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 Phelan164/codex-howto --skill build-backendgit clone --depth 1 https://github.com/Phelan164/codex-howtoWrote 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/phelan164/codex-howto/build-backend)<a href="https://agentmods.dev/skills/phelan164/codex-howto/build-backend"><img src="https://agentmods.dev/badge/skills/phelan164/codex-howto/build-backend.svg" alt="Measured on agentmods" 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 | $0.00062 | $0.00306 |
| Opus 5 | $0.00031 | $0.00153 |
| Sonnet 5 | $0.00012 | $0.00061 |
| Haiku 4.5 | $0.00006 | $0.00031 |
Grade A, and why
build-backend 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 8d 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.
What it actually says
Build Backend
Domain contract
- Trace changed behavior from input through validation, authorization, domain logic, persistence, side effects, and response mapping.
- Preserve published contracts unless the request explicitly changes them.
- Enforce authorization at a trustworthy server-side boundary.
- Validate untrusted input before domain or persistence operations.
- Keep multi-write operations atomic where partial state would violate invariants.
- Design external calls for timeouts, retries, idempotency, and duplicate delivery where applicable.
- Do not query or mutate production data without explicit authorization.
- Do not fabricate migration, framework, or API behavior; verify version-sensitive details.
- Avoid broad service abstractions that are used once.
Execute and verify
Read references/contracts-and-data.md when the task changes a public API, schema, transaction, job, or integration.
Implement the smallest change at the owning layer. Cover success, invalid input, authorization, and material failure paths. Prefer:
reproducing/focused test → contract/integration test → type/lint → build
For migrations, validate forward behavior and document rollback or roll-forward strategy.
What ships with it
2 files 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.
- 8d ago First seen · 33 lines · 62 tokens per session scan A ada1a71cf76d
build-backend is a skill published in the GitHub repository Phelan164/codex-howto (3 stars, last pushed 3d ago), licensed MIT. It adds 62 tokens to every session and 306 once invoked, about $0.0003 per session on Opus 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.
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