Greptile Summary

This PR documents that the streamTo parameter on sessions_spawn requires runtime="acp", adds a runtime guard that returns an error instead of silently ignoring streamTo for non-ACP callers, and backs it with two new tests: one for the guard behaviour and one as a schema regression test. The changes are narrow and well-targeted.

Confidence Score: 5/5

Safe to merge — the changes are additive (documentation + guard + tests) with no risk to existing call paths.

All findings are P2 or lower. The guard logic is correct, the schema description is accurate, and both new tests cover the intended behaviour. No regressions are introduced.

No files require special attention.

_{Reviews (1): Last reviewed commit: "fix(agent): hint streamTo is acp-only" | Re-trigger Greptile}

Related work from PRtags group living-dingo-54mf

Title: Open PR duplicate: sessions_spawn: streamTo schema lacks runtime=acp hint, causes 2x-token retry loop for subagent…

* This PR

Closing this as duplicate or superseded after Codex automated review.

PR #69170 is a focused, plausible schema-hint fix, but it is now superseded by maintainer PR #72331. Current main still lacks the streamTo schema description and still rejects runtime="subagent" plus streamTo, so this is not implemented-on-main. The remaining work is already carried forward in #72331, which explicitly credits #69170 and broadens the fix to tolerate ACP-only fields for subagent runtime.

Best possible solution:

Close #69170 as superseded and continue review/landing through maintainer PR #72331, preserving credit for #69170 there. The best product direction is the broader #72331 fix: keep the schema hint and tolerate ACP-only fields on subagent runtime so strict schema-following providers do not brick spawns.

What I checked:

• Current main lacks the schema hint: SessionsSpawnToolSchema still defines streamTo as optionalStringEnum(SESSIONS_SPAWN_ACP_STREAM_TARGETS) with no description on current main. (src/agents/tools/sessions-spawn-tool.ts:160, edf40ab6c944)
• Current main still has the underlying rejection behavior: The executor still returns streamTo is only supported for runtime=acp when streamTo is present and runtime is not acp. (src/agents/tools/sessions-spawn-tool.ts:264, edf40ab6c944)
• Current tests lock in rejection but not the new schema description: The current test suite asserts rejection for runtime="subagent" with streamTo: "parent", and the ACP-affordance schema test only checks that streamTo is defined, not that it carries a runtime hint. (src/agents/tools/sessions-spawn-tool.test.ts:532, edf40ab6c944)
• Maintainer replacement PR supersedes this work: Provided PR context shows open MEMBER PR fix(sessions_spawn): tolerate ACP-only fields for subagent runtime #72331, labeled maintainer, explicitly carries forward the schema-hint work from fix(agent): document that sessions_spawn streamTo is ACP-only #69170 and fix(sessions-spawn): document runtime="acp" requirement on streamTo schema #69203 while adding the broader ACP-only field tolerance path. git ls-remote confirms the referenced PR heads exist, including fix(sessions_spawn): tolerate ACP-only fields for subagent runtime #72331 at 7266af9ef5d7bdb83c18c64af533796dfdcaf2d3. (7266af9ef5d7)

So I’m closing this here and keeping the remaining discussion on the canonical linked item.

Codex Review notes: model gpt-5.5, reasoning high; reviewed against edf40ab6c944.