a child of the client span (shared trace_id, parent-child span_id). Follow the exact pattern of test_trace_context_propagation in tests/client/telemetry/test_client_tracing.py:288-339.

Example for list_tools:

async def test_list_tools_trace_context_propagation(
    self, trace_exporter: InMemorySpanExporter
):
    server = FastMCP("test-server")

    @server.tool()
    def greet(name: str) -> str:
        return f"Hello, {name}!"

    client = Client(server)
    async with client:
        await client.list_tools()

    spans = trace_exporter.get_finished_spans()
    tools_list_spans = [s for s in spans if s.name == "tools/list"]

    client_span = next(
        (s for s in tools_list_spans
         if s.attributes and "fastmcp.server.name" not in s.attributes),
        None,
    )
    server_span = next(
        (s for s in tools_list_spans
         if s.attributes and "fastmcp.server.name" in s.attributes),
        None,
    )

    assert client_span is not None
    assert server_span is not None
    assert server_span.context.trace_id == client_span.context.trace_id
    assert server_span.parent is not None
    assert server_span.parent.span_id == client_span.context.span_id

6. Run validation

uv sync
uv run pytest tests/client/telemetry/ -n auto
uv run prek run --all-files

Related Issues and Pull Requests

Issue	Relevance
#3887 — Instrument all MCP methods	Parent tracking issue for broader OTel coverage; initialize propagation should live there

Related Files

File	Relevance
src/fastmcp/client/mixins/tools.py L40-67	list_tools_mcp — missing propagation
src/fastmcp/client/mixins/resources.py L35-62	list_resources_mcp — missing propagation
src/fastmcp/client/mixins/resources.py L111-138	list_resource_templates_mcp — missing propagation
src/fastmcp/client/mixins/prompts.py L36-63	list_prompts_mcp — missing propagation
src/fastmcp/telemetry.py L50-73	inject_trace_context() — the utility to use
src/fastmcp/client/mixins/resources.py L219-237	read_resource_mcp — reference implementation of the send_request pattern
tests/client/telemetry/test_client_list_tracing.py	Existing list tracing tests (needs propagation assertions)
tests/client/telemetry/test_client_tracing.py L288-339	test_trace_context_propagation — the pattern to replicate for list tests

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