diff --git a/packages/cli/src/config/config.test.ts b/packages/cli/src/config/config.test.ts
index 422f6cd2ac..995be3fc61 100644
--- a/packages/cli/src/config/config.test.ts
+++ b/packages/cli/src/config/config.test.ts
@@ -1773,7 +1773,7 @@ describe('loadCliConfig model selection', () => {
   });
 
   it('always prefers model from argv', async () => {
-    process.argv = ['node', 'script.js', '--model', 'gemini-2.5-flash-preview'];
+    process.argv = ['node', 'script.js', '--model', 'gemini-2.5-flash'];
     const argv = await parseArguments(createTestMergedSettings());
     const config = await loadCliConfig(
       createTestMergedSettings({
@@ -1785,11 +1785,11 @@ describe('loadCliConfig model selection', () => {
       argv,
     );
 
-    expect(config.getModel()).toBe('gemini-2.5-flash-preview');
+    expect(config.getModel()).toBe('gemini-2.5-flash');
   });
 
   it('selects the model from argv if provided', async () => {
-    process.argv = ['node', 'script.js', '--model', 'gemini-2.5-flash-preview'];
+    process.argv = ['node', 'script.js', '--model', 'gemini-2.5-flash'];
     const argv = await parseArguments(createTestMergedSettings());
     const config = await loadCliConfig(
       createTestMergedSettings({
@@ -1799,7 +1799,7 @@ describe('loadCliConfig model selection', () => {
       argv,
     );
 
-    expect(config.getModel()).toBe('gemini-2.5-flash-preview');
+    expect(config.getModel()).toBe('gemini-2.5-flash');
   });
 
   it('selects the default auto model if provided via auto alias', async () => {
diff --git a/packages/cli/src/config/config.ts b/packages/cli/src/config/config.ts
index 010fb8e17f..e910d47546 100755
--- a/packages/cli/src/config/config.ts
+++ b/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
diff --git a/packages/core/src/config/models.test.ts b/packages/core/src/config/models.test.ts
index d62827ed91..b3f5db9430 100644
--- a/packages/core/src/config/models.test.ts
+++ b/packages/core/src/config/models.test.ts
@@ -22,6 +22,7 @@ import {
   GEMINI_MODEL_ALIAS_PRO,
   GEMINI_MODEL_ALIAS_FLASH,
   GEMINI_MODEL_ALIAS_AUTO,
+  GEMINI_MODEL_ALIAS_FLASH_LITE,
   PREVIEW_GEMINI_FLASH_MODEL,
   PREVIEW_GEMINI_MODEL_AUTO,
   DEFAULT_GEMINI_MODEL_AUTO,
@@ -30,6 +31,10 @@ import {
   PREVIEW_GEMINI_3_1_CUSTOM_TOOLS_MODEL,
   isPreviewModel,
   isProModel,
+  isValidModelOrAlias,
+  getValidModelsAndAliases,
+  VALID_GEMINI_MODELS,
+  VALID_ALIASES,
 } from './models.js';
 
 describe('isPreviewModel', () => {
@@ -389,3 +394,62 @@ describe('isActiveModel', () => {
     ).toBe(false);
   });
 });
+
+describe('isValidModelOrAlias', () => {
+  it('should return true for valid model names', () => {
+    expect(isValidModelOrAlias(DEFAULT_GEMINI_MODEL)).toBe(true);
+    expect(isValidModelOrAlias(PREVIEW_GEMINI_MODEL)).toBe(true);
+    expect(isValidModelOrAlias(DEFAULT_GEMINI_FLASH_MODEL)).toBe(true);
+    expect(isValidModelOrAlias(DEFAULT_GEMINI_FLASH_LITE_MODEL)).toBe(true);
+    expect(isValidModelOrAlias(PREVIEW_GEMINI_FLASH_MODEL)).toBe(true);
+    expect(isValidModelOrAlias(PREVIEW_GEMINI_3_1_MODEL)).toBe(true);
+    expect(isValidModelOrAlias(PREVIEW_GEMINI_3_1_CUSTOM_TOOLS_MODEL)).toBe(
+      true,
+    );
+  });
+
+  it('should return true for valid aliases', () => {
+    expect(isValidModelOrAlias(GEMINI_MODEL_ALIAS_AUTO)).toBe(true);
+    expect(isValidModelOrAlias(GEMINI_MODEL_ALIAS_PRO)).toBe(true);
+    expect(isValidModelOrAlias(GEMINI_MODEL_ALIAS_FLASH)).toBe(true);
+    expect(isValidModelOrAlias(GEMINI_MODEL_ALIAS_FLASH_LITE)).toBe(true);
+    expect(isValidModelOrAlias(PREVIEW_GEMINI_MODEL_AUTO)).toBe(true);
+    expect(isValidModelOrAlias(DEFAULT_GEMINI_MODEL_AUTO)).toBe(true);
+  });
+
+  it('should return true for custom (non-gemini) models', () => {
+    expect(isValidModelOrAlias('gpt-4')).toBe(true);
+    expect(isValidModelOrAlias('claude-3')).toBe(true);
+    expect(isValidModelOrAlias('my-custom-model')).toBe(true);
+  });
+
+  it('should return false for invalid gemini model names', () => {
+    expect(isValidModelOrAlias('gemini-4-pro')).toBe(false);
+    expect(isValidModelOrAlias('gemini-99-flash')).toBe(false);
+    expect(isValidModelOrAlias('gemini-invalid')).toBe(false);
+  });
+});
+
+describe('getValidModelsAndAliases', () => {
+  it('should return a sorted array', () => {
+    const result = getValidModelsAndAliases();
+    const sorted = [...result].sort();
+    expect(result).toEqual(sorted);
+  });
+
+  it('should include all valid models and aliases', () => {
+    const result = getValidModelsAndAliases();
+    for (const model of VALID_GEMINI_MODELS) {
+      expect(result).toContain(model);
+    }
+    for (const alias of VALID_ALIASES) {
+      expect(result).toContain(alias);
+    }
+  });
+
+  it('should not contain duplicates', () => {
+    const result = getValidModelsAndAliases();
+    const unique = [...new Set(result)];
+    expect(result).toEqual(unique);
+  });
+});
diff --git a/packages/core/src/config/models.ts b/packages/core/src/config/models.ts
index ffbf597793..59e7e4b457 100644
--- a/packages/core/src/config/models.ts
+++ b/packages/core/src/config/models.ts
@@ -32,6 +32,15 @@ export const GEMINI_MODEL_ALIAS_PRO = 'pro';
 export const GEMINI_MODEL_ALIAS_FLASH = 'flash';
 export const GEMINI_MODEL_ALIAS_FLASH_LITE = 'flash-lite';
 
+export const VALID_ALIASES = new Set([
+  GEMINI_MODEL_ALIAS_AUTO,
+  GEMINI_MODEL_ALIAS_PRO,
+  GEMINI_MODEL_ALIAS_FLASH,
+  GEMINI_MODEL_ALIAS_FLASH_LITE,
+  PREVIEW_GEMINI_MODEL_AUTO,
+  DEFAULT_GEMINI_MODEL_AUTO,
+]);
+
 export const DEFAULT_GEMINI_EMBEDDING_MODEL = 'gemini-embedding-001';
 
 // Cap the thinking at 8192 to prevent run-away thinking loops.
@@ -283,3 +292,37 @@ export function isActiveModel(
     );
   }
 }
+
+/**
+ * Checks if the model name is valid (either a valid model or a valid alias).
+ *
+ * @param model The model name to check.
+ * @returns True if the model is valid.
+ */
+export function isValidModelOrAlias(model: string): boolean {
+  // Check if it's a valid alias
+  if (VALID_ALIASES.has(model)) {
+    return true;
+  }
+
+  // Check if it's a valid model name
+  if (VALID_GEMINI_MODELS.has(model)) {
+    return true;
+  }
+
+  // Allow custom models (non-gemini models)
+  if (!model.startsWith('gemini-')) {
+    return true;
+  }
+
+  return false;
+}
+
+/**
+ * Gets a list of all valid model names and aliases for error messages.
+ *
+ * @returns Array of valid model names and aliases.
+ */
+export function getValidModelsAndAliases(): string[] {
+  return [...new Set([...VALID_ALIASES, ...VALID_GEMINI_MODELS])].sort();
+}