Support context injection via SessionStart hook. (#15746)

packages/cli/src/gemini.tsx

@@ -634,6 +634,16 @@ export async function main() {
     await config.initialize();
     startupProfiler.flush(config);
 
+    // If not a TTY, read from stdin
+    // This is for cases where the user pipes input directly into the command
+    let stdinData: string | undefined = undefined;
+    if (!process.stdin.isTTY) {
+      stdinData = await readStdin();
+      if (stdinData) {
+        input = input ? `${stdinData}\n\n${input}` : stdinData;
+      }
+    }
+
     // Fire SessionStart hook through MessageBus (only if hooks are enabled)
     // Must be called AFTER config.initialize() to ensure HookRegistry is loaded
     const hooksEnabled = config.getEnableHooks();
@@ -642,7 +652,23 @@ export async function main() {
       const sessionStartSource = resumedSessionData
         ? SessionStartSource.Resume
         : SessionStartSource.Startup;
-      await fireSessionStartHook(hookMessageBus, sessionStartSource);
+      const result = await fireSessionStartHook(
+        hookMessageBus,
+        sessionStartSource,
+      );
+
+      if (result) {
+        if (result.systemMessage) {
+          writeToStderr(result.systemMessage + '\n');
+        }
+        const additionalContext = result.getAdditionalContext();
+        if (additionalContext) {
+          // Prepend context to input (System Context -> Stdin -> Question)
+          input = input
+            ? `${additionalContext}\n\n${input}`
+            : additionalContext;
+        }
+      }
 
       // Register SessionEnd hook for graceful exit
       registerCleanup(async () => {
@@ -650,14 +676,6 @@ export async function main() {
       });
     }
 
-    // If not a TTY, read from stdin
-    // This is for cases where the user pipes input directly into the command
-    if (!process.stdin.isTTY) {
-      const stdinData = await readStdin();
-      if (stdinData) {
-        input = `${stdinData}\n\n${input}`;
-      }
-    }
     if (!input) {
       debugLogger.error(
         `No input provided via stdin. Input can be provided by piping data into gemini or using the --prompt option.`,

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(),
+      );
+    }
   },
 };