Give retained sessions their own retention cap (CL-7007)

pruneCompleted's maxCompleted is a TUI display cap (default 20); CL-7002
correctly removed retained sessions' unbounded exemption from it, but that
folded reusable-session retention into the same 20-item cap, so resume_agent
on an early worker failed with a bare not_found past 20 spawned workers.

Open retained sessions (retained:true, lifecycleStatus completed/interrupted)
are now excluded from maxCompleted and bounded instead by a separate
maxRetained cap (default 50, sized for fan-out), evicting least-recently-used
first and never evicting a running session. Eviction still releases sidecars/
reactor/lock entry via releaseHandles, and leaves a tombstone so resume_agent/
followup_task/close_agent report an actionable terminal status plus a
read_agent_trace pointer instead of not_found.

Updated three CL-7001/CL-7002 tests that had encoded "retained sessions share
the display cap" as correct behavior to instead assert the new maxRetained
bound.

Author: TheGreatAxios

CHANGELOG.md

@@ -11,6 +11,18 @@ matching `## [X.Y.Z]` section (plus install instructions). Do not maintain
 parallel copies under `docs/` or `scripts/notes/`. At cut time: rename
 `## [Unreleased]` to `## [X.Y.Z] - YYYY-MM-DD`, then run the release script.
 
+## [Unreleased]
+
+### Fixed
+
+- Retained worker sessions (`spawn_agent`, resumable via `resume_agent`/`followup_task`) now have
+  their own retention cap, separate from the TUI's finished-session display cap. Previously they
+  shared that 20-item cap, so `resume_agent` on an early worker failed with a bare `not_found` once
+  a fan-out of more than 20 workers had finished. A session dropped by the retention cap still
+  releases its sidecars/reactor/lock entry, always evicts least-recently-used first, and never
+  evicts a running session. `resume_agent`/`followup_task` against an evicted session now report
+  its terminal status plus a pointer to `read_agent_trace`, instead of `not_found`.
+
 ## [0.3.0] - 2026-08-24
 
 ### Breaking

src/subagent/agent-fleet.test.ts

@@ -196,12 +196,17 @@ describe("spawn_agent + wait_agents", () => {
     // DEFAULT_MAX_COMPLETED on SubAgentSessionStore is 20 finished sessions;
     // spawn (and complete) enough workers to blow well past it before any of
     // them is collected, proving fleetRecords does not depend on the store's
-    // cap. CL-7001: a retained session is bounded by this same cap too now
-    // (it used to be exempt with no separate cap or TTL, which is exactly
-    // why every spawn_agent worker leaked by default) — so unlike the
-    // pre-CL-7001 version of this test, the store itself may have already
-    // evicted (and released) the earliest ones; wait_agents/fleetRecords is
-    // the durable source of truth this test actually cares about.
+    // cap.
+    //
+    // CL-7007: this test previously asserted (as CL-7001's fix left it) that
+    // the store itself had already evicted and released the earliest
+    // session, because a retained session shared the 20-item display cap
+    // with every other finished session — exactly the shipped defect this
+    // ticket fixes (resume_agent/followup_task failed with a bare
+    // "not_found" past 20 spawned workers, blaming the caller for nothing).
+    // Open retained sessions now have their own cap (`maxRetained`, default
+    // 50), so 25 of them all stay resumable; fleetRecords/wait_agents is
+    // still asserted below as the durable source of truth regardless.
     const COUNT = 25;
     const deps = makeDeps(async () => ({ report: "irrelevant", agentRetained: true }));
     const spawn = createSpawnAgentTool(deps);
@@ -220,10 +225,9 @@ describe("spawn_agent + wait_agents", () => {
     // Let every spawn's run() resolve and complete() land before collecting.
     await new Promise((resolve) => setTimeout(resolve, 20));
 
-    // The store's own bound may have already evicted (and released) the
-    // earliest session — fleetRecords below is what wait_agents actually
-    // depends on, and it is never subject to this cap.
-    expect(deps.sessions.get(ids[0]!)).toBeUndefined();
+    // 25 open retained sessions is under the default maxRetained (50), so
+    // the earliest is still present and resumable — not evicted.
+    expect(deps.sessions.get(ids[0]!)).toBeDefined();
 
     // Every single one is retrievable through wait_agents too.
     const waited = await callTool(wait, { targets: ids, timeout_ms: 5000 });

src/subagent/lifecycle-tools.ts

@@ -136,9 +136,10 @@ export function createResumeAgentTool(deps: LifecycleToolDeps): AgentTool {
       const target = parsed.target.trim();
       const outcome = deps.sessions.resumeOne(target);
       if (!outcome.ok) {
+        const hint = outcome.hint !== undefined ? ` ${outcome.hint}` : "";
         return lifecycleResult(
           call.id,
-          `Error: cannot resume "${target}" (status: ${outcome.status}).`,
+          `Error: cannot resume "${target}" (status: ${outcome.status}).${hint}`,
         );
       }
       return lifecycleResult(call.id, JSON.stringify({ agent_id: target, status: "running" }));
@@ -235,9 +236,10 @@ export function createFollowupTaskTool(deps: LifecycleToolDeps): AgentTool {
       }
       const outcome = await deps.sessions.followupOne(target, message);
       if (!outcome.ok) {
+        const hint = outcome.hint !== undefined ? ` ${outcome.hint}` : "";
         return lifecycleResult(
           call.id,
-          `Error: cannot send followup to "${target}" (status: ${outcome.status}).`,
+          `Error: cannot send followup to "${target}" (status: ${outcome.status}).${hint}`,
         );
       }
       return lifecycleResult(

src/subagent/retain-salvage.test.ts

@@ -32,13 +32,23 @@ describe("retained session lifecycle", () => {
     expect(closed).toBe(true);
   });
 
-  test("retained completed sessions are exempt from the display cap without bound", () => {
-    const store = createSubAgentSessionStore({ maxCompleted: 3 });
+  // CL-7007: retained completed sessions are no longer bounded by
+  // `maxCompleted` (the TUI display cap) at all — that was CL-7002's fix,
+  // and it created a new bug: resume_agent/followup_task started failing
+  // with a bare "not_found" once more than `maxCompleted` (default 20)
+  // workers had spawned in a turn, even though every one of them was still
+  // perfectly reusable. Open retained sessions now get their own explicit
+  // cap, `maxRetained`, sized for fan-out rather than a sidebar list — this
+  // test moved from asserting `maxCompleted` bounds them to asserting
+  // `maxRetained` does (still bounded, still no leak, just the right knob).
+  test("retained completed sessions are bounded by maxRetained, not the display cap", () => {
+    const store = createSubAgentSessionStore({ maxCompleted: 3, maxRetained: 3 });
     for (let i = 0; i < 50; i++) {
       const s = store.start({ description: `w${i}`, agentId: "build", brief: "b", retained: true });
+      store.registerClose(s.id, async () => {});
       store.complete(s.id, "done");
     }
-    console.log("sessions retained despite maxCompleted=3:", store.list().length);
+    console.log("sessions retained despite maxRetained=3:", store.list().length);
     expect(store.list().length).toBeLessThanOrEqual(3);
   });
 

src/subagent/session-store.test.ts

@@ -358,13 +358,19 @@ describe("CL-6943 reusable worker sessions", () => {
     expect(store.resumeOne(session.id)).toEqual({ ok: false, status: "shutdown" });
   });
 
-  // CL-7001: a retained, still-open session used to be exempt from this cap
-  // entirely — no separate cap or TTL — which is exactly why every
-  // spawn_agent worker leaked by default. maxCompleted is now the one bound
-  // the store owns for every finished session, retained or not, and
-  // eviction releases the session's close handle instead of abandoning it.
-  test("pruneCompleted evicts a retained, still-open session past maxCompleted and releases it", () => {
-    const store = createSubAgentSessionStore({ maxCompleted: 1 });
+  // CL-7001 originally folded a retained, still-open session into
+  // maxCompleted (the TUI display cap) with no separate bound at all,
+  // fixing the unbounded leak but creating a new bug: resume_agent /
+  // followup_task fail once more than `maxCompleted` (default 20) workers
+  // have spawned, even though every one of them is still perfectly
+  // reusable. CL-7007 gives open retained sessions their own cap
+  // (`maxRetained`) instead — this test changed from asserting that
+  // `maxCompleted` evicts a retained session (no longer true: retained
+  // sessions are excluded from that cap, see isOpenRetained) to asserting
+  // that `maxRetained` does, with the same "handles still get released"
+  // guarantee.
+  test("pruneRetained evicts a retained, still-open session past maxRetained and releases it", () => {
+    const store = createSubAgentSessionStore({ maxCompleted: 1, maxRetained: 1 });
     const retained = store.start({
       description: "keep-me",
       agentId: "a",
@@ -378,14 +384,114 @@ describe("CL-6943 reusable worker sessions", () => {
     store.complete(retained.id, "## Summary\nDone.");
 
     for (let i = 0; i < 3; i++) {
-      const s = store.start({ description: `fill-${i}`, agentId: "a", brief: "b" });
+      const s = store.start({
+        description: `fill-${i}`,
+        agentId: "a",
+        brief: "b",
+        retained: true,
+      });
+      store.registerClose(s.id, async () => {});
       store.complete(s.id, "## Summary\nDone.");
     }
 
     expect(store.get(retained.id)).toBeUndefined();
     expect(closed).toBe(true);
   });
 
+  // CL-7002's fix (retained sessions are no longer exempt from any cap) must
+  // survive CL-7007: a non-retained finished session still obeys
+  // maxCompleted exactly as before.
+  test("maxCompleted still evicts an ordinary (non-retained) finished session", () => {
+    const store = createSubAgentSessionStore({ maxCompleted: 1 });
+    const first = store.start({ description: "first", agentId: "a", brief: "b" });
+    store.complete(first.id, "## Summary\nDone.");
+
+    for (let i = 0; i < 3; i++) {
+      const s = store.start({ description: `fill-${i}`, agentId: "a", brief: "b" });
+      store.complete(s.id, "## Summary\nDone.");
+    }
+
+    expect(store.get(first.id)).toBeUndefined();
+  });
+
+  test("resume_agent on a retention-evicted session returns an actionable status, not not_found", () => {
+    const store = createSubAgentSessionStore({ maxRetained: 1 });
+    const retained = store.start({
+      description: "keep-me",
+      agentId: "a",
+      brief: "b",
+      retained: true,
+    });
+    store.registerClose(retained.id, async () => {});
+    store.complete(retained.id, "## Summary\nDone.");
+
+    for (let i = 0; i < 3; i++) {
+      const s = store.start({
+        description: `fill-${i}`,
+        agentId: "a",
+        brief: "b",
+        retained: true,
+      });
+      store.registerClose(s.id, async () => {});
+      store.complete(s.id, "## Summary\nDone.");
+    }
+
+    const outcome = store.resumeOne(retained.id);
+    expect(outcome.ok).toBe(false);
+    if (!outcome.ok) {
+      expect(outcome.status).toBe("completed");
+      expect(outcome.hint).toMatch(/read_agent_trace/);
+    }
+  });
+
+  test("a running session is never evicted by maxRetained even when the cap is exceeded", () => {
+    const store = createSubAgentSessionStore({ maxRetained: 1 });
+    const running = store.start({
+      description: "keep-me",
+      agentId: "a",
+      brief: "b",
+      retained: true,
+    });
+    store.markRunning(running.id);
+    // Resume it back to "running" so it is an open, actively-driven session.
+    store.registerClose(running.id, async () => {});
+    store.complete(running.id, "## Summary\nDone.");
+    store.resumeOne(running.id);
+    expect(store.get(running.id)?.lifecycleStatus).toBe("running");
+
+    for (let i = 0; i < 5; i++) {
+      const s = store.start({
+        description: `fill-${i}`,
+        agentId: "a",
+        brief: "b",
+        retained: true,
+      });
+      store.registerClose(s.id, async () => {});
+      store.complete(s.id, "## Summary\nDone.");
+    }
+
+    expect(store.get(running.id)).toBeDefined();
+    expect(store.get(running.id)?.lifecycleStatus).toBe("running");
+  });
+
+  test("maxRetained bounds memory: many spawned-and-completed retained sessions do not grow without limit", () => {
+    const store = createSubAgentSessionStore({ maxRetained: 5 });
+    for (let i = 0; i < 50; i++) {
+      const s = store.start({
+        description: `worker-${i}`,
+        agentId: "a",
+        brief: "b",
+        retained: true,
+      });
+      store.registerClose(s.id, async () => {});
+      store.complete(s.id, "## Summary\nDone.");
+    }
+    const openRetained = store
+      .list()
+      .filter((s) => s.retained === true && s.lifecycleStatus === "completed");
+    expect(openRetained.length).toBeLessThanOrEqual(5);
+  });
+
   test("once closed, a retained session becomes a normal finished record subject to the cap", async () => {
     const store = createSubAgentSessionStore({ maxCompleted: 1 });
     const retained = store.start({