Write A Spec

write-a-spec

A spec is the technical-brief sidecar to an issue. It sits between the approved story (what the user wants) and the lane builders (/work-issues --lane … via /feature), and it must answer two questions:

1. What changes — data model, API shape, files touched per lane, tests required, risks.
2. Where the lane allowlists come from — the spec's File-by-file change list is the source work-issues-lib.sh::allowlist_for parses (via /feature's orchestrator) to scope each lane's /work-issues invocation.

A vague spec produces a confused builder and a broken lane scope. This skill exists to keep specs sharp enough that the parser can deterministically extract per-lane allowlists and a builder can implement without re-asking.

Source-of-truth for the protocol: [docs/plans/feature-factory.md](../../../../docs/plans/feature-factory.md) §C ("Spec → allowlist format") + ADR [0001-fenced-paths-blocks-for-lane-allowlists](../../../../docs/adr/0001-fenced-paths-blocks-for-lane-allowlists.md). Project glossary: [CONTEXT.md](../../../../CONTEXT.md).

When to run

• Step 4 of the /feature chain — orchestrator hands the approved story + research dump in.
• Direct user invocation — /write-a-spec issues/NNN-foo.md works the same way; missing research dump is handled (see below).
• After a story is approved at /feature's Checkpoint 1 but before the user reviews the spec at Checkpoint 2.

Inputs (priority order)

1. Path to an issues/NNN-*.md file (the approved story). Required.
2. Path to issues/research/NNN-*.md (the research dump from /feature's Explore subagent pass). Optional. If missing, proceed and add a Risks bullet — see "Missing-research handling" below.
3. CLAUDE.md at the repo root — required to look up lane vocabulary. Read its ## Lane boundaries section if present (see "Lane discovery").
4. ARCHITECTURE.md at the repo root — optional, read if present to ground module / dependency claims.

If invoked from an issue, also read issues/prd.md for parent context if it exists.

Output

| Source | Output | |---|---| | issues/NNN-.md | issues/NNN-.spec.md (sidecar; mirrors filename) |

The output is a new file. The skill does not modify the source issue, the PRD, or any other existing file. The only Write-tool use is to create the spec at the output path above.

Lane discovery

Before drafting the File-by-file change list, read CLAUDE.md and look for a ## Lane boundaries section. That section is the canonical lane vocabulary for the project (e.g. Backend, Frontend, CLI, Worker). Lane H3 headings in the spec MUST match labels defined there — the orchestrator's parser compares H3 text to allowlist names character-for-character.

If CLAUDE.md has no ## Lane boundaries section, ask the user once for the project's lane vocabulary, then proceed. Don't invent lanes silently.

AFK fallback (no user available — /feature running headlessly, claude -p invocation, etc.): instead of stalling on "ask the user," write a sibling issues/NNN-.QUESTIONS.md file listing the lane-vocabulary question explicitly. Use a best-guess lane vocabulary derived from top-level directory names in the repo (e.g. src/api → Backend, web/ → Frontend, cmd/ → CLI) AND clearly mark this guess in the spec's Risks section: "Lane vocabulary inferred from top-level dirs; CLAUDE.md ## Lane boundaries is missing. Confirm before any lane build runs."

If CLAUDE.md is missing entirely, treat it the same as "has no Lane boundaries section" — write QUESTIONS.md, infer from top-level dirs, mark in Risks.

Single-lane handling

If the feature only touches one lane (e.g. a backend cron job, a frontend-only UI tweak, a CLI subcommand), the spec omits the unused lane's H3 entirely — do not emit an empty paths block. The orchestrator's parser treats a missing lane as "skip this lane's invocation," which is the correct behavior for single-lane features. Empty paths blocks would be parsed as "this lane exists but has no edits," which is a different (and confusing) signal.

Missing-research handling

If the expected issues/research/NNN-*.md file is missing (e.g. the user invoked the skill directly without running /feature's Explore pass), proceed anyway. Add an explicit Risks bullet:

> No research pass performed; consider running /feature for fuller context. The file/module assumptions in this spec are based on the issue text and CLAUDE.md only.

Direct user invocation must work — this skill is not exclusively coupled to /feature. Do not refuse to draft a spec because the research dump is absent.

Process

1. Read input. Issue, research dump (if present), CLAUDE.md, ARCHITECTURE.md.
2. Resolve lane vocabulary from CLAUDE.md ## Lane boundaries.
3. Identify the deliverable. What artifact is produced, where it lives, in what shape.
4. Draft each template section. Use the template below verbatim — sections that are genuinely N/A still appear with body _N/A — _ (forces conscious consideration; doesn't silently skip a category the builder might have missed).
5. Self-critique pass. For each paths line: is this an actual literal path the builder will touch, or a placeholder? For each Tests required bullet: is this a testable behavior with an observable outcome, or a wish?
6. Stop check. All seven template sections present; every lane that has changes has an H3 + fenced paths block; every path is literal (no globs); every test bullet names an observable behavior.
7. Write the file at issues/NNN-.spec.md.
8. Print the closing one-liner:

Next: review the spec at issues/NNN-.spec.md, then run write-a-rubric and feature.sh continue --accept (or feed to /feature's Checkpoint 2 if orchestrated).

Spec template

Use this template verbatim. All seven H2 sections must appear in the produced spec, in this order.

````markdown

— Spec

Source story: issues/NNN-.md Source research: issues/research/NNN-.md (or _N/A — not run_)

Deliverable

Data model

If genuinely no data model change: _N/A — _.

API

If genuinely no API change: _N/A — _.

File-by-file change list

src/api/handlers/.ts
src/services/.ts
tests/services/.test.ts

web/components/.tsx
web/hooks/.ts
tests/components/.test.tsx

If the feature is single-lane, omit the unused lane's H3 entirely — do not emit an empty paths block.

Tests required

Each bullet becomes one criterion in the downstream rubric (write-a-rubric). NOT test file paths and NOT coverage percentages — behavior-level only.

Risks & open questions

Tenant/timezone concerns

If genuinely no concerns: _N/A — _. Most features have at least one concern worth pinning here, even if "no — single-tenant only" or "no — internal admin tool, no user-facing TZ."

````

Paths block convention

The File-by-file change list section's lane subsections must follow this format strictly — work-issues-lib.sh::allowlist_for parses it character-by-character. The parser fails silently (empty output + exit 0) on most mismatches, treating them as "lane not found / skip" — so a typo or formatting deviation produces a silently-skipped lane, not an error:

• Lane labels are H3 headings of the form ### — a single bareword from CLAUDE.md's ## Lane boundaries, with no markdown styling. Specifically:
• No bold: ### **Backend** is wrong; parser sees lane name **Backend**, never matches lookup for Backend.
• No links: ### [Backend](https://...) is wrong for the same reason.
• No trailing ### (CommonMark ATX-close): ### Backend ### puts ### inside the lane name. Use ### Backend only.
• No leading indentation: ### Backend is not recognised as an H3 by the parser.
• No parenthetical annotations: ### Backend (cron) becomes lane name Backend (cron) — fails the lookup.
• Case-sensitive — copy from CLAUDE.md verbatim. The parser does exact-string match against the lowercased lane name feature.sh passes in (per the recent lane_normalize fix). If ## Lane boundaries declares backend (lowercase) and you write ### Backend (capitalized), the parser sees no match — lane is skipped silently with status empty. Use the same case as the bareword in ## Lane boundaries, character-for-character. When in doubt, copy-paste from the source.
• Trailing whitespace IS tolerated (parser trims) — but leading whitespace anywhere in the heading is not.
• Paths blocks are fenced code blocks whose info string is literally paths (lowercase, no attributes). Variants the parser silently rejects: \\\paths bash, \\\Paths, \\\paths-list, ~~~paths` (tilde fence), indented fences.
• Each line inside a paths block is one literal file path — no globs (*, **, ?, [), no brace expansion ({a,b}), no shell metacharacters. The parser rejects all of these with exit 1 and stderr names the offender.
• Paths are repo-root-relative. Never write just a basename (crap.py) when the file lives at plugins/agentic-engineering/skills/crap/crap.py. The lane builder concatenates the path to the repo root and tries to edit; a bare basename either fails to locate the file or creates a new file at the wrong place. Even if the upstream issue/PRD used a shorthand basename, the spec MUST expand it to the full repo-root-relative path. Self-check: for each line in a paths block, the file should exist at / today (or be a path you're explicitly creating — note "new" in surrounding prose so a reader knows).
• A path may appear in only one lane within a single spec. route_findings fails loud on cross-lane duplicates (per plan §C and ADR 0001).
• Lane sub-sections appear directly under the File-by-file change list H2 with their paths block following (blank lines OK, prose OK between H3 and fence).
• Self-check before writing: after drafting, run grep -c '^`paths$' — the count MUST equal the number of lanes you intended. If you intended 2 lanes but the count is 1, you have a fence typo. Run grep '^### ' — every line must be exactly ### with no styling. Run awk '/^\\\paths$/,/^\\\$/' | grep -v '^\\\' to dump every path line, and test -e /` each one — anything that fails the existence check is either misnamed or genuinely new (which you should call out in surrounding prose).

Worked example

Assumes the project's CLAUDE.md ## Lane boundaries declares backend and frontend (lowercase). If your repo declares them as Backend/Frontend (or api/web, or any other case/name), substitute accordingly — the H3 must match CLAUDE.md character-for-character, including case.

````markdown

File-by-file change list

backend

src/api/handlers/invoices.ts
src/services/invoice-reminder.ts
src/jobs/reminder-job.ts
tests/services/invoice-reminder.test.ts

frontend

web/components/billing/ReminderCard.tsx
web/hooks/useInvoiceReminders.ts
tests/components/ReminderCard.test.tsx

````

What this does right:

• Two distinct H3 lanes, each with its own paths block.
• Every line is a literal path (no src/api/**/*.ts).
• Test paths live in the lane that owns them — backend tests under ### backend, frontend tests under ### frontend.
• No path appears in both lanes.
• H3 labels match CLAUDE.md ## Lane boundaries character-for-character (here: lowercase).

Anti-patterns

• Globs in paths blocks. src/api/**/*.ts is not a literal path. The parser will reject the spec. List every file explicitly.
• Same path in multiple lanes. src/shared.ts cannot appear under both ### Backend and ### Frontend. If you need a shared file, that's a design problem — the file probably wants splitting, or one lane owns the file and the other reads from it.
• Taste-based criteria in Tests required. "looks good", "is clean", "well-structured", "is elegant" — unscorable. Replace with an observable: "function X returns Y given Z."
• Conversation-dependent items. "as we discussed", "the case Sarah mentioned", "what we talked about Monday" — the rubric writer and the builder can't read the conversation. Make it explicit in the spec.
• File paths in Tests required. That's the builder's call. The spec says what behavior must be tested; the builder picks the file layout.
• Coverage percentages in Tests required. "80% coverage" doesn't tell the builder what to test. Behavior list does.
• Inventing lane labels not in CLAUDE.md. The orchestrator's parser will get lane-not-found and skip the lane silently. Match the vocabulary.
• Refusing on missing research. Direct-user invocation is supported. Add a Risks bullet and proceed.

Hard rules

• One spec file per issue. Output path: issues/NNN-.spec.md (sidecar to issues/NNN-.md).
• Do not modify the source issue or PRD or any existing file.
• This skill produces text only. It does not call any external API or service.
• All seven template H2 sections (Deliverable, Data model, API, File-by-file change list, Tests required, Risks & open questions, Tenant/timezone concerns) must appear in the output. Sections that are genuinely N/A use _N/A — _ as the body.
• Lane H3 headings must match labels from CLAUDE.md's ## Lane boundaries. If no such section exists, ask the user once for the project's lane vocabulary; don't invent.
• If the source story is too thin to produce a meaningful spec (e.g. acceptance criteria are vague placeholders), ask the user — don't fabricate behavior. If no user is available (AFK / autonomous run), write a sibling QUESTIONS.md listing what you'd have asked, then produce a best-guess spec with assumptions clearly marked.

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: belchman
• Source: belchman/claude-skills
• License: MIT

Install and usage instructions live in the source repository linked above.