diff --git a/src/host/hostMain.ts b/src/host/hostMain.ts
index 6b25fc4..ff56591 100644
--- a/src/host/hostMain.ts
+++ b/src/host/hostMain.ts
@@ -245,6 +245,7 @@ export async function runHost(sessionId: string): Promise<void> {
     rows: manifest.rows,
     env: manifest.env ?? {},
     term: manifest.term ?? 'xterm-256color',
+    sessionId,
   });
 
   invariant(
diff --git a/src/pty/createPty.ts b/src/pty/createPty.ts
index fbf5005..97c5897 100644
--- a/src/pty/createPty.ts
+++ b/src/pty/createPty.ts
@@ -16,6 +16,7 @@ export interface PtyOptions {
   rows: number;
   env: Record<string, string>;
   term: string;
+  sessionId?: string;
 }
 
 const EXECUTABLE_PERMISSION_MASK = 0o111;
@@ -96,6 +97,7 @@ export function resolvePtyEnv(
   env: Record<string, string>,
   term: string,
   baseEnv: Record<string, string | undefined> = process.env,
+  sessionId?: string,
 ): Record<string, string> {
   const resolved: Record<string, string> = {};
   for (const [key, value] of Object.entries(baseEnv)) {
@@ -113,6 +115,12 @@ export function resolvePtyEnv(
 
   Object.assign(resolved, env);
   resolved.TERM = term;
+
+  resolved.AGENT_TTY_ACTIVE = 'true';
+  if (sessionId) {
+    resolved.AGENT_TTY_SESSION_ID = sessionId;
+  }
+
   return resolved;
 }
 
@@ -144,6 +152,6 @@ export function createPty(options: PtyOptions): IPty {
     cwd,
     cols,
     rows,
-    env: resolvePtyEnv(env, term),
+    env: resolvePtyEnv(env, term, process.env, options.sessionId),
   });
 }
diff --git a/test/unit/pty/createPty.test.ts b/test/unit/pty/createPty.test.ts
index 62c0e71..477069b 100644
--- a/test/unit/pty/createPty.test.ts
+++ b/test/unit/pty/createPty.test.ts
@@ -64,4 +64,24 @@ describe('resolvePtyEnv', () => {
     expect(resolved.BAZ).toBe('qux');
     expect(Object.prototype.hasOwnProperty.call(resolved, 'EMPTY')).toBe(false);
   });
+
+  it('unconditionally sets AGENT_TTY_ACTIVE to true', () => {
+    const resolved = resolvePtyEnv({}, 'xterm-256color', {});
+    expect(resolved.AGENT_TTY_ACTIVE).toBe('true');
+  });
+
+  it('sets AGENT_TTY_SESSION_ID when sessionId is provided', () => {
+    const resolved = resolvePtyEnv(
+      {},
+      'xterm-256color',
+      {},
+      'test-session-123',
+    );
+    expect(resolved.AGENT_TTY_SESSION_ID).toBe('test-session-123');
+  });
+
+  it('does not set AGENT_TTY_SESSION_ID when sessionId is not provided', () => {
+    const resolved = resolvePtyEnv({}, 'xterm-256color', {});
+    expect(resolved.AGENT_TTY_SESSION_ID).toBeUndefined();
+  });
 });