feat(hooks): display hook system messages in UI (#24616)

packages/core/src/config/config.ts

@@ -908,8 +908,8 @@ export class Config implements McpContext, AgentLoopContext {
   private readonly acceptRawOutputRisk: boolean;
   private readonly dynamicModelConfiguration: boolean;
   private pendingIncludeDirectories: string[];
-  private readonly enableHooks: boolean;
   private readonly enableHooksUI: boolean;
+  private readonly enableHooks: boolean;
 
   private hooks: { [K in HookEventName]?: HookDefinition[] } | undefined;
   private projectHooks:

packages/core/src/hooks/hookEventHandler.ts

@@ -458,6 +458,15 @@ export class HookEventHandler {
       );
 
       logHookCall(this.context.config, hookCallEvent);
+
+      // Emit structured system message event for UI display
+      if (result.output?.systemMessage && result.outputFormat === 'json') {
+        coreEvents.emitHookSystemMessage({
+          hookName,
+          eventName,
+          message: result.output.systemMessage,
+        });
+      }
     }
 
     // Log individual errors

packages/core/src/hooks/hookRunner.test.ts

@@ -204,7 +204,11 @@ describe('HookRunner', () => {
       };
 
       it('should execute command hook successfully', async () => {
-        const mockOutput = { decision: 'allow', reason: 'All good' };
+        const mockOutput = {
+          decision: 'allow',
+          reason: 'All good',
+          format: 'json',
+        };
 
         // Mock successful execution
         mockSpawn.mockStdoutOn.mockImplementation(
@@ -623,6 +627,7 @@ describe('HookRunner', () => {
         hookSpecificOutput: {
           additionalContext: 'Context from hook 1',
         },
+        format: 'json',
       };
 
       let hookCallCount = 0;
@@ -803,6 +808,7 @@ describe('HookRunner', () => {
       expect(result.success).toBe(true);
       expect(result.exitCode).toBe(0);
       // Should convert plain text to structured output
+      expect(result.outputFormat).toBe('text');
       expect(result.output).toEqual({
         decision: 'allow',
         systemMessage: invalidJson,
@@ -835,6 +841,7 @@ describe('HookRunner', () => {
       );
 
       expect(result.success).toBe(true);
+      expect(result.outputFormat).toBe('text');
       expect(result.output).toEqual({
         decision: 'allow',
         systemMessage: malformedJson,
@@ -868,6 +875,7 @@ describe('HookRunner', () => {
 
       expect(result.success).toBe(false);
       expect(result.exitCode).toBe(1);
+      expect(result.outputFormat).toBe('text');
       expect(result.output).toEqual({
         decision: 'allow',
         systemMessage: `Warning: ${invalidJson}`,
@@ -901,6 +909,7 @@ describe('HookRunner', () => {
 
       expect(result.success).toBe(false);
       expect(result.exitCode).toBe(2);
+      expect(result.outputFormat).toBe('text');
       expect(result.output).toEqual({
         decision: 'deny',
         reason: invalidJson,
@@ -936,7 +945,11 @@ describe('HookRunner', () => {
     });
 
     it('should handle double-encoded JSON string', async () => {
-      const mockOutput = { decision: 'allow', reason: 'All good' };
+      const mockOutput = {
+        decision: 'allow',
+        reason: 'All good',
+        format: 'json',
+      };
       const doubleEncodedJson = JSON.stringify(JSON.stringify(mockOutput));
 
       mockSpawn.mockStdoutOn.mockImplementation(

packages/core/src/hooks/hookRunner.ts

@@ -447,6 +447,7 @@ export class HookRunner {
 
         // Parse output
         let output: HookOutput | undefined;
+        let outputFormat: 'json' | 'text' | undefined;
 
         const textToParse = stdout.trim() || stderr.trim();
         if (textToParse) {
@@ -460,6 +461,7 @@ export class HookRunner {
             if (parsed && typeof parsed === 'object') {
               // eslint-disable-next-line @typescript-eslint/no-unsafe-type-assertion
               output = parsed as HookOutput;
+              outputFormat = 'json';
             }
           } catch {
             // Not JSON, convert plain text to structured output
@@ -467,6 +469,7 @@ export class HookRunner {
               textToParse,
               exitCode || EXIT_CODE_SUCCESS,
             );
+            outputFormat = 'text';
           }
         }
 
@@ -475,6 +478,7 @@ export class HookRunner {
           eventName,
           success: exitCode === EXIT_CODE_SUCCESS,
           output,
+          outputFormat,
           stdout,
           stderr,
           exitCode: exitCode || EXIT_CODE_SUCCESS,
@@ -523,7 +527,7 @@ export class HookRunner {
     exitCode: number,
   ): HookOutput {
     if (exitCode === EXIT_CODE_SUCCESS) {
-      // Success - treat as system message or additional context
+      // Success
       return {
         decision: 'allow',
         systemMessage: text,

packages/core/src/hooks/types.ts

@@ -734,6 +734,8 @@ export interface HookExecutionResult {
   exitCode?: number;
   duration: number;
   error?: Error;
+  /** The format of the output provided by the hook */
+  outputFormat?: 'json' | 'text';
 }
 
 /**

packages/core/src/utils/events.ts

@@ -109,6 +109,13 @@ export interface HookEndPayload extends HookPayload {
   success: boolean;
 }
 
+/**
+ * Payload for the 'hook-system-message' event.
+ */
+export interface HookSystemMessagePayload extends HookPayload {
+  message: string;
+}
+
 /**
  * Payload for the 'retry-attempt' event.
  */
@@ -183,6 +190,7 @@ export enum CoreEvent {
   SettingsChanged = 'settings-changed',
   HookStart = 'hook-start',
   HookEnd = 'hook-end',
+  HookSystemMessage = 'hook-system-message',
   AgentsRefreshed = 'agents-refreshed',
   AdminSettingsChanged = 'admin-settings-changed',
   RetryAttempt = 'retry-attempt',
@@ -217,6 +225,7 @@ export interface CoreEvents extends ExtensionEvents {
   [CoreEvent.SettingsChanged]: never[];
   [CoreEvent.HookStart]: [HookStartPayload];
   [CoreEvent.HookEnd]: [HookEndPayload];
+  [CoreEvent.HookSystemMessage]: [HookSystemMessagePayload];
   [CoreEvent.AgentsRefreshed]: never[];
   [CoreEvent.AdminSettingsChanged]: never[];
   [CoreEvent.RetryAttempt]: [RetryAttemptPayload];
@@ -339,6 +348,13 @@ export class CoreEventEmitter extends EventEmitter<CoreEvents> {
     this.emit(CoreEvent.HookEnd, payload);
   }
 
+  /**
+   * Notifies subscribers that a hook has provided a system message.
+   */
+  emitHookSystemMessage(payload: HookSystemMessagePayload): void {
+    this.emit(CoreEvent.HookSystemMessage, payload);
+  }
+
   /**
    * Notifies subscribers that agents have been refreshed.
    */