Extract and test: module-first features, test-first fixes

This repo is built and maintained almost entirely by AI agents and grows by many small contributions. The thing that keeps it scalable, and keeps open-source merge conflicts small, is that new behavior lands as a focused module behind a known seam, not as another block appended to a 10k-line file. This skill is the detailed how-to behind the root CLAUDE.md "Modularity" section. Apply it whenever you implement a feature or fix a bug.

The one decision: sibling module or monolith edit

The four logic monoliths (src/ui/hud.ts ~10k, src/sim/sim.ts ~7.5k, src/main.ts ~6.4k, src/render/renderer.ts ~4.5k) are coordinators, not a license to grow them: do not split them to hit a line count, and do not rewrite them as a side effect of your task, but never GROW one either. src/main.ts especially is a firewall, not a home (its client-bootstrap helpers belong in src/game/ or src/ui/ siblings). Before you add a block of new logic to one, ask:

Does this behavior need the monolith's private mutable state (the live Sim entity loop, the Hud DOM and per-frame state, the renderer's scene graph)?

• No: it is a sibling module. Write it as its own file with a named export and a Vitest, then wire it in with a few lines (a call, a registration, a consume).
• Yes, partly: extract the pure part (the math, the formatting, the id/state resolution) into a host-agnostic module a test imports directly, and leave the stateful side a thin consumer that calls it. This is the pure-core + thin-consumer split (reference: src/ui/unit_portrait.ts core + src/ui/unit_portrait_painter.ts, shared by the player and target frames; src/ui/xp_bar.ts is a pure xpBarView() that a snapshot test drives with no DOM).

If your edit to a monolith is more than a thin wiring of something defined elsewhere, you are probably appending behavior that wants its own module.

Use the seams this repo already has

Do not invent a new architecture. Pick the seam that matches the work:

• render or ui needs new data or an action: extend IWorld in src/world_api.ts first, implement it in BOTH the offline Sim (src/sim/sim.ts) and the online ClientWorld (src/net/online.ts), then consume it through IWorld. render and ui never import a concrete world. This is the load-bearing parity rule; the cross-platform-sync agent audits it.
• New game content (mob, quest, item, ability, zone, talent): a declarative record in src/sim/content/, merged into the flat tables by src/sim/data.ts. Never inline a content table in sim.ts.
• New visual system: a new src/render/<thing>.ts exporting a build*() that returns a *View the renderer owns and calls. Not a new method bank on renderer.ts (templates: terrain.ts, props.ts, foliage.ts).
• New self-contained HUD window or panel: its own module the HUD composes, rather than a new banner section inside hud.ts. This is the direction the HUD modularization is heading; follow it for new windows.
• New server command: validate every field in dispatchMessage (server/game.ts), then call the sim.* method that owns the rule. The outcome resolves in the Sim, never on the server outside it.
• A multi-file subsystem: a directory with an index.ts barrel that exports only its public surface, plus its own short CLAUDE.md (templates: src/render/characters/, src/ui/i18n.catalog/).

When to extract, and when not to

• Extract on the rule of three. Two similar blocks: leave them. A third copy, or a single block whose responsibility you can name in one sentence with no "and", earns its own module.
• Do not abstract ahead of need. No helper, base class, options bag, or indirection for a single caller or a hypothetical future requirement. The right amount of structure is the minimum the current task needs. A wrong abstraction is more expensive than a little duplication.
• Name for the behavior, not the layer. threat_table.ts, loot_roll.ts, coords.ts, not helpers.ts or utils.ts. The file name should tell a reader what one thing it owns.
• Keep new modules host-aware. Anything reused by src/sim/ must stay DOM-free and Three-free (the tests/architecture.test.ts guard enforces this for src/sim/). Pure logic that both the sim and the UI need lives sim-side or in a neutral module both can import without breaking the import direction in src/CLAUDE.md.

Build a new module

1. Create src/<area>/<behavior>.ts with a small, explicit public surface (one or a few named exports). Keep internals private.
2. Add a Vitest at tests/<behavior>.test.ts that imports the module directly and asserts real behavior (not "it runs"). Tests live in tests/, not beside the source (see tests/CLAUDE.md for the idioms). For sim logic, add a determinism assertion: same seed gives the same result (expect(run()).toEqual(run())).
3. Wire it into its consumer with the smallest possible edit (a call, a registration, a barrel re-export). The consumer stays thin.
4. If the module is the public face of a new directory, add an index.ts barrel and a local CLAUDE.md describing only that directory's conventions.

Fix bugs test-first

1. Reproduce in a failing test before touching the fix. Write a Vitest that exercises the real code path and fails, and confirm it fails for the reason the bug describes, not an unrelated setup error. If the buggy logic is buried in a monolith and hard to test in place, that is the signal to extract the unit under test into its own module first, then test it.
2. Make the smallest change that turns the test green. Fix the root cause, not the symptom. Never special-case the test inputs or hard-code the expected value into the implementation.
3. Generalize the assertion, not the fix. Add a couple of nearby cases (boundary, empty, the mirror host) so the test pins the behavior, not one example.
4. For a high-risk or subtle fix, isolate the grader from the implementer: have one subagent write the reproducing test, a second implement the fix, and a fresh subagent review the diff for coverage (every correctness and requirement gap), so the fix is not validated by the same reasoning that produced it.

Verify, and keep the diff honest

After an extraction or fix, these stay green (run the subset your change touches):

• npx tsc --noEmit
• npx vitest run tests/<affected>.test.ts (or npm test for broad changes)
• npx vitest run tests/architecture.test.ts if you touched src/sim/, or added / renamed a src/ui or src/render *_view / *_core pure core (the completeness sweep also checks UI_PURE_CORES / RENDER_PURE_CORES registration)
• npx vitest run tests/localization_fixes.test.ts if any player-visible text or a src/sim/server emit changed (the S3 i18n guard)
• npm run ci:changed (Biome on the files you changed; this is what the .githooks/pre-push floor runs, so clear it here, not at push time). If it flags formatting on your own files, fix with a SCOPED npx @biomejs/biome check --write <file> per touched file, never a whole-tree --write (the repo defers global Biome debt, so a whole-tree write buries your change in thousands of unrelated reformats).
• npm run build before a merge

When you extract, the diff should read as move plus import, not rewrite. If you "improved" the moved code in the same change, that is scope creep: split it into a follow-up so the extraction stays reviewable. Delete the code you replaced; leave no dead duplicate, commented-out block, or unused import behind.

Effort by model (the doctrine here is identical, only the effort scales): on Opus 4.8, after the extraction fan out a fresh subagent (or the architecture-reviewer for a src/sim/ move) to review your move-diff for COVERAGE, every parity and correctness gap, before calling it done. On the Sonnet baseline, take small verifiable steps and lean on one investigator.

Repo anti-patterns to avoid

• Appending a new system as another // ---- banner section in sim.ts or hud.ts when it does not need that file's private state.
• Reaching past IWorld into Sim/ClientWorld from render/ or ui/.
• Adding a content table or balance number inline in sim.ts instead of src/sim/content/ and the tuning const blocks.
• A helpers.ts/utils.ts grab-bag, or an abstraction with exactly one caller.
• Splitting a monolith purely to reduce its line count, with no seam and no test.