Getting it into your agent
One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.
npx skills add borhen68/SkillEngine --skill spec-driven-developmentgit clone --depth 1 https://github.com/borhen68/SkillEngineWrote 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.
[](https://agentmods.dev/skills/borhen68/skillengine/spec-driven-development)<a href="https://agentmods.dev/skills/borhen68/skillengine/spec-driven-development"><img src="https://agentmods.dev/badge/skills/borhen68/skillengine/spec-driven-development/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.
<a href="https://agentmods.dev/skills/borhen68/skillengine/spec-driven-development"><img src="https://agentmods.dev/badge/skills/borhen68/skillengine/spec-driven-development.svg" alt="Reviewed on agentmods" width="80" height="20"></a>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 | $0.00066 | $0.02334 |
| Opus 5.5 | $0.00026 | $0.00934 |
| Sonnet 5.5 | $0.00013 | $0.00467 |
| Haiku 4.5 | $0.00007 | $0.00233 |
Grade A, and why
spec-driven-development 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 1mo 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.
How it starts
The opening of the file, as written. The whole thing — 246 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Spec-Driven Development
Overview
The most expensive bug is the one that never should have been built. When you write code without a spec, you're not just risking implementation errors — you're risking building the wrong thing entirely. A two-hour spec saves two weeks of rework.
The spec-driven contract: No code is written until both the agent and the human agree on what "done" looks like. The spec is the shared source of truth — it defines objectives, constraints, acceptance criteria, and boundaries. Code without a spec is expensive guessing.
Real-world impact: Teams that spec before coding ship 40% faster and have 60% fewer post-launch bugs. The time "saved" by skipping the spec is spent debugging, refactoring, and apologizing to users.
When to Use
- Starting a new project or feature
- Requirements are ambiguous or incomplete
- The change touches multiple files or modules
- You're about to make an architectural decision
- The task would take more than 30 minutes to implement
When NOT to use: Single-line fixes, typo corrections, or changes where requirements are unambiguous and self-contained.
Iron Rules
These target the specification failure modes specific to AI agents. Each is absolute.
- Never fill a gap silently. Every requirement the user didn't state but the implementation needs is an assumption — list each one and get a reaction before it hardens into code. The gaps are where the real requirements live.
- A spec must be disagreeable. If the spec only paraphrases the request back in more words, it's spec theater. A real spec contains decisions the user could object to — chosen trade-offs, named non-goals, explicit exclusions. No possible objection means no decision was made.
- Untestable criteria are not criteria. "Fast", "intuitive", "robust" cannot fail, so they cannot gate anything. Every success criterion needs a number, a command, or an observable behavior that a third party could check without asking you.
- Gates need an explicit yes. "Sounds good", "sure", or silence is not approval — re-ask, offering something concrete to disagree with. Code written past an unconfirmed gate is expensive guessing with a spec-shaped fig leaf.
- Discovering means re-specifying, not improvising. When implementation reveals the spec is wrong, stop. Update the spec, get it re-approved, then continue. Silent divergence makes the spec a lie and every future decision built on it wrong.
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.
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.
- 1mo ago First seen · 246 lines · 66 tokens per session scan A 0f09a08db201
spec-driven-development is a skill published in the GitHub repository borhen68/SkillEngine (17 stars, last pushed 4mo ago), licensed MIT. It adds 66 tokens to every session and 2,334 once invoked, about $0.0003 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 2026-08-30.
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