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+# Classifiers
+
+## Overview
+
+Classifiers categorize and label eval outputs. Unlike scorers (numeric 0-1), classifiers produce structured classification items with optional metadata. Both receive the same arguments (`output`, `expected`, `input`, `metadata`) and run in parallel during evaluations.
+
+Classifications are stored as `Record<string, ClassificationItem[]>` -- a dictionary keyed by classifier name, where each value is an array of items. This supports multiple classifiers producing independent label sets, a single classifier producing multiple labels, and multiple classifiers contributing to the same key.
+
+---
+
+## Public API
+
+### Evaluator Interface
+
+An evaluator **MUST** include at least one of `scores` or `classifiers` (or both). SDKs **MUST** validate this at runtime and raise a clear error if neither is provided, even if the constraint is also enforced at the type level.
+
+```typescript
+interface EvaluatorBase<Input, Output, Expected, Metadata> {
+  data: () => Dataset<Input, Expected, Metadata>;
+  task: (input: Input, hooks: Hooks) => Output | Promise<Output>;
+}
+
+type Evaluator<Input, Output, Expected, Metadata> =
+  | EvaluatorBase<Input, Output, Expected, Metadata> & {
+      scores: EvalScorer<Input, Output, Expected, Metadata>[];
+      classifiers?: EvalClassifier<Input, Output, Expected, Metadata>[];
+    }
+  | EvaluatorBase<Input, Output, Expected, Metadata> & {
+      scores?: EvalScorer<Input, Output, Expected, Metadata>[];
+      classifiers: EvalClassifier<Input, Output, Expected, Metadata>[];
+    };
+```
+
+### EvalClassifier
+
+A classifier function accepts the same arguments as a scorer. It **MAY** return a single `Classification`, an array (multi-label), or `null`. It **MAY** be synchronous or asynchronous.
+
+```typescript
+type OneOrMoreClassifications = Classification | Classification[] | null;
+
+type EvalClassifier<Input, Output, Expected, Metadata> = (
+  args: EvalScorerArgs<Input, Output, Expected, Metadata>,
+) => OneOrMoreClassifications | Promise<OneOrMoreClassifications>;
+```
+
+### Classification
+
+Returned by classifier functions. The `name` field is used as the grouping key in the results dictionary and is omitted when converting to the storage format.
+
+```typescript
+interface Classification {
+  name: string;
+  id: string;
+  label?: string;
+  metadata?: Record<string, unknown>;
+}
+```
+
+### EvalResult
+
+The `classifications` field **MUST** be omitted (not an empty object) when no classifiers are defined or all return `null`.
+
+```typescript
+interface EvalResult {
+  input: unknown;
+  output: unknown;
+  expected?: unknown;
+  scores?: Record<string, number | null>;
+  classifications?: Record<string, ClassificationItem[]>;
+  metadata?: Record<string, unknown>;
+}
+```
+
+---
+
+## Behavior
+
+### Execution
+
+SDKs **MUST** run classifiers in parallel with scorers (e.g., `Promise.all`).
+
+Each classifier **MUST** run inside a traced span with `type: "classifier"` and `purpose: "scorer"`. The traced span name is resolved from the classifier function name (or fallback) rather than from returned classification items:
+
+```typescript
+rootSpan.traced(
+  (classifierSpan) => {
+    const result = await classifierFn({ input, output, expected, metadata });
+    classifierSpan.log({ output: result });
+    return result;
+  },
+  {
+    name: resolvedClassifierSpanName,
+    span_attributes: {
+      type: "classifier",
+      purpose: "scorer",
+    },
+  },
+);
+```
+
+### Name Resolution
+
+SDKs **MUST** resolve names in two places:
+
+1. Classification span name: `.name` property of the classifier function, then fallback `classifier_${index}`
+2. Classification result grouping key: `name` field on each returned `Classification` object
+
+Each returned classification item **MUST** include a non-empty string `name`; items with the same returned `name` **MUST** be appended to the same array.
+
+### Validation
+
+Each classification result **MUST** have:
+- A `name` that is a non-empty string
+- An `id` that is a non-empty string
+
+If validation fails, treat the classifier as failed.
+
+Additional field rules:
+- `metadata` is an unconstrained `Record<string, unknown>`. SDKs **MUST NOT** impose size limits.
+- Duplicate `{name, id}` pairs are allowed. Multiple items with the same `id` under the same name key **MUST** all be stored. Deduplication, if needed, is handled at the display layer.
+- Order is stable. Items **MUST** be stored in the order they are returned by the classifier.
+
+### Conversion to ClassificationItem
+
+When storing results, SDKs **MUST** convert `Classification` to `ClassificationItem`:
+
+1. Copy `id` as-is
+2. Default `label` to `id` if not provided
+3. Include `metadata` only if present
+4. Omit `name` (it becomes the dictionary key)
+
+### Error Handling
+
+Classifier failures **MUST NOT** abort the evaluation or affect other classifiers/scorers.
+
+On failure:
+1. Record the error under `classifier_errors` in eval metadata (maps classifier name to error message)
+2. Log the error to the root span's metadata
+3. **SHOULD** emit a debug warning
+
+This mirrors the `scorer_errors` pattern.
+
+---
+
+## Wire Format
+
+### ClassificationItem
+
+The storage format for a single classification. Derived from `Classification` by dropping `name` and defaulting `label` to `id` when omitted.
+
+| Field | Type | Required | Description |
+|---|---|---|---|
+| `id` | String | **REQUIRED** | Stable identifier for filtering and grouping. |
+| `label` | String | **REQUIRED** | Display label. Defaults to `id` during conversion. |
+| `metadata` | Record\<string, unknown\> | **OPTIONAL** | Arbitrary metadata. |
+
+### Classifications on Events
+
+Stored as a top-level `classifications` field on experiment and log events. **MUST** be `Record<string, ClassificationItem[]>`. **MUST** be omitted when empty.
+
+```json
+{
+  "classifications": {
+    "category": [
+      { "id": "greeting", "label": "Greeting" }
+    ],
+    "sentiment": [
+      { "id": "positive", "label": "Positive" },
+      { "id": "enthusiastic", "label": "Enthusiastic" }
+    ]
+  }
+}
+```
+
+---
+
+## Examples
+
+### Basic
+
+```javascript
+Eval("my-project", {
+  data: () => [{ input: "Hello!", expected: "Hi there!" }],
+  task: async (input) => callMyModel(input),
+  scores: [
+    ({ output, expected }) => ({
+      name: "exact_match",
+      score: output === expected ? 1 : 0,
+    }),
+  ],
+  classifiers: [
+    ({ output }) => ({
+      name: "category",
+      id: "greeting",
+      label: "Greeting",
+    }),
+  ],
+});
+```
+
+### Classifiers Only (No Scores)
+
+```javascript
+Eval("my-project", {
+  data: () => [{ input: "Hello!", expected: "Hi there!" }],
+  task: async (input) => callMyModel(input),
+  classifiers: [categoryClassifier, sentimentClassifier],
+});
+```
+
+### Multi-Label
+
+```javascript
+const sentimentClassifier = ({ output }) => [
+  { name: "sentiment", id: "positive", label: "Positive" },
+  { name: "sentiment", id: "enthusiastic", label: "Enthusiastic" },
+];
+```
+
+### Error Output
+
+When a classifier fails, the result includes:
+
+```json
+{
+  "metadata": {
+    "classifier_errors": {
+      "broken_classifier": "must return classifications with a non-empty string name"
+    }
+  }
+}
+```