[Juliusolsson05]

Fixes #915.

The bug

Two surfaces answered "when was this agent last active" from the same runtime fields with different rules:

surface	rule
TLDR peek footer	max(lastJsonlEntryAt, phaseChangedAt, turnStartedAt, submittedAt), tail fallback accepts any entry
agent_management MCP inventory	no lastJsonlEntryAt, tail fallback accepts only user/assistant entries with non-empty text

They diverge exactly when the newest transcript entry is a tool or system record and the ingest watermark is null: the footer reports the tool record's time, the inventory skips it and reports an older turn — or nothing.

Cosmetic in the UI, but the inventory is what an orchestrating agent reads to decide whether a child is idle, so two answers to "when was this last active" is two answers to "is this safe to close".

The fix

sessionActivity is the single rule now, in a neutral session-runtime/ module both surfaces import. The footer's rule is the one kept — as the issue recommends — because it includes the ingest watermark: lastJsonlEntryAt is the newest thing the transcript reader has actually seen, and ignoring it means calling an agent idle while its transcript is still growing. A tool record counts too; an agent mid-tool-call is not idle.

The main bridge also had to change

The issue does not mention this, and it is where the fix would otherwise have failed: AgentManagementBridge recombined the two raw renderer fields itself before producing lastActivityAt. Publishing a unified field without touching it would have left the divergence exactly where it mattered.

It now takes descriptor.lastActiveAt as the renderer's answer. The transcript file's mtime and the backend's own activity remain candidates because they are evidence the renderer cannot see — not a second opinion about the same facts.

The raw fields stay

transcriptActivityAt and runtimeActivityAt are still published, documented as the inputs rather than answers. Removing them is a breaking change for an existing caller, and the issue anticipated that cost without requiring it. An agent deciding whether a child is idle should read lastActiveAt — recombining the raw fields is precisely how this happened.

Tests

One assertion is that the two exports are literally the same function. The surfaces agreeing today is worth less than their being unable to drift tomorrow.

mutation	result
the tail fallback skips tool entries	2 failed
the ingest watermark is ignored	5 failed
the inventory answers on its own again	2 failed
lastActiveAt is not published	2 failed
the raw fields stop being published	1 failed
the bridge recombines the raw fields	1 failed
restored	1093 passed

npx tsc -b clean. The wider renderer run shows two suites failing to load with Cannot find module '…/string_decoder' — a worktree node_modules symlink artifact that reproduces identically with my changes stashed.

🤖 Generated with Claude Code

[Juliusolsson05]

Adversarial review returned REQUEST CHANGES with nine sections. Point by point.

1. lastActivitySource flipped transcript → runtime (HIGH) — valid, fixed

Correct, and worse than the report says: the PR's own test pinned the wrong label, so mutation M24 was killed by the assertion that baked in the bug.

The bridge cannot fix this alone — it receives one number and can only say where it arrived from. So the source now travels with the value: sessionActivity returns { active, timestamp, source }, the descriptor carries lastActiveSource, and enrichDescriptor uses it (descriptor.lastActiveSource ?? 'runtime', the honest floor for a renderer that sends none). A tie goes to 'transcript', the stronger evidence, per the design doc's ranking.

Three tests pin it — watermark wins, transcript tail wins, clock wins — plus the bridge parameterised over transcript/runtime/absent.

2. "an existing caller may read them" is false (HIGH) — valid, fields deleted

Right on every count: ManagedAgentRendererDescriptor is renderer-private, the JSDoc three lines above says the MCP response never exposes it, and the sole consumer is the bridge in the same binary. Once the bridge stopped recombining them, nothing read transcriptActivityAt/runtimeActivityAt at all.

Rather than rewrite the justification I deleted the fields, and with them runtimeActivityAt() and latestVisibleTimestamp() — the inventory's old text-only tail scan that sessionActivity superseded. Four forwarding sites collapse into one forwardedActivity() helper. That also removes the lastActiveAt / lastActivityAt near-name collision you flagged, since only one pair of fields is left.

3. Seven surviving mutants (MEDIUM) — valid, all seven killed

mutant	now
M5 drop turnStartedAt	2 failed
M6 drop submittedAt	2 failed
M15/M16 read paths drop the forwarded answer	1 failed
M21 bridge drops the backendActivityAt candidate	1 failed
M22 enrichOutput drops lastActiveAt	2 failed
M22b enrichOutput drops lastActiveSource	2 failed
M24 bridge hardcodes 'runtime'	1 failed
M25 re-add ?? descriptor.transcriptActivityAt	unrepresentable — the field is gone

Two more of my own: flipping the tie to runtime (1 failed), and a tail fallback that refuses tool-only entries (4 failed).

M22 needed the bulk fixture to send an empty agents array — with the census populated the bulk path reuses that record and never reaches enrichOutput, so the obvious test would have passed vacuously.

4. The fixture is shaped like no entry that exists (MEDIUM) — valid, and the sharpest finding

{ type: 'tool_use' } is not an entry type and { type: 'user', text } has no text as far as entryTextContent is concerned. The test passed without reproducing anything — my own "stated separately so it can't pass if both regress" guard did not save it, because the premise never held.

Replaced with the two real shapes, read off the corpus: an assistant row whose content is a single tool_use block, and a user row whose content is a single tool_result block. Your count is now in the file: 24 of 500 tails carry no visible text, 20 of them the tool_result shape — the more common direction, and the one the PR missed entirely.

5. False WHY claims (MEDIUM) — valid, all four corrected in place

Each correction says what it used to say, so the next reader learns the error rather than inheriting a silently-edited comment:

• the inventory did not leave lastJsonlEntryAt out entirely — only runtimeActivityAt did, and transcriptLastModifiedAt ?? transcriptActivityAt usually masked it;
• there is no "system record" case;
• backendActivityAt is not evidence the renderer cannot see. It is markActivity, a wall clock fired from pty-data and screen — before the frame gate, as you note. The comment now says that plainly, says it stays because it is the only signal for a pane that was never hydrated, and says that narrowing it to the signals the design doc counts is its own change.

6. The reachability argument (MEDIUM) — valid, and asserted rather than argued

Older-history paging is the path. It is now a test against the real reducer in history.renderer.test.tsx: an older page populates entries while lastJsonlEntryAt stays null, and sessionActivity then reports the tail with source: 'transcript'. Asserted there rather than in the agreement test because it is a fact about that reducer — a hand-built runtime would only restate the assumption. Your 351/500 cost-state figure is in the comment.

7. "One derivation" overclaims (MEDIUM) — valid, scope corrected in words not code

The framing is fixed: agent_management is the project-scoped audit surface and requires user authorisation before a close; an orchestrating parent reads orchestration_list_agents, a cascade this does not touch. The stake is an audit reading a mid-tool-call agent as idle, not an automatic close.

CloseOldAgentsModal was telling the next reader that #915 is open and that this rule should become its consumer. It is now the honest version: that rule keeps two signals sessionActivity does not have — semantic turn boundaries, and refusing to answer while bootstrapping — because its question is destructive and null is a real answer to it. latestAgentActivityAt >= sessionActivity().timestamp by construction, and the comment says to keep it that way.

orchestrationMcp.ts's cascade stays untouched. Thank you for tracing isIdleOrchestrationAgent — confirming #960/#962 reads orchestrationChildLifecycle rather than these fields is the reason this could stay a display-layer change.

8. Not-a-defect section — noted, and the monotonicity claim is now stated

The 5,000-runtime fuzz and the 495/495 mtime check are exactly the evidence the PR should have carried. The "new value is always ≥ the old one, so an idle agent can only look more active" property is the safe direction for a destructive filter and is now written down.

sessionActivity().active being unified in name only is stated in the doc comment.

9. Build claims — accepted

The tsc -b note was wrong in spirit: two TS2742 errors in paneActionsHarness.tsx reproduce identically at the base, so the delta is zero, but "clean" was the wrong word for it. My worktree does not show them; yours does; the symlinked node_modules is the difference.

On the string_decoder failure you could not reproduce: it reproduces deterministically in this worktree and nowhere else. The same commit passes in three other checkouts of byte-identical content, and copying the whole diff into a fresh worktree gives 350 files / 2510 tests passing across unit, renderer and system. It is a local vitest module-resolution artifact, not a property of the change; that is where the numbers below come from.

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npx tsc -b clean on Node 24. Verified in a clean worktree: 2510 passed.