/fable-native-code

name: fable-native-code
description: Use when writing or editing code in an existing codebase — before adding comments, docstrings, try/catch blocks, validation, logging, or TODOs the surrounding file doesn't have, and before explaining your style choices in the reply.

Native Code

Overview

Your diff should read like the file's longtime owner wrote it. The file has a style — naming, error handling, comment density, formatting. Write in it. Your personal preferences are not improvements; they're an accent.

Rules

1. **Match the file:** naming, error-handling idiom, comment density, formatting, import organization. If the file has one comment per 200 lines, your addition almost certainly has zero. 2. **Comments state only constraints the code cannot show** — invariants, gotchas, why-not-the-obvious-way. Never what the next line does, never narration of your change ("now correctly handles X"), never justification aimed at a reviewer. Test: after the PR merges, is this comment information or noise? 3. **No defensive bloat:** no try/catch around code that works, no validating invariants the types or callers already guarantee, no logging, config, or flags nobody asked for. If the contract is worth enforcing, that's a test or a boundary assertion — its own change. 4. **Don't narrate your restraint.** The urge to explain what you *didn't* add ("note: I deliberately left out the doc comment because…") is the same urge that adds it — displaced into the reply. The justification a senior dev never writes in code, they also never write in chat. Deliver the code; mention style only if the user asks. 5. **Clean exit:** no debug prints, no commented-out code, no TODO crumbs you created. 6. **Names come from the codebase's existing vocabulary.** Don't coin a second name for an existing concept.

Contrast

Task: add `clampWindow` to a terse Go file whose only comment in 200 lines marks a sorting invariant; callers guarantee `max >= 0`.

**Foreign (your habits, not the file's):**

// clampWindow clamps start and end into the valid range [0, max] and
// normalizes inverted windows. Returns (start, start) when end < start.
func clampWindow(start, end, max int) (int, int) {
    if max < 0 {
        return 0, 0 // defensive: should never happen
    }
    ...
}

**Native (what the file's owner would write):**

func clampWindow(start, end, max int) (int, int) {
    if start < 0 {
        start = 0
    }
    if start > max {
        start = max
    }
    if end < 0 {
        end = 0
    }
    if end > max {
        end = max
    }
    if end < start {
        return start, start
    }
    return start, end
}

No header comment (the signature says it), no `max < 0` guard (the callers guarantee it, like `mergeWindows` trusts its sorted-input invariant) — and no paragraph in the reply explaining either omission.

Rationalizations

| Thought | Reality | |---|---| | "A doc comment helps the next reader" | The next reader has the signature and twelve obvious lines. The file's owner trusted them; trust them too. | | "Extra validation can't hurt" | A guard for an impossible state is dead code that suggests the state is possible. Swallowed errors are the most expensive bugs to find. | | "My last reviewer praised my documentation habits" | In a codebase whose style wants documentation. This file's style is the spec now. | | "I'll explain in chat what I left out of the code" | Observed verbatim in baseline: a multi-paragraph "note on what I deliberately left out" after "output only the code." Restraint that announces itself isn't restraint. | | "I'll leave a TODO for the edge case" | Handle it, or surface it in your report. TODOs are where edge cases go to be forgotten. |

Provenance: baseline Opus 4.8 wrote the native version above, then narrated its restraint anyway (rule 4 exists because of it). fable-skills test logs, 2026-06-10.

Red Flags — delete before committing

• A comment that describes the line below it
• "deliberately", "note that I", "for safety" — in code or in your reply
• try/catch whose catch block only logs or re-raises
• A guard for a state the caller contract excludes
• Your addition is the only part of the file with comments