## What
- Idempotency cache for write deduplication with SHA256 body hashing
- Query coalescing for identical concurrent search requests
- Config options for TTL, max entries, coalescing window, max subscribers
## Why
HTTP retries, SDK retry loops, and at-least-once delivery produce duplicate writes.
Hot identical search queries waste caching opportunities.
## Details
- Accept Idempotency-Key header for writes
- Return cached mtask ID on hit, 409 conflict on key reuse with different body
- Query fingerprint includes canonical JSON + index UID + settings version
- Settings change invalidates in-flight coalesce (settings_version in fingerprint)
- 50ms default coalescing window closes at response time
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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