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API Design · gsd-build | DecimalAI

codingby gsd-build

API Design

gsd-build/api-design

Source gsd-build/gsd-2 ↗License MIT

Design or review an HTTP/REST/GraphQL API for versioning, pagination, error shapes, idempotency, auth, and evolvability.

SkillScore91Maintained

Automatic

Coding pack

Last verified 9/27/2026

What it changes

Passed 20 of 22 test cases with the skill, and 16 of 22 without it.

Without skill73%

With skill91%

Checks it passed that the model alone missed

Append architectural decisions to `.gsd/DECISIONS.md`.
Record architectural decisions under `.gsd/DECISIONS.md`.
Append architectural decision records to `.gsd/DECISIONS.md`.

and 2 more in the full report

5 fixed15 passed either way1 still failing1 broke

Pass-rate lift: +18 pp
Tokens: +65%
Agent turns: 0%
Response time: -5%

Model gemini-3.6-flashMeasured 9/27/2026v120 of 22 cases passed with skill

Tokens, turns and time describe resource use, not a dollar price.

Use this skill

Save it into your project's skills folder. Claude Code picks it up in its next session.

curl -fsSL --create-dirs -o .claude/skills/gsd-build-api-design/SKILL.md https://app.decimal.ai/s/gsd-build-api-design/SKILL.md

Want it pinned and updatable? pip install "decimalai>=0.15", then decimalai skills add gsd-build-api-design, and start Claude Code with claude --plugin-dir "$(git rev-parse --show-toplevel)/.agents"

1 · One-time — install the CLI (docs ↗)

pip install "decimalai>=0.15"

2 · Get the skill — writes .agents/skills/gsd-build-api-design/ and pins it in .decimal/skills.lock

decimalai skills add gsd-build-api-design

No CLI? Get the same file — curl · raw file

curl -fsSL --create-dirs -o .claude/skills/gsd-build-api-design/SKILL.md https://app.decimal.ai/s/gsd-build-api-design/SKILL.md

Raw file: /s/gsd-build-api-design/SKILL.md ↗ · machine-readable summary at /s/gsd-build-api-design.json ↗

Safety checks: 3 of 3 passed

Code scan CleanIntent PassedContent Clean

See it in action

One of 5 test cases the skill fixed. Same model, same prompt, with and without the skill.

1 of 5

Prompt

We are finalizing the design specification for a new Notification Gateway REST API. Beyond documenting endpoints and payload shapes, what repository record must be updated to capture these design choices?

What was checked, without and with the skill
Checked	Without skill	With skill
Append architectural decisions to `.gsd/DECISIONS.md`.	Missed	Passed

case-17 · gemini-3.6-flash · 9/27/2026 · answers are capped and redacted

What's inside

The instructions your model reads when the skill loads.

SKILL.md185 linessingle file

Invocation points:

Adding a new public API endpoint
Redesigning an internal API boundary between services
Code review of a PR that introduces HTTP handlers
A slice whose acceptance criteria include "the API works"
A GraphQL schema change

<core_principle> CALLERS OUTLIVE YOUR ASSUMPTIONS. An API you ship today has to keep working when your internals change, when the mobile app version two is still in use, and when a third party integrates against it. Design for extension, not just for the current caller.

HONEST STATUS CODES. 200 OK with {"error": "not found"} is a lie. 404 says not found. Use the HTTP semantics the protocol offers — HTTP clients, caches, and intermediaries rely on them.

PAGINATION IS NON-OPTIONAL. Any list endpoint that doesn't paginate will eventually get a request for "all records" that kills your database. </core_principle>

Step 1: Gather the contract

Answer, or ask (one round, 1–3 questions):

Who are the callers? Internal service / mobile app / public third-party / same-repo frontend.
What's the versioning stance? None / URL-path (/v1/) / header-based / GraphQL schema evolution.
Auth model? Public / API key / OAuth / session cookie / mTLS / none-but-internal-only.
Idempotency expectation? Is a retry safe? Required?
Consistency model? Read-your-writes, eventual, serializable?

Step 2: Resource and operation naming

REST

Nouns not verbs in URLs: POST /users, not POST /createUser.

Track record

Benchmark

20 / 22

test cases passed, vs 16 / 22 without

Adoption

Users: 0
Times used: 0

Safety and source details

SkillSafety

✓ 3 of 3 passed

Code scan15 of 15 checks clean

0 critical · 0 warning · 0 info — scanner v5 — 8/31/2026

View 15 scanner checks

live_secretremote_code_executionreverse_shelldata_exfiltrationssrf_metadataagent_snoopinganti_refusaldestructive_commandsobfuscated_unicodeunicode_homoglyphopaque_blobsuspicious_urltool_overreachtrigger_abuseprompt_injection_phrasing

IntentSafe

LLM judge · does the skill do what it claims

ContentClean

LLM judge · policy & harmful-content review

Source

Imported

gsd-build / gsd-2

src/resources/skills/api-design/SKILL.md

View on GitHub ↗

License MITImported 8/31/2026Version v1

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