Improve error handling and logging in evaluation judge

pkg/evaluation/eval_test.go

@@ -156,23 +156,29 @@ func TestParseJudgeResponse(t *testing.T) {
 		text       string
 		wantPassed bool
 		wantReason string
+		wantErr    bool
 	}{
-		{"simple pass", `{"result": "pass", "reason": "good"}`, true, "good"},
-		{"simple fail", `{"result": "fail", "reason": "bad"}`, false, "bad"},
-		{"pass uppercase", `{"result": "PASS", "reason": "good"}`, true, "good"},
-		{"fail uppercase", `{"result": "FAIL", "reason": "bad"}`, false, "bad"},
-		{"pass mixed case", `{"result": "Pass", "reason": "good"}`, true, "good"},
-		{"invalid json returns false", `not json at all`, false, "failed to parse judge response"},
-		{"empty result returns false", `{"result": "", "reason": "empty"}`, false, "empty"},
-		{"missing result field", `{"reason": "no result field"}`, false, "no result field"},
+		{"simple pass", `{"result": "pass", "reason": "good"}`, true, "good", false},
+		{"simple fail", `{"result": "fail", "reason": "bad"}`, false, "bad", false},
+		{"pass uppercase", `{"result": "PASS", "reason": "good"}`, true, "good", false},
+		{"fail uppercase", `{"result": "FAIL", "reason": "bad"}`, false, "bad", false},
+		{"pass mixed case", `{"result": "Pass", "reason": "good"}`, true, "good", false},
+		{"invalid json returns error", `not json at all`, false, "", true},
+		{"empty result returns false", `{"result": "", "reason": "empty"}`, false, "empty", false},
+		{"missing result field", `{"reason": "no result field"}`, false, "no result field", false},
 	}
 
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
 			t.Parallel()
-			got := parseJudgeResponse(tt.text)
-			assert.Equal(t, tt.wantPassed, got.passed)
-			assert.Equal(t, tt.wantReason, got.reason)
+			passed, reason, err := parseJudgeResponse(tt.text)
+			if tt.wantErr {
+				require.Error(t, err)
+			} else {
+				require.NoError(t, err)
+				assert.Equal(t, tt.wantPassed, passed)
+				assert.Equal(t, tt.wantReason, reason)
+			}
 		})
 	}
 }

pkg/evaluation/judge.go

@@ -3,7 +3,10 @@ package evaluation
 import (
 	"context"
 	"encoding/json"
+	"errors"
 	"fmt"
+	"io"
+	"log/slog"
 	"strings"
 	"sync"
 
@@ -164,18 +167,42 @@ func (j *Judge) checkSingle(ctx context.Context, response, criterion string) (pa
 	defer stream.Close()
 
 	var fullResponse strings.Builder
+	var streamErr error
 	for {
 		resp, err := stream.Recv()
 		if err != nil {
+			if !errors.Is(err, io.EOF) {
+				streamErr = err
+			}
 			break
 		}
 		for _, choice := range resp.Choices {
 			fullResponse.WriteString(choice.Delta.Content)
 		}
 	}
 
-	result := parseJudgeResponse(fullResponse.String())
-	return result.passed, result.reason, nil
+	if streamErr != nil {
+		return false, "", fmt.Errorf("streaming judge response: %w", streamErr)
+	}
+
+	raw := fullResponse.String()
+	passed, reason, err = parseJudgeResponse(raw)
+	if err != nil {
+		slog.Warn("Failed to parse judge response",
+			"criterion", criterion,
+			"raw_response", raw,
+			"error", err,
+		)
+		return false, "", fmt.Errorf("parsing judge response (length=%d): %w", len(raw), err)
+	}
+
+	slog.Debug("Judge response parsed successfully",
+		"criterion", criterion,
+		"passed", passed,
+		"reason", reason,
+	)
+
+	return passed, reason, nil
 }
 
 // judgeResponse represents the structured response from the judge model.
@@ -184,23 +211,22 @@ type judgeResponse struct {
 	Reason string `json:"reason"`
 }
 
-// parsedJudgeResult contains the parsed result from the judge.
-type parsedJudgeResult struct {
-	passed bool
-	reason string
-}
-
-func parseJudgeResponse(text string) parsedJudgeResult {
+// parseJudgeResponse parses a JSON judge response and returns whether the check
+// passed, the reason, and any parse error.
+func parseJudgeResponse(text string) (passed bool, reason string, err error) {
 	text = strings.TrimSpace(text)
 
 	var resp judgeResponse
 	if err := json.Unmarshal([]byte(text), &resp); err != nil {
-		// With structured output this should not happen, but handle gracefully
-		return parsedJudgeResult{passed: false, reason: "failed to parse judge response"}
+		return false, "", fmt.Errorf("invalid JSON: %w", err)
 	}
 
-	return parsedJudgeResult{
-		passed: strings.EqualFold(resp.Result, "pass"),
-		reason: resp.Reason,
+	if resp.Result == "" {
+		slog.Warn("Judge response has empty result field",
+			"raw_response", text,
+			"reason_field", resp.Reason,
+		)
 	}
+
+	return strings.EqualFold(resp.Result, "pass"), resp.Reason, nil
 }