Evaluating conversational agents typically requires expensive human participants or pre-recorded dialogues that don’t adapt to agent behavior. EP can simulate end-users in multi-turn evaluations, enabling full conversational loops without a human in the loop. This is powered by a lightweight user simulator derived from 𝜏²-bench and integrated into EP’s rollout manager. EP can simulate end-users in multi-turn evaluations, enabling full conversational loops without a human in the loop. This is powered by a lightweight user simulator derived from 𝜏²-bench and integrated into EP’s rollout manager.

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What It Does

  • Generates realistic user turns based on scenario instructions and global guidelines.
  • Interleaves with the agent’s tool-using turns to create full conversations.
  • Signals when to stop (e.g., task complete, transfer, or out-of-scope) via a special termination token.
Under the hood, EP uses UserSimulator. Rollout orchestration is handled by ExecutionManager. The simulator:
  • Builds a system prompt from global guidelines + your scenario instructions.
  • Optionally uses tool schemas to steer requests.
  • Provides a is_stop(...) check that EP maps to termination_reason = "user_stop".

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Enabling Simulation

Provide dataset_info.user_simulation in your EvaluationRow (or dataset) to turn on the simulator for that row. 
{
  "messages": [
    { "role": "system", "content": "You are an assistant that uses tools." }
  ],
  "input_metadata": {
    "dataset_info": {
      "user_prompt_template": "Observation: {observation}",
      "environment_context": { "seed": 42 },
      "user_simulation": {
        "enabled": true,
        "system_prompt": "You are a shopper trying to find a red jacket under $100.",
        "llm": "gpt-4.1",
        "llm_args": { "temperature": 0.0 }
      }
    }
  }
}
Fields and defaults:
  • enabled: boolean flag; if true, EP uses the simulator for the conversation.
  • system_prompt: scenario instructions appended to global guidelines.
  • llm: backing model for the user simulation (default: gpt-4.1).
  • llm_args: sampling args for the simulator (default: { "temperature": 0.0 }).

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Conversation Flow

When user_simulation.enabled is true:
  • EP seeds the conversation with the simulator’s first user message.
  • The agent policy receives tool schemas and responds with tool calls or a final answer.
  • After each agent turn, the simulator may produce the next user message.
  • If the simulator emits a stop intent, EP ends the episode with termination_reason = user_stop.
Step counting:
  • Without simulation: each tool call increments the step counter.
  • With simulation: EP increments the step counter after a full agent↔user turn, and records a consolidated control-plane step (reward, termination, tool calls).

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Minimal End-to-End

import eval_protocol as ep
from eval_protocol.models import EvaluationRow, Message

rows = [
    EvaluationRow(
        messages=[Message(role="system", content="Use tools to help the user.")],
        input_metadata={
            "dataset_info": {
                "user_prompt_template": "Obs: {observation}",
                "environment_context": {"seed": 7},
                "user_simulation": {
                    "enabled": True,
                    "system_prompt": "Book a table for two tonight at 7pm.",
                    "llm": "gpt-4.1",
                    "llm_args": {"temperature": 0.0}
                }
            }
        },
    )
]

envs = ep.make("http://localhost:8000/mcp", evaluation_rows=rows, model_id="my-model")
policy = ep.OpenAIPolicy(model_id="gpt-4o-mini")

async def run():
    async for row in ep.rollout(envs, policy=policy, steps=64):
        print(row.rollout_status.termination_reason)

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Tips

  • Keep scenario instructions specific and outcome-oriented to guide the simulator.
  • Set temperature low for reproducible behavior (or use record/playback).
  • Use rewards and control-plane summaries to assess task success rather than only length of the dialogue.

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Troubleshooting

  • Simulator does nothing: ensure user_simulation.enabled is true and you have at least a system message.
  • Episode never ends: check that your environment’s rewards/termination are wired, or set a sensible steps limit.
  • Unexpected termination: the simulator may have emitted a stop intent; inspect termination_reason and conversation history.

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GitHub References