Evaluate with pydantic_evals and Logfire

How This Works

pydantic_evals runs your actual function or agent against a Dataset of Cases (input + expected output + metadata), scores each with one or more Evaluators, and produces a report. It depends on logfire itself (the datasets extra pulls in the real SDK, not a mock), so whether logfire.configure() has run determines only whether results also upload to Logfire's Datasets & Experiments UI — omitting it keeps results entirely local and printed to the terminal, silently, not an error.

Agentic evaluators (tool-call correctness, trajectory matching) need more than that: they read the task's own execution span tree, so without a working logfire.configure() they don't just fail to upload — every case reports "No span tree available" and the check never ran at all.

Step 1: Check for an Existing Braintrust Suite First

Cheap check, before anything else: does this repo already have an existing Braintrust suite — actual Eval(...) calls or from braintrust import Eval in source, not just a braintrust dependency listed without any real usage? This path needs no CLI auth at all — don't run Step 2 for it.

Keep the existing Eval() code (Python braintrust>=0.30.1 / TypeScript braintrust>=3.24.0 — verified versions) and redirect its next run to Logfire by changing environment variables only, no pydantic_evals involved:

export BRAINTRUST_APP_URL="https://logfire-us.pydantic.dev/v1/braintrust"  # EU: logfire-eu.pydantic.dev
export BRAINTRUST_API_KEY="<logfire-project-write-token>"                  # Project -> Settings -> Write tokens
unset BRAINTRUST_API_URL BRAINTRUST_PROXY_URL  # these override the endpoint above if set — the #1 "it still hit Braintrust" cause

This is a compatibility preview, not full parity: covers inline/callable data, local tasks and scorers, multiple scores, one label per name, and normal summary finalization. It does not cover Braintrust-hosted datasets/prompts/functions, BTQL, the model proxy, server-side scoring, or post-finalization feedback — and summarize_scores=False, a manual flush() without a comparison, or the Rust SDK never request the summary this endpoint needs, so nothing lands even though the run appears to succeed. Full detail and the concept-translation table (Braintrust "project" → Logfire dataset name, "scorer" → evaluator, etc.): https://pydantic.dev/docs/logfire/get-started/comparisons/migrate-from-braintrust/.

Skip straight to Step 5 (Verify) — the SDK's own printed result URL also opens directly in Logfire, and nothing else here (auth, dataset definition) applies to this path.

No existing Braintrust suite? Continue to Step 2 now, before the more detailed identification in Step 3 — nothing past this point requires knowing the function/agent or dataset shape yet.

Step 2: Authenticate When the Run Needs Logfire

Skip authentication and continue to Step 3 only when the user explicitly wants a local-only pydantic_evals run using evaluators that do not need span data; omit logfire.configure() in Step 4 so results stay in the terminal. Uploading results, using a hosted dataset, or running a span-based evaluator such as ToolCorrectness requires Logfire, so authenticate before opening or running evaluation files and target the exact project first.

For a Logfire-backed run, check first — uvx logfire --non-interactive whoami (JS: npx logfire whoami) — and skip to Step 3 if it already reports the right project and region. Otherwise, follow the full command sequence, flags, and gotchas (the --non-interactive requirement, why auth won't open a browser for you, the LOGFIRE_TOKEN-vs-credentials-file conflict, and token-file safety) in Authenticate and Select the Exact Project. This CLI flow is for logfire.configure(); Step 3's hosted-dataset operations use a separate API key with different scopes.

Step 3: Detect What to Evaluate

Identify the function or agent under test (a PydanticAI agent, an LLM-calling function, any callable that takes an input and returns an output) and whether a dataset already exists:

• In-code dataset: a Python module defining Case/Dataset directly — the default for an agent-driven workflow.
• Hosted/managed dataset: cases live in the Logfire UI, edited by non-engineers, pulled/pushed via a separate LogfireAPIClient (from logfire.experimental.api_client import LogfireAPIClient). client.get_dataset(name) with no type arguments returns a raw dict, not something push_dataset or .evaluate_sync() can take — pass the input/output (and metadata, if used) types to get back a real pydantic_evals.Dataset: client.get_dataset(name, MyInputType, MyOutputType). If the stored dataset contains custom evaluators, also pass their classes with custom_evaluator_types=[ExactMatch] (and custom report evaluators with custom_report_evaluator_types=[...]) so they can be deserialized. Push with client.push_dataset(dataset). This needs its own API key from Settings → API Keys (scoped project:read_datasets/project:write_datasets), not Step 2's CLI auth flow. Only relevant if the user specifically wants case editing outside code.

Step 4: Define the Dataset and Run It

uv add 'logfire[datasets]'

from dataclasses import dataclass

import logfire
from pydantic_evals import Case, Dataset
from pydantic_evals.evaluators import Evaluator, EvaluatorContext, IsInstance

logfire.configure()  # omit this and results stay local only, silently


@dataclass
class ExactMatch(Evaluator[str, str]):
    def evaluate(self, ctx: EvaluatorContext[str, str]) -> bool:
        return ctx.output == ctx.expected_output


def classify_sentiment(text: str) -> str:
    ...  # the function under test


dataset = Dataset[str, str, None](
    name='sentiment-eval',
    cases=[
        Case(name='positive', inputs='I love this', expected_output='positive'),
        Case(name='negative', inputs='This is terrible', expected_output='negative'),
    ],
    evaluators=[ExactMatch(), IsInstance(type_name='str')],
)

report = dataset.evaluate_sync(classify_sentiment)  # or `await dataset.evaluate(...)`
report.print(include_input=True, include_output=True)

Before running the full dataset, run a smoke test on 2-3 cases if the dataset is large or uses LLMJudge/any evaluator that makes a real, billed model call — a bug caught on 3 cases costs 3 model calls, the same bug caught on 300 costs 300:

smoke = Dataset(
    name=dataset.name,
    cases=dataset.cases[:3],
    evaluators=dataset.evaluators,
    report_evaluators=dataset.report_evaluators,
)
smoke_report = smoke.evaluate_sync(classify_sentiment)
smoke_report.print(include_input=True, include_output=True)

Confirm the smoke run has zero unexpected errors and the assertions that should pass do. Then, if the full dataset is large or uses paid model calls, tell the user the case count and which evaluators will make model calls, and get explicit confirmation before running the full dataset — don't run an expensive full pass on the strength of a clean smoke test alone without saying so.

Custom evaluators must be @dataclass subclasses — a plain class raises at run time. Case names must be unique within a dataset. The evaluators reached for most:

Also available: Contains, MaxDuration, TrajectoryMatch, ArgumentCorrectness, MaxToolCalls, MaxModelRequests — same span-tree dependency as ToolCorrectness for the tool/trajectory ones; see pydantic_evals.evaluators for the full set. These five agentic (span-based) evaluators need pydantic-evals>=2.4.0 — on an older pin, check pyproject.toml/uv.lock and upgrade before reaching for them, since the import itself is what fails, not a silent no-op.

The Python evaluator (arbitrary code execution) was removed for security reasons — don't reach for it even if an older example references it.

If editing a hosted dataset: client.push_dataset(dataset) overwrites server-side evaluators on every push, including removing ones you deleted locally — don't push a stale local copy over a dataset others have edited in the UI.

Step 5: Verify

A report printing to the terminal isn't proof it reached Logfire — confirm the run actually landed. Never report a case as passed, a score, or a run as complete without having actually checked it in this session — if a run fails, cancels, or produces no scores, report that failure plainly; never substitute an invented score or a manual guess at what the result "should" be.

Came from the Step 1 Braintrust path (Step 2 skipped)? There's no whoami-resolved project to look up here — use the SDK's own printed result URL instead, which already opens directly in the right Logfire project. Confirm the same things below (completion, pass mix, case detail) from that page rather than searching by name.

1. Query for the run directly, if a Logfire MCP server or API is connected — the root span for a run is named evaluate {name} and carries gen_ai.operation.name = 'experiment', dataset_name, and task_name attributes; find the most recent one matching your dataset's name and confirm logfire.experiment.metadata shows the case count and pass rate you expect. Otherwise, open AI Evaluations → Datasets & Experiments → Experiments in Logfire for the exact project from Step 2, and find the run by name/timestamp.
2. Read the Overview tab (or the queried metadata) first: completion count, assertion pass mix, task errors, average duration. If completion says "Not reported," the run sent case data but never signaled it finished — treat that as a broken run, not a passing one.
3. Open the Cases tab, starting from Needs Review / Failed / Errors, not the full list.
4. Drill into a failing case's trace in Live view for the actual evidence, rather than trusting the summary score alone.
5. Fix and re-run until the cases that should pass do, and any tool-call/trajectory checks show real span data, not "No span tree available."

Close with a final report built from what you just confirmed — the run name, exact case count and pass rate you queried, and which evaluators ran — not a template. Include the direct link to this experiment (the SDK's own printed result URL, or the Datasets & Experiments page you opened it from), so the user can see the run without having to ask where to look.