cw-slice

The expensive mistake in this repo is not a bad implementation — it is a second implementation. A new model_*, *_config, provider_*, or "bridge" module beside the one that already does the job ships two systems and a comment that is no longer true. This skill is the ponytail ladder's rung 2 with commands attached: find the thing that already exists, then edit it.

Stage 2 of the loop: cw-orient → slice → cw-gates → cw-dogfood → cw-land → cw-handoff.

When to use

• Any feature, upgrade, refactor, or "make X work like Y" request.
• Before creating a new module, trait, config struct, or command.
• When a plan or issue tells you to build something and you have not yet confirmed it does not already exist.

Workflow

1. Walk the ladder before opening an editor (AGENTS.md, "The ponytail method" — not restated here, since a second copy is what rung 2 forbids). Stop at the first rung that answers. The ladder runs after reading the code, never instead of it: a short diff written without reading the call sites is a guess, not a small change. State the rung only when the choice isn't obvious from the diff itself.

2. Grep for the predecessor. This is the step that gets skipped and the one that costs the most:

   grep -rn "<the concept, in the words the code would use>" crates --include='*.rs' | head -40
   ls crates
   grep -rln 'model_\|_config\|provider_' crates/*/src --include='*.rs' | head -30
   

   Search behavior and symbols, not just filenames. If you find an owner, edit it. If you are still adding a new layer, its commit message must name the predecessor it replaces — otherwise you are editing the wrong file.

3. Check the contracts you are about to walk into. One of these is guard-tested, the rest are convention — either way, move them with their code, not around it:

   • One turn loop: crates/tui/src/core/engine/turn_loop.rs, guarded by crates/core/tests/single_turn_loop.rs. A second loop fails the guard; changing the shape means changing the guard with it.
   • One base prompt: BASE_PROMPT in crates/tui/src/prompts/text.rs.
   • The subagent tool is agent; agent_open / agent_eval / agent_close / delegate_to_agent are removed surfaces. If the shape must move, move the code and add the guard test that judges the new shape — the convention is not a fence around the area.
   • The system prompt + tool catalog are a session-pinned KV-cache prefix (docs/CACHE.md). Any new session-context contributor must state its cache effect — frozen prefix vs. append-only history. Never splice a volatile fact into the prefix.
   • crates/tui/src/core/ is a module inside the TUI crate. crates/core is a different crate that runs no turns. Do not confuse them.
   • Repeatedly misidentified as dead, verify consumers before removing: runtime/src/context_budget.rs, tui/src/model_registry.rs, runtime/src/prompt_zones.rs, tui/src/tools/remember.rs, config/src/route/.

4. Read the scoped guidance for the files you will touch. crates/tui/AGENTS.md owns the UI contracts (one owner per fact, codewhale_palette::grammar semantics, typed state enums, toast routing, tr(locale, MessageId::...) for user prose). crates/localization/locales/AGENTS.md owns string changes. web/AGENTS.md owns the site. docs/MOTION_CONTRACT.md owns motion. Design law lives in docs/design/, not in a prototype file someone left in a sibling directory.

5. Bound the slice. One coherent change, reviewable in one sitting, that leaves the tree building and green. AGENTS.md's two corollaries keep slices honest: an abstraction must delete caller code, and a migration ships its last consumer or does not start.

6. Fix the evidence bar now, not after. Decide before writing code what will prove this works, and write it into your plan:

   • the focused test or existing check that covers the behavior (scripts/dev-test.sh --list maps an area to its fastest invocation);
   • whether the change is visible enough to need cw-dogfood;
   • whether it is cross-cutting enough to need the full sweep in cw-gates.

7. Write the implementation, then prove it runs. No new tests or code comments unless explicitly asked (AGENTS.md); update or delete existing tests the change makes wrong.

Red flags / don't

• Don't add a module that "bridges", "mirrors", "stages", or "wraps" something that already exists without naming that thing in the commit message.
• Don't fork a singleton (turn loop, base prompt, delegation axis, lifecycle system) without moving its guard test and consumers with it. The repo has one of each on purpose; a silent second one is the failure mode, not the refactor.
• Don't write new tests unless explicitly asked, and never first. Don't add new code comments.
• Don't contort production code to keep a brittle assertion green. A test that only encodes old behavior is evidence, not a veto: change it with the code.
• Don't cut trust-boundary validation, data-loss handling, security, or accessibility to make a diff shorter. Brevity is never a reason to drop a guard.
• Don't leave a #[allow(dead_code)] behind as the cost of an incomplete migration — scripts/check-dead-code-budget.py is the running receipt.

Output

Default shape before the first edit — compress when trivial (a one-line change gets a one-line note, not four bullets):

• which rung of the ladder you stopped at and why;
• the existing owner you found (path/to/file.rs:line), or the predecessor your new module names;
• the bounded slice, in one sentence;
• the evidence bar you will meet, chosen in advance.