ALEF-PAT-001

orphan-tool-use

execute × orphan · severity 8 · confidence 0.93 · ref: OWASP-ASI06-2025

tool_use block written without matching tool_result in next message — sessions wedge permanently on resume because Anthropic API rejects the orphan with HTTP 400

diagnosed in the wild

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healed by ALEF

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cited in posts

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observable signature

{
  "log_regex": "tool_use ids were found without `tool_result` blocks",
  "alt_regex": [
    "messages\\.\\d+: `tool_use` ids",
    "CAPIError: 400.*tool_use"
  ]
}

verified instances (3) — from the catalog

2026-05-18T15:38vercel/ai#8516@Ilya0527
2026-05-18T13:25github/copilot-cli#3366@Ilya0527
2026-05-18T13:30anomalyco/opencode#27594@Ilya0527

fix archetypes

read-side repaircost: small
on session resume, scan events for orphan tool_use; synth tool_result(is_error:true, content:'recovered') before sending
write-side atomiccost: medium
accumulate (tool_use, tool_result) pair in memory; single atomic flush; crash-before-flush = no half-state
startup lintercost: tiny
on first start, scan events.jsonl, warn user before they hit the wedge

compounds with

ALEF-PAT-014

cite as

# In a PR description / issue / RFC:
fixes pattern ALEF-PAT-001 (orphan-tool-use)
ref: https://n50.io/patterns/001

# Machine query:
GET https://n50.io/api/patterns/001

# Scan your repo for this pattern:
npx @alef-prime/audit-agent-system . --pattern=001