LLMs as 5x Faster Sandboxes

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GitHub - experientiallabs/world-model-harness: World-model-as-a-harness for simulating AI agent environments · GitHub

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World Model Harness

Docker as an LLM. Simulate an agent environment from traces instead of standing up a sandbox.

A frontier LLM acts as the environment your agent steps against, reconstructed from OpenTelemetry<br>traces. The harness ingests recorded (state, action) -> observation steps, builds a retrieval index,<br>evolves the base environment prompt with GEPA, and serves the resulting world model locally.

How It Works

Build from OTel traces: ingest, normalize, split train/held-out, index the replay buffer, and<br>optimize the environment prompt.

Serve or play the built model: agents call WorldModel.step(action) in-process or through the<br>local HTTP backend.

Evaluate reconstruction fidelity with wmh eval against trace files.

Quickstart

uv sync<br>wmh providers verify<br>wmh build --name airline --file examples/tau-bench/traces.otel.jsonl<br>wmh list<br>wmh eval examples/tau-bench/traces.otel.jsonl<br>wmh eval list<br>wmh eval run tau-bench<br>wmh eval results<br>wmh examples list<br>wmh examples run tau-bench -- --trace 0<br>wmh serve<br>wmh demo --name airline<br>wmh play --name airline

wmh build with no flags launches a guided creation wizard on an interactive terminal. Pass<br>--file and related flags, or --no-interactive, for scriptable runs.

World models are named and stored under .wmh/models//. wmh list, wmh serve, wmh demo,<br>and wmh play only use models built locally in that directory.

CLI Reference

Command<br>What it does

wmh build<br>Builds a named world model from OTel traces or a vendor trace pull. It ingests traces, normalizes them, splits train/held-out data, builds the retrieval index, runs GEPA prompt optimization, and writes the artifact to .wmh/models//. With no required inputs on a TTY, it opens the guided wizard.

wmh list<br>Lists world models found under the selected root's models/ directory, including provider, held-out score, rollout count, and frontier size when those metrics exist. By default, the selected root is .wmh/, so plain wmh list does not read committed example artifacts.

wmh eval<br>Scores reconstruction fidelity on one or more OTel trace files. It performs a deterministic train/held-out split, replays held-out steps through the base or supplied prompt, grades predicted observations against recorded observations, and prints per-file plus overall fidelity.

wmh eval list<br>Lists named eval suites from examples//evals/*.toml. Suites are example-local definitions for repeatable reconstruction-fidelity runs.

wmh eval run<br>Runs a named eval suite, using its configured trace files and split/scoring settings. Results are written as local JSON under .wmh/evals/// unless --out is supplied. The default suite for an example can be selected by task name, e.g. wmh eval run tau-bench.

wmh eval results [suite]<br>Summarizes locally saved named eval results from .wmh/evals/. These are generated artifacts and should not be committed.

wmh serve<br>Starts the local FastAPI backend on 127.0.0.1:8000 by default. It serves all locally built models, or only the repeated --name selections, through /world_models/... HTTP routes.

wmh demo<br>Runs a short demo against a built model. A throwaway LLM agent proposes an action from sampled trace examples, the world model predicts the environment observation, and the CLI prints the action, environment prompt, and observation.

wmh play<br>Opens an interactive REPL for a built model. You type tool calls or free-text actions, and the...

eval model world examples from traces

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