[donei003]

Follow-up to #63, now merged. Brings the scorer return contract in line with ai-sdks-monorepo TESTING.md §8.8.4, which was tightened after #63 was written.

Problem

A scorer's contract was "a bool or a finite number in 0–1", with True/False coerced to 1.0/0.0. Two problems with the coercion:

It put two score types on the wire. A threshold then meant different things depending on which type a scorer happened to return, even though LaunchDarkly rules on score-vs-threshold identically for both — so the comparison a caller reasoned about and the one ingest performed could differ.

It hid bugs, and the failure mode was a green run. Every non-empty value is truthy, so a scorer that returned "high", or a stray object, scored 1.0 and passed:

# Before: scored 1.0, row passed, no error anywhere.
Scorer(name="quality", fn=lambda row, output: grade(output))  # grade() returns "high"

A false pass is the one failure mode an evaluation harness must not have — the point of the gate is that a broken check fails loudly.

Change

The contract is now a finite number in 0–1, and nothing else. A scorer answering a yes/no question returns 1.0 or 0.0 itself:

def mentions_policy(row: DatasetRow, output: str | None) -> float:
    return 1.0 if "refund policy" in (output or "").lower() else 0.0

A bool, a non-finite number, or one out of range is an invalid_score result — a per-criterion ERROR event, never a raise, since the row's generation has already been paid for.

numeric_score() already excluded bool (Python's bool is an int subclass, which is exactly the trap), so the fix deletes the coercion branch rather than adding a check — the two error paths collapse into one whose message names the offending value.

ScorerFn drops bool from its signature. This documents the contract, but type checkers still accept a bool here because bool is a subclass of int. The runtime check in numeric_score() is the guard.

The first invalid_score from each scorer in a run also logs a warning that names the scorer and the returned value. A caller who upgrades with bool scorers sees the cause in their own logs.

⚠️ Breaking

A scorer returning a bool now produces an invalid_score result instead of 1.0/0.0. Callers return the number directly. This is deliberate per §8.8.4 ("there is no boolean shortcut and nothing is coerced on the caller's behalf") — but it is a behavior change and worth a release note.

Every scorer in the examples repo is converted to match in launchdarkly-labs/ai-sdk-evaluations-example#4.

Validation

• uv run pytest -q — 1264 passed, 11 skipped
• uv run mypy .../evaluations — clean; ruff check / ruff format --check — clean
• New parametrized coverage: True, False, NaN, Infinity, 3, -1, "high", None all produce invalid_score with the value named and no score field, and the run still completes, flushes, and polls
• Boundary coverage: 0, 1, 0.0, 1.0, 0.25 stay valid, ints included — only bool is excluded
• The suite's own scorers were converted from bool to numeric, which is the change callers make

🤖 Generated with Claude Code

---

Note

Overview
Breaking: evaluation Scorer functions must return a finite number in 0–1; True/False are no longer coerced to 1.0/0.0 and instead emit per-criterion invalid_score ERROR events (the run still completes).

The runner drops the boolean coercion path and validates all scorer returns through numeric_score, with a once-per-scorer-per-run warning when a scorer returns something invalid (bools, strings, NaN/Inf, out-of-range). ScorerFn typing and README/Scorer docs now describe the numeric-only contract and show yes/no scorers returning 1.0/0.0 explicitly. Tests were updated and expanded for invalid vs boundary-valid scores.

^{Reviewed by Cursor Bugbot for commit 52f69be. Bugbot is set up for automated code reviews on this repo. Configure here.}

[devin-ai-integration]

Note

Newer findings are available below. Devin Review posted a newer report on this PR, in addition to the findings presented here.

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.

[jeffdupont]

Reviewed with the 1.0 freeze in mind. Nothing blocking from me. This matches ai-sdks-monorepo TESTING.md §8.8.4 as tightened in 1402609 ("there is no boolean shortcut and nothing is coerced on the caller's behalf"), and the invalid_score row in the §8.8.4 failure table. Tested on 0bc9544: make test exit 0 (1410 passed, 11 skipped), make typecheck exit 0. That head is 2 commits behind main, and both are release commits.

On the contract question, reject is the right choice for 1.0. I went through the three options:

• Coerce is the bug this PR fixes. return "high" scored a pass.
• Drop doesn't work. The backend needs one result per (row, criterion) to finish the run, so a dropped result stalls it until the §8.9 timeout.
• Raise throws away a generation that has already been paid for.

A per-criterion ERROR is what §8.8.3 already does for judge scores, so judges and scorers now behave the same way. It is also the right direction to freeze. If we want a boolean shortcut later, loosening the contract is additive. Tightening it after 1.0 would be a breaking change.

JS parity: there's nothing to port. JS has no evaluations on main (deferred to 1.1 in the GA plan), and its judge path already rejects booleans, because isFiniteScore checks typeof score === 'number' (packages/client/src/judges.ts:68). When evals land in JS, §8.8.4 covers it.

Smaller notes:

• The PR body says dropping bool from ScorerFn means "a caller who annotates their scorer sees this at type-check time". That isn't true. I ran mypy --strict against this branch: a def f(...) -> bool scorer and lambda row, output: True both pass, and only a -> str scorer is flagged. bool is a subclass of int, and type checkers accept int where float is expected. The runtime check is the only guard. Narrowing the type is still worth doing, but the claim should come out of the PR body and the squash message. Details inline.
• Please mark it as breaking when you squash: fix(evaluations)!: or a BREAKING CHANGE: footer. The coercion shipped in launchdarkly-ai-server 0.2.4. As titled, release-please will list this under Bug Fixes as a patch, and a caller with bool scorers will see every row go ERROR with no note telling them why.
• Optional: on that upgrade path the only signal is passed=False and error_rows in the summary. _criterion_error_result (runner.py:818) doesn't log anything locally. A logger.warning once per scorer on invalid_score that names the returned value would make the bool case obvious from the caller's own output.

[jeffdupont]

@donei003 this is approved but conflicts with main, so it can't merge yet. The two conflicts are packages/client/tests/test_evaluations_run.py (this PR's tests still use the old run(project_key=) call) and packages/ai/README.md. A rebase onto main should be all it needs; I'll re-approve after.

[jeffdupont]

Pushed bc596c0: merged origin/main, keeping both sides of the test_evaluations_run.py conflict. The new scorer tests now use tagged(handler) and a mode, which main's handler routing requires.

Not done: the invalid_score message and warning still use repr(). The JSON _describe_value helper lands with #209, which is not on main yet. Switch them once #209 merges.

The PR body makes no claim that mypy catches a bool scorer (it says the opposite), so it is unchanged. Needs a human reviewer.

[jeffdupont]

@aknight-ld, could you re-approve? There are new commits since your approval.

• 52f69be: merged main in (it was conflicting after feat(evaluations): normalize text behaviors #209). The conflict was in the scorer validation in evaluations/runner.py. I kept this PR's rule that a bool is an invalid score, against main's 1.0/0.0 coercion. The invalid_score message and the one-time warning now name the value with feat(evaluations): normalize text behaviors #209's _describe_value (true, null, "high", NaN) instead of repr(), and test_scorer_must_return_a_finite_number_in_range asserts that form.
• bc596c0 (earlier): merged main and added tagged(handler) and a mode to the new tests, as main requires.

Locally the full suite passes (3379 passed), along with lint, format, typecheck and the API report check. @donei003, FYI since it's your branch: I merged main in rather than rebasing, so nothing was force-pushed.

[jeffdupont]

Re-approving after the main merge in 52f69be: bools still reject, and the invalid_score message now uses _describe_value.

[devin-ai-integration]

Devin Review found 1 new potential issue.

[devin-ai-integration]

🔴 Invalid scorer values abort evaluation runs

When a scorer returns a mapping with tuple keys, _describe_value raises instead of recording invalid_score. The exception escapes _run_scorer_for_result, aborting the run after generation.

Learn more

The scorer's return value is supplied by customer code. An invalid return must become a per-criterion ERROR, leaving the run free to emit events for all rows. Here numeric_score rejects a mapping, but _describe_value calls canonical_json, whose json.dumps step rejects mappings with tuple keys. The exception occurs after the scorer invocation's exception handler and propagates through _run_criteria_for_results, preventing criterion event emission.

Example: A scorer returns {('precision', 'recall'): 0.8} instead of a score. The expected invalid_score event never appears; run() raises TypeError after generating the rows.

Recommended fix: Make _describe_value safely fall back to repr(value) when JSON serialization fails, while retaining canonical formatting for ordinary JSON-like values. Test a scorer returning a mapping with tuple keys and assert it produces an invalid_score event and completes the run.

---

Was this helpful? React with 👍 or 👎 to provide feedback.