Migrate MCP Apps support from insiders mode to feature flag with insiders opt-in

docs/server-configuration.md

@@ -14,6 +14,7 @@ We currently support the following ways in which the GitHub MCP Server can be co
 | Dynamic Mode | Not available | `--dynamic-toolsets` flag or `GITHUB_DYNAMIC_TOOLSETS` env var |
 | Lockdown Mode | `X-MCP-Lockdown` header | `--lockdown-mode` flag or `GITHUB_LOCKDOWN_MODE` env var |
 | Insiders Mode | `X-MCP-Insiders` header or `/insiders` URL | `--insiders` flag or `GITHUB_INSIDERS` env var |
+| Feature Flags | `X-MCP-Features` header | `--features` flag |
 | Scope Filtering | Always enabled | Always enabled |
 | Server Name/Title | Not available | `GITHUB_MCP_SERVER_NAME` / `GITHUB_MCP_SERVER_TITLE` env vars or `github-mcp-server-config.json` |
 
@@ -390,7 +391,7 @@ Lockdown mode ensures the server only surfaces content in public repositories fr
 
 **Best for:** Users who want early access to experimental features and new tools before they reach general availability.
 
-Insiders Mode unlocks experimental features, such as [MCP Apps](./insiders-features.md#mcp-apps) support. We created this mode to have a way to roll out experimental features and collect feedback. So if you are using Insiders, please don't hesitate to share your feedback with us! Features in Insiders Mode may change, evolve, or be removed based on user feedback. 
+Insiders Mode unlocks experimental features, such as [MCP Apps](#mcp-apps) support. We created this mode to have a way to roll out experimental features and collect feedback. So if you are using Insiders, please don't hesitate to share your feedback with us! Features in Insiders Mode may change, evolve, or be removed based on user feedback.
 
 <table>
 <tr><th>Remote Server</th><th>Local Server</th></tr>
@@ -443,6 +444,62 @@ See [Insiders Features](./insiders-features.md) for a full list of what's availa
 
 ---
 
+### MCP Apps
+
+[MCP Apps](https://modelcontextprotocol.io/docs/extensions/apps) is an extension to the Model Context Protocol that enables servers to deliver interactive user interfaces to end users. Instead of returning plain text that the LLM must interpret and relay, tools can render forms, profiles, and dashboards right in the chat.
+
+MCP Apps is enabled by [Insiders Mode](#insiders-mode), or independently via the `remote_mcp_ui_apps` feature flag.
+
+**Supported tools:**
+
+| Tool | Description |
+|------|-------------|
+| `get_me` | Displays your GitHub user profile with avatar, bio, and stats in a rich card |
+| `issue_write` | Opens an interactive form to create or update issues |
+| `create_pull_request` | Provides a full PR creation form to create a pull request (or a draft pull request) |
+
+**Client requirements:** MCP Apps requires a host that supports the [MCP Apps extension](https://modelcontextprotocol.io/docs/extensions/apps). Currently tested with VS Code (`chat.mcp.apps.enabled` setting).
+
+<table>
+<tr><th>Remote Server</th><th>Local Server</th></tr>
+<tr valign="top">
+<td>
+
+```json
+{
+  "type": "http",
+  "url": "https://api.githubcopilot.com/mcp/",
+  "headers": {
+    "X-MCP-Features": "remote_mcp_ui_apps"
+  }
+}
+```
+
+</td>
+<td>
+
+```json
+{
+  "type": "stdio",
+  "command": "go",
+  "args": [
+    "run",
+    "./cmd/github-mcp-server",
+    "stdio",
+    "--features=remote_mcp_ui_apps"
+  ],
+  "env": {
+    "GITHUB_PERSONAL_ACCESS_TOKEN": "${input:github_token}"
+  }
+}
+```
+
+</td>
+</tr>
+</table>
+
+---
+
 ### Scope Filtering
 
 **Automatic feature:** The server handles OAuth scopes differently depending on authentication type:

internal/ghmcp/server.go

@@ -114,8 +114,8 @@ func NewStdioMCPServer(ctx context.Context, cfg github.MCPServerConfig) (*mcp.Se
 		return nil, fmt.Errorf("failed to create GitHub clients: %w", err)
 	}
 
-	// Create feature checker
-	featureChecker := createFeatureChecker(cfg.EnabledFeatures)
+	// Create feature checker — resolves explicit features + insiders expansion
+	featureChecker := createFeatureChecker(cfg.EnabledFeatures, cfg.InsidersMode)
 
 	// Create dependencies for tool handlers
 	obs, err := observability.NewExporters(cfg.Logger, metrics.NewNoopMetrics())
@@ -144,8 +144,7 @@ func NewStdioMCPServer(ctx context.Context, cfg github.MCPServerConfig) (*mcp.Se
 		WithTools(github.CleanTools(cfg.EnabledTools)).
 		WithExcludeTools(cfg.ExcludeTools).
 		WithServerInstructions().
-		WithFeatureChecker(featureChecker).
-		WithInsidersMode(cfg.InsidersMode)
+		WithFeatureChecker(featureChecker)
 
 	// Apply token scope filtering if scopes are known (for PAT filtering)
 	if cfg.TokenScopes != nil {
@@ -162,10 +161,12 @@ 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 available (requires running script/build-ui).
-	// We check availability to allow Insiders mode to work for non-UI features
-	// even when UI assets haven't been built.
-	if cfg.InsidersMode && github.UIAssetsAvailable() {
+	// 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)
 	}
 
@@ -334,15 +335,11 @@ func RunStdioServer(cfg StdioServerConfig) error {
 	return nil
 }
 
-// createFeatureChecker returns a FeatureFlagChecker that checks if a flag name
-// is present in the provided list of enabled features. For the local server,
-// this is populated from the --features CLI flag.
-func createFeatureChecker(enabledFeatures []string) inventory.FeatureFlagChecker {
-	// Build a set for O(1) lookup
-	featureSet := make(map[string]bool, len(enabledFeatures))
-	for _, f := range enabledFeatures {
-		featureSet[f] = true
-	}
+// createFeatureChecker returns a FeatureFlagChecker that resolves features
+// using the centralized ResolveFeatureFlags function. For the local server,
+// features are resolved once at startup from --features CLI flag + insiders mode.
+func createFeatureChecker(enabledFeatures []string, insidersMode bool) inventory.FeatureFlagChecker {
+	featureSet := github.ResolveFeatureFlags(enabledFeatures, insidersMode)
 	return func(_ context.Context, flagName string) (bool, error) {
 		return featureSet[flagName], nil
 	}

pkg/github/feature_flags.go

@@ -1,7 +1,54 @@
 package github
 
+// MCPAppsFeatureFlag is the feature flag name for MCP Apps (interactive UI forms).
+const MCPAppsFeatureFlag = "remote_mcp_ui_apps"
+
+// AllowedFeatureFlags is the allowlist of feature flags that can be enabled
+// by users via --features CLI flag or X-MCP-Features HTTP header.
+// Only flags in this list are accepted; unknown flags are silently ignored.
+// This is the single source of truth for which flags are user-controllable.
+var AllowedFeatureFlags = []string{
+	MCPAppsFeatureFlag,
+}
+
+// InsidersFeatureFlags is the list of feature flags that insiders mode enables.
+// When insiders mode is active, all flags in this list are treated as enabled.
+// This is the single source of truth for what "insiders" means in terms of
+// feature flag expansion.
+var InsidersFeatureFlags = []string{
+	MCPAppsFeatureFlag,
+}
+
 // FeatureFlags defines runtime feature toggles that adjust tool behavior.
 type FeatureFlags struct {
 	LockdownMode bool
 	InsidersMode bool
 }
+
+// ResolveFeatureFlags computes the effective set of enabled feature flags by:
+//  1. Taking explicitly enabled features (from CLI flags or HTTP headers)
+//  2. Adding insiders-expanded features when insiders mode is active
+//  3. Validating all features against the AllowedFeatureFlags allowlist
+//
+// Returns a set (map) for O(1) lookup by the feature checker.
+func ResolveFeatureFlags(enabledFeatures []string, insidersMode bool) map[string]bool {
+	allowed := make(map[string]bool, len(AllowedFeatureFlags))
+	for _, f := range AllowedFeatureFlags {
+		allowed[f] = true
+	}
+
+	effective := make(map[string]bool)
+	for _, f := range enabledFeatures {
+		if allowed[f] {
+			effective[f] = true
+		}
+	}
+	if insidersMode {
+		for _, f := range InsidersFeatureFlags {
+			if allowed[f] {
+				effective[f] = true
+			}
+		}
+	}
+	return effective
+}

pkg/github/feature_flags_test.go

@@ -136,6 +136,70 @@ func TestHelloWorld_ConditionalBehavior_Featureflag(t *testing.T) {
 	}
 }
 
+func TestResolveFeatureFlags(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name            string
+		enabledFeatures []string
+		insidersMode    bool
+		expectedFlags   []string
+		unexpectedFlags []string
+	}{
+		{
+			name:            "no features, no insiders",
+			enabledFeatures: nil,
+			insidersMode:    false,
+			expectedFlags:   nil,
+			unexpectedFlags: []string{MCPAppsFeatureFlag},
+		},
+		{
+			name:            "explicit feature enabled",
+			enabledFeatures: []string{MCPAppsFeatureFlag},
+			insidersMode:    false,
+			expectedFlags:   []string{MCPAppsFeatureFlag},
+		},
+		{
+			name:            "insiders mode enables insiders flags",
+			enabledFeatures: nil,
+			insidersMode:    true,
+			expectedFlags:   InsidersFeatureFlags,
+		},
+		{
+			name:            "unknown flags are filtered out",
+			enabledFeatures: []string{"unknown_flag", "another_unknown"},
+			insidersMode:    false,
+			unexpectedFlags: []string{"unknown_flag", "another_unknown"},
+		},
+		{
+			name:            "mix of known and unknown flags",
+			enabledFeatures: []string{MCPAppsFeatureFlag, "unknown_flag"},
+			insidersMode:    false,
+			expectedFlags:   []string{MCPAppsFeatureFlag},
+			unexpectedFlags: []string{"unknown_flag"},
+		},
+		{
+			name:            "explicit plus insiders deduplicates",
+			enabledFeatures: []string{MCPAppsFeatureFlag},
+			insidersMode:    true,
+			expectedFlags:   []string{MCPAppsFeatureFlag},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+			result := ResolveFeatureFlags(tt.enabledFeatures, tt.insidersMode)
+			for _, flag := range tt.expectedFlags {
+				assert.True(t, result[flag], "expected flag %q to be enabled", flag)
+			}
+			for _, flag := range tt.unexpectedFlags {
+				assert.False(t, result[flag], "expected flag %q to not be enabled", flag)
+			}
+		})
+	}
+}
+
 func TestHelloWorld_ConditionalBehavior_Config(t *testing.T) {
 	t.Parallel()
 

pkg/github/issues.go

@@ -1073,12 +1073,12 @@ Options are:
 				return utils.NewToolResultError(err.Error()), nil, nil
 			}
 
-			// When insiders mode is enabled and the client supports MCP Apps UI,
+			// When the MCP Apps feature flag is enabled and the client supports MCP Apps UI,
 			// check if this is a UI form submission. The UI sends _ui_submitted=true
 			// to distinguish form submissions from LLM calls.
 			uiSubmitted, _ := OptionalParam[bool](args, "_ui_submitted")
 
-			if deps.GetFlags(ctx).InsidersMode && clientSupportsUI(ctx, req) && !uiSubmitted {
+			if deps.IsFeatureEnabled(ctx, MCPAppsFeatureFlag) && clientSupportsUI(ctx, req) && !uiSubmitted {
 				if method == "update" {
 					// Skip the UI form when a state change is requested because
 					// the form only handles title/body editing and would lose the

pkg/github/issues_test.go

@@ -932,9 +932,9 @@ func Test_CreateIssue(t *testing.T) {
 	}
 }
 
-// Test_IssueWrite_InsidersMode_UIGate verifies the insiders mode UI gate
+// Test_IssueWrite_MCPApps_UIGate verifies the MCP Apps feature flag UI gate
 // behavior: UI clients get a form message, non-UI clients execute directly.
-func Test_IssueWrite_InsidersMode_UIGate(t *testing.T) {
+func Test_IssueWrite_MCPApps_UIGate(t *testing.T) {
 	t.Parallel()
 
 	mockIssue := &github.Issue{
@@ -949,11 +949,17 @@ func Test_IssueWrite_InsidersMode_UIGate(t *testing.T) {
 		PostReposIssuesByOwnerByRepo: mockResponse(t, http.StatusCreated, mockIssue),
 	}))
 
-	deps := BaseDeps{
-		Client:    client,
-		GQLClient: githubv4.NewClient(nil),
-		Flags:     FeatureFlags{InsidersMode: true},
+	mcpAppsChecker := func(_ context.Context, flag string) (bool, error) {
+		return flag == MCPAppsFeatureFlag, nil
 	}
+	deps := NewBaseDeps(
+		client, githubv4.NewClient(nil), nil, nil,
+		translations.NullTranslationHelper,
+		FeatureFlags{},
+		0,
+		mcpAppsChecker,
+		stubExporters(),
+	)
 	handler := serverTool.Handler(deps)
 
 	t.Run("UI client without _ui_submitted returns form message", func(t *testing.T) {
@@ -1066,11 +1072,14 @@ func Test_IssueWrite_InsidersMode_UIGate(t *testing.T) {
 			),
 		))
 
-		closeDeps := BaseDeps{
-			Client:    closeClient,
-			GQLClient: closeGQLClient,
-			Flags:     FeatureFlags{InsidersMode: true},
-		}
+		closeDeps := NewBaseDeps(
+			closeClient, closeGQLClient, nil, nil,
+			translations.NullTranslationHelper,
+			FeatureFlags{},
+			0,
+			mcpAppsChecker,
+			stubExporters(),
+		)
 		closeHandler := serverTool.Handler(closeDeps)
 
 		request := createMCPRequestWithSession(t, ClientNameVSCodeInsiders, true, map[string]any{

pkg/github/pullrequests.go

@@ -601,12 +601,12 @@ func CreatePullRequest(t translations.TranslationHelperFunc) inventory.ServerToo
 				return utils.NewToolResultError(err.Error()), nil, nil
 			}
 
-			// When insiders mode is enabled and the client supports MCP Apps UI,
+			// When the MCP Apps feature flag is enabled and the client supports MCP Apps UI,
 			// check if this is a UI form submission. The UI sends _ui_submitted=true
 			// to distinguish form submissions from LLM calls.
 			uiSubmitted, _ := OptionalParam[bool](args, "_ui_submitted")
 
-			if deps.GetFlags(ctx).InsidersMode && clientSupportsUI(ctx, req) && !uiSubmitted {
+			if deps.IsFeatureEnabled(ctx, MCPAppsFeatureFlag) && clientSupportsUI(ctx, req) && !uiSubmitted {
 				return utils.NewToolResultText(fmt.Sprintf("Ready to create a pull request in %s/%s. IMPORTANT: The PR has NOT been created yet. Do NOT tell the user the PR was created. The user MUST click Submit in the form to create it.", owner, repo)), nil, nil
 			}
 

pkg/github/pullrequests_test.go

@@ -2312,9 +2312,9 @@ func Test_CreatePullRequest(t *testing.T) {
 	}
 }
 
-// Test_CreatePullRequest_InsidersMode_UIGate verifies the insiders mode UI gate
+// Test_CreatePullRequest_MCPApps_UIGate verifies the MCP Apps feature flag UI gate
 // behavior: UI clients get a form message, non-UI clients execute directly.
-func Test_CreatePullRequest_InsidersMode_UIGate(t *testing.T) {
+func Test_CreatePullRequest_MCPApps_UIGate(t *testing.T) {
 	t.Parallel()
 
 	mockPR := &github.PullRequest{
@@ -2332,11 +2332,17 @@ func Test_CreatePullRequest_InsidersMode_UIGate(t *testing.T) {
 		PostReposPullsByOwnerByRepo: mockResponse(t, http.StatusCreated, mockPR),
 	}))
 
-	deps := BaseDeps{
-		Client:    client,
-		GQLClient: githubv4.NewClient(nil),
-		Flags:     FeatureFlags{InsidersMode: true},
+	mcpAppsChecker := func(_ context.Context, flag string) (bool, error) {
+		return flag == MCPAppsFeatureFlag, nil
 	}
+	deps := NewBaseDeps(
+		client, githubv4.NewClient(nil), nil, nil,
+		translations.NullTranslationHelper,
+		FeatureFlags{},
+		0,
+		mcpAppsChecker,
+		stubExporters(),
+	)
 	handler := serverTool.Handler(deps)
 
 	t.Run("UI client without _ui_submitted returns form message", func(t *testing.T) {

pkg/http/handler.go

@@ -253,7 +253,9 @@ func DefaultInventoryFactory(_ *ServerConfig, t translations.TranslationHelperFu
 }
 
 // InventoryFiltersForRequest applies filters to the inventory builder
-// based on the request context and headers
+// based on the request context and headers.
+// MCP Apps UI metadata is handled by the builder via the feature checker —
+// no need to check headers here.
 func InventoryFiltersForRequest(r *http.Request, builder *inventory.Builder) *inventory.Builder {
 	ctx := r.Context()