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+# Agentic system design — multi-model audit-to-code pipeline
+
+**Purpose:** turn the static plan in [`finding-orchestration.md`](finding-orchestration.md)
+into a running **multi-agent system** that works the planned sessions, interleaves
+**reviews and fresh audits**, and lands findings in the code **cleanly** (atomic,
+attributed, tested, parity-safe).
+
+This design is grounded in what Session 1+2 actually did: Haiku renamed/constants,
+Sonnet did validation/error-handling, **Opus** did the numerics + the **review gate**
+that validated the cheaper models' work, then diagnosed a CI OOM and sequenced the
+#36→#39 rebase. The system below generalises exactly that flow.
+
+---
+
+## 1. Design principles (the lessons that shape the architecture)
+
+1. **Assign by cognitive load, not severity.** A 🔴 one-line switch is Haiku/Sonnet
+   work; a 🟡 numerical reformulation is Opus. The dispatcher routes on *"how much
+   must be understood to get it right"*.
+2. **Reviewer ≠ implementer, and the reviewer is the strong model.** Every change
+   passes an **independent Opus review gate**. Self-review misses parity/numeric
+   regressions; a fresh high-capability pass catches them (it caught nothing wrong
+   this session — which is the point: it *certified* the cheap work).
+3. **The plan doc is the blackboard.** Agents are stateless sessions that
+   read → act → write `finding-orchestration.md`. No message bus; the shared state
+   is human-inspectable markdown.
+4. **One finding → one commit → one test → one attribution.** Traceability and clean
+   revertability. Renames ship with `[[deprecated]]` aliases; refactors ship with
+   value-identity assertions.
+5. **Gate everything irreversible to a human.** Legal/provenance (G0), public-API
+   stabilization (A4/A5), and the final merge are human decisions the system
+   *surfaces*, never makes.
+6. **Separate code failures from infra failures.** A CI red is not automatically a
+   code bug — a dedicated diagnostic step distinguishes `cc1plus Killed` (OOM) from a
+   real defect before anyone touches the code.
+7. **Exploit warm context.** Batch findings that touch the same code while a session
+   is warm (S2's I1/H1/N7 were done by Opus *immediately* after the `newton_core`
+   refactor instead of as a cold pickup).
+
+---
+
+## 2. Roles
+
+| Role | Model | Owns | Triggered by | Output |
+|---|---|---|---|---|
+| **Orchestrator / Planner** | Sonnet | the backlog + dispatch | new findings, finished session | next session spec + launch prompt; updated status board |
+| **Mechanic** | **Haiku** | renames, named constants, doc/citation/format fixes | dispatched mechanical findings | atomic commits + `[[deprecated]]` aliases |
+| **Engineer** | **Sonnet** | tests, error handling, CI, small/safe API changes, robustness | dispatched standard findings | commits + per-finding tests |
+| **Specialist** | **Opus** | numerics, math correctness, architecture refactors, public-API/irreversible calls | dispatched judgment findings | commits + tests + design notes |
+| **Reviewer (Gatekeeper)** | **Opus** | the review gate after every implementation session | session marked "implemented" | APPROVE / CHANGES-REQUESTED + fixes |
+| **Auditor (External reviewer)** | **Opus** | generating *new* audits with `file:line` + fix + acceptance | new module, schedule, or "re-audit" | new audit doc → new backlog findings |
+| **Integrator / Release** | Sonnet | git mechanics: branch, PR, rebase/conflict, CI watch, merge sequencing | approved session | PR; rebases; merge (after human gate) |
+| **Infra Diagnostician** | Sonnet→Opus | CI-failure triage (code vs runner/OOM), infra fixes | any CI red | "code vs infra" verdict + fix (e.g. `-j1`) |
+| **Human (Owner)** | — | legal/provenance, API stabilization, final merge | surfaced decision | decision recorded in the plan |
+
+> Tiering rule of thumb: **Haiku ≈ "transcribe the fix"**, **Sonnet ≈ "engineer the
+> fix from a clear spec"**, **Opus ≈ "decide what the fix should be" + "judge someone
+> else's fix"**.
+
+---
+
+## 3. Workflow (state machine, per session)
+
+```
+            ┌──────────────────────────────────────────────────────────┐
+            │  BACKLOG  (audit findings, status ⬜/⏸/⛔ in the plan doc) │
+            └───────────────┬──────────────────────────────────────────┘
+                            │  Orchestrator picks next ⬜, routes by load
+                            ▼
+   ┌────────── PLAN ──────────┐     emits a session-prompts.md block
+   │ model + findings + branch │────────────────────────────────────────┐
+   └───────────┬──────────────┘                                         │
+               ▼                                                        │
+        IMPLEMENT (Mechanic | Engineer | Specialist)                    │
+        · atomic commit per finding  · test per finding                 │
+        · deprecated aliases / value-identity asserts                   │
+               ▼                                                        │
+        SELF-VERIFY  (build + full test suite green locally)            │
+               ▼                                                        │
+   ┌──── REVIEW GATE (Opus, independent) ────┐                          │
+   │ build · parity · value-identity ·       │── CHANGES-REQUESTED ─────┘
+   │ public-surface · attribution · plan ✅  │
+   └───────────┬─────────────────────────────┘  APPROVE
+               ▼
+        INTEGRATE (Integrator) → open PR
+               ▼
+        CI ──red──► INFRA DIAGNOSTICIAN ──code?──► back to IMPLEMENT
+               │                          └─infra?─► fix infra (e.g. -j1), re-run
+              green
+               ▼
+        HUMAN MERGE GATE ──► MERGE ──► Orchestrator marks ✅, updates plan
+               ▼
+        (every N sessions / per module)  AUDITOR re-audit ──► new BACKLOG findings
+```
+
+Key edges proven this session: the **CHANGES-REQUESTED loop** (none needed — Sonnet/Haiku
+work was clean), the **INFRA branch** (the `cc1plus` OOM → `-j1`, *not* a code change),
+and the **multi-PR sequencing** at INTEGRATE (#36 merged first, #39 rebased to drop the
+duplicate A1–A3).
+
+---
+
+## 4. Shared state & artifacts (the blackboard)
+
+- **`finding-orchestration.md`** — backlog + status board + per-finding model/session.
+  The single source of truth every agent reads/writes.
+- **Audit docs** (`*-audit-*.md`) — the finding *specs*: `file:line`, fix, acceptance
+  criteria. The Auditor writes them; implementers consume them.
+- **`session-prompts.md`** — launch templates the Orchestrator fills in per session.
+- **PRs + commits** — the audit trail; commit trailer `Co-Authored-By: Claude <Model>`
+  records *which* model did *which* finding.
+- **Decision log** (a section in the plan) — human gates: G0 status ("authors emailed,
+  awaiting reply"), API-stabilization decisions, merge approvals.
+
+Why blackboard over a chat-bus: sessions are short-lived and stateless; the durable
+state is the markdown; humans can read and override it at any point.
+
+---
+
+## 5. Gates & guardrails
+
+**Automated gates (must pass to advance):**
+- Build clean; **full test suite green, no count regression**.
+- **No parity/golden-vector test perturbed** (HardJava-style defaults intact).
+- New public surface (result types, enums, file formats) is intentional + documented.
+- Each finding has a commit + a test; renames carry `[[deprecated]]` aliases.
+
+**Human gates (system surfaces, human decides):**
+- **Legal/provenance (G0)** — porting/relicensing rights. Blocks G1–G12, D1/D2, A4/A5.
+- **Public-API stabilization** — once in a released/CGAL API, renames need a deprecation
+  cycle; lock names only when the owner says so.
+- **Final merge** — the agent prepares a green, mergeable PR; the human clicks merge
+  (or explicitly delegates it).
+
+**Blocked-dependency tracking:** findings gated by an open human decision are marked ⛔
+in the plan and never dispatched (S6 stays blocked until G0 resolves).
+
+---
+
+## 6. How phases, reviews and audits interleave (cadence)
+
+- **Per session:** implement → **review gate** (always). The review is *inline* with
+  the cadence, not a separate phase you forget.
+- **Per PR:** CI + infra-triage + human merge gate.
+- **Per module / every N sessions / on request:** an **Auditor** pass produces a fresh
+  audit, refilling the backlog. This is the loop that keeps the system honest as the
+  code grows — new code gets audited, new findings get planned, the cycle repeats.
+- **Continuous:** the Orchestrator keeps the plan's status board current so any human
+  glance shows "done / in-flight / blocked".
+
+So the rhythm is: **audit → plan → (implement → review)\* → integrate → merge → re-audit.**
+
+---
+
+## 7. How to actually run it with Claude
+
+Three implementation levels, cheapest first:
+
+1. **Manual sessions (today).** The Orchestrator (you, or a Sonnet session) hands a
+   `session-prompts.md` block to a fresh session set to the named model. The review
+   gate is a second session set to Opus. This already works — it is exactly Session 1+2.
+2. **Claude Code subagents.** Define each role as a subagent with a pinned model and a
+   tight tool allow-list (Mechanic: edit+bash; Reviewer: read+bash, no write-to-main;
+   Integrator: bash/git/PR API). A top-level "Orchestrator" session dispatches via the
+   Task tool and reconciles the plan doc.
+3. **Claude Agent SDK (autonomous).** A supervisor process loops the state machine:
+   reads the plan, spawns role-agents (model per role), runs the gates as code
+   (build/test/CI checks), and stops at human gates. The blackboard (the markdown +
+   git) is the durable state across runs.
+
+Pin models per role explicitly; do **not** let a cheap role silently escalate — that is
+what the review gate and the dispatcher's load-routing are for.
+
+---
+
+## 8. Cost & efficiency strategy
+
+- **Route down aggressively, gate up always.** Most findings go to Haiku/Sonnet; only
+  the Opus review gate and genuine-judgment findings pay for Opus.
+- **Batch by warm context.** Group findings touching the same files into one session
+  (S2's three findings shared `newton_core`); avoid cold re-derivation.
+- **Make the reviewer cheap to satisfy.** Atomic commits + per-finding tests + a fixed
+  checklist make the Opus review fast (it reads a small, well-scoped diff).
+- **Fail fast on infra.** The diagnostician prevents wasted Opus cycles "fixing" a
+  phantom code bug that was really an OOM.
+
+---
+
+## 9. Worked example (this session, mapped to the roles)
+
+| What happened | Role(s) |
+|---|---|
+| A1–A3 renames, N4/N6 constants, M-citations | **Mechanic (Haiku)** |
+| V1–V4 error handling, C2/C3 coverage gate, I-tests | **Engineer (Sonnet)** |
+| B1 inv-dist block-FD port, N3/N5, `newton_core` (H2+B2/B4/B5), S2 (I1/H1/N7) | **Specialist (Opus)** |
+| Validated the Haiku/Sonnet commits (value-identity, alias forwarding, coverage logic) | **Reviewer (Opus)** |
+| Built the plan, model assignment, session prompts | **Orchestrator** |
+| `#36` merge → rebase `#39` (drop dup A1–A3) → PRs | **Integrator** |
+| `test-fast` red → proved `cc1plus` OOM via a pure-main probe → `-j1` fix | **Infra Diagnostician** |
+| G0 porting rights: "authors emailed, awaiting reply"; merge approval | **Human gate** |
+
+The audits themselves (the 11 docs) were a prior **Auditor (Opus)** pass — the input the
+whole pipeline consumes.
+
+---
+
+## 10. Failure modes the design absorbs
+
+- **Cheap model gets it subtly wrong** → caught by the Opus review gate.
+- **Refactor silently changes numerics** → caught by parity tests + value-identity asserts.
+- **CI red panic** → diagnostician separates infra (OOM) from code before any "fix".
+- **Two PRs overlap** (#36/#39) → Integrator sequences merge + rebase deterministically.
+- **A finding needs a human/legal call** → marked ⛔, never auto-dispatched.
+- **Backlog goes stale as code grows** → periodic Auditor re-audit refills it.
+```