Support context injection via SessionStart hook. (#15746)

packages/cli/src/ui/AppContainer.tsx

@@ -300,7 +300,31 @@ export const AppContainer = (props: AppContainerProps) => {
         const sessionStartSource = resumedSessionData
           ? SessionStartSource.Resume
           : SessionStartSource.Startup;
-        await fireSessionStartHook(hookMessageBus, sessionStartSource);
+        const result = await fireSessionStartHook(
+          hookMessageBus,
+          sessionStartSource,
+        );
+
+        if (result) {
+          if (result.systemMessage) {
+            historyManager.addItem(
+              {
+                type: MessageType.INFO,
+                text: result.systemMessage,
+              },
+              Date.now(),
+            );
+          }
+
+          const additionalContext = result.getAdditionalContext();
+          const geminiClient = config.getGeminiClient();
+          if (additionalContext && geminiClient) {
+            await geminiClient.addHistory({
+              role: 'user',
+              parts: [{ text: additionalContext }],
+            });
+          }
+        }
       }
 
       // Fire-and-forget: generate summary for previous session in background
@@ -321,6 +345,12 @@ export const AppContainer = (props: AppContainerProps) => {
         await fireSessionEndHook(hookMessageBus, SessionEndReason.Exit);
       }
     });
+    // Disable the dependencies check here. historyManager gets flagged
+    // but we don't want to react to changes to it because each new history
+    // item, including the ones from the start session hook will cause a
+    // re-render and an error when we try to reload config.
+    //
+    // eslint-disable-next-line react-hooks/exhaustive-deps
   }, [config, resumedSessionData]);
 
   useEffect(

packages/cli/src/ui/commands/clearCommand.ts

@@ -4,6 +4,7 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
+import type { DefaultHookOutput } from '@google/gemini-cli-core';
 import {
   uiTelemetryService,
   fireSessionEndHook,
@@ -14,6 +15,7 @@ import {
 } from '@google/gemini-cli-core';
 import type { SlashCommand } from './types.js';
 import { CommandKind } from './types.js';
+import { MessageType } from '../types.js';
 import { randomUUID } from 'node:crypto';
 
 export const clearCommand: SlashCommand = {
@@ -52,8 +54,9 @@ export const clearCommand: SlashCommand = {
     }
 
     // Fire SessionStart hook after clearing
+    let result: DefaultHookOutput | undefined;
     if (config?.getEnableHooks() && messageBus) {
-      await fireSessionStartHook(messageBus, SessionStartSource.Clear);
+      result = await fireSessionStartHook(messageBus, SessionStartSource.Clear);
     }
 
     // Give the event loop a chance to process any pending telemetry operations
@@ -68,5 +71,15 @@ export const clearCommand: SlashCommand = {
 
     uiTelemetryService.setLastPromptTokenCount(0);
     context.ui.clear();
+
+    if (result?.systemMessage) {
+      context.ui.addItem(
+        {
+          type: MessageType.INFO,
+          text: result.systemMessage,
+        },
+        Date.now(),
+      );
+    }
   },
 };