SemVer Advisor

The wrong call here is silent until a consumer's build breaks. "It compiled, ship a minor" is how breaking changes escape — a tightened validation rule, a changed default, or a removed export looks small in the diff but breaks every downstream caller. This skill makes the bump a defensible decision: it pins down what your public API surface actually is, diffs the release range against it, classifies each change, and applies the SemVer rules — including the pre-1.0 exception people forget.

When to use this skill

• You are about to tag a release and are unsure whether the changes are breaking.
• Someone proposed minor or patch and you want to verify it against the real diff.
• You changed behavior without changing a signature and need to know if that is still breaking (it often is).
• You maintain a 0.x library and keep forgetting that SemVer treats pre-1.0 differently.
• A CI/release gate failed on version mismatch and you need the correct bump with a rationale.

Instructions

1. Define the public API surface first — it is narrower or wider than you think. Enumerate every contract a consumer can depend on, not just the language exports:

   • Code exports: the package entry points (exports/main in package.json, __all__, pub/public symbols). Anything reachable from the documented entry point is public; deep imports into internal paths usually are not (unless your docs/exports map expose them).
   • CLI: command names, flags, positional args, env vars they read, and exit codes.
   • Config: accepted keys, their types, defaults, and required-ness in config files / schema.
   • Wire contracts: HTTP routes, request/response shapes, status codes, GraphQL schema, event/message payloads.
   • File formats: on-disk formats you read or write, serialization versions, migration outputs.

   # entry points and public surface clues
   git show HEAD:package.json | grep -E '"(main|module|types|exports|bin)"' -A3
   grep -rEn '__all__|^export (default |const |function |class |\{)' src | head -50

2. Diff the release range, scoped to the surface. Use the last released tag as the lower bound; review only files that touch the surface from step 1.

   LAST_TAG=$(git describe --tags --abbrev=0)
   git diff "$LAST_TAG"..HEAD --stat
   git diff "$LAST_TAG"..HEAD -- <surface paths: src/index.*, cli/, openapi.*, *.schema.json>

3. Classify each surface change into exactly one bucket.

   • Breaking (forces major): removed or renamed export/flag/route/config key; changed function signature, required arg added, or narrowed/changed return type; changed default behavior a consumer relied on; stricter validation that rejects previously-valid input; changed error type/exit code/status code; removed config default; changed file-format output that old readers can't parse.
   • Additive (minor): new export, flag, optional config key with a safe default, new route, new optional response field — all 100% backward compatible.
   • Fix (patch): bug fix that restores documented behavior with no API change, internal refactor, perf, docs, deps that don't change the public contract.

4. Apply the rule, then handle the pre-1.0 caveat. Take the highest-severity bucket present: any breaking → major; else any additive → minor; else patch. Then check the current version:

   • >= 1.0.0: apply the rule directly.
   • 0.y.z (pre-1.0): SemVer special-cases this. A breaking change bumps the minor (0.y → 0.(y+1)), and additive/fix changes bump the patch. State explicitly that you are using pre-1.0 semantics.

5. Re-check every "no signature change" item before finalizing. A change with an identical signature can still be breaking — search the diff for default-value changes, validation tightening, altered side effects, and changed return values (not just types). These are the ones that get mislabeled as patches.

6. Output the recommendation with receipts. Give the bump, the resulting version number, the one-line rule that decided it, and the itemized changes per bucket — with each breaking change named explicitly so a reviewer can challenge it.

Output

A bump recommendation, reproducible from the diff:

## SemVer recommendation: MAJOR  (1.4.2 → 2.0.0)
 
Rule applied: contains ≥1 breaking change → major (current version ≥ 1.0.0).
 
### Breaking (forces major)
- Removed export `parseLegacy()` from package entry — consumers importing it will fail to resolve.
- `loadConfig()` now throws on unknown keys (was: ignored) — stricter validation rejects previously-valid config.
- Default of `--timeout` changed 0 (infinite) → 30000ms — changes runtime behavior for callers relying on the old default.
 
### Additive (would be minor on its own)
- New optional flag `--format json`.
 
### Fix (would be patch on its own)
- Fixed off-by-one in `splitRange()` matching documented behavior.
 
Note: if this were a 0.x package, the same set would be a MINOR bump (0.y → 0.(y+1)).