diff --git a/.github/workflows/pyrefly-diff-comment.yml b/.github/workflows/pyrefly-diff-comment.yml
index 7f82942e7e..8e16baf933 100644
--- a/.github/workflows/pyrefly-diff-comment.yml
+++ b/.github/workflows/pyrefly-diff-comment.yml
@@ -77,10 +77,28 @@ jobs:
             }
 
             if (diff.trim()) {
-              await github.rest.issues.createComment({
+              const body = '### Pyrefly Diff\n<details>\n<summary>base → PR</summary>\n\n```diff\n' + diff + '\n```\n</details>';
+              const marker = '### Pyrefly Diff';
+              const { data: comments } = await github.rest.issues.listComments({
                 issue_number: prNumber,
                 owner: context.repo.owner,
                 repo: context.repo.repo,
-                body: '### Pyrefly Diff\n<details>\n<summary>base → PR</summary>\n\n```diff\n' + diff + '\n```\n</details>',
               });
+              const existing = comments.find((comment) => comment.body.startsWith(marker));
+
+              if (existing) {
+                await github.rest.issues.updateComment({
+                  comment_id: existing.id,
+                  owner: context.repo.owner,
+                  repo: context.repo.repo,
+                  body,
+                });
+              } else {
+                await github.rest.issues.createComment({
+                  issue_number: prNumber,
+                  owner: context.repo.owner,
+                  repo: context.repo.repo,
+                  body,
+                });
+              }
             }
diff --git a/.github/workflows/pyrefly-diff.yml b/.github/workflows/pyrefly-diff.yml
index 0cf54e3585..386bd25751 100644
--- a/.github/workflows/pyrefly-diff.yml
+++ b/.github/workflows/pyrefly-diff.yml
@@ -103,9 +103,26 @@ jobs:
                 ].join('\n')
               : '### Pyrefly Diff\nNo changes detected.';
 
-            await github.rest.issues.createComment({
+            const marker = '### Pyrefly Diff';
+            const { data: comments } = await github.rest.issues.listComments({
               issue_number: prNumber,
               owner: context.repo.owner,
               repo: context.repo.repo,
-              body,
             });
+            const existing = comments.find((comment) => comment.body.startsWith(marker));
+
+            if (existing) {
+              await github.rest.issues.updateComment({
+                comment_id: existing.id,
+                owner: context.repo.owner,
+                repo: context.repo.repo,
+                body,
+              });
+            } else {
+              await github.rest.issues.createComment({
+                issue_number: prNumber,
+                owner: context.repo.owner,
+                repo: context.repo.repo,
+                body,
+              });
+            }