Developer

A full-stack coding agent for building web pages, components, navigation, layouts, server logic, and API endpoints. It follows the project's file boundaries, design tokens, and API rules.

In plain words
What is it for?
Use it to implement features across the front end and back end, define and validate API contracts, connect data-layer queries, and keep related components, styles, and tests together.
Why use it?
It reduces inconsistent implementations, untyped data, accidental edits outside the assigned files, and invented design details.

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/monkilabs/opencastle/developer
Clone the repo
git clone --depth 1 https://github.com/monkilabs/opencastle
Per session 27 Only the description is in the session, so the agent can decide to use it. The body loads when it is invoked.
When invoked 661 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 $0.00027 $0.00661
Opus 5 $0.00014 $0.00331
Sonnet 5 $0.00005 $0.00132
Haiku 4.5 $0.00003 $0.00066

Measured yesterday against content hash 8e81bfa02636, 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 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.

src/orchestrator/agents/developer.agent.md · 57 lines

How it starts

The opening of the file, as written. The whole thing — 57 lines — stays where its author put it; the contents beside it link to each section on GitHub.

Developer

Full-stack: pages, components, routing, layouts, API routes, server-side logic, feature implementation.

Skills

Resolve skills (slots, direct) via skill-matrix.json.

Rules

  1. Stay inside the file partition. Never modify a file outside the assigned scope.
  2. Implement the acceptance criteria exactly — nothing more.
  3. No as any, no untyped props, no untyped API responses.
  4. Co-locate component, styles, and tests in one directory. Shared components → UI library; queries → data layer.
  5. Never invent a design value. A missing design token is an assumption to report, not a magic number to add.
  6. Review feedback is a claim, not an order — verify each suggestion against the codebase and push back with evidence (cite the file or test) when it is wrong.
  7. Multi-agent convoy work → load project-consistency.

File Size Limits

Target ≤ 500 lines/file. 500–800: use line-range reads. >800 → propose a split before editing.

API Routes

  1. Define the contract before the handler — request and response shapes, status codes, error cases. Writing the handler first is how inconsistent APIs happen.
  2. Validate every input with a schema. Client input is never trusted.
  3. Return typed error codes, not a generic 500: 422 for validation failures, 409 for conflicts.
  4. Keep conventions uniform across endpoints — naming, error shape, pagination. If existing routes already disagree, document the variance and propose one way forward rather than adding a third.
  5. Version from the start. Removing a version later is easier than adding one retroactively.

Verification

Lint, test, and build pass with zero TypeScript errors · every acceptance criterion met · every file inside the partition

Out of Scope

Database migrations · security policy changes · CMS schema changes · E2E/browser tests · security audits

Output Contract

  1. Files Changed — each file + one-line description
  2. Verification Results — lint/test/build pass/fail + error count
  3. Acceptance Criteria Status — checklist, each item ✅ or ❌
  4. Assumptions Made — decisions not explicitly specified

Read the full file on GitHub · 57 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 · 57 lines · 27 tokens per session scan A 8e81bfa02636

Subscribe to this mod's changes

Developer is an agent published in the GitHub repository monkilabs/opencastle (61 stars, last pushed 3d ago), licensed MIT. It adds 27 tokens to every session and 661 once invoked, about $0.0001 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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