> For clean Markdown of any page, append .md to the page URL.
> For a complete documentation index, see https://docs.nvidia.com/nemo/gym/llms.txt.
> For AI client integration (Claude Code, Cursor, etc.), connect to the MCP server at https://docs.nvidia.com/nemo/gym/_mcp/server.

# Build Verifiers

> Build the resources server that scores rollouts — the verifier, state, and verification patterns.

A verifier scores an agent's behavior on a task. In NeMo Gym you implement it in a **resources server** — the component that owns the verifier, per-task state, and any environment-specific tools.

New to the concepts? Start with [Environments](/about/concepts/environments) and [Architecture](/about/architecture) to see how the resources server, agent server, and model server fit together.

## Resources Server Interfaces

You build a resources server by subclassing one of two base classes. At runtime it runs as a web server that the agent drives over HTTP:

* **`SimpleResourcesServer`** — the agent calls `/seed_session` at the start to initialize per-task state (optional) and `/verify` at the end to score the rollout. Tools are exposed as additional endpoints or over MCP.
* **`GymnasiumServer`** — scores incrementally via `/step` (with `/reset` at the start) instead of a single `/verify`, for environments that need per-turn observations and rewards.

How to score a rollout, from execution and state checks to LLM-as-Judge.

Understand the full resources server structure that hosts your verifier alongside tools and session state.