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 OutlineDriven/outline-driven-development --skill api-and-interface-designgit clone --depth 1 https://github.com/OutlineDriven/outline-driven-developmentWrote 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/outlinedriven/outline-driven-development/api-and-interface-design)<a href="https://agentmods.dev/skills/outlinedriven/outline-driven-development/api-and-interface-design"><img src="https://agentmods.dev/badge/skills/outlinedriven/outline-driven-development/api-and-interface-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/outlinedriven/outline-driven-development/api-and-interface-design"><img src="https://agentmods.dev/badge/skills/outlinedriven/outline-driven-development/api-and-interface-design.svg" alt="Reviewed on agentmods" width="80" height="20"></a>- NVIDIA SkillSpector pass
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 | $0.00039 | $0.00895 |
| Opus 5.5 | $0.00016 | $0.00358 |
| Sonnet 5.5 | $0.00008 | $0.00179 |
| Haiku 4.5 | $0.00004 | $0.00089 |
Grade A, and why
api-and-interface-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 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.
Copies of this mod
1 near-identical copy found in the catalogue:
- api-and-interface-design — 100% identical, 0 lines differ
How it starts
The opening of the file, as written. The whole thing — 39 lines — stays where its author put it; the contents beside it link to each section on GitHub.
API and interface design
Contract
| Field | Bound contract |
|---|---|
| Trigger | Designing or changing a public API, route, CLI flag, or module boundary. |
| Authority | Reversible local: writes only named local interface definitions and contract docs, edits consumer files onto the new contract, and deletes legacy paths (old signatures, aliases, re-exports, deprecated entry points) during cutover; rollback is undo (discard the uncommitted draft) or version control. No remote mutation. No credential, paid, published, or deployed change, and no VCS history rewrite. |
| Side effect | Interface definitions and contract docs written before implementation; consumer files edited onto the new contract and legacy paths (old signatures, aliases, re-exports, deprecated entry points) deleted during cutover; no build, publish, or remote mutation. |
| Done | Contract is documented with semantics and errors, every consumer is migrated, and no legacy path remains. |
Inputs
The interface being designed or changed: its name and kind (API endpoint, route, CLI flag, or module boundary), and whether it is new or a change to an existing interface. The current contract text, when changing an existing interface. The list of known consumers, found by search over the codebase. Optional: target language or runtime conventions for type and error spelling.
Procedure
- Bound scope before any mutation: name the exact interface and whether it is new or a change. Search the codebase for every consumer and record the list; record any consumer that cannot be inspected as an unmigrated risk. Done when: every consumer is listed and uninspectable ones are marked as risks.
- Write the contract before implementation. For each operation, field, or flag, document its name, input types, output type, error cases, and side effects. State semantics explicitly: idempotent or not, ordering, nullability, encoding, and concurrency. Done when: every operation, field, and flag has documented semantics and errors.
- For a change to an existing interface, classify it as breaking or non-breaking. If breaking, design the cutover in one change: the new contract, the per-consumer migration, and the removal of the legacy path. Done when: the cutover is designed as one change.
- Validate inputs at the trust boundary per the documented contract: reject malformed input with a documented error; do not silently coerce or default undocumented values. Done when: malformed input is rejected with a documented error.
- Migrate every consumer to the new contract. Update each consumer so it compiles or type-checks against the new signature; record a consumer as migrated only after it is updated. Done when: every consumer is updated and recorded as migrated.
- Remove the legacy path: delete the old signature, alias, re-export, and deprecated entry point. No compatibility shim, alias, or fallback remains. Done when: a search for the old signature returns no live reference.
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 Changed · -21 tokens per session ad6adaf639f0
- 1mo ago First seen · 39 lines · 60 tokens per session scan A d48c68f1e634
api-and-interface-design is a skill published in the GitHub repository OutlineDriven/outline-driven-development (53 stars, last pushed 22d ago), licensed Apache-2.0. It adds 39 tokens to every session and 895 once invoked, about $0.0002 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 2026-09-03.
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