The end of a milestone (the improve-loop skill calls this skill there).
Before a big feature. Ask: "What change to the code would make this feature easy?"
A Swift file over 600 lines (act at 1,000), a type over 400 lines, or "unable to type-check this expression".
One feature touched more than 6 files or more than 2 targets.
The same area shows up in 3 fix-log entries, or the same kind of bug happens twice.
State that changes more than 10 times a second sits on a model that many views observe.
A dependency cannot be replaced in a test.

Vocabulary

Use these words exactly, so proposals stay comparable:

Module: anything with an interface and an implementation (a type, a file group, a target).
Interface: everything a caller must know: signatures, ordering, error modes, threading, performance.
Depth: how much a module does per unit of interface. Deep modules hide a lot behind a small interface. Shallow modules are pass-throughs.
Seam: a place where you can swap behavior without editing callers.
Adapter: one implementation behind a seam. One adapter is a hypothetical seam. Two (production and test) make it real.
Locality: a change touches one place. Leverage: callers get a lot for little.

1. Survey

Change no code in this step.

Read CLAUDE.md, ARCHITECTURE.md, docs/adr/, and docs/FIXLOG.md where they exist.

Find hot spots:

git log --since=30.days --name-only --format= | sort | uniq -c | sort -rn | head -20
find Sources -name '*.swift' | xargs wc -l | sort -n | tail -20

Map the structure: targets and their dependencies, protocols and their conformers, @MainActor types that do I/O, and what runs on which queue.
Look for Swift friction:
- A view model that owns everything (a god model).
- Wrappers that only forward calls.
- App code that reaches into a library's internals.
- Test hooks in the app target.
- Fast-changing state published to every view.
- Slow work on the main thread or a real-time queue.
- Protocols with one conformer and no test adapter.
- Drifted copies: the same private struct <Name> defined in two files (grep -rn 'private struct' Sources/ | awk '{print $3}' | sort | uniq -d).
- A settings or detail page that lives far from its feature. Pages live in the feature's folder.
- Pure logic inside a @MainActor store, which tests cannot compile alone. Move it into a plain type that takes plain values.
Deletion test for each candidate: if you deleted this module, would its complexity concentrate in one place (keep it) or spread to every caller (it earns its place)? Pass-throughs fail the test.

2. Propose

Write docs/architecture-review-<YYYY-MM-DD>.md, one card per candidate:

## <Title in domain nouns>  [Strong | Worth exploring | Speculative]

Files: Sources/App/EditorModel.swift (1,750 lines), ...
Signal: <which trigger above>
Problem: <what is hard to change today, with one example>
Proposal: <the new module and its interface, 15 lines of Swift at most>
Benefits: <locality and leverage; which tests get simpler>
Proof of same behavior: <tests, golden files, snapshots, perf baseline>
Plan: <one commit per step>

End with a top recommendation and a short "considered, not recommended" list. Saying "the code is fine" is a valid result: then every card is Speculative or there are none.

Stop and ask the user which candidate to do. In an unattended loop, do the top recommendation only if it is Strong. Leave the rest in the file.

3. Refactor

Before: record the proof: tests at the interface, golden outputs (file bytes or hashes, snapshot PNGs, old files that must still load), and a performance baseline where speed matters.
One refactor per commit. Behavior stays the same. No features mixed in.
New types go in new files that compile first. Then move callers over. The project builds after every step.
Replacing an engine: keep the old path behind a flag for one milestone, then delete it.
Replace tests, do not layer them: delete the old shallow tests that the new interface tests cover.
If a diff passes about 10 files, split it.
If the interface is open, design it twice: sketch two or three interfaces under different constraints (fewest entry points, simplest common caller, ports and adapters) and pick one with a reason.

4. Record

Update ARCHITECTURE.md: targets, layers, dependency direction, seams.
Add an ADR in docs/adr/ only when the decision is hard to reverse, surprising without context, and a real trade-off.
Add a docs/FIXLOG.md entry for any bug the refactor found. Every entry ends with a Rule: line.

Done when the review file exists, the chosen refactor is committed in steps, the proof gives the same results before and after, and ARCHITECTURE.md matches the code.

improve-architecture

Improve architecture

When to run