fix(core): Allow model to be set from `settings.json` (#5527)

packages/cli/src/config/config.test.ts

@@ -917,6 +917,68 @@ describe('loadCliConfig extensions', () => {
   });
 });
 
+describe('loadCliConfig model selection', () => {
+  it('selects a model from settings.json if provided', async () => {
+    process.argv = ['node', 'script.js'];
+    const argv = await parseArguments();
+    const config = await loadCliConfig(
+      {
+        model: 'gemini-9001-ultra',
+      },
+      [],
+      'test-session',
+      argv,
+    );
+
+    expect(config.getModel()).toBe('gemini-9001-ultra');
+  });
+
+  it('uses the default gemini model if nothing is set', async () => {
+    process.argv = ['node', 'script.js']; // No model set.
+    const argv = await parseArguments();
+    const config = await loadCliConfig(
+      {
+        // No model set.
+      },
+      [],
+      'test-session',
+      argv,
+    );
+
+    expect(config.getModel()).toBe('gemini-2.5-pro');
+  });
+
+  it('always prefers model from argvs', async () => {
+    process.argv = ['node', 'script.js', '--model', 'gemini-8675309-ultra'];
+    const argv = await parseArguments();
+    const config = await loadCliConfig(
+      {
+        model: 'gemini-9001-ultra',
+      },
+      [],
+      'test-session',
+      argv,
+    );
+
+    expect(config.getModel()).toBe('gemini-8675309-ultra');
+  });
+
+  it('selects the model from argvs if provided', async () => {
+    process.argv = ['node', 'script.js', '--model', 'gemini-8675309-ultra'];
+    const argv = await parseArguments();
+    const config = await loadCliConfig(
+      {
+        // No model provided via settings.
+      },
+      [],
+      'test-session',
+      argv,
+    );
+
+    expect(config.getModel()).toBe('gemini-8675309-ultra');
+  });
+});
+
 describe('loadCliConfig ideModeFeature', () => {
   const originalArgv = process.argv;
   const originalEnv = { ...process.env };

packages/cli/src/config/config.ts

@@ -79,7 +79,7 @@ export async function parseArguments(): Promise<CliArgs> {
       alias: 'm',
       type: 'string',
       description: `Model`,
-      default: process.env.GEMINI_MODEL || DEFAULT_GEMINI_MODEL,
+      default: process.env.GEMINI_MODEL,
     })
     .option('prompt', {
       alias: 'p',
@@ -444,7 +444,7 @@ export async function loadCliConfig(
     cwd: process.cwd(),
     fileDiscoveryService: fileService,
     bugCommand: settings.bugCommand,
-    model: argv.model!,
+    model: argv.model || settings.model || DEFAULT_GEMINI_MODEL,
     extensionContextFilePaths,
     maxSessionTurns: settings.maxSessionTurns ?? -1,
     experimentalAcp: argv.experimentalAcp || false,

packages/cli/src/config/settings.ts

@@ -86,6 +86,8 @@ export interface Settings {
   bugCommand?: BugCommandSettings;
   checkpointing?: CheckpointingSettings;
   autoConfigureMaxOldSpaceSize?: boolean;
+  /** The model name to use (e.g 'gemini-9.0-pro') */
+  model?: string;
 
   // Git-aware file filtering settings
   fileFiltering?: {