diff --git a/docs/superpowers/plans/2026-06-20-cyclone-edi-features.md b/docs/superpowers/plans/2026-06-20-cyclone-edi-features.md
new file mode 100644
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@@ -0,0 +1,379 @@
+# Sub-project 3 — Backend EDI Features: Implementation Plan
+
+**Date:** 2026-06-20
+**Status:** Approved
+**Branch:** `edi-features`
+**Spec:** `docs/superpowers/specs/2026-06-20-cyclone-edi-features-design.md`
+
+30 tasks across 5 phases. Follows the same task pattern as the SP2 plan (`[ ] Step 1 → Step N → commit`).
+
+## Phase 1 — 837P extra validation rules
+
+### Task 1: REF*G1 enforcement rule (R032)
+
+**Files:**
+- Modify: `backend/src/cyclone/parsers/validator.py`
+
+- [ ] **Step 1: Add failing tests**
+
+Append to `backend/tests/test_validator.py`:
+
+```python
+def test_r032_ref_g1_required_when_freq_7_no_ref_g1_emits_error(co_medicaid_payer_with_ref_g1_required):
+    # ... claim with frequency_code=7, no REF*G1
+    # expect validation error
+
+def test_r032_ref_g1_required_when_freq_8_no_ref_g1_emits_error(co_medicaid_payer_with_ref_g1_required):
+    # ... claim with frequency_code=8, no REF*G1
+    # expect validation error
+
+def test_r032_ref_g1_required_when_freq_7_with_ref_g1_passes(co_medicaid_payer_with_ref_g1_required):
+    # ... claim with frequency_code=7, REF*G1 present
+    # expect no error
+
+def test_r032_ref_g1_required_when_freq_1_no_ref_g1_passes(co_medicaid_payer_with_ref_g1_required):
+    # ... claim with frequency_code=1, no REF*G1
+    # expect no error (rule only fires on 7/8)
+```
+
+- [ ] **Step 2: Run tests to verify they fail**
+- [ ] **Step 3: Implement `_r032_ref_g1_required` in `validator.py`**
+- [ ] **Step 4: Register in `_RULES` list**
+- [ ] **Step 5: Commit**
+
+### Task 2: BHT06 transaction-set-purpose-code rule (R033)
+
+**Files:**
+- Modify: `backend/src/cyclone/parsers/validator.py`
+
+- [ ] **Step 1: Add failing tests**
+- [ ] **Step 2: Run tests to verify they fail**
+- [ ] **Step 3: Implement `_r033_bht06_allowed` in `validator.py`**
+- [ ] **Step 4: Register in `_RULES` list**
+- [ ] **Step 5: Commit**
+
+### Task 3: Verify existing R031 (REF*G1 optional) keeps warning behavior
+
+**Files:**
+- Verify: `backend/src/cyclone/parsers/validator.py:54-57`
+
+- [ ] **Step 1: Read current `_r031_ref_g1_optional`**
+- [ ] **Step 2: Confirm it stays as a warning for default configs**
+- [ ] **Step 3: (No commit — verification task)**
+
+### Task 4: Wire `require_ref_g1_for_adjustments` into CO Medicaid factory
+
+**Files:**
+- Verify: `backend/src/cyclone/parsers/payer.py:67`
+
+- [ ] **Step 1: Confirm factory sets the flag for CO**
+- [ ] **Step 2: Run existing co_medicaid fixture through new R032**
+- [ ] **Step 3: Adjust fixture if false positives appear**
+- [ ] **Step 4: Commit if any change**
+
+### Task 5: Phase 1 commit + full suite
+
+```bash
+.venv/bin/pytest -q
+```
+Expected: 205 + 6 = ~211 pass.
+
+---
+
+## Phase 2 — 835 CAS deep-parsing (reason-code lookup)
+
+### Task 6: `parsers/cas_codes.py` with CARC dict
+
+**Files:**
+- Create: `backend/src/cyclone/parsers/cas_codes.py`
+- Create: `backend/tests/test_cas_codes.py`
+
+- [ ] **Step 1: Add failing tests** for known/unknown lookup, count > 100
+- [ ] **Step 2: Run tests to verify they fail**
+- [ ] **Step 3: Implement `reason_label` + `all_known_codes` with ~150 CARC codes**
+- [ ] **Step 4: Run tests to verify they pass**
+- [ ] **Step 5: Commit**
+
+### Task 7: Extend `GET /api/remittances/{id}` to surface `adjustments`
+
+**Files:**
+- Modify: `backend/src/cyclone/store.py`
+- Modify: `backend/src/cyclone/api.py`
+- Modify: `backend/tests/test_api_gets.py`
+
+- [ ] **Step 1: Add failing tests** (empty adjustments returns `[]`, populated returns `{group, reason, label, amount, quantity}`)
+- [ ] **Step 2: Run tests to verify they fail**
+- [ ] **Step 3: Add a `get_remit_with_adjustments` method to `CycloneStore`**
+- [ ] **Step 4: Update `to_ui_remittance` (or add `to_ui_remittance_detail`) to include adjustments array**
+- [ ] **Step 5: Update endpoint to use the new mapper**
+- [ ] **Step 6: Run tests to verify they pass**
+- [ ] **Step 7: Commit**
+
+### Task 8: Frontend Remit type + Remittances page adjustments expansion
+
+**Files:**
+- Modify: `src/types/index.ts`
+- Modify: `src/pages/Remittances.tsx`
+
+- [ ] **Step 1: Add `adjustments?: { group, reason, label, amount, quantity }[]` to `Remittance`**
+- [ ] **Step 2: Update Remittances page to render adjustments expansion**
+- [ ] **Step 3: Add test** in Remittances.test.tsx
+- [ ] **Step 4: Verify build + tests pass**
+- [ ] **Step 5: Commit**
+
+### Task 9: Phase 2 commit + full suite
+
+```bash
+.venv/bin/pytest -q && npm test -- --run
+```
+Expected: ~215 backend, ~9 frontend.
+
+---
+
+## Phase 3 — 999 ACK transaction set
+
+### Task 10: New Pydantic models for 999
+
+**Files:**
+- Create: `backend/src/cyclone/parsers/models_999.py`
+- Modify: `backend/src/cyclone/parsers/__init__.py`
+
+- [ ] **Step 1: Implement model classes per spec §4.1**
+- [ ] **Step 2: Wire lazy re-exports**
+- [ ] **Step 3: Verify import + simple instantiation works** (1 sanity test)
+- [ ] **Step 4: Commit**
+
+### Task 11: 999 parser
+
+**Files:**
+- Create: `backend/src/cyclone/parsers/parse_999.py`
+
+- [ ] **Step 1: Add failing tests** with minimal_999.txt fixture (basic happy path)
+- [ ] **Step 2: Run tests to verify they fail**
+- [ ] **Step 3: Implement orchestrator** following `parse_835.py` pattern
+- [ ] **Step 4: Run tests to verify they pass**
+- [ ] **Step 5: Commit**
+
+### Task 12: 999 serializer (outbound)
+
+**Files:**
+- Create: `backend/src/cyclone/parsers/serialize_999.py`
+
+- [ ] **Step 1: Add failing round-trip test** (parse → serialize → parse → assert equal)
+- [ ] **Step 2: Run test to verify it fails**
+- [ ] **Step 3: Implement `serialize_999`** (ISA/GS/ST/AK1/[AK2/AK3/AK4/AK5*]/AK9/SE/GE/IEA)
+- [ ] **Step 4: Run test to verify it passes**
+- [ ] **Step 5: Commit**
+
+### Task 13: `acks` table + ORM + migration
+
+**Files:**
+- Create: `backend/src/cyclone/migrations/0002_acks.sql`
+- Modify: `backend/src/cyclone/db.py`
+
+- [ ] **Step 1: Add migration SQL** per spec §4.4
+- [ ] **Step 2: Add ORM model `Ack` to `db.py`**
+- [ ] **Step 3: Add `CycloneStore.add_ack(...)` method**
+- [ ] **Step 4: Run migration on a fresh DB; verify table exists**
+- [ ] **Step 5: Commit**
+
+### Task 14: `POST /api/parse-999` endpoint
+
+**Files:**
+- Modify: `backend/src/cyclone/api.py`
+
+- [ ] **Step 1: Add failing tests** (parse + persist, returns summary)
+- [ ] **Step 2: Run tests to verify they fail**
+- [ ] **Step 3: Add endpoint** mirroring `parse-835` JSON path
+- [ ] **Step 4: Run tests to verify they pass**
+- [ ] **Step 5: Commit**
+
+### Task 15: Auto-generate 999 on 837 ingest via `?ack=true`
+
+**Files:**
+- Modify: `backend/src/cyclone/api.py`
+- Modify: `backend/tests/test_api_parse_persists.py`
+
+- [ ] **Step 1: Add 3 failing tests** (default no ack, ack=true happy path, ack=true with rejected claims)
+- [ ] **Step 2: Run tests to verify they fail**
+- [ ] **Step 3: Add `ack: bool = Query(False)` param to `parse_837_endpoint`**
+- [ ] **Step 4: Build `BatchAckBuilder` that converts validation report to 999**
+- [ ] **Step 5: Persist `Ack` row, attach to response body**
+- [ ] **Step 6: Run tests to verify they pass**
+- [ ] **Step 7: Commit**
+
+### Task 16: Frontend `/acks` page + sidebar entry
+
+**Files:**
+- Modify: `src/lib/api.ts`
+- Modify: `src/hooks/` (new `useAcks.ts`)
+- Modify: `src/types/index.ts`
+- Create: `src/pages/Acks.tsx`
+- Create: `src/pages/Acks.test.tsx`
+- Modify: `src/components/Sidebar.tsx`
+
+- [ ] **Step 1: Add `Ack` type to `src/types/index.ts`**
+- [ ] **Step 2: Add `api.listAcks()` + `api.getAck(id)` to `src/lib/api.ts`**
+- [ ] **Step 3: Create `useAcks` hook**
+- [ ] **Step 4: Create `Acks.tsx` page** with empty state, populated list, download button
+- [ ] **Step 5: Add sidebar nav entry with `CheckCircle2` icon**
+- [ ] **Step 6: Add tests** (1-2 cases: empty + populated)
+- [ ] **Step 7: Verify typecheck + build + tests pass**
+- [ ] **Step 8: Commit**
+
+### Task 17: Phase 3 commit + full suite
+
+```bash
+.venv/bin/pytest -q && npm test -- --run
+```
+Expected: ~225 backend, ~10 frontend.
+
+---
+
+## Phase 4 — 270/271 eligibility
+
+### Task 18: 270 request models
+
+**Files:**
+- Create: `backend/src/cyclone/parsers/models_270.py`
+
+- [ ] **Step 1: Implement model classes per spec §4.2**
+- [ ] **Step 2: Wire lazy re-exports**
+- [ ] **Step 3: Sanity test**
+- [ ] **Step 4: Commit**
+
+### Task 19: 271 response models + service-type-code dict
+
+**Files:**
+- Create: `backend/src/cyclone/parsers/models_271.py`
+
+- [ ] **Step 1: Implement model classes per spec §4.3**
+- [ ] **Step 2: Add inline `SERVICE_TYPE_CODES` dict** (~50 codes)
+- [ ] **Step 3: Add `service_type_description(code)` helper**
+- [ ] **Step 4: Sanity test**
+- [ ] **Step 5: Commit**
+
+### Task 20: 270 request parser
+
+**Files:**
+- Create: `backend/src/cyclone/parsers/parse_270.py`
+
+- [ ] **Step 1: Add failing tests** with minimal_270.txt fixture
+- [ ] **Step 2: Run tests to verify they fail**
+- [ ] **Step 3: Implement orchestrator**
+- [ ] **Step 4: Run tests to verify they pass**
+- [ ] **Step 5: Commit**
+
+### Task 21: 271 response parser
+
+**Files:**
+- Create: `backend/src/cyclone/parsers/parse_271.py`
+
+- [ ] **Step 1: Add failing tests** with minimal_271.txt fixture (2 EB segments)
+- [ ] **Step 2: Run tests to verify they fail**
+- [ ] **Step 3: Implement orchestrator** with EB segment handling
+- [ ] **Step 4: Run tests to verify they pass**
+- [ ] **Step 5: Commit**
+
+### Task 22: 270 request serializer (outbound)
+
+**Files:**
+- Create: `backend/src/cyclone/parsers/serialize_270.py`
+
+- [ ] **Step 1: Add failing round-trip test**
+- [ ] **Step 2: Run test to verify it fails**
+- [ ] **Step 3: Implement `serialize_270`**
+- [ ] **Step 4: Run test to verify it passes**
+- [ ] **Step 5: Commit**
+
+### Task 23: `POST /api/eligibility/request` endpoint
+
+**Files:**
+- Modify: `backend/src/cyclone/api.py`
+- Create: `backend/tests/test_api_eligibility.py`
+
+- [ ] **Step 1: Add failing test**
+- [ ] **Step 2: Run test to verify it fails**
+- [ ] **Step 3: Implement endpoint**
+- [ ] **Step 4: Run test to verify it passes**
+- [ ] **Step 5: Commit**
+
+### Task 24: `POST /api/eligibility/parse-271` endpoint
+
+**Files:**
+- Modify: `backend/src/cyclone/api.py`
+
+- [ ] **Step 1: Add 2 failing tests** (happy path + parse error)
+- [ ] **Step 2: Run tests to verify they fail**
+- [ ] **Step 3: Implement endpoint**
+- [ ] **Step 4: Run tests to verify they pass**
+- [ ] **Step 5: Commit**
+
+### Task 25: Fixtures + cross-parser round-trip test
+
+**Files:**
+- Create: `backend/tests/fixtures/minimal_270.txt`
+- Create: `backend/tests/fixtures/minimal_271.txt`
+
+- [ ] **Step 1: Write minimal_270.txt fixture** (small valid 270 request)
+- [ ] **Step 2: Write minimal_271.txt fixture** (small valid 271 response with 2 EB segments)
+- [ ] **Step 3: Add round-trip integration test** (parse 270 → serialize → parse → assert equal)
+- [ ] **Step 4: Verify tests pass**
+- [ ] **Step 5: Commit**
+
+### Task 26: Phase 4 commit + full suite
+
+```bash
+.venv/bin/pytest -q
+```
+Expected: ~229 backend, ~10 frontend.
+
+---
+
+## Phase 5 — Docs + smoke
+
+### Task 27: Spec doc already committed (T0)
+
+Verify `docs/superpowers/specs/2026-06-20-cyclone-edi-features-design.md` is on branch.
+
+### Task 28: Update root README
+
+**Files:**
+- Modify: `README.md`
+
+- [ ] **Step 1: Update Roadmap section** to mark SP3 shipped and describe 4 areas
+- [ ] **Step 2: Commit**
+
+### Task 29: End-to-end smoke
+
+- [ ] **Step 1: Clean DB**, start server, parse 837 with `?ack=true`
+- [ ] **Step 2: Confirm 999 in response + `acks` row in DB**
+- [ ] **Step 3: Parse 999 fixture**, confirm endpoint round-trips
+- [ ] **Step 4: Build 270 request** via API, paste into parser
+- [ ] **Step 5: Commit empty** `smoke: end-to-end SP3 flow passes`
+
+### Task 30: Final cleanup + merge worktree to main
+
+- [ ] **Step 1: Verify worktree is clean**
+- [ ] **Step 2: Discard main's stale uncommitted changes** (if any)
+- [ ] **Step 3: Fast-forward merge `edi-features` into `main`**
+- [ ] **Step 4: Remove worktree + delete branch**
+
+---
+
+## Test count targets
+
+| Phase | New backend | New frontend | Running total |
+|-------|-------------|--------------|---------------|
+| Start | 205 | 8 | — |
+| P1 end | +6 | 0 | 211 / 8 |
+| P2 end | +4 | +1 | 215 / 9 |
+| P3 end | +10 | +2 | 225 / 11 |
+| P4 end | +4 | 0 | 229 / 11 |
+
+## Risk reminders
+
+- 270/271 scope creep — keep tight to EQ/EB surface.
+- 999 auto-gen semantics — "rejected" = validation errors.
+- CARC codes drift — `LAST_UPDATED` constant for tracking.
+- Migration ordering — `0002_acks.sql` after `0001_initial.sql`, idempotent.
\ No newline at end of file
diff --git a/docs/superpowers/specs/2026-06-20-cyclone-edi-features-design.md b/docs/superpowers/specs/2026-06-20-cyclone-edi-features-design.md
new file mode 100644
index 0000000..9de0e00
--- /dev/null
+++ b/docs/superpowers/specs/2026-06-20-cyclone-edi-features-design.md
@@ -0,0 +1,216 @@
+# Sub-project 3 — Backend EDI Features: Design Spec
+
+**Date:** 2026-06-20
+**Status:** Approved
+**Branch:** `edi-features`
+
+## 1. Scope
+
+Four backend EDI feature areas extending cyclone beyond the 837P parser and 835 ERA parser already shipped:
+
+1. **Additional 837P validation rules** — REF\*G1 enforcement for adjustment-frequency claims, BHT06 transaction-set-purpose-code allowlist.
+2. **835 CAS deep-parsing** — surface CARC/RARC reason-code labels alongside the existing CAS persistence.
+3. **999 ACK** — parse inbound 999s, serialize outbound 999s, optionally auto-generate a 999 in response to an inbound 837P.
+4. **270/271 eligibility** — parse 270 requests and 271 responses, serialize 270 requests.
+
+## 2. Out of scope (deferred to future sub-projects)
+
+- TA1 / 997 transaction sets (Cyclone doesn't generate them today; deferred)
+- Real-time payer eligibility round-trip (would require an outbound HTTP client + payer credentials)
+- 999 ACK UI drilldown beyond a list page
+- Per-claim ACK row linking in the DB
+
+## 3. Locked decisions
+
+### 3.1 CAS reason-code lookup = inline Python dict
+
+A new `parsers/cas_codes.py` module with ~150 CARC codes (the most common in Colorado Medicaid remits: CO-45, CO-97, PR-1, PR-2, PR-3, CO-16, CO-29, CO-50, CO-109, CO-119, etc.). No external CSV dependency — same shape as `claim_status_label(code)` at `parsers/models_835.py:55-71`.
+
+```python
+def reason_label(group_code: str, reason_code: str) -> str
+def all_known_codes() -> list[tuple[str, str, str]]  # (group, reason, label)
+```
+
+Includes a `LAST_UPDATED = "2026-06-20"` module constant for refresh tracking.
+
+### 3.2 999 ACKs get a dedicated `acks` table
+
+A new migration `0002_acks.sql` adds the table rather than reusing `batches` with a `kind="999"` + `parent_batch_id` FK. Cleaner symmetry with the `cas_adjustments` / `matches` patterns. The `acks` table holds the *response*, not the source batch.
+
+```sql
+CREATE TABLE acks (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    source_batch_id TEXT NOT NULL REFERENCES batches(id) ON DELETE CASCADE,
+    accepted_count INTEGER NOT NULL DEFAULT 0,
+    rejected_count INTEGER NOT NULL DEFAULT 0,
+    received_count INTEGER NOT NULL DEFAULT 0,
+    ack_code TEXT NOT NULL,
+    parsed_at DATETIME NOT NULL,
+    raw_json TEXT
+);
+CREATE INDEX ix_acks_source_batch_id ON acks(source_batch_id);
+```
+
+### 3.3 Auto-999 generation gated by `?ack=true`
+
+`POST /api/parse-837` gains a new `ack: bool = Query(False)` parameter. When True, after `store.add(record)`, build a 999 from the validation report:
+
+- For each claim: AK2 → AK5 with `A` (accepted) for passed validation, `R` (rejected) for failed, `E` (accepted-with-errors) for warnings.
+- Serialize via `serialize_999`, persist the resulting `Ack` row.
+- Add `body["ack"] = { id, accepted_count, rejected_count, raw_999_text }` to the JSON response.
+
+Default behavior unchanged (no auto-ACK).
+
+### 3.4 270/271 = API-only, no UI, no DB persistence
+
+Ephemeral request/response round-trip. The operator pastes a 270 query, gets a printable 270 file, submits manually, pastes the 271 back. v1 captures the minimum useful surface (EQ inquiry + EB coverage-benefit response); anything more is a separate sub-project.
+
+## 4. Schemas
+
+### 4.1 999 models (`parsers/models_999.py`)
+
+```python
+class ParseResult999(BaseModel):
+    envelope: Envelope
+    functional_groups: list[FunctionalGroupResponse]  # AK1/AK2/AK9
+    
+class FunctionalGroupResponse(BaseModel):
+    ak1: AcknowledgmentHeader                # identifies the source functional group
+    sets: list[SetFunctionalGroupResponse]   # one per ST/SE set
+    
+class AcknowledgmentHeader(BaseModel):
+    functional_id_code: str                  # e.g. "HC"
+    group_control_number: str               # from GS06
+    
+class SetFunctionalGroupResponse(BaseModel):
+    ak2: SetHeader | None
+    segment_errors: list[SegmentError]
+    element_errors: list[ElementError]
+    ak5: SegmentAcceptReject                 # A / R / E / W
+    
+class SegmentError(BaseModel):
+    ak3: SegmentContext
+    loop_id: str | None
+    segment_position: int
+    
+class ElementError(BaseModel):
+    ak3_ref: int  # FK by row id
+    element_position: int
+    
+class SegmentAcceptReject(BaseModel):
+    code: str  # A=accepted, R=rejected, E=accepted-with-errors, W=warning, X=rejected-all
+    
+class FunctionalGroupAck(BaseModel):
+    ak1: AcknowledgmentHeader
+    received_count: int     # AK905
+    accepted_count: int     # AK906
+    rejected_count: int     # AK907
+    ack_code: str           # AK909
+```
+
+### 4.2 270 request models (`parsers/models_270.py`)
+
+```python
+class ParseResult270(BaseModel):
+    envelope: Envelope
+    information_source: InformationSource270       # 2100A — payer
+    information_receiver: InformationReceiver270   # 2100B — receiver
+    subscriber: Subscriber270                       # 2000C
+    patient: Patient270 | None                      # 2100C
+    inquiries: list[EligibilityBenefitInquiry]      # EQ segments
+    
+class EligibilityBenefitInquiry(BaseModel):
+    service_type_code: str  # EQ01
+    coverage_info: str | None
+    
+class Patient270(BaseModel):
+    first_name: str | None
+    last_name: str | None
+    member_id: str | None
+    dob: date | None
+    gender: str | None
+    relationship: str | None
+```
+
+### 4.3 271 response models (`parsers/models_271.py`)
+
+```python
+class ParseResult271(BaseModel):
+    envelope: Envelope
+    information_source: InformationSource271
+    information_receiver: InformationReceiver271
+    subscriber: Subscriber271
+    patient: Patient271 | None
+    coverage_benefits: list[CoverageBenefit]   # EB segments, the heart of 271
+    
+class CoverageBenefit(BaseModel):
+    service_type_code: str       # EB01
+    coverage_level: str          # EB02 (INDividual, FAMily, etc.)
+    service_type_description: str | None  # resolved from inline dict
+    benefit_amount: Decimal | None
+    in_plan_network: bool | None
+    authorization_required: bool | None
+    plan_coverage_description: str | None
+    time_qualifier: str | None
+    benefit_date: date | None
+    additional_info: list[BenefitAdditionalInfo]
+```
+
+### 4.4 `acks` ORM model
+
+```python
+class Ack(Base):
+    __tablename__ = "acks"
+    id: Mapped[int] = mapped_column(Integer, primary_key=True, autoincrement=True)
+    source_batch_id: Mapped[str] = mapped_column(
+        String(64), ForeignKey("batches.id", ondelete="CASCADE"), nullable=False,
+    )
+    accepted_count: Mapped[int] = mapped_column(Integer, nullable=False, default=0)
+    rejected_count: Mapped[int] = mapped_column(Integer, nullable=False, default=0)
+    received_count: Mapped[int] = mapped_column(Integer, nullable=False, default=0)
+    ack_code: Mapped[str] = mapped_column(String(8), nullable=False)
+    parsed_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), nullable=False)
+    raw_json: Mapped[Optional[dict]] = mapped_column(JSONText, nullable=True)
+```
+
+## 5. API additions
+
+| Method | Path | Body / Query | Returns |
+|--------|------|--------------|---------|
+| POST | `/api/parse-999` | file upload | `ParseResult999` + persisted `acks` row |
+| POST | `/api/parse-837?ack=true` | file upload | existing response + `body["ack"]` |
+| GET | `/api/acks` | — | list of `Ack` records |
+| GET | `/api/acks/{id}` | — | `Ack` + parsed 999 detail |
+| POST | `/api/eligibility/request` | JSON body | raw 270 text |
+| POST | `/api/eligibility/parse-271` | text body or file upload | `ParseResult271` summary |
+
+## 6. UI additions
+
+- New `/acks` page (`src/pages/Acks.tsx`) — list view grouped by source batch with "Download 999" button.
+- `useAcks` hook (`src/hooks/useAcks.ts`).
+- `api.listAcks()` + `api.getAck(id)` in `src/lib/api.ts`.
+- Sidebar nav entry with `CheckCircle2` icon.
+- Remittances page gains an "Adjustments" expansion row that lists CAS rows with reason-code labels.
+
+No new UI for 270/271 (API-only per §3.4).
+
+## 7. Migration ordering
+
+`0002_acks.sql` runs after `0001_initial.sql`. `db_migrate.run()` is idempotent — re-running on a fresh DB is a no-op. `create_all` is also a no-op when migration is current (per SP2 pattern). No data backfill needed (acks table is new).
+
+## 8. Test coverage targets
+
+| Phase | New tests | Total target |
+|-------|-----------|--------------|
+| P1 — 837P rules | ~6 | ~211 backend |
+| P2 — CAS deep-parsing | ~4 | ~215 backend |
+| P3 — 999 ACK | ~10 backend + ~2 frontend | ~225 backend / ~10 frontend |
+| P4 — 270/271 | ~4 backend | ~229 backend / ~10 frontend |
+| P5 — smoke | — | (verification) |
+
+## 9. Risk areas
+
+1. **270/271 scope creep.** X12 specs are large; we capture only the minimum useful surface.
+2. **999 auto-gen semantics.** "Rejected" is loosely defined; we use validation errors as the rejection signal. Document this in the OpenAPI/scalar docs.
+3. **CARC codes drift.** Inline dict is a snapshot dated 2026-06-20. Plan to refresh annually.
+4. **Schema migration ordering.** Idempotent on fresh DBs; no data backfill. Confirmed safe.
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