invariant

Included with Lifetime

$97 forever

Use to discover implicit invariants/contracts in code and make them explicit. "What does this function silently assume", "find the contracts". Outputs are runtime asserts, type narrowings, or doc additions.

Writing & Docs

What this skill does


# invariant

Mines hidden contracts. Finds the rules code follows but never states. Surfaces them as asserts, types, or docs.

## Method

1. **Read.** The function/class/module under analysis.
2. **Find the seams** — places where the code:
   - Indexes without bounds-checking
   - Casts a type without narrowing
   - Calls a method without checking presence
   - Assumes ordering of inputs
   - Assumes uniqueness of keys
   - Assumes timezone / locale / encoding
   - Assumes a config value is present
   - Assumes a global state is initialised
3. **For each seam, infer the invariant** ("this assumes X").
4. **Decide enforcement**:
   - **Compile-time**: tighten types (remove `any`, narrow unions, add literal types)
   - **Runtime**: assert in dev, log in prod
   - **Documentation**: JSDoc/`@param` clarification when above are too heavy
5. **Apply** the chosen enforcement, smallest first.

## Output shape

```
Subject:   <file:line — fn/class>

Invariants found (implicit → explicit):

  I1: <statement>
      Where: <file:line>
      Enforcement: <type | assert | doc>
      Diff: <patch>

  I2: ...

Action:
  ✓ Type narrowing applied: 3
  ✓ Asserts added: 2
  ✓ Docstrings updated: 1
  Tests: still green
```

## Rules

- **Prefer types over runtime.** Compile-time check costs zero at runtime.
- **Asserts are dev-loud, prod-quiet.** `assert(x !== null)` in dev; metric/log in prod.
- **Docs are last resort.** Future readers may not read them.
- **One invariant per change.** Easy to revert if it surfaces a real bug.
- **Don't add asserts for impossible cases.** If the type system already proves it, don't double-check.

## Anti-patterns

- Defensive checks for cases the caller cannot trigger
- `assert(true)` placeholders
- Doc-only invariants where types would suffice
- Stripping `any` casts without checking what they were hiding (often a real bug)

## When NOT to use

- Generated code (regen pipeline overrides changes)
- Library/SDK with strict ABI commitments — invariant changes might be breaking
- Code with no runtime characterisation tests — assert additions might break unstated existing behaviour

## Subagent dispatch

- `Explore` to map callers (asserts must hold for all callers)
- `general-purpose` for the diff
- `tester` to add tests that exercise the invariant boundary

## Value over native CC

CC will add type annotations or asserts on request. CC won't systematically mine implicit invariants and surface them in priority order. The mining IS the value.

Files: 1

Size: 2.7 KB

Complexity: 6/100

Category: Writing & Docs

Source: https://github.com/ialameh/sift-coder/tree/main/skills/quality/invariant

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