What is prospec-backfill-spec?

A tool that creates a draft feature specification from existing code in an older or partially documented project. It records observed behavior, not guessed reasons, and leaves the draft for human verification.

Use it to document existing capabilities that have no feature specification. It is useful when reconstructing behavior in a brownfield project—a codebase built and changed over time.

It documents how an existing feature actually works without pretending to know the original intent. Unclear behavior is marked for clarification instead of being invented.

How benwu95/prospec configures Claude Code and Codex

Reuse

Borrowing it

Nothing to install: this file belongs to benwu95/prospec. Take a copy, put it at the same path in your own repository, and replace the rules that are about this project with yours.

Copy the file

curl -O https://raw.githubusercontent.com/benwu95/prospec/main/.agents/skills/prospec-backfill-spec/SKILL.md

Clone the repo

git clone --depth 1 https://github.com/benwu95/prospec

Made for: Claude Code, Codex.

Wrote this? Show the measurements

A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.

README.md

[![agentmods](https://agentmods.dev/badge/skills/benwu95/prospec/prospec-backfill-spec/github.svg)](https://agentmods.dev/skills/benwu95/prospec/prospec-backfill-spec)

Your own site

<a href="https://agentmods.dev/skills/benwu95/prospec/prospec-backfill-spec"><img src="https://agentmods.dev/badge/skills/benwu95/prospec/prospec-backfill-spec/github.svg" alt="Measured on agentmods" height="20"></a>

Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.

Your own site · 80×15

<a href="https://agentmods.dev/skills/benwu95/prospec/prospec-backfill-spec"><img src="https://agentmods.dev/badge/skills/benwu95/prospec/prospec-backfill-spec.svg" alt="Reviewed on agentmods" width="80" height="20"></a>

Per session 109 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.

When invoked 3,191 The whole file, excluding the scripts and references it only reads on demand.

Security scan A 0 findings. A grade says what 26 rules found in the file — not that it is safe.

Origin original No closer match found in the catalogue.

Token cost

What it costs to keep this loaded

Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.

Model	Per session	Once invoked
Fable 5.1	$0.00109	$0.03191
Opus 5.5	$0.00044	$0.01276
Sonnet 5.5	$0.00022	$0.00638
Haiku 4.5	$0.00011	$0.00319

Measured 4d ago against content hash 533c9bc8942e, method: parsed. Prices are Anthropic first-party input rates as of 2026-10-07, from the pricing page.

Security

Grade A, and why

prospec-backfill-spec scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 4d ago.

A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.

Nothing flagged

None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.

.agents/skills/prospec-backfill-spec/SKILL.md · 138 lines

How it starts

The opening of the file, as written. The whole thing — 138 lines — stays where its author put it; the contents beside it link to each section on GitHub.

Prospec Backfill Spec Skill

Activation

When triggered, briefly describe:

That you'll backfill a behavioral Feature Spec draft from existing code (not a design tool) for brownfield features/capabilities that have no Feature Spec coverage
That a feature is a vertical slice (one end-to-end capability across the modules that implement it) — not a module; one module can host several features and infrastructure modules host none
The draft records behavior, never intent — un-inferable intent is marked [NEEDS CLARIFICATION], never fabricated
The draft is staged for human verify-and-promote and is never written to the trust zone (prospec/specs/features/)

Language Policy

Write each generated document in the language the Constitution's Language Policy rule assigns to its path — change artifacts and their archived summaries in the project's artifact language, the trust zone (Knowledge base, Feature Specs, index) in English. One skill run may write both. Free text you pass the CLI that lands in a change artifact follows the artifact language too, leaving its paths, commands, enum values and the fixed prefixes the CLI matches untranslated. Keep code, identifiers, technical terms, and git commit messages in English.

CLI Prerequisite (required)

The prospec CLI is a required file for this skill — its deterministic steps call prospec commands. Probe BEFORE any other step; there is no manual fallback.

Run prospec --version (Bash).
Command not found / not executable → STOP. Ask the user to install the prospec standalone executable — the one-click installer script from the project README (macOS/Linux install.sh, Windows install.ps1) or a release binary from GitHub Releases; prospec is NOT published to npm. Then re-run this skill.
Version older than 2.10.0 → STOP. Report the installed vs required version and ask the user to upgrade, then re-run this skill.

Read the full file on GitHub · 138 lines

Files

What ships with it

1 file beside SKILL.md in the same directory: the scripts, references and assets a skill reads on demand. Not counted in the per-session cost; read them before you install if any of them is executable.

references/feature-boundary-criteria.md 3.6 KB

Changes

What this file has done since we first saw it

Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.

4d ago Changed 533c9bc8942e
9d ago Changed 1c793d122265
13d ago Changed 152df15f6f9e
20d ago Changed 9c1db8dca54d
23d ago Changed 6af2ac7cf5aa
28d ago Changed 9a3dd80f58da
1mo ago Changed 392ee8e4310c
1mo ago Changed 0cbf6bf5098e
2mo ago Changed 10dbe2249983
2mo ago First seen · 138 lines · 109 tokens per session scan A 59b440b79cd7

Subscribe to this mod's changes

prospec-backfill-spec is a skill published in the GitHub repository benwu95/prospec (6 stars, last pushed yesterday), licensed MIT. It adds 109 tokens to every session and 3,191 once invoked, about $0.0004 per session on Opus 5.5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-31.

Planning Writing and docs agentic-coding ai-coding-assistant ai-knowledge progressive-disclosure sdd spec-driven-development

prospec-backfill-spec next to 6 skills from other repositories
Mod	Type	Stars	Last push	Licence	Scan	Tokens	Works with
prospec-backfill-specbenwu95/prospec	Skill	6	yesterday	MIT	A	109	Claude Code, Codex
ospec-goalclawplays/ospec	Skill	390	2mo ago	MIT	A	29	Codex
ospecclawplays/ospec	Skill	390	2mo ago	MIT	A	26	Claude Code, Codex
ospec-changeclawplays/ospec	Skill	390	2mo ago	MIT	A	18	Codex
review-spdzhu1090093659/spec_driven_develop	Skill	993	3mo ago	MIT	A	72	Claude Code
deep-discusszhu1090093659/spec_driven_develop	Skill	993	3mo ago	MIT	A	181	Claude Code
spec-driven-developzhu1090093659/spec_driven_develop	Skill	993	3mo ago	MIT	A	148	Claude Code

prospec: Skill for Claude Code