Use SpanKind enum directly instead of string conversion

Author: Kludex

src/mcp/server/lowlevel/server.py

@@ -45,7 +45,7 @@ async def main():
 from typing import Any, Generic, cast
 
 import anyio
-from opentelemetry.trace import StatusCode
+from opentelemetry.trace import SpanKind, StatusCode
 from starlette.applications import Starlette
 from starlette.middleware import Middleware
 from starlette.middleware.authentication import AuthenticationMiddleware
@@ -457,7 +457,7 @@ async def _handle_request(
 
         with otel_span(
             span_name,
-            kind="SERVER",
+            kind=SpanKind.SERVER,
             attributes={"mcp.method.name": req.method, "jsonrpc.request.id": message.request_id},
             context=parent_context,
         ) as span:

src/mcp/shared/_otel.py

@@ -17,13 +17,12 @@
 def otel_span(
     name: str,
     *,
-    kind: str = "INTERNAL",
+    kind: SpanKind,
     attributes: dict[str, Any] | None = None,
     context: Context | None = None,
 ) -> Iterator[Any]:
     """Create an OTel span."""
-    span_kind = getattr(SpanKind, kind, SpanKind.INTERNAL)
-    with _tracer.start_as_current_span(name, kind=span_kind, attributes=attributes, context=context) as span:
+    with _tracer.start_as_current_span(name, kind=kind, attributes=attributes, context=context) as span:
         yield span
 
 

src/mcp/shared/session.py

@@ -9,6 +9,7 @@
 
 import anyio
 from anyio.streams.memory import MemoryObjectSendStream
+from opentelemetry.trace import SpanKind
 from pydantic import BaseModel, TypeAdapter
 from typing_extensions import Self
 
@@ -274,7 +275,7 @@ async def send_request(
 
             with otel_span(
                 span_name,
-                kind="CLIENT",
+                kind=SpanKind.CLIENT,
                 attributes={"mcp.method.name": request.method, "jsonrpc.request.id": request_id},
             ):
                 # Inject W3C trace context into _meta (SEP-414).