Register MCP App UI resources in shared server constructor

SamMorrowDrums · Copilot · SamMorrowDrums · commit 3fbf64f7acf1 · 2026-05-29T12:00:05.000+02:00
The remote/HTTP server never called RegisterUIResources, so when the remote_mcp_ui_apps feature flag was enabled per-request, tools like issue_write and create_pull_request would advertise a ui:// resource URI in their _meta.ui block but the resource itself was not registered. The client's follow-up resources/read call then failed with -32002 'Resource not found' (the error surfaced as 'Error loading MCP App: MPC -32002: Resource not found' in VS Code). The stdio bootstrap also gated registration on featureChecker called with context.Background(), which can't see per-request flag overrides. Move RegisterUIResources into pkg/github.NewMCPServer (the shared constructor used by both stdio and HTTP), gated only on UIAssetsAvailable(). The resources are inert static HTML; the inventory still strips _meta.ui from tools per-request via stripMCPAppsMetadata, so the URI is only advertised to clients when the flag is on for that request. Fixes #2467 Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
diff --git a/internal/ghmcp/server.go b/internal/ghmcp/server.go
@@ -173,15 +173,6 @@ func NewStdioMCPServer(ctx context.Context, cfg github.MCPServerConfig) (*mcp.Se
 		return nil, fmt.Errorf("failed to create GitHub MCP server: %w", err)
 	}
 
-	// Register MCP App UI resources if the remote_mcp_ui_apps feature flag is enabled
-	// and UI assets are available (requires running script/build-ui).
-	// We check availability to allow the feature flag to be enabled without
-	// requiring a UI build (graceful degradation).
-	mcpAppsEnabled, _ := featureChecker(context.Background(), github.MCPAppsFeatureFlag)
-	if mcpAppsEnabled && github.UIAssetsAvailable() {
-		github.RegisterUIResources(ghServer)
-	}
-
 	ghServer.AddReceivingMiddleware(addUserAgentsMiddleware(cfg, clients.restUATransp, clients.gqlHTTP))
 
 	return ghServer, nil
diff --git a/pkg/github/server.go b/pkg/github/server.go
@@ -102,6 +102,18 @@ func NewMCPServer(ctx context.Context, cfg *MCPServerConfig, deps ToolDependenci
 	// Register GitHub tools/resources/prompts from the inventory.
 	inv.RegisterAll(ctx, ghServer, deps)
 
+	// Register MCP App UI resources whenever the embedded UI assets are
+	// available. The resources are static HTML and are only referenced by
+	// tools when the remote_mcp_ui_apps feature flag is enabled for the
+	// request (the inventory strips the _meta.ui block otherwise via
+	// stripMCPAppsMetadata), so registering them unconditionally is safe.
+	// Registering here &mdash; rather than in the stdio bootstrap &mdash; ensures the
+	// remote/HTTP server also serves them, fixing the "-32002 Resource not
+	// found" error clients hit after the tool returns a ui:// URI.
+	if UIAssetsAvailable() {
+		RegisterUIResources(ghServer)
+	}
+
 	return ghServer, nil
 }
 
diff --git a/pkg/github/ui_resources_test.go b/pkg/github/ui_resources_test.go
@@ -0,0 +1,123 @@
+package github
+
+import (
+	"context"
+	"testing"
+
+	"github.com/github/github-mcp-server/pkg/inventory"
+	"github.com/modelcontextprotocol/go-sdk/mcp"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// TestRegisterUIResources_ReadableViaClient verifies that each UI resource URI
+// advertised by an MCP App-enabled tool (e.g. issue_write, create_pull_request,
+// get_me) actually resolves to a registered resource on the server.
+//
+// Regression test for the "Error loading MCP App: MPC -32002: Resource not
+// found" bug reported in issue #2467, where the HTTP/remote server returned a
+// resource URI in the tool's _meta.ui block but never registered the matching
+// resource &mdash; so the follow-up resources/read call from the client failed.
+func TestRegisterUIResources_ReadableViaClient(t *testing.T) {
+	t.Parallel()
+
+	if !UIAssetsAvailable() {
+		t.Skip("UI assets not built; run script/build-ui to enable this test")
+	}
+
+	srv := mcp.NewServer(&mcp.Implementation{Name: "test", Version: "0.0.1"}, nil)
+	RegisterUIResources(srv)
+
+	// Connect an in-memory client/server pair and read each advertised URI.
+	st, ct := mcp.NewInMemoryTransports()
+
+	type clientResult struct {
+		session *mcp.ClientSession
+		err     error
+	}
+	clientCh := make(chan clientResult, 1)
+	go func() {
+		client := mcp.NewClient(&mcp.Implementation{Name: "test-client"}, nil)
+		cs, err := client.Connect(context.Background(), ct, nil)
+		clientCh <- clientResult{session: cs, err: err}
+	}()
+
+	ss, err := srv.Connect(context.Background(), st, nil)
+	require.NoError(t, err)
+	t.Cleanup(func() { _ = ss.Close() })
+
+	got := <-clientCh
+	require.NoError(t, got.err)
+	t.Cleanup(func() { _ = got.session.Close() })
+
+	uris := []string{
+		GetMeUIResourceURI,
+		IssueWriteUIResourceURI,
+		PullRequestWriteUIResourceURI,
+	}
+	for _, uri := range uris {
+		t.Run(uri, func(t *testing.T) {
+			res, err := got.session.ReadResource(context.Background(), &mcp.ReadResourceParams{URI: uri})
+			require.NoError(t, err, "resource %s should be registered (got -32002 means it isn't)", uri)
+			require.NotNil(t, res)
+			require.NotEmpty(t, res.Contents)
+			assert.Equal(t, uri, res.Contents[0].URI)
+			assert.Equal(t, MCPAppMIMEType, res.Contents[0].MIMEType)
+			assert.NotEmpty(t, res.Contents[0].Text, "UI resource should return HTML body")
+		})
+	}
+}
+
+// TestNewMCPServer_RegistersUIResources verifies that NewMCPServer &mdash; the
+// shared constructor used by both the stdio and HTTP entry points &mdash; registers
+// the UI resources when UI assets are embedded. Previously this registration
+// only happened in the stdio bootstrap, so remote/HTTP clients hit -32002.
+func TestNewMCPServer_RegistersUIResources(t *testing.T) {
+	t.Parallel()
+
+	if !UIAssetsAvailable() {
+		t.Skip("UI assets not built; run script/build-ui to enable this test")
+	}
+
+	srv, err := NewMCPServer(context.Background(), &MCPServerConfig{
+		Version:    "test",
+		Translator: stubTranslator,
+	}, stubDeps{t: stubTranslator}, mustEmptyInventory(t))
+	require.NoError(t, err)
+
+	st, ct := mcp.NewInMemoryTransports()
+
+	type clientResult struct {
+		session *mcp.ClientSession
+		err     error
+	}
+	clientCh := make(chan clientResult, 1)
+	go func() {
+		client := mcp.NewClient(&mcp.Implementation{Name: "test-client"}, nil)
+		cs, err := client.Connect(context.Background(), ct, nil)
+		clientCh <- clientResult{session: cs, err: err}
+	}()
+
+	ss, err := srv.Connect(context.Background(), st, nil)
+	require.NoError(t, err)
+	t.Cleanup(func() { _ = ss.Close() })
+
+	got := <-clientCh
+	require.NoError(t, got.err)
+	t.Cleanup(func() { _ = got.session.Close() })
+
+	res, err := got.session.ReadResource(context.Background(), &mcp.ReadResourceParams{URI: IssueWriteUIResourceURI})
+	require.NoError(t, err)
+	require.NotNil(t, res)
+	require.NotEmpty(t, res.Contents)
+	assert.Equal(t, MCPAppMIMEType, res.Contents[0].MIMEType)
+}
+
+// mustEmptyInventory builds an empty inventory for tests that only care about
+// resources/prompts registered outside the inventory (such as the UI resources).
+func mustEmptyInventory(t *testing.T) *inventory.Inventory {
+	t.Helper()
+	inv, err := NewInventory(stubTranslator).WithToolsets([]string{}).Build()
+	require.NoError(t, err)
+	return inv
+}
