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Writing an eval task

A task is a directory. Everything about it is a file you can read, diff and argue with.

tasks/fix-chunk-off-by-one/
├── task.json      metadata, budget, graders
├── prompt.md      what the agent is told
├── fixture/       copied into the sandbox; this is the agent's whole world
├── holdout/       staged in at grading time only — the agent never sees it
└── solution/      a reference fix, used to prove the graders are load-bearing

Run one while you are writing it:

endstate eval --suite tasks/ --task fix-chunk-off-by-one --sandbox local

The manifest

{
  "id": "fix-chunk-off-by-one",
  "category": "bug-fix",
  "description": "chunk() silently drops the final partial chunk.",
  "max_steps": 20,
  "timeout_s": 600.0,
  "budget": { "max_context_tokens": 60000, "reserve_output_tokens": 4000 },
  "graders": [
    {
      "name": "endstate.evals.graders:command_succeeds",
      "args": { "command": "python -m unittest discover -s tests -t . -q" }
    },
    {
      "name": "endstate.evals.graders:files_unchanged",
      "args": { "paths": ["tests/test_chunk.py"] }
    }
  ]
}

graders is a list and the list is a conjunction: every check from every grader must pass. That is the shape the interesting task definitions have — did the work and did not cheat to get there — and making it the only shape available means nobody has to remember to write the second half as a separate step.

prompt.md wins over a prompt key in the manifest, because prose belongs in a file someone will actually edit.

The four assertions every task should make

A task graded only by "the test suite passes" has swapped a fluency exploit for a test-editing exploit. The shipped tasks all make these four, and this repo's own test suite fails if a task tries to get by on a command alone.

[
  { "name": "endstate.evals.graders:command_succeeds",
    "args": { "command": "python -m unittest discover -s tests -t . -q" } },

  { "name": "endstate.evals.graders:files_unchanged",
    "args": { "paths": ["tests/test_chunk.py"] } },

  { "name": "endstate.evals.graders:no_new_skips" },

  { "name": "endstate.evals.graders:changed_paths_within",
    "args": { "allowed": ["chunker/*", "*.md"] } }
]

The fifth assertion is not a grader: it is the holdout/ directory. Put tests there that the agent never sees, and special-casing the visible inputs stops working.

The grader contract

def grade(sandbox: Sandbox) -> Verdict: ...

One argument, and it is the sandbox. A grader is resolved from its dotted path before it runs, and resolution inspects the signature: the first parameter must be sandbox and positional, and anything named messages, transcript, result or history is refused. See Evaluation for why that is enforced rather than documented.

Arguments beyond the sandbox come from args and are bound before the grader is called, so the callable the runner holds really does take only the sandbox.

Writing your own

Put it anywhere importable and point at it with module:function:

# mytasks/graders.py
from endstate.evals import Check, Sandbox, Verdict


def no_secrets_committed(sandbox: Sandbox, *, pattern: str = r"sk-[A-Za-z0-9]{20,}") -> Verdict:
    import re

    offenders = [
        path for path, text in sandbox.end_state_files().items() if re.search(pattern, text)
    ]
    return Verdict.from_checks(
        [Check(name="no secrets in the tree", passed=not offenders, detail=", ".join(offenders))]
    )

{ "name": "mytasks.graders:no_secrets_committed" }

Return every check you ran, not just the failing one. "The tests pass but you edited them" and "the tests fail" are different findings, and a verdict that collapses to one boolean cannot say which.

What the sandbox gives a grader

Method Answers
run(command) Did it work? Returns exit code, stdout, stderr.
read_text(path) What is in the file now? Missing reads as "".
fixture_text(path) What was in it before the agent started?
end_state_files() Every text file in the end state, keyed by relative path.
changed_paths() What did the agent add, edit or delete?
tree_hash() Is this sandbox byte-for-byte what it was?
exists(path) Is it there?

fixture_text is the one worth noticing. The pristine fixture is kept beside the live tree, so "is this file byte-for-byte what we shipped?" is answerable — which is what catches an agent editing the tests instead of the code.

Note that these read the sealed end state, frozen when the agent stopped and before the held-out tests were staged in. A grader cannot accidentally assert on its own side effects.

Fixtures

  • Standard library only. The sandbox has no network, so a fixture that needs pip install cannot run. The shipped tasks use unittest rather than pytest for exactly this reason.
  • tests/ needs an __init__.py for unittest discover -s tests -t . to import it.
  • Ship the failing test. For a bug-fix task the suite should be red on arrival: that is the signal the agent works from, and a task whose tests already pass is a task with no feedback loop.
  • No version control history. .git, .hg and .svn are pruned when the fixture is copied in, so you do not have to remember — but do not build a fixture whose answer is only hidden by that.

The solution directory

solution/ is overlaid on the fixture to check that the graders discriminate. It is never given to an agent, and the test that uses it asserts both directions:

  • the graders fail on the untouched fixture, and
  • the graders pass on the reference solution.

Write it. A grader that passes either way is measuring nothing, and it is much easier to write one of those than most people expect — the failure is silent and looks exactly like a well-behaved suite.

Asserting on the run, not just the tree

Some things the filesystem cannot answer. "Compaction fired" is not a property of the end state, and neither is "the policy refused something" — an agent that ignored the request leaves the same tree as one the harness stopped. Those go in requires, which the runner checks and conjoins with the grader's verdict:

"requires": {
  "denied_calls": { "min": 1 },
  "compaction_events": { "min": 1 },
  "steps": { "max": 8 },
  "total_tokens": { "max": 120000 },
  "stop_reason": "end_turn"
}

Available counters: compaction_events, denied_calls, unsettled_calls, steps, input_tokens, output_tokens, total_tokens, and stop_reason. Each bound takes min, max, or both.

That list is closed on purpose, and unknown keys are rejected rather than ignored — both the field names and the min/max inside a bound. A {"minimum": 1} typo would otherwise parse as a bound with no ends and pass for any value, so the task file would read as asserting something while asserting nothing.

Killing a run on purpose

A recovery task adds:

"recovery": { "crash_at_call": 1, "after_side_effect": false }

crash_at_call counts tool calls across the whole run rather than steps, because the interesting kill points are inside a batch. after_side_effect picks the window, and the two are different problems: false is the honest kill where the work never happened, true is the irreducible one where it landed and nothing recorded it.

The runner then runs the task, lets it die at that call, builds a new loop over the reloaded session, and resumes. The task's ordinary graders decide the verdict — the question being asked is whether the work still lands.

Categories

bug-fix, feature, refactor, compaction, permissioning, recovery, cost. Categories are validated at load time, so a typo is an error rather than a task that quietly never runs in any filter.

A permissioning task is the one exception to the solution/ rule: its correct outcome is that nothing happened, so an overlay of reference files would contradict the tree_unchanged assertion it grades on. Leave solution/ out; the requires block is what discriminates.