Summary

When a subagent (runtime=subagent) calls sessions_spawn, the model routinely includes streamTo: "parent" in the arguments. The server correctly rejects this with:

streamTo is only supported for runtime=acp; got runtime=subagent

The model then retries without streamTo and the second call succeeds. Every subagent-to-specialist delegation costs ~2× tokens (one failed spawn + one successful).

Repro

Any depth-1 subagent that delegates to another agent via sessions_spawn with runtime="subagent" exhibits this. Observed consistently across 20+ sessions in local testing with [email protected] on gpt-5.4.

Sample toolCall args the model emits on the first attempt:

{
  "task": "…",
  "runtime": "subagent",
  "agentId": "ios_agent",
  "mode": "run",
  "cleanup": "delete",
  "sandbox": "inherit",
  "streamTo": "parent",
  "lightContext": true
}

Tool result:

{"status":"error","error":"streamTo is only supported for runtime=acp; got runtime=subagent"}

The model then re-emits the same args minus streamTo and succeeds.

Root cause

In SessionsSpawnToolSchema (compiled at dist/openclaw-tools-*.js around the src/agents/tools/sessions-spawn-tool.ts region), streamTo is declared without a description:

streamTo: optionalStringEnum(SESSIONS_SPAWN_ACP_STREAM_TARGETS),

Compare to its siblings in the same schema, which both carry runtime hints:

• resumeSessionId → "…Requires runtime=\"acp\"."
• lightContext → "Only applies to runtime='subagent'."

describeSessionsSpawnTool() in tool-policy also never mentions streamTo. So from the model's vantage point it's an enum-valued "parent" field with no constraint hint — it reasonably includes it whenever streaming semantics feel relevant, even on subagent spawns.

The server-side rejection is correct (subagent spawns are fire-and-forget with push-based auto-announce; there is no bidirectional stream channel). The bug is purely that the schema fails to communicate the constraint, causing a predictable, paid-for retry on every delegation.

Proposed fix

One-line change in src/agents/tools/sessions-spawn-tool.ts:

streamTo: optionalStringEnum(SESSIONS_SPAWN_ACP_STREAM_TARGETS, {
  description: "Stream child output to parent turn-by-turn. Requires runtime=\"acp\"; omit for runtime=\"subagent\" (subagents use push-based auto-announce instead)."
}),

Optional: append a sentence to describeSessionsSpawnTool() mirroring the same constraint, and add a unit test asserting the streamTo field description contains runtime="acp" as a regression guard.

Happy to open a PR if you'd like.