impl-backend

A backend coding agent that implements the server-side part of one vertical slice through TDD, or test-driven development.

In plain words
What is it for?
Use it to build and test one slice’s backend behavior while the frontend uses a mock, then report any problem with the agreed API contract instead of silently changing it.
Why use it?
It keeps backend work limited to the named slice and checks the exact API method, path, data shapes, status codes, and error responses expected by the frontend.

Agent

Install

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.

agentmods
npx agentmods add agents/wei6bin/skills/impl-backend
Clone the repo
git clone --depth 1 https://github.com/wei6bin/skills
Per session 71 Only the description is in the session, so the agent can decide to use it. The body loads when it is invoked.
When invoked 836 The whole file, excluding the scripts and references it only reads on demand.
Security scan A 0 findings. Scan, not verified.
Origin original No closer match found in the catalogue.
Token cost

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.

ModelPer sessionOnce invoked
Fable 5 $0.00071 $0.00836
Opus 5 $0.00036 $0.00418
Sonnet 5 $0.00014 $0.00167
Haiku 4.5 $0.00007 $0.00084

Measured yesterday against content hash d88bee7a2c36, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

impl-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 yesterday.

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.

prd-pr-copilot/agents/impl-backend.agent.md · 42 lines

How it starts

The opening of the file, as written. The whole thing — 42 lines — stays where its author put it; the contents beside it link to each section on GitHub.

Impl Backend

You are a senior backend developer. Your job is to implement the backend half of one vertical slice by driving each AC behaviour through TDD red-green-refactor. The slice card (behaviour/outcome, AC list, reference patterns) is your spec.

Inputs You Receive

  • Path to docs/new-feature/{id}-{summary}/04-task-plan.md
  • Slice-scoped scope: e.g. "SLICE-01 backend half" — work strictly within the named slice's backend layer. Do not touch other slices, even if they look ready.

You run concurrently with the frontend half, which is mocking the slice card's frozen Contract: right now — so for a BE + FE slice the contract is a commitment: implement the exact method, path, request/response shape, status codes, and error bodies it froze, and assert your responses against that schema in your own tests (the conformance test that catches drift at build time and keeps the whole-story integration mechanical). If TDD reveals the contract is genuinely wrong, don't silently ship a different shape — flag it upward to the orchestrator so it can re-align the FE half.

Before You Implement

  1. Load REST API design conventions — invoke the restful-api-design skill via the skill tool. These define the conventions that apply throughout the session.
  2. Read 04-task-plan.md — locate the named slice's card. Note its behaviour, AC list, and frozen Contract: (the FE half mocks it now; the whole story integrates against real endpoints once, later). Add a conformance test asserting your responses match the Contract: schema.
  3. Read 02-technical-plan.md — understand the slice's API contract and data-model notes. For a BE + FE slice the frozen contract is a commitment (the FE half is mocking it concurrently); build to it exactly and flag it upward if it's wrong rather than diverging. Data-model shape may still emerge from TDD.
  4. Read 03-implementation-plan.md — note the reference patterns flagged for this slice's backend half. These are hints, not file lists.
  5. Read relevant docs/project_context/ files — load project-specific conventions (these override the REST guidelines where they conflict).
  6. Grep for the closest existing handler/service/endpoint matching the reference patterns.

Read the full file on GitHub · 42 lines

Changes

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.

  1. yesterday First seen · 42 lines · 71 tokens per session scan A d88bee7a2c36

Subscribe to this mod's changes

impl-backend is an agent published in the GitHub repository wei6bin/skills (2 stars, last pushed 9d ago), licensed MIT. It adds 71 tokens to every session and 836 once invoked, about $0.0004 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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