diff --git a/cli/CHANGELOG.md b/cli/CHANGELOG.md
index 14483448..60dd0f8a 100644
--- a/cli/CHANGELOG.md
+++ b/cli/CHANGELOG.md
@@ -1,6 +1,7 @@
 # Mops CLI Changelog
 
 ## Next
+- Fix `mops check --fix` crashing with `TypeError: Cannot read properties of undefined (reading 'split')` when `moc`'s diagnostic run resolves without buffered stdout. `parseDiagnostics` now treats a missing/`undefined` stdout as "no diagnostics" instead of calling `.split` on it, so the autofix pass degrades gracefully and the authoritative `mops check` still runs. Previously this unhandled throw aborted the whole command (and any deploy pipeline invoking `mops check --fix`).
 - Fix `mops check --fix` and `mops lint --fix` corrupting source files when two `mops` processes run concurrently in the same project (e.g. two coding agents on the same checkout). Concurrent runs could apply stale `moc` byte offsets to a sibling's already-mutated file, leaving source like `let nat = identity` (with the type-arg and call dropped) or `list.sortInPlace(` with an unclosed paren. `--fix` invocations now acquire a project-root advisory lock at `.mops/fix.lock` and serialize, cargo-style ("Waiting for another `mops --fix` run to finish..."). Read-only `mops check` and `mops lint` are unchanged.
 
 - Deprecate the `dfx` replica in `mops bench`, `mops test --mode replica`, and `mops watch`. Behavior is unchanged — `--replica dfx`, the implicit `dfx` fallback when no `[toolchain.pocket-ic]` is set, and the dfx-bundled PocketIC fallback all still work — but each now prints a warning. Run `mops toolchain use pocket-ic <version>` to silence it. The `dfx` paths will be removed and the default flipped to PocketIC in mops v3 — `dfx` is being deprecated upstream and PocketIC is a better fit for benchmarks and replica tests (deterministic, in-process, no background daemon).
diff --git a/cli/helpers/autofix-motoko.ts b/cli/helpers/autofix-motoko.ts
index ee6992d1..7fbb6aee 100644
--- a/cli/helpers/autofix-motoko.ts
+++ b/cli/helpers/autofix-motoko.ts
@@ -26,7 +26,10 @@ export interface MocDiagnostic {
   notes: string[];
 }
 
-export function parseDiagnostics(stdout: string): MocDiagnostic[] {
+export function parseDiagnostics(stdout: string | undefined): MocDiagnostic[] {
+  if (!stdout) {
+    return [];
+  }
   return stdout
     .split("\n")
     .filter((l) => l.trim())
diff --git a/cli/tests/check-fix.test.ts b/cli/tests/check-fix.test.ts
index f0ad6e27..e4793c90 100644
--- a/cli/tests/check-fix.test.ts
+++ b/cli/tests/check-fix.test.ts
@@ -14,6 +14,30 @@ function countCodes(stdout: string): Record<string, number> {
   return counts;
 }
 
+describe("parseDiagnostics", () => {
+  test("returns [] for undefined stdout", () => {
+    // execa can resolve with `result.stdout === undefined` (e.g. output not
+    // buffered). The autofix path passes it straight in, so this must not throw.
+    expect(parseDiagnostics(undefined)).toEqual([]);
+  });
+
+  test("returns [] for empty stdout", () => {
+    expect(parseDiagnostics("")).toEqual([]);
+  });
+
+  test("skips non-JSON lines and parses diagnostics", () => {
+    const diag = {
+      message: "m",
+      code: "M0223",
+      level: "warning",
+      spans: [],
+      notes: [],
+    };
+    const stdout = `not json\n${JSON.stringify(diag)}\n`;
+    expect(parseDiagnostics(stdout)).toEqual([diag]);
+  });
+});
+
 describe("check --fix", () => {
   const fixDir = path.join(import.meta.dirname, "check/fix");
   const runDir = path.join(fixDir, "run");