About
The `spec` skill is the exclusive mutator for a project's SPEC.md file, handling creation, amendments, and backpropagation of bugs. It triggers on commands like writing a new spec, amending specific sections (§G, §C, etc.), distilling a spec from code, or logging a bug. It follows the formatting rules defined in FORMAT.md for all writes.
Quick Install
Claude Code
Recommendednpx skills add JuliusBrussee/blueprint -a claude-code/plugin add https://github.com/JuliusBrussee/blueprintgit clone https://github.com/JuliusBrussee/blueprint.git ~/.claude/skills/specCopy and paste this command in Claude Code to install this skill
Documentation
spec — spec mutator
Read FORMAT.md at repo root if not already loaded. Caveman skill applies to all writes here.
DISPATCH
Inspect user request and project state:
- No
SPEC.mdat repo root AND args describe idea → NEW - No
SPEC.mdANDfrom-codein args → DISTILL SPEC.mdexists AND args startbug:→ BACKPROPSPEC.mdexists AND args startamend→ AMENDSPEC.mdexists, no args → ask user which mode
NEW — idea → spec
Input: user idea.
Steps:
- Extract goal (1 line, caveman). → §G.
- List constraints user stated or implied. → §C.
- List external surfaces user named. → §I.
- Propose initial invariants. → §V (numbered V1…).
- Break goal into ordered tasks. → §T pipe table, all status
., ids T1… - §B section with header row only (
id|date|cause|fix).
Write to SPEC.md. Show user full file. Ask: "spec OK? suggest edits or invoke build."
DISTILL — code → spec
Walk repo. Produce §G (infer from README/package.json/main entry), §C (infer from stack), §I (enumerate public APIs/CLIs/configs), §V (derive from tests and assertions), §T (one task per known TODO or missing test), §B (empty).
Caveman everywhere. Flag uncertain items with ? in text so user can confirm.
BACKPROP — bug → §B + §V
Input: bug: <description>.
Steps:
- Parse bug description.
- Find root cause (read relevant code).
- Decide: would a new invariant catch recurrence? If yes → draft
V<next>. - Append §B row:
B<next>|<date>|<cause>|V<N>. - Append new invariant to §V.
- If fix also changes behavior → add/update §T rows.
- Show diff. Apply only on user OK.
Rule: every bug gets a §B entry. Invariant optional but preferred.
AMEND — targeted edit
Input: amend §V.3 or amend §T etc.
Read that section. Show current. Ask user what changes. Write. Show diff.
Never silently rewrite sections user did not name.
OUTPUT RULES
- Caveman format per
FORMAT.md. - Preserve identifiers, paths, code verbatim.
- Numbering monotonic — never reuse §V.N or §B.N.
- §T row
citescolumn ! list §V/§I deps:T5|.|impl auth mw|V2,I.api.
NON-GOALS
- No sub-agents. Main thread writes.
- No dashboards, no logs, no state files beyond SPEC.md itself.
- No auto-build after spec. User invokes build explicitly.
GitHub Repository
Frequently asked questions
What is the spec skill?
spec is a Claude Skill by JuliusBrussee. Skills package instructions and resources that Claude loads on demand, so Claude can perform spec-related tasks without extra prompting.
How do I install spec?
Use the install commands on this page: add spec to Claude Code as a plugin, or clone its repository into your skills directory, then restart Claude so it picks up the skill.
What category does spec belong to?
spec is in the Meta category, tagged general.
Is spec free to use?
Yes. spec is listed on AIMCP and free to install. It runs inside Claude, so no separate service account is required to use the skill itself.
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