SKILL: Orphaned Tool Call Sanitizer

Domain: messaging Trigger: When replaying history for a subagent, make sure assistant tool_use blocks never refer to tool_use_ids that lack results. Source Pattern: Distilled from reviewed subagent orchestration, isolation, and lifecycle implementations.

Core Method

Collect every tool_use_id emitted inside tool_result blocks on user messages, then drop any assistant message whose content includes a tool_use block that references an ID missing from that set. Non-assistant messages and assistant messages without tool_use blocks pass through unchanged, so only the incomplete invocations are pruned before query runs.

Key Rules

• Build the tool use ids with results set exclusively from user-side tool_result blocks, since assistant messages can mention tool_use IDs that never resolved.
• Treat every assistant message containing a tool_use entry whose id is absent from the result set as unsafe and remove it entirely.
• Preserve progress, system, and clean assistant messages so the downstream agent still sees the rest of the context.
• Run this sanitizer before providing initial messages to run agent so the async call path never sees an orphaned invocation.

Example Application

When the main agent forks to an Explore or Plan subagent, sanitize the shared history first so the subagent never inherits an assistant tool_use record that was never followed by a tool_result. The downstream query call then succeeds without rejecting the payload.

Anti-Patterns (What NOT to do)

• Forwarding assistant messages with tool_use blocks that lack corresponding results; such payloads trigger API validation errors.
• Waiting until after the query to drop the offending message, which already fails or leaves the transcript poisoned.