Greptile Summary

This PR fixes a regression introduced in 2026.3.7 where streamTo appearing on the shared sessions_spawn surface caused subagent spawns to fail with a hard error when the field was present but runtime was not "acp". The fix converts that error into a silent ignore for non-ACP runtimes, keeping ACP behavior intact.

Key changes:

• sessions-spawn-tool.ts: Resolves streamTo to undefined for non-ACP runtimes, eliminating the guard that previously returned a runtime error.
• sessions-spawn-tool.test.ts: Adds a test verifying successful subagent execution when streamTo is present but runtime is not "acp".
• docs/tools/index.md: Marks streamTo as ACP-only and notes that subagent runtime ignores it for compatibility.

The fix is minimal, targeted, and safe—it simply makes the system tolerant of a field that shouldn't affect subagent behavior, while preserving full ACP support.

Confidence Score: 5/5

• Safe to merge — the change is minimal and targeted, converting a hard error into a silent ignore for non-ACP runtimes while preserving full ACP support.
• The implementation is straightforward: streamTo resolution now depends on runtime, yielding undefined for subagent spawns. The removed guard was redundant since streamTo is not forwarded to spawnSubagentDirect anyway. Test suite covers both the new subagent-ignore path and existing ACP-passes-streamTo path. No correctness issues identified.
• No files require special attention

_{Last reviewed commit: 88cae38}

This pull request has been automatically marked as stale due to inactivity.
Please add updates or it will be closed.

Closing this as duplicate or superseded after Codex automated review.

Close #40102 as superseded by open PR #68397. Current main still has the runtime=subagent failure, so this is not implemented-on-main, but #68397 is the broader canonical fix: it strips both ACP-only streamTo and the adjacent resumeSessionId field for subagent spawns while preserving ACP behavior and updating regression tests.

Best possible solution:

Close #40102 as superseded and keep the remaining review/landing work on #68397, which is the broader canonical fix for ACP-only fields on runtime="subagent" while preserving ACP semantics.

What I checked:

• Current main still rejects streamTo for subagent runtime: sessions_spawn reads streamTo before dispatch and returns status: "error" when streamTo is present and runtime !== "acp", so fix: ignore ACP-only streamTo for subagent spawns #40102 is not already implemented on main. (src/agents/tools/sessions-spawn-tool.ts:264, d54d2d6b9b8a)
• Current tests assert the old rejection: The current regression test is still named rejects streamTo when runtime is not "acp" and asserts the error plus no spawnSubagentDirect call. (src/agents/tools/sessions-spawn-tool.test.ts:532, d54d2d6b9b8a)
• Adjacent ACP-only field has the same current failure shape: Current main also rejects resumeSessionId for non-ACP runtime, and the test suite asserts that rejection, which is why the broader fix covers more of the same shared-schema failure. (src/agents/tools/sessions-spawn-tool.ts:272, d54d2d6b9b8a)
• PR fix: ignore ACP-only streamTo for subagent spawns #40102 is a narrow streamTo-only fix: GitHub API shows fix: ignore ACP-only streamTo for subagent spawns #40102 is open, author association NONE, with non-protected labels, and its patch only drops streamTo for non-ACP runtimes plus a streamTo test and docs note. The Greptile comment reviewed that narrow change positively as minimal and safe. (src/agents/tools/sessions-spawn-tool.ts, 88cae38fd7d9)
• PR fix(sessions_spawn): silently strip ACP-only fields for runtime=subagent #68397 supersedes the same work: GitHub API shows fix(sessions_spawn): silently strip ACP-only fields for runtime=subagent #68397 is open with title fix(sessions_spawn): silently strip ACP-only fields for runtime=subagent; its patch removes both streamTo and resumeSessionId non-ACP guards and updates tests to assert both fields are silently dropped for runtime=subagent. Its Greptile review also confirms the broader scope and preserved ACP behavior. (src/agents/tools/sessions-spawn-tool.ts, e5c2067d2fc0)

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 d54d2d6b9b8a.