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+# Round 13: WU 2.4 — Schema Change Detection
+
+**Work Unit:** 2.4  
+**Objective:** Detect and alert on API schema changes — the #1 unsolved problem in autonomous agent infrastructure  
+**Status:** KICKOFF  
+**Depends on:** WU 2.1 (proxy core), WU 2.2 (agent identity), WU 2.3 (metering)  
+**Assigned to:** Codex sub-agent (`model="codex53"`)  
+**Timeline:** ~11–13 hours (thin-slice execution, 4 slices, 12-15 min each)  
+**Target metrics:** 40+ tests, 5 modules, 0 regressions
+
+---
+
+## Why This Work
+
+Schema changes are the #1 unsolved problem. From research:
+- **Cost cited:** $2.1K–$45M in losses per incident
+- **Frequency:** Weekly for actively-developed APIs (OpenAI, Stripe, HubSpot)
+- **Current status:** No standard detection mechanism
+- **Impact:** Agents silently fail or return garbled data when schemas drift
+
+Rhumb's unique advantage: we see every call through the proxy. We can fingerprint schema, detect diffs, and alert **before agents break**.
+
+---
+
+## Deliverables
+
+### Module 1: Schema Fingerprinting Engine
+**File:** `packages/api/services/schema_fingerprint.py`  
+**Responsibility:** Capture and normalize API response schemas
+
+**Key Components:**
+- `SchemaFingerprint` dataclass: captures structure of response (field names, types, nested structure)
+- `fingerprint_response()` function: parse response → structural hash (content-agnostic)
+- Deep structural comparison: field add/remove, type change, nesting depth, cardinality (singular vs array)
+- Semantic drift detection: detect common renames (parse similarity, levenshtein distance on field names)
+- Metadata extraction: latency, status code, headers (content-type, cache-control)
+
+**Tests (10 tests):**
+- Fingerprint stable response → consistent hash
+- Add field → detected as change
+- Remove field → detected as change
+- Type change (string → int) → detected
+- Nested object structure change → detected
+- Same fields, different order → same fingerprint (order-insensitive)
+- Semantic rename (old_field → new_field) → similarity score >0.8 flags as likely rename
+- Null/optional fields → treated as cardinality change
+- Array → single object change → detected
+- Complex nested structure (3+ levels) → fingerprinted correctly
+
+---
+
+### Module 2: Schema Change Detector
+**File:** `packages/api/services/schema_change_detector.py`  
+**Responsibility:** Detect, classify, and track schema drift
+
+**Key Components:**
+- `SchemaChangeDetector` class: compares current vs baseline fingerprint
+- `detect_changes()` method: returns list of `SchemaChange` (add, remove, rename, type_change, nesting_change)
+- `classify_severity()` method: breaking vs non-breaking vs advisory
+  - **Breaking:** field removal, type change, nesting change
+  - **Non-breaking:** field addition, optional field changes
+  - **Advisory:** naming convention changes
+- `alert_required()` method: returns bool (breaking changes → alert always, non-breaking → only if configured)
+- Redis-backed baseline tracking: `schema:baseline:{service}:{endpoint}` → latest fingerprint hash + timestamp
+
+**Tests (12 tests):**
+- No changes detected when responses identical
+- Field addition flagged as non-breaking
+- Field removal flagged as breaking
+- Type change flagged as breaking
+- Multiple changes in single response → detected and classified
+- Empty response → handled gracefully
+- Breaking change + advisory change in same diff → both surfaced
+- Baseline update flow (store new fingerprint after validation)
+- Stale baseline (>7 days old) → handled with age warning
+- Non-JSON response (HTML error) → graceful fallback
+- Rate limit response (429) → not treated as schema drift
+- Error response schema (500) → separate from success schema
+
+---
+
+### Module 3: Alert Pipeline
+**File:** `packages/api/services/schema_alert_pipeline.py`  
+**Responsibility:** Route schema change alerts to operators
+
+**Key Components:**
+- `AlertDispatcher` class: routes breaking changes to webhook + email + in-app
+- `webhook_dispatch()`: POST to operator's configured webhook URL (auth token in header)
+  - Payload: service name, endpoint, change detail, severity, timestamp, fingerprint diff
+  - Retry logic: exponential backoff (3x, max 1h)
+  - Error handling: webhook failure logged, alert marked retry_pending
+- `email_dispatch()`: Slack/email notification (mock for now, real integration in Phase 3)
+- `inapp_dispatch()`: Store schema alert in `schema_alerts` table, queryable via `/v1/admin/schema-alerts`
+- Alert deduplication: same change on same endpoint → only alert once per 24h (unless severity escalates)
+
+**Tests (8 tests):**
+- Breaking change → webhook dispatched
+- Non-breaking change → no alert (unless configured)
+- Webhook success (200 OK) → logged, alert marked sent
+- Webhook failure (500) → retry scheduled, alert marked pending
+- Alert deduplication: same change 2x in 1h → only one webhook call
+- Payload shape validation: includes all required fields
+- Email dispatch (mock) → logged with recipient
+- Alert query: `/v1/admin/schema-alerts?service=stripe&limit=10` → returns recent alerts
+
+---
+
+### Module 4: Proxy Integration
+**File:** `packages/api/routes/proxy.py` (extension)  
+**Responsibility:** Integrate schema detection into the proxy call path
+
+**Key Components:**
+- Extend `POST /proxy/` to call `schema_change_detector.detect_changes()` after every proxied call
+- Store fingerprint in `schema_events` table (lightweight, no blocking)
+- If breaking change detected: dispatch alert asynchronously (don't block response)
+- New endpoint: `GET /v1/admin/schema/{service}/{endpoint}` → returns latest fingerprint + change history
+- Leaderboard integration: schema freshness feeds into AN Score confidence (if schema stable 30 days, freshness bonus)
+
+**Integration tests (10 tests):**
+- Proxy call + schema stable → response unaffected, fingerprint stored
+- Proxy call + breaking change detected → response unaffected, alert dispatched async
+- Multiple calls, same schema → fingerprint reused (no re-computation)
+- Admin endpoint `/v1/admin/schema/stripe/create-payment-intent` → returns current fingerprint + last 5 changes
+- Leaderboard: service with stable schema (30d no changes) → freshness multiplier applied
+- Operator receives webhook on breaking change → webhook payload well-formed
+- Schema drift on error response (500) → not treated as drift on success schema
+- High-volume endpoint (1,000 calls/sec) → fingerprinting doesn't block, stored async
+- Multi-tenant: agent A's schema alert doesn't leak to agent B
+- Admin schema-alerts query: filters by service, date range, severity
+
+---
+
+### Module 5: Supabase Migration
+**File:** `packages/api/migrations/0007_schema_detection.sql`  
+**Responsibility:** Database schema for schema tracking
+
+**Tables:**
+```sql
+CREATE TABLE schema_fingerprints (
+  id BIGSERIAL PRIMARY KEY,
+  service_id BIGINT NOT NULL,
+  endpoint TEXT NOT NULL,
+  fingerprint_hash TEXT NOT NULL,
+  updated_at TIMESTAMP DEFAULT now(),
+  UNIQUE(service_id, endpoint)
+);
+
+CREATE TABLE schema_events (
+  id BIGSERIAL PRIMARY KEY,
+  service_id BIGINT NOT NULL,
+  endpoint TEXT NOT NULL,
+  fingerprint_hash TEXT NOT NULL,
+  change_type TEXT, -- add, remove, type_change, rename, nesting_change
+  severity TEXT, -- breaking, non_breaking, advisory
+  captured_at TIMESTAMP DEFAULT now(),
+  CONSTRAINT fk_service FOREIGN KEY (service_id) REFERENCES services(id)
+);
+
+CREATE INDEX idx_schema_events_service_endpoint ON schema_events(service_id, endpoint);
+CREATE INDEX idx_schema_events_captured_at ON schema_events(captured_at DESC);
+
+CREATE TABLE schema_alerts (
+  id BIGSERIAL PRIMARY KEY,
+  service_id BIGINT NOT NULL,
+  endpoint TEXT NOT NULL,
+  change_detail JSONB,
+  severity TEXT,
+  alert_sent_at TIMESTAMP,
+  webhook_url TEXT,
+  webhook_status INT,
+  retry_count INT DEFAULT 0,
+  retry_at TIMESTAMP,
+  CONSTRAINT fk_service FOREIGN KEY (service_id) REFERENCES services(id)
+);
+
+CREATE INDEX idx_schema_alerts_service_pending ON schema_alerts(service_id) WHERE webhook_status IS NULL;
+CREATE INDEX idx_schema_alerts_created_at ON schema_alerts(created_at DESC);
+```
+
+---
+
+## Acceptance Criteria
+
+### Functional
+- ✅ Fingerprint captures schema structure (fields, types, nesting)
+- ✅ Detector identifies changes (add, remove, type change, rename, nesting)
+- ✅ Severity classification (breaking vs non-breaking vs advisory)
+- ✅ Alert dispatch (webhook + email + in-app) for breaking changes
+- ✅ Deduplication (same change only alerts once per 24h)
+- ✅ Proxy integration (non-blocking, async alert dispatch)
+- ✅ Admin endpoint: `/v1/admin/schema/{service}/{endpoint}` returns fingerprint + change history
+- ✅ Leaderboard integration: schema stability feeds AN Score freshness
+
+### Quality
+- ✅ 40+ integration tests (fingerprint, detector, alerts, proxy, admin)
+- ✅ Type annotations complete (mypy clean)
+- ✅ Linting clean (flake8, isort)
+- ✅ Zero regressions from Phase 2 (239 tests still passing)
+- ✅ Supabase migration idempotent (no side effects)
+
+### Performance
+- ✅ Fingerprinting: <5ms per response (streaming, not blocking)
+- ✅ Change detection: <10ms per call (O(n) in field count, not response size)
+- ✅ Alert dispatch: async (webhook calls don't block proxy response)
+- ✅ Baseline lookups: O(1) Redis (or in-memory cache)
+
+### Operability
+- ✅ Config: operator can set webhook URL + alert preferences per service
+- ✅ Logging: all fingerprints + changes logged (queryable via admin endpoint)
+- ✅ Monitoring: alert success/failure rates visible in dashboards
+- ✅ Graceful degradation: if Redis unavailable, in-memory fallback (with limits)
+
+---
+
+## Thin-Slice Decomposition
+
+| Slice | Focus | Deliverables | Tests | Approx Time |
+|-------|-------|---------------|----|------------|
+| A | Fingerprinting + comparison | Module 1 + tests | 10 | 12 min |
+| B | Change detection + classification | Module 2 + tests | 12 | 13 min |
+| C | Alert pipeline + integration | Module 3 + Module 4 integration | 10 | 14 min |
+| D | Admin endpoints + E2E | Module 4 completion + Module 5 + integration tests | 10+ | 12 min |
+
+**Total target:** 40+ tests, 4–5 modules, 50–60 min execution time (single sub-agent run)
+
+---
+
+## Dependencies
+
+**Runtime:** Depends on Phase 2 (WU 2.1–2.3) being live
+- Proxy router must be operational (for call interception)
+- Agent identity must be in place (for per-agent alert routing)
+- Billing/metering must exist (for schema events to include cost context)
+
+**No blocking dependencies:** All modules can be developed in parallel slices
+
+---
+
+## Success Signal
+
+By end of Round 13:
+- ✅ Proxy detects schema changes on every call (non-blocking)
+- ✅ Breaking changes trigger webhook alerts within 1 second
+- ✅ Operators can query schema history per endpoint
+- ✅ Leaderboard scores account for schema stability (freshness bonus for stable services)
+- ✅ 239 Phase 2 tests still passing + 40+ new tests
+- ✅ Ready for Phase 3 GTM launch with complete Access Layer
+
+---
+
+## Questions / Clarifications
+
+**Q: How do we handle schema versioning (e.g., API v1 → v2)?**  
+A: Currently treat as separate endpoint. Future: add version parameter to fingerprint key.
+
+**Q: What if an endpoint returns different schemas based on auth level?**  
+A: Each (agent + endpoint) pair gets separate baseline. Schema changes scoped per agent if needed.
+
+**Q: Real Stripe integration for alerts?**  
+A: No. Phase 3 work. For now: webhook dispatch + in-app alerts. Email is mocked.
+
+**Q: How do we avoid false positives (e.g., timestamps, UUIDs)?**  
+A: Fingerprint ignores scalar values, only captures structure. Timestamps/UUIDs don't trigger alerts.
+
+**Q: Can operators opt-out of alerts for non-breaking changes?**  
+A: Yes. Config: `alert_severity: breaking_only` (default) or `all`. Per-service setting.