test: add context propagation test for StreamableClientTransport

Adds TestStreamableClientContextPropagation to verify that context values
are properly propagated to background HTTP operations (SSE GET and cleanup
DELETE requests) in StreamableClientTransport.

The test uses a custom HTTP handler to capture request contexts and verify
that context values from the parent context are accessible in both the
SSE connection establishment and session cleanup requests.

Author: manuelibar

mcp/streamable_test.go

@@ -1430,3 +1430,106 @@ func TestStreamableGET(t *testing.T) {
 		t.Errorf("GET with session ID: got status %d, want %d", got, want)
 	}
 }
+
+// contextCapturingHandler wraps fakeStreamableServer and captures request contexts
+type contextCapturingHandler struct {
+	capturedGetContext    *context.Context
+	capturedDeleteContext *context.Context
+	mu                    *sync.Mutex
+	server                *fakeStreamableServer
+}
+
+func (h *contextCapturingHandler) ServeHTTP(w http.ResponseWriter, req *http.Request) {
+	h.mu.Lock()
+	switch req.Method {
+	case http.MethodGet:
+		*h.capturedGetContext = req.Context()
+	case http.MethodDelete:
+		*h.capturedDeleteContext = req.Context()
+	}
+	h.mu.Unlock()
+
+	// Delegate to the fake server
+	h.server.ServeHTTP(w, req)
+}
+
+func TestStreamableClientContextPropagation(t *testing.T) {
+	// Test that context values are propagated to background HTTP requests
+	// (SSE GET and cleanup DELETE requests) in StreamableClientTransport.
+
+	type contextKey string
+	const testKey contextKey = "test-key"
+	const testValue = "test-value"
+
+	ctx := context.WithValue(context.Background(), testKey, testValue)
+
+	var capturedGetContext, capturedDeleteContext context.Context
+	var mu sync.Mutex
+
+	fake := &fakeStreamableServer{
+		t: t,
+		responses: fakeResponses{
+			{"POST", "", methodInitialize}: {
+				header: header{
+					"Content-Type":  "application/json",
+					sessionIDHeader: "123",
+				},
+				body: jsonBody(t, initResp),
+			},
+			{"POST", "123", notificationInitialized}: {
+				status:              http.StatusAccepted,
+				wantProtocolVersion: latestProtocolVersion,
+			},
+			{"GET", "123", ""}: {
+				header: header{
+					"Content-Type": "text/event-stream",
+				},
+				optional:            true,
+				wantProtocolVersion: latestProtocolVersion,
+				callback: func() {
+					// This captures the context when GET request is made
+					// Note: We can't directly access req.Context() here, but
+					// the test verifies that the fix enables context propagation
+				},
+			},
+			{"DELETE", "123", ""}: {},
+		},
+	}
+
+	handler := &contextCapturingHandler{
+		capturedGetContext:    &capturedGetContext,
+		capturedDeleteContext: &capturedDeleteContext,
+		mu:                    &mu,
+		server:                fake,
+	}
+
+	httpServer := httptest.NewServer(handler)
+	defer httpServer.Close()
+
+	streamTransport := &StreamableClientTransport{Endpoint: httpServer.URL}
+	mcpClient := NewClient(testImpl, nil)
+	session, err := mcpClient.Connect(ctx, streamTransport, nil)
+	if err != nil {
+		t.Fatalf("client.Connect() failed: %v", err)
+	}
+
+	if err := session.Close(); err != nil {
+		t.Errorf("closing session: %v", err)
+	}
+
+	mu.Lock()
+	defer mu.Unlock()
+
+	if capturedGetContext == nil {
+		t.Error("GET request context was not captured")
+	} else if got := capturedGetContext.Value(testKey); got != testValue {
+		t.Errorf("GET request context value: got %v, want %v", got, testValue)
+	}
+
+	if capturedDeleteContext == nil {
+		t.Error("DELETE request context was not captured")
+	} else if got := capturedDeleteContext.Value(testKey); got != testValue {
+		t.Errorf("DELETE request context value: got %v, want %v", got, testValue)
+	}
+
+}