extract_trace_context: return None when carrier has no valid traceparent

A non-empty _meta without a traceparent key (e.g. only a progressToken)
made extract() return an empty Context, which orphans the span when
passed explicitly to start_as_current_span. Check the extracted context
carries a valid span and return None otherwise so callers fall through
to ambient parenting.

Author: maxisbey

src/mcp/shared/_otel.py

@@ -8,7 +8,7 @@
 
 from opentelemetry.context import Context
 from opentelemetry.propagate import extract, inject
-from opentelemetry.trace import SpanKind, get_tracer
+from opentelemetry.trace import SpanKind, get_current_span, get_tracer
 from opentelemetry.trace.span import Span
 
 _tracer = get_tracer("mcp-python-sdk")
@@ -44,13 +44,17 @@ def inject_trace_context(meta: dict[str, Any]) -> None:
 def extract_trace_context(meta: Mapping[str, Any] | None) -> Context | None:
     """Extract W3C trace context from a `_meta` dict.
 
-    Returns `None` when the carrier is absent or malformed so callers fall
-    through to ambient parenting; an explicit empty `Context` would orphan
-    the span instead of nesting under the current one.
+    Returns `None` when the carrier is absent, malformed, or carries no
+    valid `traceparent`, so callers fall through to ambient parenting; an
+    explicit empty `Context` would orphan the span instead of nesting under
+    the current one.
     """
     if not meta:
         return None
     try:
-        return extract(meta)
+        ctx = extract(meta)
     except (ValueError, TypeError):
         return None
+    if not get_current_span(ctx).get_span_context().is_valid:
+        return None
+    return ctx

tests/server/test_otel.py

@@ -95,6 +95,34 @@ async def wrapped(dctx: Any, method: str, params: dict[str, Any] | None) -> Any:
     assert inner.parent.span_id == outer.context.span_id
 
 
+@pytest.mark.anyio
+async def test_nests_under_ambient_span_when_meta_lacks_traceparent(server: SrvT, spans: SpanCapture):
+    """`_meta` is present but carries no `traceparent` (e.g. only a
+    `progressToken`). `extract()` would yield an empty Context here, which
+    would orphan the span; the middleware must fall through to ambient
+    parenting just as if `_meta` were absent."""
+
+    def replace_meta(call_next: Any) -> Any:
+        async def wrapped(dctx: Any, method: str, params: dict[str, Any] | None) -> Any:
+            rewritten = {**(params or {}), "_meta": {"progressToken": "tok"}}
+            return await call_next(dctx, method, rewritten)
+
+        return wrapped
+
+    server.middleware.append(OpenTelemetryMiddleware())
+    async with connected_runner(server, dispatch_middleware=[replace_meta, otel_middleware]) as (client, _):
+        spans.clear()
+        await client.send_raw_request("tools/list", None)
+    server_spans = [s for s in spans.finished() if s.kind == SpanKind.SERVER]
+    assert len(server_spans) == 2
+    [outer] = [s for s in server_spans if s.parent is None]
+    [inner] = [s for s in server_spans if s.parent is not None]
+    assert inner.context is not None and outer.context is not None
+    assert inner.parent is not None
+    assert inner.context.trace_id == outer.context.trace_id
+    assert inner.parent.span_id == outer.context.span_id
+
+
 @pytest.mark.anyio
 async def test_extracts_trace_context_from_meta(server: SrvT, spans: SpanCapture):
     meta: dict[str, Any] = {}