Summary

sessions_spawn currently exposes ACP-only fields such as streamTo and resumeSessionId in the public tool schema, even when the intended runtime is subagent.

With schema-following models, this can cause subagent spawns to fail with errors like:

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

This makes sessions_spawn(runtime="subagent") fragile or unusable in real multi-agent workflows.

Environment

• OpenClaw version: 2026.3.24
• OS: macOS (Apple Silicon)
• Runtime: subagent
• Model behavior: schema-following tool calls

What we confirmed locally

Tool schema

The public SessionsSpawnToolSchema includes both subagent and ACP fields together, including:

• subagent-side fields:
  • task
  • label
  • runtime
  • agentId
  • attachments
  • attachAs
• ACP-only fields:
  • resumeSessionId
  • cwd
  • streamTo

Execution path

In the sessions_spawn tool implementation, streamTo and resumeSessionId are validated before runtime dispatch:

• if streamTo && runtime !== "acp" → error
• if resumeSessionId && runtime !== "acp" → error

So if a model includes ACP-only fields in a runtime="subagent" call, execution fails before reaching spawnSubagentDirect(...).

Subagent runtime itself is not the problem

We verified that spawnSubagentDirect(...) itself does not require streamTo.

After locally patching the guard to ignore ACP-only fields for non-ACP runtime, subagent spawning worked again and we were able to run:

• main -> researcher
• researcher -> result push-back
• a full multi-agent drill:
  • researcher
  • writer
  • coder

Expected behavior

One of the following should happen:

1. streamTo and resumeSessionId should not be exposed in the tool schema for runtime="subagent", or
2. ACP-only fields should be silently ignored/stripped when runtime="subagent".

Why this matters

This is especially easy to trigger with schema-following models, because once ACP-only fields are visible in the same tool schema, they may be carried into subagent calls even when the intent is correct.

This blocks real multi-agent delegation workflows.

Suggested fix

Short-term:

• auto-strip or ignore ACP-only fields (streamTo, resumeSessionId) when runtime="subagent"

Long-term:

• split ACP and subagent spawn schemas more clearly, or expose separate tools/helpers for each path

Local workaround used

We locally patched the built file to stop rejecting ACP-only fields for non-ACP runtime, then restarted the gateway. After that, subagent spawning succeeded.

This suggests the failure is in schema/tool-call robustness, not in the subagent runtime itself.