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 agentmods add skills/drvoss/everything-copilot-cli/api-and-interface-designnpx skills add drvoss/everything-copilot-cli --skill api-and-interface-designgit clone --depth 1 https://github.com/drvoss/everything-copilot-cliWhat 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 | $0.00037 | $0.01274 |
| Opus 5 | $0.00018 | $0.00637 |
| Sonnet 5 | $0.00007 | $0.00255 |
| Haiku 4.5 | $0.00004 | $0.00127 |
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 2d 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 — 176 lines — stays where its author put it; the contents beside it link to each section on GitHub.
API and Interface Design
Define the contract before the implementation. This skill is for public or cross-team interfaces where downstream consumers can depend on every observable behavior, including the ones you did not mean to promise.
When to Use
- Designing a new public HTTP API, CLI command, webhook, SDK surface, or plugin contract
- Changing an existing interface that other teams, tools, or customers already consume
- Clarifying breaking-change policy before implementation starts
- Tightening the interface section of
spec-driven-developmentfor a compatibility-sensitive feature
When NOT to Use
| Instead of api-and-interface-design | Use |
|---|---|
| Internal implementation design inside one module | spec-driven-development |
| Writing user-facing docs for a finished API | api-documentation |
| Retiring or replacing an old interface | deprecation-and-migration |
| Narrow bug fixes with no contract change | do the fix directly |
Workflow
1. Identify the real contract boundary
Write down:
- Who consumes this interface? Humans, services, CLI users, integrations, third-party developers
- What is observable? Inputs, outputs, error codes, ordering, timing, idempotency, pagination, retries
- What must stay stable? Field names, response structure, exit codes, callback payloads, invariants
If the consumer or stability target is unclear, you are not ready to implement.
2. Draft the contract before code
Create a contract document or spec section before touching implementation:
## Interface Contract: createWidget
**Interface type**: REST API / CLI / SDK / webhook
**Consumers**: Internal services + external customers
**Stability**: stable / beta / experimental
### Inputs
- Method / command / function name:
- Required parameters:
- Optional parameters:
- Validation rules:
### Outputs
- Success shape:
- Error shape:
- Ordering / pagination guarantees:
### Invariants
- [Behavior that must remain true]
### Non-Goals
- [What this interface deliberately does not promise]
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.
- 2d ago First seen · 176 lines · 37 tokens per session scan A 4058267e2acc
api-and-interface-design is a skill published in the GitHub repository drvoss/everything-copilot-cli (45 stars, last pushed 5d ago), licensed MIT. It adds 37 tokens to every session and 1,274 once invoked, about $0.0002 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 2026-08-30.
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