Agent Skills: Coding Rules (Compressed, Practical)

Coding Rules (Compressed, Practical)

1) Testing and bug fixes

• Every bug fix must include a test that fails before the fix and passes after.
• If a test is genuinely not feasible (rare), state why and how you verified the fix.
• Prefer minimal tests that reproduce the bug and lock in the expected behavior.

2) Async consistency

• If a function is async, all I/O inside it must be async.
• Avoid blocking calls in async code (requests, open, psycopg2, time.sleep).
• Sync is acceptable for CPU-bound or purely in-memory work.
• Use async equivalents: httpx.AsyncClient, aiofiles, asyncpg, asyncio.sleep.

3) Input validation at boundaries

• Validate all external input: request bodies, query params, path params, file uploads, webhooks, and external API responses.
• Prefer schema-based validation (Pydantic/Zod); keep validation at boundaries.

4) No code injection

• Never execute user-provided code or paths.
• No eval, exec, new Function, dynamic imports, or shell=True with user input.
• Sanitize paths; verify ownership before serving resources.

5) No secrets access

• Never read or print secrets in conversation context.
• Do not cat/grep .env files or secret stores.
• Refer to secret names only; check presence without echoing values.

6) SQL safety and query limits

• Always use parameterized queries (no string concatenation or formatting).
• Every SELECT must be bounded (LIMIT, pagination, or a single-row predicate).
• Exceptions: COUNT(*), aggregation with bounded cardinality, or WHERE id = ?.

7) Avoid race conditions

• Avoid check-then-act and read-modify-write without atomic guards.
• Use transactions, ON CONFLICT, SELECT FOR UPDATE, and constraints.

8) Error handling

• Never swallow exceptions. If you catch, log and re-raise or explain why continuing is safe.
• No empty or silent except blocks.

9) Configuration and constants

• Do not hardcode magic numbers, URLs, or config values.
• Centralize settings and use named constants.
• Avoid scattered os.getenv() calls outside the settings module.

10) Architecture and design

• Prefer composition over inheritance; inheritance only for true IS-A or framework requirements.
• Avoid circular imports; keep dependency direction one-way.
• Apply DRY and SOLID: small, focused functions and interfaces.

DRY and SOLID quick reference

• DRY: Extract repeated logic into reusable functions
• SRP: One reason to change per function/class
• OCP: Extend via abstraction, don't modify existing code
• LSP: Subtypes must be substitutable for base types
• ISP: Small, focused interfaces - don't force unused methods
• DIP: Depend on abstractions, inject dependencies

Checklist:

1. Is this logic duplicated? → Extract it
2. Does this do one thing? → Split if not
3. Am I modifying existing code to extend? → Use abstraction
4. Could I use Protocol instead of ABC? → Prefer Protocol

Composition over inheritance

Prefer composition (HAS-A) over inheritance (IS-A). Inject dependencies instead of extending classes.

Acceptable inheritance:

• ABCs for interface definition only
• Framework requirements (Django models, Exception subclasses)
• True IS-A relationships (rare)

Red flags:

• Hierarchy > 2 levels deep
• Overriding methods with different behavior
• Mixin classes
• isinstance() checks to determine behavior

11) Frontend safety

• No client-side console logging in production code.
• Frontend must not access databases or secrets; route privileged ops through a backend API.
• Do not use filesystem, process spawning, or dynamic code execution on the client.

12) Database changes

• Schema changes must update both schema and documentation.
• Avoid destructive migrations without explicit confirmation.
• Prefer reversible, incremental changes (add → migrate → remove).

13) Supabase local safety (if applicable)

• Never rename, delete, or edit applied migrations.
• Destructive commands (db reset, db push --force) require explicit confirmation.
• Create new migration files for changes instead of editing applied ones.

14) Type hints (Python)

• All Python functions require parameter and return type hints.
• Use modern syntax (list[str], dict[str, int], X | None).

15) Documentation hygiene

• Do not create docs files unless explicitly requested.