diff --git a/src/commands/config.ts b/src/commands/config.ts
index 5811aa7..21f6c29 100644
--- a/src/commands/config.ts
+++ b/src/commands/config.ts
@@ -101,11 +101,16 @@ function applyProviderChange(config: Config, provider: Config['provider']): Conf
 }
 
 function updateConfigFromRawValue(config: Config, key: ConfigSetKey, rawValue: string): Config {
-  if (key === 'provider') {
-    return applyProviderChange(config, parseConfigSetValue('provider', rawValue));
+  const updatedConfig =
+    key === 'provider'
+      ? applyProviderChange(config, parseConfigSetValue('provider', rawValue))
+      : updateConfigField(config, key, parseConfigSetValue(key, rawValue));
+
+  if (updatedConfig.provider === CUSTOM_PROVIDER_KEY && !updatedConfig.baseUrl) {
+    throw new Error('Custom provider requires a configured baseUrl. Set baseUrl before selecting __custom__.');
   }
 
-  return updateConfigField(config, key, parseConfigSetValue(key, rawValue));
+  return updatedConfig;
 }
 
 /** Masks a stored API key while leaving enough prefix to identify which key is configured. */
diff --git a/tests/config-command.test.mjs b/tests/config-command.test.mjs
index 30ac976..d2adc37 100644
--- a/tests/config-command.test.mjs
+++ b/tests/config-command.test.mjs
@@ -332,6 +332,22 @@ test('config set clears stale baseUrl when switching between built-in providers'
   });
 });
 
+test('config set requires a baseUrl before switching to the custom provider', async () => {
+  await withTempHome(async (homeDir) => {
+    writeConfig(homeDir, { baseUrl: undefined, provider: 'openai' });
+
+    await assert.rejects(
+      () => runConfigWithArgs(homeDir, ['set', 'provider', '__custom__']),
+      (error) => {
+        assert.equal(error.code, 1);
+        assert.match(error.stdout + error.stderr, /Custom provider requires a configured baseUrl/);
+        assert.equal(readConfig(homeDir).provider, 'openai');
+        return true;
+      },
+    );
+  });
+});
+
 test('config set preserves baseUrl when switching to custom provider', async () => {
   await withTempHome(async (homeDir) => {
     writeConfig(homeDir, {