diff --git a/mcp/streamable.go b/mcp/streamable.go
index 8ff9cd1f..470bc7f7 100644
--- a/mcp/streamable.go
+++ b/mcp/streamable.go
@@ -1553,6 +1553,30 @@ func (c *streamableServerConn) servePOST(w http.ResponseWriter, req *http.Reques
 	done := make(chan struct{})
 	stream.done = done
 	stream.protocolVersion = effectiveVersion
+
+	// Reject any call whose ID is already in flight on this session,
+	// atomically and without partial registration.
+	c.mu.Lock()
+	for reqID := range calls {
+		if _, ok := c.requestStreams[reqID]; ok {
+			c.mu.Unlock()
+			writeJSONRPCError(w, http.StatusBadRequest, reqID, &jsonrpc.Error{
+				Code:    jsonrpc.CodeInvalidRequest,
+				Message: fmt.Sprintf("duplicate in-flight request ID %v", reqID.Raw()),
+			})
+			return
+		}
+	}
+	c.streams[stream.id] = stream
+	for reqID := range calls {
+		c.requestStreams[reqID] = stream.id
+	}
+	c.mu.Unlock()
+
+	// TODO(rfindley): if we have no event store, we should really cancel all
+	// remaining requests here, since the client will never get the results.
+	defer stream.release()
+
 	if c.jsonResponse {
 		// JSON mode: collect messages in pendingJSONMessages until done.
 		// Set pendingJSONMessages to a non-nil value to signal that this is an
@@ -1581,20 +1605,6 @@ func (c *streamableServerConn) servePOST(w http.ResponseWriter, req *http.Reques
 		}
 	}
 
-	// TODO(rfindley): if we have no event store, we should really cancel all
-	// remaining requests here, since the client will never get the results.
-	defer stream.release()
-
-	// The stream is now set up to deliver messages.
-	//
-	// Register it before publishing incoming messages.
-	c.mu.Lock()
-	c.streams[stream.id] = stream
-	for reqID := range calls {
-		c.requestStreams[reqID] = stream.id
-	}
-	c.mu.Unlock()
-
 	// Publish incoming messages.
 	for _, msg := range incoming {
 		select {
diff --git a/mcp/streamable_test.go b/mcp/streamable_test.go
index 2efee20e..f3bc98db 100644
--- a/mcp/streamable_test.go
+++ b/mcp/streamable_test.go
@@ -3716,3 +3716,67 @@ func TestStreamableHTTP_E2E_DiscoverSuccess(t *testing.T) {
 		t.Errorf("CallTool result[0] = %+v, want TextContent{Text:\"hello\"}", res.Content[0])
 	}
 }
+
+// A POST whose call ID is already in flight on the same session must be rejected
+// with a JSON-RPC error, must not overwrite the existing request-to-stream
+// mapping, and must not publish the duplicate message to the session's incoming
+// channel.
+func TestStreamableServerRejectsDuplicateInFlightRequestID(t *testing.T) {
+	id := jsonrpc2.Int64ID(1)
+	conn := &streamableServerConn{
+		logger:   ensureLogger(nil),
+		incoming: make(chan jsonrpc.Message, 1),
+		done:     make(chan struct{}),
+		streams: map[string]*stream{
+			"existing": {
+				id:       "existing",
+				logger:   ensureLogger(nil),
+				requests: map[jsonrpc.ID]struct{}{id: {}},
+			},
+		},
+		requestStreams: map[jsonrpc.ID]string{id: "existing"},
+	}
+
+	data, err := jsonrpc2.EncodeMessage(req(1, methodPing, &PingParams{}))
+	if err != nil {
+		t.Fatalf("EncodeMessage() error = %v", err)
+	}
+	ctx, cancel := context.WithTimeout(t.Context(), 20*time.Millisecond)
+	defer cancel()
+	httpReq := httptest.NewRequestWithContext(ctx, http.MethodPost, "/", bytes.NewReader(data))
+	httpReq.Header.Set("Content-Type", "application/json")
+	httpReq.Header.Set("Accept", "application/json, text/event-stream")
+
+	rec := httptest.NewRecorder()
+	conn.servePOST(rec, httpReq)
+
+	if rec.Code != http.StatusBadRequest {
+		t.Fatalf("status = %d, want %d; body = %s", rec.Code, http.StatusBadRequest, rec.Body.String())
+	}
+	msg, err := jsonrpc2.DecodeMessage(rec.Body.Bytes())
+	if err != nil {
+		t.Fatalf("DecodeMessage() error = %v; body = %s", err, rec.Body.String())
+	}
+	resp, ok := msg.(*jsonrpc.Response)
+	if !ok {
+		t.Fatalf("response type = %T, want *jsonrpc.Response", msg)
+	}
+	if got := resp.ID.Raw(); got != int64(1) {
+		t.Fatalf("response ID = %v, want 1", got)
+	}
+	var jerr *jsonrpc.Error
+	if !errors.As(resp.Error, &jerr) {
+		t.Fatalf("response error = %v, want *jsonrpc.Error", resp.Error)
+	}
+	if jerr.Code != jsonrpc.CodeInvalidRequest {
+		t.Fatalf("error code = %d, want %d", jerr.Code, jsonrpc.CodeInvalidRequest)
+	}
+	if got := conn.requestStreams[id]; got != "existing" {
+		t.Fatalf("requestStreams[%v] = %q, want %q", id, got, "existing")
+	}
+	select {
+	case msg := <-conn.incoming:
+		t.Fatalf("duplicate request was published to incoming: %v", msg)
+	default:
+	}
+}