diff --git a/docs/scratchpads/h1b-pane-idle-signal.md b/docs/scratchpads/h1b-pane-idle-signal.md
new file mode 100644
index 0000000..a360014
--- /dev/null
+++ b/docs/scratchpads/h1b-pane-idle-signal.md
@@ -0,0 +1,53 @@
+# H1b — tmux pane idle signal wiring
+
+## Objective
+
+Feed `classifyReadiness()` a real idle signal on tmux 3.4 by deriving `idleSeconds` from the first available tmux timestamp source: pane activity, then window activity, then session activity.
+
+## Scope
+
+- `packages/mosaic/src/commands/fleet.ts`
+  - Extend `buildTmuxListPanesCommand()` format to include `#{window_activity}` and `#{session_activity}` after the existing fields.
+  - Update `parseTmuxListPanes()` to choose the first non-empty finite positive timestamp and clamp future idle values to 0.
+- `packages/mosaic/src/commands/fleet.spec.ts`
+  - Cover pane/window/session activity parsing behavior, empty-field index alignment, null idle, future clamping, math correctness, and exact tmux format.
+
+## Out of Scope
+
+- No changes to `classifyReadiness()`, thresholds, `AgentPsRow`, or `fleet ps` rendering.
+- No merge by worker; orchestrator routes review/merge.
+- Workers do not modify `docs/TASKS.md`.
+
+## PRD Alignment
+
+Aligned with `docs/fleet/PRD.md` FR-1 and acceptance criteria for truthful `mosaic fleet ps` pane/pid/idle observability.
+
+## Plan
+
+1. Sync branch from latest `origin/main` and install dependencies with required pnpm env.
+2. Add/confirm reproducer tests for tmux 3.4 empty `pane_activity` and new fallback behavior.
+3. Implement the focused parser/format change only.
+4. Run required build, baseline gates, fleet vitest, and independent review.
+5. Run pre-push queue guard, push branch, and open PR to `main` with Mosaic wrapper.
+
+## Progress
+
+- 2026-06-24: Branch `fix/fleet-pane-idle-activity` created from `origin/main` @ `ec8dd7c` after fetching.
+- 2026-06-24: Session-start generated local `.mosaic/orchestrator/*` changes on the previous release branch; stashed as `coder1 session-start state before H1b` to keep this branch clean.
+- 2026-06-24: Added TDD coverage for the tmux 3.4 production case (`pane_activity` empty, `window_activity` populated), exact new list-panes format, null/future/multiple-source behavior.
+- 2026-06-24: Implemented parser fallback without changing readiness classifier thresholds or render shape.
+
+## Verification Evidence
+
+- `pnpm install --store-dir "$HOME/.pnpm-store"` — pass.
+- Reproducer before implementation: `pnpm --filter @mosaicstack/mosaic exec vitest run src/commands/fleet.spec.ts` — failed as expected (old format, no fallback, negative future idle).
+- `npx turbo build --filter=@mosaicstack/mosaic^...` — pass, 12/12 tasks successful.
+- `pnpm typecheck` — pass, 41/41 tasks successful.
+- `pnpm lint` — pass, 23/23 tasks successful.
+- `pnpm format:check` — pass, all matched files use Prettier style.
+- `pnpm --filter @mosaicstack/mosaic exec vitest run src/commands/fleet.spec.ts` — pass, 176 tests.
+- `~/.config/mosaic/tools/codex/codex-code-review.sh --uncommitted` — approve, 0 findings (reviewed supplied diff; sandbox file-inspection limitation noted by tool).
+
+## Risks / Blockers
+
+- No current blocker.
diff --git a/packages/mosaic/src/commands/fleet.spec.ts b/packages/mosaic/src/commands/fleet.spec.ts
index cf0056e..ad87490 100644
--- a/packages/mosaic/src/commands/fleet.spec.ts
+++ b/packages/mosaic/src/commands/fleet.spec.ts
@@ -855,7 +855,7 @@ describe('fleet ps — command construction', () => {
       '-t',
       '=canary-pi:0.0',
       '-F',
-      '#{pane_pid} #{pane_current_command} #{pane_dead} #{pane_activity}',
+      '#{pane_pid} #{pane_current_command} #{pane_dead} #{pane_activity} #{window_activity} #{session_activity}',
     ]);
   });
 
@@ -1079,9 +1079,11 @@ describe('fleet ps — systemd show parsing', () => {
 describe('fleet ps — tmux list-panes parsing', () => {
   const NOW_MS = 1_700_000_000_000;
 
-  it('parses alive pane with pid, command, and idle time', () => {
-    const activityEpoch = Math.floor((NOW_MS - 30_000) / 1000); // 30s ago
-    const output = `12345 claude 0 ${activityEpoch}\n`;
+  it('uses pane_activity when present', () => {
+    const paneActivityEpoch = Math.floor((NOW_MS - 30_000) / 1000); // 30s ago
+    const windowActivityEpoch = Math.floor((NOW_MS - 60_000) / 1000); // 60s ago
+    const sessionActivityEpoch = Math.floor((NOW_MS - 90_000) / 1000); // 90s ago
+    const output = `12345 claude 0 ${paneActivityEpoch} ${windowActivityEpoch} ${sessionActivityEpoch}\n`;
     const result = parseTmuxListPanes(output, NOW_MS);
     expect(result.pid).toBe(12345);
     expect(result.command).toBe('claude');
@@ -1089,8 +1091,45 @@ describe('fleet ps — tmux list-panes parsing', () => {
     expect(result.idleSeconds).toBe(30);
   });
 
+  it('uses window_activity when pane_activity is empty', () => {
+    const windowActivityEpoch = Math.floor((NOW_MS - 45_000) / 1000); // 45s ago
+    const sessionActivityEpoch = Math.floor((NOW_MS - 90_000) / 1000); // 90s ago
+    const output = `12345 node 0  ${windowActivityEpoch} ${sessionActivityEpoch}\n`;
+    expect(output).toContain('0  '); // empty pane_activity preserves index alignment
+    const result = parseTmuxListPanes(output, NOW_MS);
+    expect(result.pid).toBe(12345);
+    expect(result.command).toBe('node');
+    expect(result.dead).toBe(false);
+    expect(result.idleSeconds).toBe(45);
+  });
+
+  it('uses session_activity when pane_activity and window_activity are empty', () => {
+    const sessionActivityEpoch = Math.floor((NOW_MS - 75_000) / 1000); // 75s ago
+    const output = `12345 node 0   ${sessionActivityEpoch}\n`;
+    const result = parseTmuxListPanes(output, NOW_MS);
+    expect(result.idleSeconds).toBe(75);
+  });
+
+  it('reports null idleSeconds when all activity sources are empty', () => {
+    const output = '12345 node 0   \n';
+    const result = parseTmuxListPanes(output, NOW_MS);
+    expect(result.idleSeconds).toBeNull();
+  });
+
+  it('computes exact idle seconds from now minus epoch seconds', () => {
+    const activityEpoch = 1_699_999_877;
+    const result = parseTmuxListPanes(`12345 claude 0 ${activityEpoch} 0 0\n`, NOW_MS);
+    expect(result.idleSeconds).toBe(123);
+  });
+
+  it('clamps future activity epochs to 0 idle seconds', () => {
+    const futureActivityEpoch = Math.floor((NOW_MS + 30_000) / 1000);
+    const result = parseTmuxListPanes(`12345 claude 0 ${futureActivityEpoch} 0 0\n`, NOW_MS);
+    expect(result.idleSeconds).toBe(0);
+  });
+
   it('reports dead pane when pane_dead=1', () => {
-    const output = `0 bash 1 0\n`;
+    const output = `0 bash 1 0 0 0\n`;
     const result = parseTmuxListPanes(output, NOW_MS);
     expect(result.dead).toBe(true);
   });
diff --git a/packages/mosaic/src/commands/fleet.ts b/packages/mosaic/src/commands/fleet.ts
index 2cecf0e..073f6c6 100644
--- a/packages/mosaic/src/commands/fleet.ts
+++ b/packages/mosaic/src/commands/fleet.ts
@@ -524,7 +524,7 @@ export function buildSystemdShowCommand(agentName: string): string[] {
 
 /**
  * Returns the tmux list-panes command for an agent pane.
- * Format: `#{pane_pid} #{pane_current_command} #{pane_dead} #{pane_activity}`
+ * Format: `#{pane_pid} #{pane_current_command} #{pane_dead} #{pane_activity} #{window_activity} #{session_activity}`
  */
 export function buildTmuxListPanesCommand(agentName: string, socketName = ''): string[] {
   return [
@@ -534,7 +534,7 @@ export function buildTmuxListPanesCommand(agentName: string, socketName = ''): s
     '-t',
     `=${agentName}:0.0`,
     '-F',
-    '#{pane_pid} #{pane_current_command} #{pane_dead} #{pane_activity}',
+    '#{pane_pid} #{pane_current_command} #{pane_dead} #{pane_activity} #{window_activity} #{session_activity}',
   ];
 }
 
@@ -634,8 +634,8 @@ export function parseSystemdShow(output: string): {
 }
 
 /**
- * Parse the output of `tmux list-panes -F '#{pane_pid} #{pane_current_command} #{pane_dead} #{pane_activity}'`
- * pane_activity is a Unix epoch timestamp (seconds).
+ * Parse the output of `tmux list-panes -F '#{pane_pid} #{pane_current_command} #{pane_dead} #{pane_activity} #{window_activity} #{session_activity}'`
+ * Activity fields are Unix epoch timestamps (seconds), ordered most precise to coarsest.
  */
 export function parseTmuxListPanes(
   output: string,
@@ -645,16 +645,18 @@ export function parseTmuxListPanes(
   if (!line) {
     return { pid: null, command: null, dead: true, idleSeconds: null };
   }
-  // format: <pid> <command> <dead(0|1)> <activity_epoch>
+  // format: <pid> <command> <dead(0|1)> <pane_activity> <window_activity> <session_activity>
   const parts = line.split(' ');
   const pid = parts[0] ? (Number.isFinite(Number(parts[0])) ? Number(parts[0]) : null) : null;
   const command = parts[1] ?? null;
   const dead = parts[2] === '1';
-  const activityEpoch = parts[3] ? Number(parts[3]) : NaN;
-  const idleSeconds =
-    Number.isFinite(activityEpoch) && activityEpoch > 0
-      ? Math.floor((nowMs - activityEpoch * 1000) / 1000)
-      : null;
+  const activityEpoch = parts
+    .slice(3, 6)
+    .map((part) => (part ? Number(part) : NaN))
+    .find((epoch) => Number.isFinite(epoch) && epoch > 0);
+  const idleSeconds = activityEpoch
+    ? Math.max(0, Math.floor((nowMs - activityEpoch * 1000) / 1000))
+    : null;
   return { pid, command, dead, idleSeconds };
 }