diff --git a/packages/mosaic/src/fleet/promotion-transport.spec.ts b/packages/mosaic/src/fleet/promotion-transport.spec.ts
index c37dcb9c..a59d9187 100644
--- a/packages/mosaic/src/fleet/promotion-transport.spec.ts
+++ b/packages/mosaic/src/fleet/promotion-transport.spec.ts
@@ -58,4 +58,47 @@ describe('TmuxPromotionTransport', () => {
       'Enter',
     ]);
   });
+
+  // Regression for #1124: the launcher runs claude as a spawnSync CHILD of
+  // node(mosaic), so the lease env is on the child, not the tmux pane pid. The
+  // transport must WALK the subtree. This test exercises the real walk (no
+  // full mock of the resolution) — the seam the original unit test hid.
+  it('walks the pane subtree to the claude child that carries the lease id', async () => {
+    const runner = vi.fn<CommandRunner>().mockResolvedValueOnce(result('1234 claude 0 0 0 0\n'));
+    // pane pid 1234 = node(mosaic): NO lease env. child 5678 = claude: carries it.
+    const environmentReader = vi.fn(async (pid: number) =>
+      pid === 5678 ? `FOO=bar\0MOSAIC_LEASE_SESSION_ID=${sessionId}\0` : `FOO=bar\0`,
+    );
+    const childrenReader = vi.fn(async (pid: number) => (pid === 1234 ? [5678] : []));
+    const transport = new TmuxPromotionTransport({
+      environmentReader,
+      childrenReader,
+      mosaicHome: '/mosaic',
+      rosterLoader: async () => roster,
+      runner,
+    });
+
+    const target = await transport.resolve('claude-seat');
+
+    expect(target.sessionId).toBe(sessionId);
+    expect(environmentReader).toHaveBeenCalledWith(1234); // pane pid: no lease
+    expect(environmentReader).toHaveBeenCalledWith(5678); // walked to the child
+    expect(childrenReader).toHaveBeenCalledWith(1234); // walk actually ran
+  });
+
+  it('fails closed when no process in the pane subtree carries a lease id', async () => {
+    const runner = vi.fn<CommandRunner>().mockResolvedValueOnce(result('1234 claude 0 0 0 0\n'));
+    const environmentReader = vi.fn(async () => `FOO=bar\0`);
+    const childrenReader = vi.fn(async (pid: number) => (pid === 1234 ? [5678] : []));
+    const transport = new TmuxPromotionTransport({
+      environmentReader,
+      childrenReader,
+      mosaicHome: '/mosaic',
+      rosterLoader: async () => roster,
+      runner,
+    });
+
+    await expect(transport.resolve('claude-seat')).rejects.toThrow('no readable lease session');
+    expect(childrenReader).toHaveBeenCalledWith(1234);
+  });
 });
diff --git a/packages/mosaic/src/fleet/promotion-transport.ts b/packages/mosaic/src/fleet/promotion-transport.ts
index 4a3f3015..2e8fa408 100644
--- a/packages/mosaic/src/fleet/promotion-transport.ts
+++ b/packages/mosaic/src/fleet/promotion-transport.ts
@@ -29,18 +29,27 @@ export interface PromotionTransport {
 
 export interface TmuxPromotionTransportOptions {
   environmentReader?: (pid: number) => Promise<string>;
+  childrenReader?: (pid: number) => Promise<number[]>;
   mosaicHome: string;
   rosterLoader?: () => Promise<FleetRoster>;
   runner: CommandRunner;
 }
 
+// The launcher runs the runtime as a spawnSync CHILD of node(mosaic) (see
+// launch.ts:99 — deliberate, so the parent survives to propagate signals), so
+// MOSAIC_LEASE_SESSION_ID lives on the claude child, NOT on the tmux pane's root
+// pid. Bound the descendant search so a hung/large process tree can't stall it.
+const MAX_SUBTREE_PIDS = 128;
+
 /** Local, roster-bound transport for the in-seat promotion command. */
 export class TmuxPromotionTransport implements PromotionTransport {
   private readonly environmentReader: (pid: number) => Promise<string>;
+  private readonly childrenReader: (pid: number) => Promise<number[]>;
   private readonly rosterLoader: () => Promise<FleetRoster>;
 
   constructor(private readonly options: TmuxPromotionTransportOptions) {
     this.environmentReader = options.environmentReader ?? readPaneEnvironment;
+    this.childrenReader = options.childrenReader ?? readChildPids;
     this.rosterLoader =
       options.rosterLoader ??
       (() => loadFleetRoster(resolveFleetPaths(options.mosaicHome).rosterPath));
@@ -69,7 +78,7 @@ export class TmuxPromotionTransport implements PromotionTransport {
     ) {
       throw new Error(`Promotion seat runtime identity mismatch: ${seat}.`);
     }
-    const sessionId = parseLeaseSessionId(await this.environmentReader(pane.pid));
+    const sessionId = await this.resolveLeaseSessionId(pane.pid);
     if (sessionId === null) {
       throw new Error(`Promotion seat has no readable lease session: ${seat}.`);
     }
@@ -80,6 +89,39 @@ export class TmuxPromotionTransport implements PromotionTransport {
     };
   }
 
+  /**
+   * Find the lease session id in the pane's process subtree. The pane's root pid
+   * is node(mosaic), which has no lease env; the id lives on the claude child.
+   * BFS from the root, bounded, returning the first descendant that carries a
+   * valid MOSAIC_LEASE_SESSION_ID. Fail-closed (null) if none is found.
+   */
+  private async resolveLeaseSessionId(rootPid: number): Promise<string | null> {
+    const queue: number[] = [rootPid];
+    const seen = new Set<number>();
+    while (queue.length > 0 && seen.size < MAX_SUBTREE_PIDS) {
+      const pid = queue.shift()!;
+      if (seen.has(pid)) continue;
+      seen.add(pid);
+      let sessionId: string | null = null;
+      try {
+        sessionId = parseLeaseSessionId(await this.environmentReader(pid));
+      } catch {
+        sessionId = null;
+      }
+      if (sessionId !== null) return sessionId;
+      let children: number[] = [];
+      try {
+        children = await this.childrenReader(pid);
+      } catch {
+        children = [];
+      }
+      for (const child of children) {
+        if (!seen.has(child)) queue.push(child);
+      }
+    }
+    return null;
+  }
+
   async sendPromotion(target: PromotionTarget): Promise<void> {
     const targetPane = `=${target.seat}:0.0`;
     const socketName = target.bundle === 'default' ? '' : target.bundle;
@@ -143,6 +185,20 @@ async function readPaneEnvironment(pid: number): Promise<string> {
   return readFile(`/proc/${pid}/environ`, 'utf8');
 }
 
+async function readChildPids(pid: number): Promise<number[]> {
+  // Linux exposes direct children of the main thread here (CONFIG_PROC_CHILDREN).
+  try {
+    const raw = await readFile(`/proc/${pid}/task/${pid}/children`, 'utf8');
+    return raw
+      .split(/\s+/)
+      .filter(Boolean)
+      .map((value) => Number.parseInt(value, 10))
+      .filter((value) => Number.isInteger(value) && value > 0);
+  } catch {
+    return [];
+  }
+}
+
 function parseLeaseSessionId(environment: string): string | null {
   const value = environment
     .split('\0')