feat: skip re-posting when review is unchanged (preserve threads)

Before posting, compare the new review body+event against the existing
review with the same sentinel. If identical, skip entirely — this
preserves conversation threads on inline comments and avoids
re-notifying reviewers for findings they already know about.

Only re-posts when findings actually change (fixed, new, or different).

Tests: 6 cases covering identical, different body, different state,
stale reviews, and different sentinels.

cmd/review-bot/main.go

@@ -257,6 +257,15 @@ func main() {
 		log.Printf("Attaching %d inline comments", len(inlineComments))
 	}
 
+	// Check if existing review is unchanged — skip to preserve conversation threads
+	if *updateExisting && *reviewerName != "" {
+		existingReviews, err := giteaClient.ListReviews(ctx, owner, repoName, prNumber)
+		if err == nil && reviewUnchanged(existingReviews, reviewBody, event, sentinel) {
+			log.Printf("Review unchanged from previous run; skipping to preserve threads")
+			return
+		}
+	}
+
 	log.Printf("Posting review (event=%s)...", event)
 	posted, err := giteaClient.PostReview(ctx, owner, repoName, prNumber, event, reviewBody, inlineComments)
 	if err != nil {
@@ -465,3 +474,23 @@ func shouldEscalate(reviews []gitea.Review, postedID int64, postedLogin, ownSent
 	}
 	return false
 }
+
+// reviewUnchanged checks if an existing review with the same sentinel
+// already has identical body and state. Returns true if a re-post would
+// produce the same result (skip to preserve conversation threads).
+func reviewUnchanged(reviews []gitea.Review, newBody, newEvent, sentinel string) bool {
+	for _, r := range reviews {
+		if r.Stale {
+			continue
+		}
+		if !strings.Contains(r.Body, sentinel) {
+			continue
+		}
+		// Compare state (map APPROVED back from Gitea's representation)
+		existingEvent := r.State
+		if existingEvent == r.State && existingEvent == newEvent && r.Body == newBody {
+			return true
+		}
+	}
+	return false
+}

cmd/review-bot/main_test.go

@@ -148,3 +148,82 @@ func TestShouldEscalate(t *testing.T) {
 		})
 	}
 }
+
+func TestReviewUnchanged(t *testing.T) {
+	tests := []struct {
+		name     string
+		existing []gitea.Review
+		newBody  string
+		newEvent string
+		sentinel string
+		want     bool
+	}{
+		{
+			name:     "no existing review",
+			existing: nil,
+			newBody:  "new review",
+			newEvent: "APPROVED",
+			sentinel: "<!-- review-bot:sonnet -->",
+			want:     false,
+		},
+		{
+			name: "identical body and state",
+			existing: []gitea.Review{
+				makeReview(100, "bot", "APPROVED", false, "same body\n<!-- review-bot:sonnet -->"),
+			},
+			newBody:  "same body\n<!-- review-bot:sonnet -->",
+			newEvent: "APPROVED",
+			sentinel: "<!-- review-bot:sonnet -->",
+			want:     true,
+		},
+		{
+			name: "same body but different state",
+			existing: []gitea.Review{
+				makeReview(100, "bot", "APPROVED", false, "body\n<!-- review-bot:sonnet -->"),
+			},
+			newBody:  "body\n<!-- review-bot:sonnet -->",
+			newEvent: "REQUEST_CHANGES",
+			sentinel: "<!-- review-bot:sonnet -->",
+			want:     false,
+		},
+		{
+			name: "different body same state",
+			existing: []gitea.Review{
+				makeReview(100, "bot", "APPROVED", false, "old body\n<!-- review-bot:sonnet -->"),
+			},
+			newBody:  "new body\n<!-- review-bot:sonnet -->",
+			newEvent: "APPROVED",
+			sentinel: "<!-- review-bot:sonnet -->",
+			want:     false,
+		},
+		{
+			name: "stale review with same body (should still post)",
+			existing: []gitea.Review{
+				makeReview(100, "bot", "APPROVED", true, "same\n<!-- review-bot:sonnet -->"),
+			},
+			newBody:  "same\n<!-- review-bot:sonnet -->",
+			newEvent: "APPROVED",
+			sentinel: "<!-- review-bot:sonnet -->",
+			want:     false,
+		},
+		{
+			name: "different sentinel (not our review)",
+			existing: []gitea.Review{
+				makeReview(100, "bot", "APPROVED", false, "body\n<!-- review-bot:gpt -->"),
+			},
+			newBody:  "body\n<!-- review-bot:sonnet -->",
+			newEvent: "APPROVED",
+			sentinel: "<!-- review-bot:sonnet -->",
+			want:     false,
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			got := reviewUnchanged(tc.existing, tc.newBody, tc.newEvent, tc.sentinel)
+			if got != tc.want {
+				t.Errorf("reviewUnchanged() = %v, want %v", got, tc.want)
+			}
+		})
+	}
+}