workspace-server/internal/handlers/a2a_proxy.go

@@ -686,11 +686,22 @@ func (h *WorkspaceHandler) resolveAgentURL(ctx context.Context, workspaceID stri
 		_ = db.CacheURL(ctx, workspaceID, agentURL)
 	}
 
-	// When the platform runs inside Docker, 127.0.0.1:{host_port} is
-	// unreachable (it's the platform container's own localhost, not the
-	// Docker host). Rewrite to the container's Docker-bridge hostname.
+	// When the platform runs inside Docker, a managed workspace's
+	// 127.0.0.1:{host_port} URL points at the Docker host and must be
+	// rewritten to the workspace container's Docker-bridge hostname.
+	// External runtimes are not managed containers; their local test/runtime
+	// URL is the target and must not be synthesized into ws-<id>:8000.
 	if strings.HasPrefix(agentURL, "http://127.0.0.1:") && h.provisioner != nil && platformInDocker {
-		agentURL = provisioner.InternalURL(workspaceID)
+		var wsRuntime string
+		if err := db.DB.QueryRowContext(ctx,
+			`SELECT COALESCE(runtime, 'langgraph') FROM workspaces WHERE id = $1`,
+			workspaceID,
+		).Scan(&wsRuntime); err != nil {
+			log.Printf("ProxyA2A: runtime lookup before Docker URL rewrite failed for %s: %v", workspaceID, err)
+		}
+		if !isExternalLikeRuntime(wsRuntime) {
+			agentURL = provisioner.InternalURL(workspaceID)
+		}
 	}
 	// SSRF defence: reject private/metadata URLs before making outbound call.
 	if err := isSafeURL(agentURL); err != nil {

workspace-server/internal/handlers/a2a_proxy_test.go

@@ -1511,6 +1511,35 @@ func TestResolveAgentURL_DockerRewrite(t *testing.T) {
 	}
 }
 
+func TestResolveAgentURL_ExternalRuntimeLoopbackNotRewrittenInDocker(t *testing.T) {
+	mock := setupTestDB(t)
+	mr := setupTestRedis(t)
+	allowLoopbackForTest(t)
+	handler := NewWorkspaceHandler(newTestBroadcaster(), nil, "http://localhost:8080", t.TempDir())
+	waitForHandlerAsyncBeforeDBCleanup(t, handler)
+	handler.provisioner = &stubLocalProv{}
+
+	restore := setPlatformInDockerForTest(true)
+	defer restore()
+
+	agentURL := "http://127.0.0.1:55555"
+	mr.Set("ws:ws-external:url", agentURL)
+	mock.ExpectQuery("SELECT COALESCE\\(runtime").
+		WithArgs("ws-external").
+		WillReturnRows(sqlmock.NewRows([]string{"runtime"}).AddRow("external"))
+
+	url, perr := handler.resolveAgentURL(context.Background(), "ws-external")
+	if perr != nil {
+		t.Fatalf("unexpected error: %+v", perr)
+	}
+	if url != agentURL {
+		t.Errorf("external runtime loopback URL must not be rewritten; got %q want %q", url, agentURL)
+	}
+	if err := mock.ExpectationsWereMet(); err != nil {
+		t.Errorf("unmet sqlmock expectations: %v", err)
+	}
+}
+
 // --- dispatchA2A direct unit tests ---
 
 func TestDispatchA2A_BuildRequestError(t *testing.T) {