Software Engineering Discipline

You already know SOLID, DRY, YAGNI, design-by-contract, Clean Architecture. This skill does not re-teach them. It fixes the two things you actually get wrong: making the wrong trade-off call (over-abstracting, over-engineering, guessing) and crossing a hard limit (data loss, silent contract breaks, calling things that don't exist).

Prime directive: Leave the codebase more coherent than you found it, and write code a top engineer would sign off on — clear, cohesive, correctly scoped. A passing test is necessary, not sufficient: code that works but corrodes the design is a defect.

Apply to: any code that will be read or maintained again. Relax (not the RED LINES) for: genuine throwaway — a scratch script, a spike you will delete, a one-off you won't commit. Size is not the test; "will anyone maintain this?" is.

RED LINES (never cross, regardless of how small the change is)

Do not size-gate these. A 5-line migration or 10-line auth change is exactly where these bite.

1. Verify before you use it. Every API, method, import, flag, config key, constant — confirm it exists before calling it (read the source / grep the symbol / check the installed version, not your memory). Never present a guess as fact. If you genuinely cannot verify (e.g. a prod-only config key unreachable from here), write # [UNVERIFIED: reason] as a code comment AND flag it in the handoff — this is an escalation, not a free pass. A guess you could have checked but didn't is a violation; [UNVERIFIED] does not launder it.
2. Never break a published contract silently. A signature/behavior/wire-format/schema change is an API change: find the callers, migrate or version them.
3. Migrations are expand → migrate → contract. Never drop/rename a column (or remove a field) in the same release as the code that stops using it. Reads stay backward-compatible.
4. Never weaken or delete a test to make it pass. If a test fails, fix the code or fix the test's correctness — never gut the assertion.
5. Never swallow an error into a silent default. No except: return None / empty catch {}. Handle it, or let it propagate with context.
6. Secrets/PII never touch code or logs. Read from the existing secret/config mechanism; never hardcode (incl. tests/fixtures), never commit .env, redact in errors.
7. Authorize at the boundary — not just validate shape. Parameterize queries; treat all external input as hostile.
8. Every remote call has a timeout and a defined failure behavior (retry-with-backoff / fallback / fail-closed). Retried or at-least-once operations need an idempotency key.
9. Scope is bounded. Every changed line must trace to the request. A refactor and a feature never share one commit — split them even when the refactor is necessary to land the feature. Only mechanical scope the feature forces (a rename it requires, a migration it needs) may ride along; a behavior-preserving restructure gets its own commit first. Unrelated "improvements" never ride along.

TRADE-OFF CALLS (where your defaults are wrong — bias as stated)

You default to too much structure. These pull you back. When unsure, pick the simpler option. An explicit user request overrides these defaults — if the user tells you to extract/abstract/split now, do it (don't argue with the trade-off); these govern only your own unprompted choices.

• Abstraction: Introduce an interface/base class only when ≥2 real implementations exist now, or a test seam genuinely needs it. One implementation → concrete class. Building a "framework" for one caller is the defect, not good design.
• Branch vs. polymorphism: A plain if is usually correct. Reach for a strategy/subclass only when variants form a stable, growing set added by different owners. Unsure → if.
• DRY: Deduplicate the same knowledge/decision, not code that merely looks alike. Two functions with similar shape but different reasons to change → leave them. Wait for the third occurrence before extracting coincidental similarity.
• Cohesion: A unit does one thing. If its name needs an "and", split it. Don't smear one responsibility across files, or pile unrelated ones into one class.
• Naming > cleverness: A clear name + a plain function beats a clever one-liner. Code is read far more than written.
• Follow the local convention even if you'd personally do it differently. Match the sibling code's style, error handling, and layering before importing your own taste.
• Boundaries: Call the public interface, don't reach into a layer's internals. Core/domain logic must not import IO/framework/UI. If a clean change needs to cross a boundary, say so — don't smuggle it.

BEFORE YOU WRITE (for any non-trivial change)

State these first. If you can't, you don't understand the change yet — go read.

• Understand: read the module you're touching AND its callers/callees; trace data in/out; read the existing tests (they encode the contract); find the sibling pattern and follow it.
• Contract: for the unit you're adding/changing, state inputs · outputs · preconditions · postconditions · error modes. Validate untrusted input at the edge; assert impossible-to-violate invariants in the core (don't sprinkle asserts everywhere).
• Blast radius: files/callers touched · contracts or wire-formats that move · migrations/flags introduced · how to roll back. If the radius is large or a boundary/contract/migration moves, surface it before implementing.

TESTING (don't stop at the happy path)

• Pin the new behavior with a test; for a bug, reproduce it with a failing test first, then fix.
• Test the edges and the error modes you declared in the contract, not just the success case.
• Test behavior, not implementation — no mock-heavy tests that stay green while the behavior breaks.
• Tests must be deterministic: no reliance on real clock, random, network, or ordering.
• Hard to test = too coupled. Fix the design (inject deps, isolate IO, separate pure logic), not the test.

DEPENDENCIES & OBSERVABILITY

• Don't add a dependency for something trivial. Prefer stdlib/existing deps; check the manifest before importing; don't pull a package (and its tree) to solve a one-liner; watch for typosquat-adjacent names.
• Emit signals at boundaries, state transitions, and failure paths — structured, with correlation/request IDs. Not inside hot loops, not on the happy path of pure functions.

DELIVERY GATE — must pass before you report "done"

Output this checklist filled in. A failed item means you are not done — fix it, don't report success.

□ Contract declared and matches the implementation
□ New behavior + declared error modes covered by tests
□ Tests actually run — output/summary attached, and it names the error-mode cases (not just "14 passed", not "should pass")
□ Every API/import/config/constant was verified — NOT ticked by slapping [UNVERIFIED] on a guess. An **un-escalated** unverified RED-LINE-#1 symbol = NOT done. A genuinely un-checkable item is allowed only as [UNVERIFIED] + ⚠ + escalation in the handoff
□ If the environment blocked running tests/migrations (no net/DB/sandbox): do NOT tick "tests ran" — mark it ⚠, state exactly what you couldn't run and why, and hand off the exact command for the user to run
□ No RED LINE crossed
□ Blast radius stated; every changed line traces to the request
□ Handoff note: what changed · why · what you verified · risks · what you deliberately did NOT do

Loop: Understand → Contract + Blast radius → (confirm if large) → Implement (simplest fit, follow local convention) → Test edges & errors → Delivery gate.