xoai/coding-principles

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Coding Principles

Seven universal principles for writing production-quality code. Apply these to every line, in every language. They are not a post-hoc checklist — they shape decisions AS you write.

Stack skills (react, flutter, nextjs, etc.) add language-specific idioms on top. These principles are the foundation.

Principle 1: Clarity Over Cleverness

Write code a stranger can read without asking you what it does.

Do:

• Name things for what they DO: fetchActiveUsers(), isExpired, MAX_RETRY_ATTEMPTS
• Functions do one thing. If you need "and" to describe it, split it.
• Comments explain WHY, not WHAT. The code says what; the comment says why it's surprising or non-obvious.
• Prefer explicit over implicit. if (user.role === "admin") not if (checkAccess(user, 2)).

Don't:

• Magic numbers: if (retries > 3) → if (retries > MAX_RETRIES)
• Clever one-liners that save a line but cost a minute to read.
• Names like data, temp, flag2, processStuff, handleIt.
• Nested ternaries. Ever.

Principle 2: Fail Loudly, Recover Gracefully

Every external call can fail. Handle it explicitly. Never swallow errors. Give the caller something useful when things go wrong.

Do:

• Every fetch, query, read, parse has error handling.
• Error messages include: what happened, what was expected, what to do about it.
• Async operations have loading, success, AND error states. No async without all three.
• Distinguish recoverable (retry, fallback) from fatal (log, alert, stop).

Don't:

• Empty catch blocks. Ever. If you truly ignore an error, comment WHY.
• console.log(err) as the only error handling.
• "Failed to load" with no context. WHO failed to load WHAT and WHY.
• Retrying fatal errors. Crashing on recoverable ones.

Principle 3: Guard the Boundaries

Validate at every entry point. Don't trust input from users, APIs, files, or even your own internal modules.

Do:

• Public function parameters: validate type, range, presence.
• API responses: check shape before accessing nested fields. response?.data?.user?.id not response.data.user.id.
• User input: validate before processing. Reject early, clearly.
• Configuration: fail at startup if config is invalid, not at 3am when the missing value is first accessed.
• Database results: handle empty results, null fields, unexpected types.

Don't:

• Trust that an API response has the shape you expect.
• Access nested properties without null checks.
• Process user input without validation.
• Assume config values exist without checking.

Principle 4: Smallest Scope, Shortest Lifetime

Variables close to where they're used. Functions close to what calls them. Reduce the blast radius of every change.

Do:

• Declare variables at first use, not at the top.
• Prefer local over global. Prefer parameters over shared state.
• Prefer pure functions (same input → same output) where practical.
• Keep functions short. If you're scrolling, it's too long. Extract.
• Modules have one reason to change.

Don't:

• Declare all variables at the top of the function.
• Use global state when a parameter would work.
• Write 200-line functions. Extract logical sections.
• Put unrelated functionality in the same module because "it's convenient."

Principle 5: Make the Right Thing Easy, the Wrong Thing Hard

Design APIs and interfaces so correct usage is obvious and misuse requires effort.

Do:

• Required parameters come first. Optional parameters have defaults.
• Return types that force the caller to handle success and failure.
• Use the type system: enum not string, branded types for IDs, non-nullable when null is invalid.
• Impossible states should be unrepresentable.

Don't:

• Return null to mean both "failed" and "empty."
• Accept any or untyped dictionaries for structured data.
• Design functions where the caller must remember to check a flag.
• Allow invalid state combinations that crash at runtime.

Principle 6: Consistency Beats Perfection

Match the existing codebase. Consistency across the project matters more than your personal preference.

Do:

• Read existing code before writing new code. Match patterns.
• Follow the project's error handling pattern.
• Use the project's existing utilities before writing new ones.
• If the project has no patterns, establish one and follow it.
• Match naming: if the project uses camelCase, use camelCase.

Don't:

• Introduce a new style in your files because "it's better."
• Write a utility function when one already exists in the project.
• Mix patterns: callbacks in one file, promises in another, async/await in a third.

Principle 7: Test What Matters, Not What's Easy

Write tests that catch bugs, not tests that inflate coverage numbers.

Do:

• Test the contract: "given X input, expect Y output."
• Test boundaries: min, max, zero, empty, one, many.
• Test error paths: invalid input, timeout, permission denied.
• One assertion per test when practical.
• Test names describe the scenario: test_expired_token_returns_401.

Don't:

• Test implementation details: "function calls helper A then B."
• Test only the happy path.
• Name tests test_1, test_auth_3, test_new.
• Write tests that pass regardless of the implementation being correct.
• Mock everything — some integration is worth testing.

How This Loads

Build-loop loads this capability at Step 3, before each task:

Sage: Loading coding principles for implementation.
Following: clarity, error handling, boundary guards, minimal scope,
safe APIs, consistency, behavior testing.

These principles are active for every line written during the task. They are NOT a post-hoc checklist — they shape the code as it's written.

Relationship to Other Capabilities

• Stack skills (react, flutter, nextjs) add language-specific idioms. Principles provide the universal foundation.
• TDD capability drives the test-first workflow. Principle 7 guides WHAT to test within that workflow.
• quality-review (Gate 3) reviews AFTER implementation. Principles guide DURING implementation. Both are needed.
• auto-QA verifies code against spec. Principles ensure the code is well-crafted regardless of spec compliance.

Rules

• Principles apply to ALL languages. No language-specific rules here.
• Principles guide, they don't block. Pragmatic exceptions are fine when explicitly justified.
• When principles conflict with project conventions, conventions win (Principle 6).
• When principles conflict with each other, clarity wins (Principle 1).