impact-check

name	impact-check
description	Analyze code changes to find related areas that may also need modification. Supports uncommitted changes, specific commits, commit ranges, and branches. Use before committing or opening a PR to catch missed updates.
allowed-tools	Bash(git *), Read, Grep, Glob, Agent

Impact Check — Code Change Impact Analysis

Repo profile — read .claude/repo-profile.md first. This skill is repo-agnostic; arc is the reference implementation. The monorepo layout, package naming, and paths below (packages/, providers/, runtimes/, blocklets/) are arc's — map them to the consuming repo's structure via the profile.

Analyze code changes and find related areas in the codebase that may also need modification.

Input Modes

Parse $ARGUMENTS to determine the diff mode:

Input	Mode	Diff Command
(empty)	working-tree	git diff HEAD
--branch	current branch vs main	git diff $(git merge-base <main> HEAD)..HEAD
--branch <name>	named branch vs main	git diff $(git merge-base <main> <name>)..<name>
--branch <name> --base <base>	branch vs custom base	git diff $(git merge-base <base> <name>)..<name>
contains ..	commit range	git diff <from>..<to>
other	single commit	git diff <ref>^..<ref>

When detecting the main branch, check which of main or master exists.

Analysis Process

Step 1: Collect Changes

1. Determine diff mode from arguments
2. Run the appropriate git diff command (with --stat for summary, and full diff for content)
3. List all changed files and summarize: N files changed, +X -Y lines
4. If no changes found, report "Nothing to analyze" and stop

Step 2: Understand What Changed

For each changed file, read the diff and identify:

• Interface/type changes: function signatures, type definitions, exported types
• Export changes: added, removed, or renamed exports
• Behavioral changes: logic changes that could affect callers
• Configuration changes: package.json, tsconfig, linter/formatter config, etc.
• Schema/model changes: data structure modifications

Step 3: Find Related Code

For each significant change identified in Step 2:

1. Import/reference search: Use Grep to find every file that references the changed module.
   • For a deleted or renamed file/export, grep the bare basename or symbol (e.g. agent-core, deriveAgentTools) to get the full candidate set — then filter comments in Step 4 by reading. Do not rely on a single-line import … from / require(…) regex: it misses multi-line imports (where the from "…" line has no import keyword) and re-exports, silently dropping real breaks.
   • Also search string / dynamic references that no import regex catches: readSource("…/foo.ts"), readFileSync / join / resolve with the file path, dynamic import(…), and worker/route path literals. Cross-package breakage most often hides here (e.g. a runtimes/node/test/** that readSource()s a deleted providers/** file).
2. Symbol usage search: For changed/removed exports, Grep for their usage across the codebase
3. Test file check: For each changed file src/foo.ts, check if test/foo.test.ts exists and whether it was also modified
4. Documentation check: If interfaces or public APIs changed, check if README or docs reference them
5. Conformance test check: If the repo's Custom Impact Checks (.claude/repo-profile.md) declare a provider conformance suite, and a provider changed, verify its conformance fixture was updated
6. Caller check (new exports): For each added export/function/param, grep its call sites repo-wide. Zero callers inside the repo — and not a public API/SDK re-export — is a speculative abstraction with no consumer; flag it. A lower layer nothing calls should not land on its own; the author should point to the consumer, or the follow-up change that adds it.
7. Parity check — mirror & registration: Two flavors, both cross-file invariants a self-contained diff won't reveal:
   • Cross-runtime mirror: multi-target builds must stay in sync — a change that touches one deployment target or a shared cross-target provider means the sibling target(s) need the same semantic. A one-sided change is behavior drift between targets unless intentional. See the repo's Deployment Environments section (.claude/repo-profile.md) for which targets exist.
   • Declaration ↔ dispatch/renderer: the diff adds a variant to an enum / union type / registry / factory (a new widget, message type, provider kind, event, …) in one package → grep the consumer that switches on it (dispatch table, switch, renderer map, handler registry — often in a different package) and confirm it gained the matching case/handler. This is the inverse hazard of reverse-reference: deleting breaks compilation loudly, but adding-without-wiring compiles clean. A default/fallback branch (renders "Unknown", silent no-op, drops the message) swallows the missing case, so check-types and build stay green and the gap only shows at runtime as a degraded/absent result. Also check the reverse — a new dispatch case for a variant the declaration side never added.

Use the Agent tool with subagent_type=Explore to parallelize searches when there are many changed files (>5).

Step 4: Evaluate Impact

For each related file found in Step 3, Read the relevant sections and assess:

• Will it break? Type errors, missing imports, incompatible API calls
• Will it behave incorrectly? Logic assumptions that no longer hold
• Is it incomplete? Tests that don't cover the new behavior, docs that are now stale
• Redefinition ≠ break: a symbol matched by name because a new or ported module now defines it (common after a rename/rewrite — e.g. runAgent moved to a new provider) is not a break. Only a dangling reference — one that still resolves to the deleted/changed definition — breaks. Confirm by reading which module the reference actually resolves to before flagging 🔴; a name match alone is noise.

Step 5: Output Report

## Impact Analysis — <scope description>

<N> files changed | +X -Y lines | <mode: working-tree / branch X vs Y / commit abc1234 / ...>

### Changes Summary
- brief bullet points of what changed and why it matters

### 🔴 MUST — Requires Modification
> Files that will break or produce incorrect behavior if not updated.

- **`path/to/file.ts:42`** — <specific reason>
  Changed: `oldThing` → `newThing`, but this file still references `oldThing`

### 🟡 SHOULD — Recommended to Check
> Files that likely need updates but won't immediately break.

- **`path/to/file.test.ts`** — test does not cover the new <behavior>
- **`README.md`** — example code references old API signature
- **`path/to/new-export.ts:12`** — new export `fooBar` has zero call sites repo-wide and isn't a public re-export (speculative — confirm a consumer exists or lands in a follow-up)
- **`runtimes/cloudflare/…`** — node side changed `<behavior>` but the CF mirror was not updated (cross-runtime parity)
- **`providers/…/dispatch.ts`** — new variant `fooKind` added to the enum/registry in `types.ts` but the consumer switch has no matching `case`; falls through to the `default` (renders "Unknown"/no-op) — compiles clean, degrades at runtime (declaration ↔ dispatch parity)

### 🟢 OK — No Action Needed
> Confirmed these related files are unaffected.

- `path/to/other.ts` — imports the module but doesn't use the changed API

Important Rules

1. Be specific: Every finding must include a file path, line number (when possible), and concrete reason
2. No false alarms: Only flag something as 🔴 MUST if you've confirmed the incompatibility by reading the code. When uncertain, use 🟡 SHOULD
3. Scope to 1 hop: Only analyze direct dependents of changed files, not transitive dependencies
4. Skip noise: Ignore changes that are purely formatting, comments, or whitespace
5. Respect project conventions: This is a <package_manager> monorepo. Cross-package impacts within the repo's monorepo layout are the highest-value findings — see the repo's Custom Impact Checks section (.claude/repo-profile.md) for which package boundaries matter
6. Fast exit: If all changes are documentation-only, test-only, or config-only with no code impact, report 🟢 OK immediately

Project-Specific Checks

On top of the universal steps above, run the repo's Custom Impact Checks — the domain-specific reverse-reference checks documented in .claude/repo-profile.md. Arc's are the AFS/AUP-specific ones (e.g. @aigne/afs importers, runProviderTests / test/conformance.test.ts, AUP widget dispatch parity, the cross-runtime mirror, joinURL from ufo) — read that section rather than assuming this list, since a different repo's profile will name entirely different checks.