Agent Skills: Senior Backend Engineer

Senior Backend Engineer

Scaffold and review backend services: API design and OpenAPI-driven code generation for Express/Fastify/Koa, PostgreSQL schema analysis and migration generation, HTTP load testing, and production security hardening. Outputs ready-to-run route handlers, Zod validators, TypeScript types, migrations with rollbacks, and load-test reports.

Core Capabilities

• API scaffolding — generate route handlers, validation middleware, TypeScript types, and OpenAPI specs across Express, Fastify, and Koa.
• Database optimization — schema analysis, missing-index detection, N+1 risk detection, and migration generation with paired rollback scripts.
• Load testing — configurable concurrency with latency percentiles (P50/P90/P95/P99), throughput, error rates, and endpoint comparison.
• Security hardening — JWT config, rate limiting, input validation (Zod), and security headers (helmet) for production readiness.
• Standardized contracts — consistent data/error/meta response envelope and HTTP status conventions.

When to Use

• Designing a new API or refactoring existing endpoints.
• Slow queries or database performance needs improvement.
• Preparing an API for production or after a security review.
• Building regression/load-test baselines for backend endpoints.

Clarify First

Before scaffolding, confirm these inputs. If any is unknown or vague, ASK — do not assume:

• [ ] Framework — Express / Fastify / Koa (--framework; changes the generated route handlers, validators, and types)
• [ ] API contract source — the OpenAPI spec or endpoint list to scaffold from (the input the scaffolder reads)
• [ ] Database intent — the schema file and whether you want analysis vs migration generation (drives database_migration_tool.py)

Stop rule: ask only the 2-3 that most change the output. If the user says "just draft it," proceed and list your assumptions at the top of the artifact.

Tools

| Tool | Purpose | Command | |------|---------|---------| | api_scaffolder.py | Generate route handlers, Zod validators, and TS types from an OpenAPI spec | python scripts/api_scaffolder.py openapi.yaml --framework express --output src/routes/ | | database_migration_tool.py | Analyze schemas, suggest indexes, and generate migrations with rollbacks | python scripts/database_migration_tool.py schema.sql --analyze | | api_load_tester.py | HTTP load test with latency percentiles, throughput, and comparison | python scripts/api_load_tester.py https://api.example.com/users --concurrency 50 --duration 30 |

References

Load the reference that matches the task — keep this file lean and pull detail on demand:

• references/tools-reference.md — full usage examples, flag tables, and sample output for the scaffolder, migration tool, and load tester, plus quick-start and common commands. Read when running any tool.
• references/workflows-and-patterns.md — the API-design, database-optimization, and security-hardening workflows, common response/index patterns, the troubleshooting table, and the success-criteria bar. Read when designing or hardening a service.
• references/api_design_patterns.md — REST vs GraphQL, versioning, error handling, pagination. Read when designing new APIs.
• references/database_optimization_guide.md — indexing strategies, query optimization, N+1 solutions. Read when fixing slow queries.
• references/backend_security_practices.md — OWASP Top 10, auth patterns, input validation. Read when hardening security.

Scope & Limitations

What this skill covers:

• REST API design, scaffolding, and OpenAPI-driven code generation for Express, Fastify, and Koa
• PostgreSQL schema analysis, index optimization, migration generation with rollback support
• HTTP load testing with latency percentile analysis, throughput measurement, and endpoint comparison
• Backend security patterns including JWT configuration, rate limiting, input validation, and security headers

What this skill does NOT cover:

• Frontend development, UI components, or client-side state management -- see senior-frontend
• Infrastructure provisioning, container orchestration, or CI/CD pipeline setup -- see senior-devops
• GraphQL schema design, resolvers, or subscriptions -- see senior-fullstack
• Application performance monitoring (APM), distributed tracing, or log aggregation -- see senior-secops

Integration Points

| Skill | Integration | Data Flow | |-------|-------------|-----------| | senior-fullstack | API routes generated here feed into fullstack project scaffolding | OpenAPI spec → fullstack scaffolder consumes as API contract | | senior-devops | Migration scripts output here are consumed by CI/CD deployment pipelines | migrations/ directory → deployment workflow applies and verifies | | senior-security | Load test results and security hardening output feed into security review | Load test JSON → security audit validates rate limiting and error handling | | senior-qa | Generated route handlers and validators provide test surface for QA automation | Route files + Zod schemas → QA generates integration test suites | | senior-frontend | TypeScript types generated by the scaffolder are shared with frontend consumers | types.ts → frontend imports API types for type-safe client code | | code-reviewer | Schema analysis issues and migration diffs feed into code review checklists | Analysis report → reviewer validates index coverage and naming conventions |