diff --git a/.changeset/plucky-wolves-run.md b/.changeset/plucky-wolves-run.md
new file mode 100644
index 000000000..ab7ba11f2
--- /dev/null
+++ b/.changeset/plucky-wolves-run.md
@@ -0,0 +1,5 @@
+---
+type: Fixed
+pr: 1664
+---
+**`frontmatter set` on an object-list field now fails closed instead of silently doing nothing** — setting `must_haves` (or another object-list field) to a value whose lossy parse projection matched the original's was a silent no-op: the command reported `{updated:true}` but the change never applied (the writer's scalar-only parser had flattened both to the same shape). `frontmatter set` now detects a no-op write for dict-valued fields and surfaces a clear error directing the user to edit the file directly. Scalars and scalar arrays round-trip faithfully, so idempotent sets of those still report `{updated:true}` (no false positive).
diff --git a/src/frontmatter.cts b/src/frontmatter.cts
index 590ed5414..5d4028736 100644
--- a/src/frontmatter.cts
+++ b/src/frontmatter.cts
@@ -503,10 +503,35 @@ function cmdFrontmatterSet(cwd: string, filePath: string, field: string | undefi
   try { parsedValue = JSON.parse(value as string); } catch { parsedValue = value; }
   fm[field as string] = parsedValue as FrontmatterValue;
   const newContent = spliceFrontmatter(content, fm);
+  // #1660: a no-op set (newContent unchanged) with a dict-valued field means the lossy
+  // frontmatter parser made the new value's projection equal the original's — the change
+  // did not apply (bites object-list fields like must_haves). Detection lives in the pure
+  // exported helper noOpObjectListSetError so the mutation gate (property/unit set) covers
+  // it — the cmd path itself is not in that set.
+  const noOpErr = noOpObjectListSetError(content, newContent, parsedValue);
+  if (noOpErr) {
+    output({ error: noOpErr, field }, raw, undefined);
+    return;
+  }
   platformWriteSync(fullPath, newContent);
   output({ updated: true, field, value: parsedValue }, raw, 'true');
 }
 
+/**
+ * #1660: detect a frontmatter `set` that would be a silent no-op on a dict-valued field.
+ * Returns an error message when the splice produced no content change but the new value
+ * is a dict (object-list fields like must_haves, whose `{path, provides}` items flatten to
+ * scalar strings under extractFrontmatter so a replacement can deep-equal the original's
+ * projection), else null. Scalars and scalar arrays round-trip faithfully, so idempotent
+ * sets of those are intentionally NOT flagged. Pure and unit-tested directly (the cmd path
+ * is not in Stryker's property/unit set, so the detection must be testable in isolation).
+ */
+function noOpObjectListSetError(originalContent: string, newContent: string, parsedValue: unknown): string | null {
+  if (newContent !== originalContent) return null;
+  if (parsedValue === null || typeof parsedValue !== 'object' || Array.isArray(parsedValue)) return null;
+  return 'frontmatter set had no effect — the supplied value is equivalent to the existing field under the frontmatter parser, which cannot faithfully round-trip object-list fields like must_haves. Edit the file directly.';
+}
+
 function cmdFrontmatterMerge(cwd: string, filePath: string, data: string | undefined, raw: boolean): void {
   if (!filePath || !data) { error('file and data required'); }
   const fullPath = path.isAbsolute(filePath) ? filePath : path.join(cwd, filePath);
@@ -543,6 +568,7 @@ export = {
   parseFrontmatter: extractFrontmatter,
   reconstructFrontmatter,
   spliceFrontmatter,
+  noOpObjectListSetError,
   parseMustHavesBlock,
   FRONTMATTER_SCHEMAS,
   cmdFrontmatterGet,
diff --git a/tests/frontmatter-cli.test.cjs b/tests/frontmatter-cli.test.cjs
index a28dea3c5..aed6987ab 100644
--- a/tests/frontmatter-cli.test.cjs
+++ b/tests/frontmatter-cli.test.cjs
@@ -402,3 +402,42 @@ describe('frontmatter set/merge preserves must_haves object-lists (#1572)', () =
     );
   });
 });
+
+// Bug #1660 — frontmatter set of an object-list field (e.g. must_haves) is a silent no-op
+// when the new value's lossy parse projection equals the original's. Folded into the owning
+// frontmatter-cli test (no new top-level bug-NNNN file).
+describe('Bug #1660: frontmatter set of an object-list field fails closed instead of a silent no-op', () => {
+  const PLAN_WITH_MUST_HAVES = [
+    '---', 'phase: 1', 'wave: 1',
+    'must_haves:', '  artifacts:', '    - path: src/foo.ts', '      provides: the foo',
+    '---', '# body', '',
+  ].join('\n');
+
+  test('setting must_haves to a value that flattens to the original projection fails closed (no silent no-op)', () => {
+    const file = writeTempFile(PLAN_WITH_MUST_HAVES);
+    const before = fs.readFileSync(file, 'utf-8');
+    // New value {artifacts:["path: src/foo.ts"]} — its extractFrontmatter projection equals
+    // the original's flattened projection, so the set would otherwise be a silent no-op.
+    const result = runGsdTools(['frontmatter', 'set', file, '--field', 'must_haves', '--value', JSON.stringify({ artifacts: ['path: src/foo.ts'] })]);
+    const parsed = JSON.parse(result.output);
+    assert.ok(parsed.error, 'a no-op set of an object-list field must surface an error, not silent {updated:true}');
+    const after = fs.readFileSync(file, 'utf-8');
+    assert.equal(after, before, 'the file must be unchanged when the set is refused (no silent partial write)');
+  });
+
+  test('an idempotent set of a scalar (wave, same value) still reports updated (no false positive)', () => {
+    const file = writeTempFile('---\nphase: 1\nwave: 1\n---\n# body\n');
+    const result = runGsdTools(['frontmatter', 'set', file, '--field', 'wave', '--value', '1']);
+    const parsed = JSON.parse(result.output);
+    assert.equal(parsed.updated, true, 'an idempotent SCALAR set must still report {updated:true} (not fail-closed)');
+    assert.ok(!parsed.error, 'an idempotent scalar set must not produce an error');
+  });
+
+  test('an idempotent set of a scalar array (tags, same value) still reports updated (no false positive)', () => {
+    const file = writeTempFile('---\nphase: 1\ntags: ["a","b"]\n---\n# body\n');
+    const result = runGsdTools(['frontmatter', 'set', file, '--field', 'tags', '--value', '["a","b"]']);
+    const parsed = JSON.parse(result.output);
+    assert.equal(parsed.updated, true, 'an idempotent scalar-ARRAY set must still report {updated:true} (arrays round-trip; not fail-closed)');
+    assert.ok(!parsed.error, 'an idempotent scalar-array set must not produce an error');
+  });
+});
diff --git a/tests/frontmatter.unit.test.cjs b/tests/frontmatter.unit.test.cjs
index a3f9cff37..14eb0505f 100644
--- a/tests/frontmatter.unit.test.cjs
+++ b/tests/frontmatter.unit.test.cjs
@@ -20,6 +20,7 @@ const {
   extractFrontmatter,
   reconstructFrontmatter,
   spliceFrontmatter,
+  noOpObjectListSetError,
   parseMustHavesBlock,
   FRONTMATTER_SCHEMAS,
 } = require('../gsd-core/bin/lib/frontmatter.cjs');
@@ -1202,3 +1203,29 @@ describe('reconstructFrontmatter: nested subval plain string', () => {
     assert.equal(result, 'meta:\n  tag: "issue#42"');
   });
 });
+
+// noOpObjectListSetError (#1660) — pure detection helper, unit-tested directly because the
+// cmdFrontmatterSet path is not in Stryker's property/unit set.
+describe('noOpObjectListSetError (#1660)', () => {
+  const ORIG = '---\nphase: 1\n---\n';
+  test('changed content (real update) → null', () => {
+    assert.equal(noOpObjectListSetError(ORIG, ORIG + 'x', { must_haves: 1 }), null);
+  });
+  test('scalar value no-op → null (idempotent scalar sets are fine)', () => {
+    for (const v of [1, 'str', true, 0, '']) assert.equal(noOpObjectListSetError(ORIG, ORIG, v), null, `scalar ${JSON.stringify(v)}`);
+  });
+  test('scalar-array value no-op → null (scalar arrays round-trip faithfully)', () => {
+    assert.equal(noOpObjectListSetError(ORIG, ORIG, ['a', 'b']), null);
+    assert.equal(noOpObjectListSetError(ORIG, ORIG, []), null);
+  });
+  test('null value no-op → null', () => {
+    assert.equal(noOpObjectListSetError(ORIG, ORIG, null), null);
+  });
+  test('dict value no-op → error message naming the object-list round-trip limit', () => {
+    const msg = noOpObjectListSetError(ORIG, ORIG, { artifacts: [{ path: 'p' }] });
+    assert.equal(typeof msg, 'string');
+    assert.ok(msg.includes('had no effect'), msg);
+    assert.ok(msg.includes('object-list'), msg);
+    assert.ok(msg.includes('Edit the file directly'), msg);
+  });
+});