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.`,