Bug Description

sessions_spawn correctly defines streamTo as an ACP-only field, and the executor explicitly rejects it when runtime !== "acp".

However, in the current tool-call path I observed sessions_spawn(runtime="subagent") receiving streamTo anyway, even though the intended call was a minimal subagent spawn.

That leads to an error like:

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

This looks like a parameter pollution / tool-call bridge issue rather than a sessions_spawn business-logic issue.

Expected Behavior

When a caller intends a plain subagent spawn, the actual executed params should not contain ACP-only fields such as streamTo.

In other words:

• runtime="subagent" calls should arrive without ACP-only routing fields
• streamTo should only appear for genuine ACP spawn flows

Actual Behavior

A minimal sessions_spawn attempt for runtime="subagent" failed before entering normal spawn flow because streamTo was present in the executed args.

The executor then correctly rejected it with:

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

What I checked

streamTo is intentionally ACP-only

Relevant code:

• src/agents/acp-spawn.ts
• src/agents/tools/sessions-spawn-tool.ts

streamTo was introduced for ACP parent-streaming behavior and has valid ACP semantics.

sessions_spawn core logic appears correct

sessions-spawn-tool.ts explicitly does:

• parse streamTo
• reject it when runtime !== "acp"

That part looks intentional and correct.

I did not find evidence that core sessions_spawn logic itself injects streamTo

Searching the core source only found streamTo in:

• ACP spawn logic
• sessions_spawn schema/tool implementation
• related tests

I did not find a separate OpenClaw core business layer that obviously injects streamTo into subagent calls.

Likely problem area

This makes the problem look more like:

• tool-call bridge / adapter layer
• parameter serialization / propagation layer
• or some higher-level invocation path that carries optional schema fields into actual tool execution

Environment

• OpenClaw version: local source checkout based on 2026.3.24
• Runtime: local Linux / WSL2
• Channel/surface: tool invocation path from active agent session
• Observed during manual runtime validation

Suggested investigation direction

Please check the current tool-call bridge / adapter path for sessions_spawn and confirm whether optional schema fields are being propagated into actual executed params even when not intentionally requested.