fix(cli): validate --model argument at startup (#21393)

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

packages/cli/src/config/config.ts

@@ -31,6 +31,8 @@ import {
   type HierarchicalMemory,
   coreEvents,
   GEMINI_MODEL_ALIAS_AUTO,
+  isValidModelOrAlias,
+  getValidModelsAndAliases,
   getAdminErrorMessage,
   isHeadlessMode,
   Config,
@@ -671,6 +673,18 @@ export async function loadCliConfig(
   const specifiedModel =
     argv.model || process.env['GEMINI_MODEL'] || settings.model?.name;
 
+  // Validate the model if one was explicitly specified
+  if (specifiedModel && specifiedModel !== GEMINI_MODEL_ALIAS_AUTO) {
+    if (!isValidModelOrAlias(specifiedModel)) {
+      const validModels = getValidModelsAndAliases();
+
+      throw new FatalConfigError(
+        `Invalid model: "${specifiedModel}"\n\n` +
+          `Valid models and aliases:\n${validModels.map((m) => `  - ${m}`).join('\n')}\n\n` +
+          `Use /model to switch models interactively.`,
+      );
+    }
+  }
   const resolvedModel =
     specifiedModel === GEMINI_MODEL_ALIAS_AUTO
       ? defaultModel