diff --git a/packages/google/src/google-prepare-tools.test.ts b/packages/google/src/google-prepare-tools.test.ts
index 0a5094196021..01876d5e2d35 100644
--- a/packages/google/src/google-prepare-tools.test.ts
+++ b/packages/google/src/google-prepare-tools.test.ts
@@ -472,3 +472,173 @@ it('should add warnings for google maps on unsupported models', () => {
     ]
   `);
 });
+
+it('should throw error for invalid tool name starting with number', () => {
+  expect(() =>
+    prepareTools({
+      tools: [
+        {
+          type: 'function',
+          name: '123invalid',
+          description: 'Invalid name',
+          inputSchema: { type: 'object', properties: {} },
+        },
+      ],
+      modelId: 'gemini-2.5-flash',
+    }),
+  ).toThrowError(/Invalid tool name/);
+});
+
+it('should accept valid tool name starting with underscore', () => {
+  const result = prepareTools({
+    tools: [
+      {
+        type: 'function',
+        name: '_validName',
+        description: 'Valid name starting with underscore',
+        inputSchema: { type: 'object', properties: {} },
+      },
+    ],
+    modelId: 'gemini-2.5-flash',
+  });
+  expect(result.tools).toEqual([
+    {
+      functionDeclarations: [
+        {
+          name: '_validName',
+          description: 'Valid name starting with underscore',
+          parameters: undefined,
+        },
+      ],
+    },
+  ]);
+  expect(result.toolWarnings).toEqual([]);
+});
+
+it('should accept valid tool names with allowed special characters', () => {
+  const result = prepareTools({
+    tools: [
+      {
+        type: 'function',
+        name: 'tool.name',
+        description: 'Name with dot',
+        inputSchema: { type: 'object', properties: {} },
+      },
+      {
+        type: 'function',
+        name: 'tool:name',
+        description: 'Name with colon',
+        inputSchema: { type: 'object', properties: {} },
+      },
+      {
+        type: 'function',
+        name: 'tool-name',
+        description: 'Name with dash',
+        inputSchema: { type: 'object', properties: {} },
+      },
+    ],
+    modelId: 'gemini-2.5-flash',
+  });
+  expect(result.tools).toEqual([
+    {
+      functionDeclarations: [
+        {
+          name: 'tool.name',
+          description: 'Name with dot',
+          parameters: undefined,
+        },
+        {
+          name: 'tool:name',
+          description: 'Name with colon',
+          parameters: undefined,
+        },
+        {
+          name: 'tool-name',
+          description: 'Name with dash',
+          parameters: undefined,
+        },
+      ],
+    },
+  ]);
+  expect(result.toolWarnings).toEqual([]);
+});
+
+it('should throw error for tool name starting with dash', () => {
+  expect(() =>
+    prepareTools({
+      tools: [
+        {
+          type: 'function',
+          name: '-invalid',
+          description: 'Invalid name',
+          inputSchema: { type: 'object', properties: {} },
+        },
+      ],
+      modelId: 'gemini-2.5-flash',
+    }),
+  ).toThrowError(/Invalid tool name/);
+});
+
+it('should throw error for tool name starting with dot', () => {
+  expect(() =>
+    prepareTools({
+      tools: [
+        {
+          type: 'function',
+          name: '.invalid',
+          description: 'Invalid name',
+          inputSchema: { type: 'object', properties: {} },
+        },
+      ],
+      modelId: 'gemini-2.5-flash',
+    }),
+  ).toThrowError(/Invalid tool name/);
+});
+
+it('should throw error for tool name starting with colon', () => {
+  expect(() =>
+    prepareTools({
+      tools: [
+        {
+          type: 'function',
+          name: ':invalid',
+          description: 'Invalid name',
+          inputSchema: { type: 'object', properties: {} },
+        },
+      ],
+      modelId: 'gemini-2.5-flash',
+    }),
+  ).toThrowError(/Invalid tool name/);
+});
+
+it('should throw error for tool name with @ symbol', () => {
+  expect(() =>
+    prepareTools({
+      tools: [
+        {
+          type: 'function',
+          name: 'tool@name',
+          description: 'Invalid name',
+          inputSchema: { type: 'object', properties: {} },
+        },
+      ],
+      modelId: 'gemini-2.5-flash',
+    }),
+  ).toThrowError(/Invalid tool name/);
+});
+
+it('should throw error for tool name with # symbol', () => {
+  expect(() =>
+    prepareTools({
+      tools: [
+        {
+          type: 'function',
+          name: 'tool#name',
+          description: 'Invalid name',
+          inputSchema: { type: 'object', properties: {} },
+        },
+      ],
+      modelId: 'gemini-2.5-flash',
+    }),
+  ).toThrowError(/Invalid tool name/);
+});
diff --git a/packages/google/src/google-prepare-tools.ts b/packages/google/src/google-prepare-tools.ts
index a94f1006821e..1c7ffda02104 100644
--- a/packages/google/src/google-prepare-tools.ts
+++ b/packages/google/src/google-prepare-tools.ts
@@ -16,25 +16,25 @@ export function prepareTools({
   modelId: GoogleGenerativeAIModelId;
 }): {
   tools:
-    | Array<
-        | {
-            functionDeclarations: Array<{
-              name: string;
-              description: string;
-              parameters: unknown;
-            }>;
-          }
-        | Record<string, any>
-      >
-    | undefined;
-  toolConfig:
-    | undefined
+  | Array<
     | {
-        functionCallingConfig: {
-          mode: 'AUTO' | 'NONE' | 'ANY';
-          allowedFunctionNames?: string[];
-        };
-      };
+      functionDeclarations: Array<{
+        name: string;
+        description: string;
+        parameters: unknown;
+      }>;
+    }
+    | Record<string, any>
+  >
+  | undefined;
+  toolConfig:
+  | undefined
+  | {
+    functionCallingConfig: {
+      mode: 'AUTO' | 'NONE' | 'ANY';
+      allowedFunctionNames?: string[];
+    };
+  };
   toolWarnings: SharedV3Warning[];
 } {
   // when the tools array is empty, change it to undefined to prevent errors:
@@ -199,6 +199,12 @@ export function prepareTools({
   for (const tool of tools) {
     switch (tool.type) {
       case 'function':
+        if (!/^[a-zA-Z_][a-zA-Z0-9_.:-]*$/.test(tool.name)) {
+          throw new Error(
+            `Invalid tool name: ${tool.name}. Tool names must start with a letter or underscore and contain only alphanumeric characters, underscores, dots, colons, or dashes.`,
+          );
+        }
+
         functionDeclarations.push({
           name: tool.name,
           description: tool.description ?? '',