Code That Fits in Your Head

Engineering heuristics for sustainable software, based on Mark Seemann's 2021 book and clearly labelled agent-era amendments.

Code examples are C#-first; every rule and the tooling tables are language-neutral.

Philosophy (Why This Skill Exists)

Software development is principally a design activity, not construction. An agent may produce most of the text, but people still review, operate, extend, and own the resulting system. These heuristics make software sustainable: understandable, resistant to architectural erosion, and cheap to change after thousands of decisions.

Core mental model from Chapter 1:

Metaphor	What it gets right	What it misses
Building a house	Plans, structure	Software endures; there's no construction phase (compiling is free); dependencies can start anywhere
Growing a garden	Pruning, refactoring, tending	Code does not improve by itself; generated code still needs stewardship
Art / craft	Skill, mastery, situational knowledge	Doesn't scale; leaves newcomers without guidance
Engineering (the target)	Heuristics, review, sign-off, checklists	We're not there yet — physical-construction calculations don't apply

"The act of describing a program in unambiguous detail and the act of programming are one and the same." — Kevlin Henney

Practical implications for a code agent:

1. Successful software endures. Prefer changes that preserve clear boundaries and keep future change affordable.
2. Complexity is the enemy, not task size. Agents can complete changes spanning tens of thousands of lines when architecture, plan, and acceptance criteria are sound. Reject needless coupling, duplication, hidden effects, and unverifiable bulk—not large scope by itself.
3. Heuristics, not laws. Understand the purpose of a rule before applying or relaxing it. Project policy overrides generic formatting and workflow conventions.
4. Verification is part of design. Types, tests, schemas, architecture checks, observability, and explicit acceptance criteria constrain both human- and agent-written code.
5. Code is a liability. Generated volume is not progress. Prefer the smallest coherent design that solves the problem without accumulating debt.

See references/foundations/ for more on sustainability, readability, and brain-limited design.

How to Use This Skill

1. Identify the user's task (writing, reviewing, debugging, security review, setting up, etc.)
2. Read guidelines.md — it maps tasks and symptoms to specific reference files
3. Load only the reference files relevant to the current task (progressive disclosure)
4. Apply the rules; when in doubt, consult references/practices-glossary/ for cross-references
5. When a rule proves repeatedly useful in a repository, operationalize it (workflows/operationalize-finding.md) so it persists as a gate and in project agent instructions.

Chapter Index

Topic	Use when...
references/foundations/	Sustainability, readability, complexity control, and code as liability
references/codebase-setup/	Starting or inheriting a code base — git, build automation, warnings-as-errors
references/outside-in-tdd/	Writing new features test-first; walking skeleton, AAA, triangulation, devil's advocate, editing tests
references/encapsulation/	Designing types with invariants; DTO vs Domain Model, always-valid, Postel's law, parse-don't-validate
references/decomposition/	Controlling method and system complexity; cyclomatic complexity, cohesion, coupling, feature envy, fractal architecture
references/api-design/	Designing a public API; affordance, poka-yoke, CQS, hierarchy of communication, naming over comments
references/separation-of-concerns/	Adding cross-cutting concerns; Decorator pattern, logging, what to log, performance vs legibility
references/teamwork-git/	Writing commits, reviewing changes, continuous integration, collective ownership
references/evolution/	Changing running systems; feature flags, Strangler pattern, versioning, regular dependency updates, Conway's law
references/troubleshooting/	Debugging a defect; scientific method, rubber ducking, reproduce-as-test, bisection, non-deterministic defects
references/security/	Threat modelling; STRIDE (spoofing, tampering, repudiation, info disclosure, DoS, elevation)
references/code-navigation/	Onboarding to a code base; big picture, file organisation, cycles, property-based testing, behavioural code analysis
references/tooling/	Executable measurement commands: complexity, cycles, duplication, dead code, hotspots, mutation testing
references/practices-glossary/	Looking up a named book practice and its current status

⚠️ Editorial amendments (NOT from the book)

The folder below is NOT content from Seemann's book. It contains our own additions covering agent-specific concerns the 2021 book does not address. Do not attribute these files to Seemann. See references/agent-native/knowledge.md.

Topic	Use when...
references/agent-native/	Agent-specific verification integrity, hallucination and dependency grounding, executable guardrails, and accountable review

Workflows

Composite step-by-step processes live in workflows/:

Task	Workflow
Review a pull request / piece of code	workflows/review-code.md
Add a new feature from scratch	workflows/add-feature-outside-in.md
Investigate and fix a defect	workflows/debug-defect.md
Threat-model a new endpoint	workflows/threat-model.md
Turn a recurring finding into an executable gate and persist it in project memory	workflows/operationalize-finding.md

See guidelines.md for the full routing layer (task → file, symptom → file).