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To scaffold an environment, run the following: 
uv run init <MY_ENV_NAME>
There are optional flags:
  • -p, --path <dir> — parent directory, default: ./environments
  • -T, --add-tool — also scaffold a vf.Toolset tool server at servers/tool.py
    • Use this option to create custom tools which are installed into the harnesses via MCP. However, not all harnesses support external tools.
  • -U, --add-user — also scaffold a vf.User simulator at servers/user.py
    • Use this when you need to simulate a user interacting with the main LLM. However, not all harnesses support user simulation.
  • -H, --add-harness — also scaffold a custom vf.Harness at harness.py, selectable via --harness.id <name>
    • In general, you should build your environments so that any of the built-in harnesses work. There are few reasons to build a custom harness.
However, for most environments, building a taskset should be enough.
For a production-scale catalog of benchmark environments, see the companion research-environments repository.

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A minimal environment

import verifiers.v1 as vf


# The data for a given task
class AdditionData(vf.TaskData):
    answer: int


# A task defines a single problem and is defined as a subclass of vf.Task
class AdditionTask(vf.Task[AdditionData]):
    # @vf.reward denotes the scoring function for the task.
    # It needs the trace, which contains the whole message graph, including function calls, user messages etc.
    # It returns the reward for the single task based on this function.
    @vf.reward
    async def exact_match(self, trace: vf.Trace) -> float:
        return float(trace.last_reply == str(self.data.answer))


# The taskset defines the tasks and needs a vf.TasksetConfig, which can be empty.
class AdditionTaskset(vf.Taskset[AdditionTask, vf.TasksetConfig]):
    def load(self) -> list[AdditionTask]:
        return [
            AdditionTask(
                AdditionData(idx=i, prompt=f"What is {i} + {i}?", answer=2 * i),
                self.config.task,
            )
            for i in range(100)
        ]


# Export the Taskset for verifiers to find it when loading
__all__ = ["AdditionTaskset"]
You can also use @vf.metric to record non-scored values and @vf.group_reward for group rewards, which might be useful for training.

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Making values configurable

If you want to make certain fields configurable for the user, subclass vf.TasksetConfig: 
# Allow the user to change the number of tasks
class AdditionConfig(vf.TasksetConfig):
    num_tasks: int = 100

class AdditionTaskset(vf.Taskset[AdditionTask, AdditionConfig]):
    def load(self) -> list[AdditionTask]:
        return [
            AdditionTask(
                AdditionData(idx=i, prompt=f"What is {i} + {i}?", answer=2 * i),
                self.config.task,
            )
            for i in range(self.config.num_tasks) # <- re-use the value here
        ]
Common usages for vf.TasksetConfig are load-time settings, such as splits (e.g., train/test), dataset names, etc. You can also use vf.TaskConfig to configure task data, such as scoring parameters: 
class AdditionTaskConfig(vf.TaskConfig):
    tolerance: float = 0.0

class AdditionTask(vf.Task[AdditionData, vf.State, AdditionTaskConfig]):
    @vf.reward
    async def exact_match(self, trace: vf.Trace) -> float:
        tolerance = self.config.tolerance  # A task-wide confgi
        ...

class AdditionConfig(vf.TasksetConfig):
    num_tasks: int = 100                             # --taskset.num-tasks
    task: AdditionTaskConfig = AdditionTaskConfig()  # --taskset.task.tolerance
The boundary: per-row data (the question, the reference answer) lives on TaskData fields; values uniform across the taskset live on the config — load-time ones directly on the TasksetConfig, task-facing ones under task. A task can also be constructed directly — AdditionTask(data, config=AdditionTaskConfig(...)) — and an omitted config defaults to the declared type’s defaults, so a standalone task works out of the box. Overriding from_trace(trace) (not implemented by default) opts a task into being derived from a finished rollout’s bare Trace — how a multi-agent step spawns a follow-up task. Only the data rides the wire: trace.task.data is the TaskData (with trace.task.type recording the producing Task class’s name), and behavior re-attaches by constructing the task class around it.

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Adding Tools

Some environments require custom tools, which are bundled as a vf.Toolset (similar to how a vf.Taskset bundles vf.Task). Tools are exposed as MCP servers to the given harness and thus need a harness which exposes MCP support (via SUPPORTS_MCP). You can create them like this (remember the bootstrapping with uv run init MY_ENV -T): 
DATABASE = None

class SearchToolset(vf.Toolset[vf.SharedToolsetConfig]):
    TOOL_PREFIX = "search"

    @vf.tool
    async def query(self, text: str) -> list[str]:
        """Search the task corpus."""
        return DATABASE.search(text)

# User-configurable knobs
class SearchConfig(vf.TasksetConfig):
    tools: vf.SharedToolsetConfig = vf.SharedToolsetConfig()

class SearchTaskset(vf.Taskset[vf.Task, SearchConfig]):
    tools = (SearchToolset,)
Taskset tools are shared by a worker’s rollouts. Tools can also be set per task.

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Using Judges

If your reward is semantic, use an LLM judge. 
import verifiers.v1 as vf
from functools import cached_property

class Task(vf.Task):
    answer: str

class CorrectnessJudge(vf.Judge[bool]):
    # The rubric for the judge
    prompt = """Question: {question}
    Answer: {answer}
    Response: {response}
    Correct? Reply yes or no."""

    # Parse the response from the judge
    def parse(self, response: vf.JudgeResponse[bool]) -> bool:
        return "yes" in response.text


class JudgedData(vf.TaskData):
    answer: str


class JudgedTaskConfig(vf.TaskConfig):
    # The judge inherits base_url and api keys from the client config
    judge: vf.JudgeConfig = vf.JudgeConfig(model="openai/gpt-5-mini")


class JudgedTask(vf.Task[JudgedData, vf.State, JudgedTaskConfig]):
    @vf.reward()
    async def correct(self, trace: vf.Trace) -> float:
        # Keeping judge configuration on TaskConfig makes it overridable from CLI/TOML.
        judge = CorrectnessJudge(self.config.judge)
        result = await judge.evaluate(
            trace=trace,
            question=self.data.prompt_text,
            answer=self.data.answer,
            # give the last assistant message to the judge
            response=trace.last_reply,
        )
        return float(result.parsed)


class SetConfig(vf.TasksetConfig):
    task: JudgedTaskConfig = JudgedTaskConfig()


class JudgeTraceTaskset(vf.Taskset[JudgedTask, SetConfig]):
    def load(self) -> list[JudgedTask]:
        return [
            JudgedTask(
                JudgedData(idx=0, prompt="What is 2+2?", answer="4"),
                self.config.task,
            )
        ]
To override the judge model, set taskset.task.judge.model in your config (it is a string).