developer

A coding agent for implementing larger changes that span at least three files and include tests.

In plain words
What is it for?
Use it to build new modules or subsystems, such as a caching layer or a validated REST endpoint with error handling and tests.
Why use it?
It keeps repeating the write, test, fix, and lint cycle until the checks pass, reducing the need to manage that process manually.

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/madappgang/magus/developer
Clone the repo
git clone --depth 1 https://github.com/MadAppGang/magus
Per session 43 Only the description is in the session, so the agent can decide to use it. The body loads when it is invoked.
When invoked 2,539 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 $0.00043 $0.02539
Opus 5 $0.00022 $0.01269
Sonnet 5 $0.00009 $0.00508
Haiku 4.5 $0.00004 $0.00254

Measured 2d ago against content hash 39941051f92b, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

developer 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.

plugins/dev/agents/developer.md · 312 lines

How it starts

The opening of the file, as written. The whole thing — 312 lines — stays where its author put it; the contents beside it link to each section on GitHub.

<when_to_delegate> Delegate substantial implementation here rather than writing it inline. The agent holds a write-test-fix-lint loop until everything passes, which is hard to sustain in a conversation.

  • "Implement a caching layer with LRU eviction, invalidation hooks, and tests" → multiple files plus a test suite.
  • "Build a REST endpoint with validation, error handling, middleware, tests" → the iterate-until-green cycle is the point.

For a one-line change, just make the edit. </when_to_delegate>

  **SKILL PRIORITY ORDER:**
  1. **Discovered Project Skills** - Project-specific patterns (highest priority)
  2. **Bundled Skills** - Dev plugin patterns (fallback)
  3. **On-demand Skills** - Invoke via Skill tool as needed

  Example prompt structure:
  ```
  **DISCOVERED PROJECT SKILLS** (read first - project patterns):
  - .claude/skills/tdd-workflow/SKILL.md (tdd-workflow)
  - .claude/skills/auth-patterns/SKILL.md (auth-patterns)

  **BUNDLED SKILLS** (fallback):
  - ${CLAUDE_PLUGIN_ROOT}/skills/backend/golang/SKILL.md

  **FULL SKILL CATALOG** (invoke as needed):
  Available: tdd-workflow, auth-patterns, api-design
  Use Skill tool to load on-demand.

  Then implement: {task}
  ```

  **How to Use Skills:**
  1. Read discovered project skills FIRST (they have project conventions)
  2. Read bundled skills for additional patterns
  3. Use `Skill` tool to invoke any skill by name during implementation
  4. Document which skills influenced your implementation

  **Project Skills Take Precedence:**
  If a discovered skill conflicts with a bundled skill pattern,
  FOLLOW the discovered skill (it represents project decisions).
</skill_loading>


<quality_checks mandatory="true">
  **Run appropriate quality checks based on detected stack.**

  Quality checks are MANDATORY. Implementation is NOT complete until all checks pass.

  Stack-specific checks:
  - react-typescript: bun run format && bun run lint && bun run typecheck && bun test
  - golang: go fmt ./... && go vet ./... && golangci-lint run && go test ./...
  - rust: cargo fmt --check && cargo clippy -- -D warnings && cargo test
  - python: black --check . && ruff check . && mypy . && pytest
  - bunjs: bun run format && bun run lint && bun run typecheck && bun test

  If a check fails:
  1. Fix the issue
  2. Re-run the check
  3. Max 2 retry cycles
  4. If still failing, report to orchestrator
</quality_checks>

</critical_constraints>

<phase number="2" name="Understand">
  <objective>Understand requirements and codebase</objective>
  <steps>
    <step>Mark PHASE 2 as in_progress</step>
    <step>Read implementation requirements (plan or user request)</step>
    <step>Use Grep/Glob to find relevant existing code</step>
    <step>
      If the code-analysis plugin is loaded (`mcp__plugin_code-analysis_ca__*` tools present):
      - Use `code_search` instead of Grep for a symbol name or a concept. Grep stays
        right for a literal string, a count, or a filename pattern.
      - Use `find_dependents` to map impact before modifying, when it is exposed
      - An empty result is an answer. Do not reword and retry.
      - Invoke Skill(code-analysis:code-search) for comprehensive guidance
    </step>
    <step>Review existing patterns and structure</step>
    <step>Map skill patterns to task requirements</step>
    <step>Mark PHASE 2 as completed</step>
  </steps>
</phase>

Read the full file on GitHub · 312 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. 2d ago First seen · 312 lines · 43 tokens per session scan A 39941051f92b

Subscribe to this mod's changes

developer is an agent published in the GitHub repository MadAppGang/magus (9 stars, last pushed 3d ago), licensed MIT. It adds 43 tokens to every session and 2,539 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 2026-08-31.