fix: check stderr for json errors in TS -> Go call

src/lib/plugins/uv/index.ts

@@ -123,14 +123,16 @@ function createDepgraphError(detail: string): CustomError {
 }
 
 function extractErrorDetail(result: GoCommandResult): string {
-  if (result.stdout) {
-    try {
-      const parsed = JSON.parse(result.stdout);
-      if (parsed.error) {
-        return parsed.error;
+  for (const output of [result.stderr, result.stdout]) {
+    if (output) {
+      try {
+        const parsed = JSON.parse(output);
+        if (parsed.error) {
+          return parsed.error;
+        }
+      } catch {
+        // not valid JSON; try next source, or fall back to returning plain stderr
       }
-    } catch {
-      // stdout wasn't valid JSON; fall through to stderr
     }
   }
   return result.stderr || 'Unable to process dependency information';

src/lib/plugins/uv/uv.spec.ts

@@ -227,6 +227,26 @@ describe('uv plugin', () => {
     expect(err.message).toBe('something went wrong');
   });
 
+  it('extracts error message from JSON in stderr when stdout is empty', async () => {
+    const stderrJson = JSON.stringify({
+      ok: false,
+      error:
+        'uv.lock is out of sync with pyproject.toml. Run `uv lock` to update the lockfile.',
+      path: '/test',
+    });
+    execGoCommandSpy.mockResolvedValueOnce(mockResult('', 2, stderrJson));
+
+    const err: CustomError = await inspect('.', 'uv.lock').catch((e) => e);
+
+    expect(err).toBeInstanceOf(CustomError);
+    expect(err.userMessage).toBe(
+      'uv.lock is out of sync with pyproject.toml. Run `uv lock` to update the lockfile.',
+    );
+    expect(err.message).toBe(
+      'uv.lock is out of sync with pyproject.toml. Run `uv lock` to update the lockfile.',
+    );
+  });
+
   it('attaches an error catalog entry for IPC propagation', async () => {
     const jsonError = JSON.stringify({
       ok: false,