Add purpose tag to Eval score spans

src/main/java/dev/braintrust/devserver/Devserver.java

@@ -607,6 +607,7 @@ private void setScoreSpanAttributes(
         Map<String, Object> scoreSpanAttrs = new LinkedHashMap<>();
         scoreSpanAttrs.put("type", "score");
         scoreSpanAttrs.put("name", scorerName);
+        scoreSpanAttrs.put("purpose", "scorer");
         if (braintrustGeneration != null) {
             scoreSpanAttrs.put("generation", braintrustGeneration);
         }

src/main/java/dev/braintrust/eval/Eval.java

@@ -230,6 +230,7 @@ private void recordScores(
         Map<String, Object> spanAttrs = new LinkedHashMap<>();
         spanAttrs.put("type", "score");
         spanAttrs.put("name", scorer.getName());
+        spanAttrs.put("purpose", "scorer");
         scoreSpan.setAttribute("braintrust.span_attributes", toJson(spanAttrs));
         var scoresJson = toJson(scorerScores);
         scoreSpan.setAttribute("braintrust.output_json", scoresJson);

src/test/java/dev/braintrust/devserver/DevserverTest.java

@@ -433,6 +433,7 @@ void testStreamingEval() throws Exception {
             assertNotNull(spanAttrsJson, "Score span should have span_attributes");
             JsonNode spanAttrs = JSON_MAPPER.readTree(spanAttrsJson);
             assertEquals("score", spanAttrs.get("type").asText());
+            assertEquals("scorer", spanAttrs.get("purpose").asText());
             assertEquals("test-gen-1", spanAttrs.get("generation").asText());
 
             // Scorer name should be either simple_scorer or the remote scorer

src/test/java/dev/braintrust/eval/EvalTest.java

@@ -104,6 +104,30 @@ public void evalOtelTraceWithProperAttributes() {
                 numRootSpans.get() * 4,
                 spans.size(),
                 "each eval case should make four spans (one per scorer)");
+
+        // All score spans should have purpose=scorer in span_attributes
+        var scoreSpans =
+                spans.stream()
+                        .filter(
+                                s -> {
+                                    var attrs =
+                                            s.getAttributes()
+                                                    .get(
+                                                            AttributeKey.stringKey(
+                                                                    "braintrust.span_attributes"));
+                                    return attrs != null && attrs.contains("\"type\":\"score\"");
+                                })
+                        .toList();
+        assertFalse(scoreSpans.isEmpty(), "should have score spans");
+        for (var scoreSpan : scoreSpans) {
+            var spanAttrsJson =
+                    scoreSpan
+                            .getAttributes()
+                            .get(AttributeKey.stringKey("braintrust.span_attributes"));
+            assertTrue(
+                    spanAttrsJson.contains("\"purpose\":\"scorer\""),
+                    "score span should have purpose=scorer in span_attributes: " + spanAttrsJson);
+        }
     }
 
     boolean isFruitOrVegetable(String str) {