fix(pty): don't leak an outer session id into session-less PTYs

docs/USAGE.md

@@ -268,7 +268,7 @@ The shell also receives two session variables, so scripts inside a session can d
 | `AGENT_TTY_ACTIVE`     | `true`.                                                   |
 | `AGENT_TTY_SESSION_ID` | The session's ID (a ULID, as printed by `create --json`). |
 
-Both replace inherited values, so a session created from inside another session reports its own ID, not the outer one. Like the `PROMPT_EOL_MARK` default, they are set at spawn time and are not stored in the session's recorded `env`.
+Both replace inherited values, so a session created from inside another session reports its own ID, not the outer one. A PTY that agent-tty spawns without a session (the `doctor` spawn probe) gets neither variable: inherited values are removed, so it never reports an outer session's ID. Like the `PROMPT_EOL_MARK` default, they are set at spawn time and are not stored in the session's recorded `env`.
 
 Any `--env` value always wins, so you can opt back into the shell's native behavior or override a session variable per session:
 

src/pty/createPty.ts

@@ -87,9 +87,11 @@ const PROMPT_EOL_MARK_ENV_KEY = 'PROMPT_EOL_MARK';
  *
  * Precedence, lowest to highest: the inherited process environment (minus
  * host-only internals), then the `PROMPT_EOL_MARK=''` default, then the
- * generated session metadata (`AGENT_TTY_ACTIVE=true` and, when `sessionId` is
- * given, `AGENT_TTY_SESSION_ID`), then the caller-supplied `env` (so a `--env`
- * value always wins — even an explicit empty one), then `TERM`. The defaults sit
+ * generated session metadata, then the caller-supplied `env` (so a `--env`
+ * value always wins — even an explicit empty one), then `TERM`. The session
+ * metadata is `AGENT_TTY_ACTIVE=true` plus `AGENT_TTY_SESSION_ID` when
+ * `sessionId` is given; without a `sessionId` both are removed instead, so a
+ * session-less PTY never claims an inherited outer session. The defaults sit
  * after the inherited environment so they also override inherited values: a
  * nested session reports its own id rather than the outer session's, and
  * captures stay deterministic regardless of the launching shell. The
@@ -117,8 +119,17 @@ export function resolvePtyEnv(
     resolved[PROMPT_EOL_MARK_ENV_KEY] = '';
   }
 
-  resolved.AGENT_TTY_ACTIVE = 'true';
-  if (sessionId) {
+  invariant(
+    sessionId === undefined || sessionId.length > 0,
+    'resolvePtyEnv: sessionId must be non-empty when provided',
+  );
+  if (sessionId === undefined) {
+    // A PTY without a session (the doctor spawn probe) must not claim one, so
+    // drop any session variables inherited from an outer session.
+    delete resolved.AGENT_TTY_ACTIVE;
+    delete resolved.AGENT_TTY_SESSION_ID;
+  } else {
+    resolved.AGENT_TTY_ACTIVE = 'true';
     resolved.AGENT_TTY_SESSION_ID = sessionId;
   }
 

test/unit/pty/createPty.test.ts

@@ -65,24 +65,58 @@ describe('resolvePtyEnv', () => {
     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', () => {
+  it('sets AGENT_TTY_ACTIVE and AGENT_TTY_SESSION_ID when sessionId is provided', () => {
     const resolved = resolvePtyEnv(
       {},
       'xterm-256color',
       {},
       'test-session-123',
     );
+    expect(resolved.AGENT_TTY_ACTIVE).toBe('true');
     expect(resolved.AGENT_TTY_SESSION_ID).toBe('test-session-123');
   });
 
-  it('does not set AGENT_TTY_SESSION_ID when sessionId is not provided', () => {
+  it('sets no session variables when sessionId is not provided', () => {
     const resolved = resolvePtyEnv({}, 'xterm-256color', {});
-    expect(resolved.AGENT_TTY_SESSION_ID).toBeUndefined();
+    expect(
+      Object.prototype.hasOwnProperty.call(resolved, 'AGENT_TTY_ACTIVE'),
+    ).toBe(false);
+    expect(
+      Object.prototype.hasOwnProperty.call(resolved, 'AGENT_TTY_SESSION_ID'),
+    ).toBe(false);
+  });
+
+  it('strips inherited outer session variables when sessionId is not provided', () => {
+    const resolved = resolvePtyEnv({}, 'xterm-256color', {
+      AGENT_TTY_ACTIVE: 'true',
+      AGENT_TTY_SESSION_ID: 'outer-session',
+      OTHER: 'kept',
+    });
+
+    expect(
+      Object.prototype.hasOwnProperty.call(resolved, 'AGENT_TTY_ACTIVE'),
+    ).toBe(false);
+    expect(
+      Object.prototype.hasOwnProperty.call(resolved, 'AGENT_TTY_SESSION_ID'),
+    ).toBe(false);
+    expect(resolved.OTHER).toBe('kept');
+  });
+
+  it('lets caller-supplied env set session variables when sessionId is not provided', () => {
+    const resolved = resolvePtyEnv(
+      { AGENT_TTY_ACTIVE: 'custom', AGENT_TTY_SESSION_ID: 'custom-id' },
+      'xterm-256color',
+      { AGENT_TTY_ACTIVE: 'true', AGENT_TTY_SESSION_ID: 'outer-session' },
+    );
+
+    expect(resolved.AGENT_TTY_ACTIVE).toBe('custom');
+    expect(resolved.AGENT_TTY_SESSION_ID).toBe('custom-id');
+  });
+
+  it('rejects an empty sessionId', () => {
+    expect(() => resolvePtyEnv({}, 'xterm-256color', {}, '')).toThrow(
+      /sessionId must be non-empty/,
+    );
   });
 
   it('replaces an inherited outer session id with the session id', () => {