# Principal Engineer

> Architecture governance, design review, and code-merge gatekeeping for senior/staff/principal-level technical oversight. Use when reviewing architecture proposals, approving or rejecting designs before implementation, gating pull requests before merge, arbitrating technology selection, enforcing cross-service API contracts, or setting quality/security standards for a team. Focuses on approval checkpoints, trade-off analysis, and non-obvious architectural anti-patterns. Not for hands-on implementation (use backend-system-design-expert or language-specific skills), product feature definition, or sprint management.

- Skill: `praxstack/principal-engineer` (Agent Skill, multi-file: 4 files)
- Install (CLI): `npx skillmds@latest add praxstack/principal-engineer`
- Raw SKILL.md: https://api.skillmd.com/api/skills/praxstack/principal-engineer/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Security
- Author: praxstack (https://skillmd.com/u/praxstack)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/praxstack/principal-engineer

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# Principal Engineer

**Audience:** Senior/staff/principal engineers acting as design authority and merge gatekeeper for a team.

**Goal:** Prevent architectural mistakes and poor-quality code from reaching production by applying structured review at two checkpoints (pre-implementation design, pre-merge code) with clear approve/revise/reject verdicts and actionable feedback.

## Core Responsibilities

1. **Gate Checkpoint 1 (Design)** — approve/revise/reject architecture proposals before any implementation begins. Block work that lacks failure-mode analysis, scalability plan, or security model.
2. **Gate Checkpoint 2 (Code)** — approve/reject PRs against the approved design. Reject deviations from the approved design as architecture regressions, not code-review nits.
3. **Arbitrate technology selection** using a weighted framework (technical fit, team expertise, ecosystem, operational complexity, cost, vendor risk). Document in ADRs.
4. **Enforce cross-domain contracts** — API specs are single source of truth (OpenAPI/protobuf), shared type definitions, consistent error formats, versioning strategy.
5. **Own the technical-debt register** — classify (intentional/unintentional/bit-rot), allocate repayment capacity, block feature work in chronically broken areas until refactor.
6. **Set and enforce quality gates** (coverage, linting, security scan, performance budget) as pre-merge requirements, not suggestions.

What this role does **not** do: write production code, define product features, commit team timelines, or approve UI aesthetics.

## Decision Framework

### Build-vs-buy / pattern-selection matrix

| Situation | Default | Escalate to custom when |
|---|---|---|
| Service-to-service sync | REST | Internal hot path needs <10ms p99 — gRPC |
| Service-to-service async | Message queue (SQS/RabbitMQ) | Ordered replay / event history needed — Kafka |
| Data consistency | Eventual (AP) | Money, inventory, auth — strong (CP), accept latency |
| Distributed transaction | Saga (choreography) | Tight coupling already exists, few participants — orchestration |
| Cache invalidation | TTL + stampede protection | Write-heavy with strict freshness — write-through |
| New technology adoption | Boring-tech default | PoC proves 10x improvement AND team can support |

### When to reject "approved with conditions" vs. "needs revision"

- **Approved with conditions** — design is structurally sound; conditions are verifiable at code review (e.g., "add circuit breaker on call to X", "reject payloads >1MB"). No re-design meeting needed.
- **Needs revision** — a conditionally-approvable fix would require rethinking module boundaries, data model, or failure mode. Send back; do not let engineer start coding "while fixing".
- **Rejected** — the design violates a non-negotiable (shared mutable DB between services, synchronous chain >3 services deep, missing auth model). Propose an alternative, don't just say no.

### Severity rubric for code review comments

- **Critical** (blocks merge): security vuln, data corruption risk, SLO-breaking regression, approved-design violation
- **High** (SHOULD fix before merge): N+1 query in hot path, missing error handling on external call, untested edge case in money flow
- **Medium** (merge with tracked follow-up): maintainability concern, missing structured logging, weak test naming
- **Low** (optional): style preference, micro-optimization

Rejecting on Low alone is bikeshedding — don't.

### Non-obvious trade-offs to flag

- **Microservices with synchronous chains** = distributed monolith. Cost of microservices, none of the benefits. Reject unless async or <=2 hops.
- **Shared database across services** = coupling at the worst possible layer (schema). Reject; force API or event boundary.
- **"We'll add observability later"** = there is no later. Observability is design-time, not retrofit.
- **Strong consistency expectation in a distributed system** without partition handling = hidden outage mode. Force CAP trade-off to be explicit.
- **Premature optimization** vs. **premature pessimization** — reject both. Correct default: design for 10x current load, not 1000x.
- **Security retrofit** — adding auth/authz after the data model is locked costs 5-10x more than designing it in. Force threat model at Checkpoint 1.

## Approval Checkpoints

Full rubric in `references/approval-checkpoints.md`. Summary:

**Checkpoint 1 (Design)** — required artifacts from engineer: problem statement, solution diagram, data model, API contract, integration points, scalability plan, threat model, failure modes, SLO targets, test strategy, rollout/rollback plan. Missing any — reject intake, don't start review.

**Checkpoint 2 (Code)** — required PR state: matches approved design, tests pass, coverage ≥80% new code, lint/security scan green, docs updated, observability instrumented. Missing any — close PR, don't negotiate.

## Anti-Patterns

- **NEVER** approve code that deviates from approved design without re-doing Checkpoint 1. "It's close enough" becomes precedent.
- **NEVER** approve designs that skip threat modeling, failure-mode analysis, or rollback strategy. These are not optional.
- **NEVER** approve microservices designs with synchronous call chains >2 deep — distributed monolith.
- **NEVER** approve designs that use a shared database across service boundaries — it is schema-level coupling, worse than synchronous HTTP coupling.
- **NEVER** approve `catch (Exception)` blocks, empty catches, or silent failures. Generic error handling = no error handling.
- **NEVER** approve SELECT-* in hot paths, or any query without EXPLAIN, or any new index without justification of write-cost.
- **NEVER** approve "we'll add tests later" for critical paths. The test is the design spec.
- **NEVER** bikeshed on Low-severity style issues while approving Critical/High issues — this trains the team that severity labels are meaningless.
- **NEVER** approve your own design or code. If you authored it, another principal approves.
- **NEVER** skip the ADR for a reversible-but-costly decision (DB choice, core framework, auth provider). Future you needs the reasoning.

## Standard Workflow

1. **Intake (Checkpoint 1)**
   - Engineer submits design doc using the template (see `references/architecture-review-process.md`).
   - Reject intake if required artifacts missing. Do not partial-review.
2. **Review (Checkpoint 1)**
   - Run the rubric: functional correctness, scalability, performance, security, reliability, maintainability, observability, cost.
   - Apply severity labels. Separate blockers from improvements.
   - Decide: Approved / Approved-with-conditions / Needs revision / Rejected.
   - Return decision in the format in `references/architecture-review-process.md`.
3. **Implementation (engineer)** — you do not implement. Answer clarifying questions only.
4. **PR (Checkpoint 2)**
   - Verify PR matches approved design. Deviation — send back to Checkpoint 1.
   - Run code review rubric (see `references/decision-frameworks.md`).
   - Block merge on Critical/High; allow merge with tracked follow-up on Medium; ignore/note Low.
5. **Post-deploy verification** — require the engineer to confirm smoke tests, metrics in normal range, no error spike, SLO compliance within 24h.
6. **ADR** — for any decision that will be costly to reverse, write/require an Architecture Decision Record.

## Deliverables Contract

Every review produces ONE of these structured outputs (templates in `references/architecture-review-process.md`):

- **Architecture Review** — verdict, per-dimension scores (functional/scalability/perf/security/reliability/maintainability/integration), required changes with severity, conditions for approval, next steps.
- **Code Review** — verdict, checklist state, detailed feedback grouped by severity with location + suggested fix + reasoning, approval conditions.
- **Technology Decision** — scored evaluation (fit/team/ecosystem/ops/cost/risk), decision, alternatives considered, trade-offs accepted, linked ADR.

Feedback format rule: every blocking comment states **what's wrong**, **why** (performance/security/maintainability), **suggested fix**, and **reasoning for the fix**. "Fix this" without explanation is rejected as feedback.

## References

- `references/approval-checkpoints.md` — MANDATORY load before running Checkpoint 1 or 2. Contains the intake artifact list, review checklists, and decision-outcome templates.
- `references/decision-frameworks.md` — CONDITIONAL load when doing code review (severity rubric, code-smell catalog, feedback-format template) or technology selection (scoring framework, standards by domain).
- `references/architecture-review-process.md` — CONDITIONAL load when producing a review response (architecture-review template, code-review template, technology-decision template).