fable-discipline

name

fable-discipline

description

Engineering discipline distilled from a forensic read of one model's work. Use when building a feature, fixing a bug, writing tests, hardening a data path, or running any task that spans multiple files, sources, or sessions. Two layers: how to RUN the task (stage it, verify each stage with a check that can fail, write done-criteria) and how to WRITE the code (recon before edit, verify against a copy with a negative self-test, single-writer chokepoints, scar-anchored why-comments). Ships a paired hook that deterministically blocks a test from touching live data, the one habit a machine can enforce.

Fable-Discipline

Coding and task discipline, distilled from reading thousands of edits by one capable model and grading them against an independent review. It is additive: it does not replace what your model already does well, it adds the habits that strong engineers have and most code generation skips.

Most of it is judgment the skill teaches. One slice is enforced by a hook, because a rule a model can talk itself out of is a suggestion, not a gate. The skill teaches; the hook makes the one checkable habit non-optional.

Position: the execution-discipline layer of prd-os

This skill is not a sibling system to prd-os; it is prd-os's execution-discipline layer (merged 2026-07-04, see the plugin CHANGELOG). Two load paths, one skill:

• PRD work: /issue-start loads this skill before the first edit of every DSSE issue. The issue's receipts (verified, reviewed, findings_triaged) are the task-level half of the same contract this skill states per-edit.
• Non-PRD work: the quick-plan fast path and the fable-discipline auto-invoke rule load it for any coding task bigger than a one-line change.

The public standalone export (github.com/assafkip/fable-discipline) mirrors this copy; scripts/export-fable-mirror.sh --check is the drift blocker.

When NOT to use this

One-line changes, typo fixes, a task with one obvious approach that fits in a single pass. Staging a trivial task buries the answer under ceremony. This earns its cost only when a one-shot attempt would plausibly miss something.

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Layer 1: running the task

How to move through complex work without shipping a confident wrong answer.

1. Stage before you act. Write the stage plan first. Number the stages, name the one checkable artifact each produces. If a stage produces nothing checkable, merge it into the next. The map is living, not a contract; update it when what you learn invalidates it.

2. Verify each stage with a check that can fail. A test that runs, a file that provably exists in the right shape, a source actually read, an output diffed against the spec. "I reviewed it and it looks right" is not a check: a model that would skip verification also passes its own introspection. If a stage has no failable check, say so and mark its output unverified so the gap is visible.

3. Say, then batch. State the one-line intent, then fire the burst of actions that executes it. The test is binary: a reader given only your intent lines can reconstruct the plan, or the intent line failed; rewrite it. It keeps you from drifting mid-burst.

4. Done is written, not felt. Define done-criteria up front. On a task that spans sessions, keep a short work log (decisions, what was tried, what failed) and re-read it before continuing, so you do not duplicate work or guess where you left off.

5. Ground before you diagnose. Confirm a perceived error actually reproduces before you act on it. Read where the data comes from before you trust what it represents. The failure mode this prevents: seeing a likely error, diagnosing it instinctively, and running with that explanation before checking it was real.

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Layer 2: writing the code

1. Recon before edit. Read reality, do not assume it. Grep the real schema and call-sites before changing anything. Never edit a file you have not read this session, unless you created that file in this same session (your own fresh write counts as read). If you already read a schema, re-read the exact field names rather than guessing them.

2. Verify against a copy, with a negative self-test. A passing gate is not trusted until it has been seen to fail. Run the reproducer against a copy of the live resource, never the live one, unless the resource is disposable by design (a regenerable fixture or sandbox environment). Then corrupt a valid input and prove the check FAILS on the violation, so a green result is not a rubber stamp.

3. Single-writer chokepoint, guarded by a grep-the-tree test. Route every mutation of a shared resource through one helper, and write a test that greps the tree to prove no caller bypasses it. Migrate existing call-sites one small, independently revertible edit at a time, not one bulk rewrite.

4. Why-comments anchored to a named scar. Comments encode the constraint and the specific past bug that motivates it, not a restatement of the code. These survive refactors because they encode an invariant.

5. Capture every out-of-scope finding; never just mention it. If you notice a real issue that is out of scope for the current work, write it to a tracked backlog (an issue, a ticket, a standing ledger your gate reads), not a bare prose mention. A mention is a silent drop; a tracked item is one a gate can keep failing until it is resolved. The paired lint blocks deferral language written into code without such a capture. Override: capture first, then ack the captured line with # spillover-skip; there is no skip-first path.

6. Build against the recurring gap classes. If the change scales or touches sensitive data, walk the gap-class block in references/checklist.md: an in-memory cap is not a disk bound; a UI hide is not access control; a gate fails closed while a filter fails open; redact at the egress edge; a new flag must reach every reader; check-then-mutate needs one lock; single-source the version; a cross-cutting invariant needs a written scope + a self-enumerating guard. Check only the classes the change touches.

7. Least-code bias. Prefer the smallest change that solves it. Reach for delete-and-reuse before you write new code; the best fix is often a line removed, not a line added. When you catch yourself writing the third near-identical thing, stop and generalize the mechanism instead of adding a third copy.

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Consistency rules

Habits that are easy to do somewhere and forget elsewhere. Make them non-optional:

• Test isolation. A test uses a temp copy, a tempfile, or :memory:, never a real data path. (This is the slice the paired hook enforces.) Override: an audit test that must NAME a live path may assert on it (assertion lines are exempt); anything else carries # fable-discipline-lint-skip in that file with a one-line reason.
• Declare and pin every new dependency the moment you import it, and keep a test that proves the manifest covers every third-party import.
• Specify degenerate cases before implementing: empty, single-element, disconnected, non-converging. Each gets defined behavior, not an implicit crash.
• Validate persisted external input. Never store arbitrary user or model JSON that, if malformed, can permanently break a render or load path, unless every read path runs a validating loader that tolerates the malformed record.
• Enumerate all call-sites when scoping a change. Grep for every site the change must reach before declaring it done.

Anti-patterns to drop

• Guessing a schema or API you already read. Re-read it.
• Acting on an unconfirmed diagnosis. Confirm the error is real first.
• Re-attempting the same failing command. Change the approach, do not retry it.
• Applying the user's stated style rules only to shipped output, not your own narration.

Communication while building

Terse mid-task, then decompress at the seam into a short "ran, not assumed" block listing what you ran and what passed. When a real choice exists, name the options, mark your pick, give the tradeoff, end with one action.

Enforcement

The deterministic slice (test isolation) is enforced by scripts/fable-discipline-lint.py, wired in hooks/hooks.json as a PostToolUse hook. Everything else is judgment and lives here. See EXAMPLE.md for a worked before/after, and references/checklist.md for the copy-paste pre-done gate.