Create only idempotent artifact write

SKILL: Create-Only Idempotent Artifact Write

Domain: tool-orchestration
Trigger: Apply when a deterministic artifact is keyed by a stable invocation ID and may be replayed across retries, compaction, or resume passes. Source Pattern: Distilled from reviewed tool-loop and result-shaping patterns.

Core Method

When an artifact path is derived from a stable identity, write it once with an atomic create-only operation instead of checking for existence first. If the write hits "already exists," treat that as proof the earlier successful write already materialized the deterministic content and continue with the normal readback or preview path. This removes the stat-then-write race, avoids useless rewrites during replay, and keeps repeated passes behaviorally identical.

Key Rules

• Derive the artifact path from a stable key such as tool_use_id so repeated calls target the same location.
• Use a create-only write primitive like flag: 'wx' to collapse "create if absent" into one filesystem operation.
• Treat EEXIST as idempotent success when the content is deterministic for that key; only unexpected filesystem errors should fail the operation.
• Continue into the downstream preview or response-building path even when the file already existed, so replayed calls return the same contract as fresh ones.
• Avoid preflight existence checks because they reintroduce races and duplicate I/O under concurrency.

Example Application

If a large tool result is persisted under tool-results/<tool_use_id>.json, write it once with a create-only flag. When transcript replay or micro-compaction revisits that same tool result, the write sees EEXIST, skips the rewrite, and still returns the same preview payload and saved path to the caller.

Anti-Patterns (What NOT to do)

• Do not stat() and then writeFile() for idempotency; another writer can win between those calls.
• Do not overwrite existing artifacts on every replay when the file path is already the canonical output for that key.
• Do not treat "file already exists" as a hard error in deterministic replay flows; it is often the success path.