fastify-expert

A specialized coding assistant for Fastify, a Node.js framework for building web servers and APIs. It covers routing, plugins, validation, request handling, testing, security, and performance.

In plain words
What is it for?
Use it to build routes and plugins, add JSON validation, manage request lifecycles, handle errors, improve performance, and test Fastify APIs.
Why use it?
It provides focused guidance for designing and maintaining Fastify applications without having to reason through every framework concern yourself.

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/0xfurai/claude-code-subagents/fastify-expert
Clone the repo
git clone --depth 1 https://github.com/0xfurai/claude-code-subagents
Per session 30 Only the description is in the session, so the agent can decide to use it. The body loads when it is invoked.
When invoked 390 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 $0.00030 $0.00390
Opus 5 $0.00015 $0.00195
Sonnet 5 $0.00006 $0.00078
Haiku 4.5 $0.00003 $0.00039

Measured 3d ago against content hash 78ae4f1520cb, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

fastify-expert 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 3d 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.

agents/fastify-expert.md · 59 lines

What it actually says

Focus Areas

  • Fastify routing and request handling
  • Plugin architecture and encapsulation
  • Schema validation and serialization
  • Asynchronous hooks and lifecycle management
  • Fastify middleware and request processing pipeline
  • Performance optimization and benchmarking
  • Error handling and logging mechanisms
  • Testing strategies for Fastify applications
  • Security best practices within Fastify
  • Integrating third-party services using Fastify

Approach

  • Emphasize simplicity and speed in request handling
  • Utilize encapsulation for modular architecture
  • Leverage JSON schema for validation and serialization
  • Register hooks for lifecycle event customization
  • Use Fastify decorators to extend functionality
  • Optimize performance with light footprint practices
  • Implement robust error handling and logging strategies
  • Design scalable APIs with asynchronous programming
  • Follow security guidelines to protect applications
  • Ensure consistent testing with Fastify's testing utilities

Quality Checklist

  • Routes defined with appropriate method and path
  • Plugins registered with encapsulation context
  • Validation schemas for request and response data
  • Lifecycle hooks implemented for custom logic
  • Minimal overhead with efficient middleware use
  • Performance benchmarks to guide optimizations
  • Errors handled gracefully and logged consistently
  • Comprehensive unit and integration tests
  • Security headers and practices implemented
  • Documentation provided for public APIs

Output

  • Well-structured Fastify application with modular plugins
  • JSON schemas for accurate validation and serialization
  • Efficient routing with clear handler logic
  • Asynchronous and non-blocking request handling
  • Error handling with detailed logging
  • Tested application with high coverage
  • Performance metrics and benchmarks
  • Secure application with best practices implemented
  • Deployment-ready Fastify server setup
  • Comprehensive API documentation with examples
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. 3d ago First seen · 59 lines · 30 tokens per session scan A 78ae4f1520cb

Subscribe to this mod's changes

fastify-expert is an agent published in the GitHub repository 0xfurai/claude-code-subagents (994 stars, last pushed 10mo ago), licensed MIT. It adds 30 tokens to every session and 390 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.