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 lugassawan/swe-workbench --skill principle-api-designgit clone --depth 1 https://github.com/lugassawan/swe-workbenchWrote 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/lugassawan/swe-workbench/principle-api-design)<a href="https://agentmods.dev/skills/lugassawan/swe-workbench/principle-api-design"><img src="https://agentmods.dev/badge/skills/lugassawan/swe-workbench/principle-api-design/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/lugassawan/swe-workbench/principle-api-design"><img src="https://agentmods.dev/badge/skills/lugassawan/swe-workbench/principle-api-design.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 | $0.00069 | $0.01218 |
| Opus 5 | $0.00034 | $0.00609 |
| Sonnet 5 | $0.00014 | $0.00244 |
| Haiku 4.5 | $0.00007 | $0.00122 |
Grade A, and why
principle-api-design 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 6d 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 — 94 lines — stays where its author put it; the contents beside it link to each section on GitHub.
API Design
An API is a contract. Breaking it breaks callers. Design for evolvability from day one.
Contract-First
Design the interface before the implementation. Write the schema or proto before any code.
- Contracts force clarity on resource modeling, field names, and error cases early.
- If the design is painful to describe, the implementation will be painful to use.
- API review at design time is cheap; API review after clients exist is very expensive.
Versioning
Every breaking change requires a new version. Communicate it, window it, enforce the schedule.
- Breaking: removing a field, changing a field type, changing semantics of a status code, renaming a resource.
- Additive (non-breaking): new optional fields, new endpoints, new enum values in response.
- URL-path versioning (
/v1/orders) — visible, cacheable, easy to route; good for major versions. - Header versioning (
API-Version: 2024-01-01) — cleaner URLs; common for date-based schemes. - Set a deprecation window (minimum 6 months for external APIs) and enforce it; sunset headers signal the date.
Idempotency
Operations must be safe to retry. Network failures happen; callers will retry.
- Safe (GET, HEAD, OPTIONS) — no side effects.
- Idempotent (PUT, DELETE) — repeating produces the same result.
- Non-idempotent (POST) — accept an idempotency key header; server deduplicates by key.
- Key scope: client-generated UUID, tied to the logical operation, expires after a defined window.
- Return the same response for a duplicate key — do not create twice, do not error.
Pagination
Never return unbounded collections. Choose the model based on query patterns.
- Offset pagination — simple; breaks when records insert or delete during traversal; fine for small, stable sets.
- Cursor pagination — stable under concurrent writes; opaque cursor encodes position; cannot jump to page N.
- Always cap page size server-side; document the default and maximum.
- Return a
next_cursorornext_pagetoken — do not expose the underlying query offset or row ID.
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.
- 6d ago First seen · 94 lines · 69 tokens per session scan A 3e5e8f175d4d
principle-api-design is a skill published in the GitHub repository lugassawan/swe-workbench (2 stars, last pushed 4d ago), licensed MIT. It adds 69 tokens to every session and 1,218 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 2026-09-03.
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