diff --git a/.changeset/cli-diff-insert-seed-note.md b/.changeset/cli-diff-insert-seed-note.md
new file mode 100644
index 00000000000..bd96436585f
--- /dev/null
+++ b/.changeset/cli-diff-insert-seed-note.md
@@ -0,0 +1,8 @@
+---
+'@objectstack/cli': patch
+---
+
+fix(cli): `os diff` now says what a replay does to a seed dataset in mode `insert`, as it already did for `replace`: a replay inserts its records again, and a row the database refuses is that row's error. The loader's `insert` arm inserts every record without looking for a match, so the plan no longer reads an `insert` dataset as harmless.
+
+Clause-②: no
+Metadata-protocol: none
diff --git a/packages/cli/src/commands/diff.ts b/packages/cli/src/commands/diff.ts
index 5a8ab9b986c..b02482c057c 100644
--- a/packages/cli/src/commands/diff.ts
+++ b/packages/cli/src/commands/diff.ts
@@ -79,6 +79,7 @@ function diffNamedArrays(
 function seedModeNote(mode: string): string {
   if (seedModeOverwritesMatchedRows(mode)) return '; a replay writes its columns over the rows it matches';
   if (mode === 'replace') return '; a replay deletes nothing and inserts these again, so rows already in the table stay';
+  if (mode === 'insert') return "; a replay inserts these again, and a row the database refuses is that row's error";
   return '';
 }
 
diff --git a/packages/cli/test/diff-upgrade-plan.test.ts b/packages/cli/test/diff-upgrade-plan.test.ts
index 8bf07d95b24..6ef764a7dbb 100644
--- a/packages/cli/test/diff-upgrade-plan.test.ts
+++ b/packages/cli/test/diff-upgrade-plan.test.ts
@@ -153,4 +153,28 @@ describe('os diff — what the upgrade plan adds', () => {
       { type: 'modified', category: 'data', name: 'acme_region', breaking: false, detail: 'seed dataset changed: records; mode ignore' },
     ]);
   });
+
+  it('an insert dataset, added or changed, says a replay inserts its records again; ignore stays silent', () => {
+    const before = lead({}, [
+      { object: 'acme_source', mode: 'insert', records: [{ name: 'web' }] },
+      { object: 'acme_region', mode: 'ignore', records: [{ name: 'emea' }] },
+    ]);
+    const after = lead({}, [
+      { object: 'acme_source', mode: 'insert', records: [{ name: 'web' }, { name: 'event' }] },
+      { object: 'acme_region', mode: 'ignore', records: [{ name: 'apac' }] },
+      { object: 'acme_channel', mode: 'insert', records: [{ name: 'email' }] },
+    ]);
+    const insertNote = "a replay inserts these again, and a row the database refuses is that row's error";
+    expect(diffStacks(before, after)).toEqual([
+      {
+        type: 'added', category: 'data', name: 'acme_channel', breaking: false,
+        detail: `seed dataset added: 1 record(s); mode insert; ${insertNote}`,
+      },
+      {
+        type: 'modified', category: 'data', name: 'acme_source', breaking: false,
+        detail: `seed dataset changed: records; mode insert; ${insertNote}`,
+      },
+      { type: 'modified', category: 'data', name: 'acme_region', breaking: false, detail: 'seed dataset changed: records; mode ignore' },
+    ]);
+  });
 });