4 files changed, 30 insertions, 10 deletions
diff --git a/packages/core/src/tools/mcp-client.ts b/packages/core/src/tools/mcp-client.ts
index beb70549..24c975c3 100644
--- a/packages/core/src/tools/mcp-client.ts
+++ b/packages/core/src/tools/mcp-client.ts
@@ -18,8 +18,9 @@ import {
 import { parse } from 'shell-quote';
 import { MCPServerConfig } from '../config/config.js';
 import { DiscoveredMCPTool } from './mcp-tool.js';
-import { FunctionDeclaration, Type, mcpToTool } from '@google/genai';
-import { sanitizeParameters, ToolRegistry } from './tool-registry.js';
+
+import { FunctionDeclaration, mcpToTool } from '@google/genai';
+import { ToolRegistry } from './tool-registry.js';
 import {
   ActiveFileNotificationSchema,
   IDE_SERVER_NAME,
@@ -275,15 +276,13 @@ export async function discoverTools(
 
       const toolNameForModel = generateValidName(funcDecl, mcpServerName);
 
-      sanitizeParameters(funcDecl.parameters);
-
       discoveredTools.push(
         new DiscoveredMCPTool(
           mcpCallableTool,
           mcpServerName,
           toolNameForModel,
           funcDecl.description ?? '',
-          funcDecl.parameters ?? { type: Type.OBJECT, properties: {} },
+          funcDecl.parametersJsonSchema ?? { type: 'object', properties: {} },
           funcDecl.name!,
           mcpServerConfig.timeout ?? MCP_DEFAULT_TIMEOUT_MSEC,
           mcpServerConfig.trust,
diff --git a/packages/core/src/tools/mcp-tool.test.ts b/packages/core/src/tools/mcp-tool.test.ts
index d972efdb..4411e674 100644
--- a/packages/core/src/tools/mcp-tool.test.ts
+++ b/packages/core/src/tools/mcp-tool.test.ts
@@ -65,7 +65,8 @@ describe('DiscoveredMCPTool', () => {
       expect(tool.name).toBe(toolNameForModel);
       expect(tool.schema.name).toBe(toolNameForModel);
       expect(tool.schema.description).toBe(baseDescription);
-      expect(tool.schema.parameters).toEqual(inputSchema);
+      expect(tool.schema.parameters).toBeUndefined();
+      expect(tool.schema.parametersJsonSchema).toEqual(inputSchema);
       expect(tool.serverToolName).toBe(serverToolName);
       expect(tool.timeout).toBeUndefined();
     });
@@ -81,6 +82,8 @@ describe('DiscoveredMCPTool', () => {
         serverToolName,
       );
       expect(tool.schema.description).toBe(baseDescription);
+      expect(tool.schema.parameters).toBeUndefined();
+      expect(tool.schema.parametersJsonSchema).toEqual(inputSchema);
     });
 
     it('should accept and store a custom timeout', () => {
diff --git a/packages/core/src/tools/mcp-tool.ts b/packages/core/src/tools/mcp-tool.ts
index cc4739a4..663ec6ee 100644
--- a/packages/core/src/tools/mcp-tool.ts
+++ b/packages/core/src/tools/mcp-tool.ts
@@ -11,7 +11,13 @@ import {
   ToolConfirmationOutcome,
   ToolMcpConfirmationDetails,
 } from './tools.js';
-import { CallableTool, Part, FunctionCall, Schema } from '@google/genai';
+import {
+  CallableTool,
+  Part,
+  FunctionCall,
+  FunctionDeclaration,
+  Type,
+} from '@google/genai';
 
 type ToolParams = Record<string, unknown>;
 
@@ -23,7 +29,7 @@ export class DiscoveredMCPTool extends BaseTool<ToolParams, ToolResult> {
     readonly serverName: string,
     readonly name: string,
     readonly description: string,
-    readonly parameterSchema: Schema,
+    readonly parameterSchemaJson: unknown,
     readonly serverToolName: string,
     readonly timeout?: number,
     readonly trust?: boolean,
@@ -32,12 +38,24 @@ export class DiscoveredMCPTool extends BaseTool<ToolParams, ToolResult> {
       name,
       `${serverToolName} (${serverName} MCP Server)`,
       description,
-      parameterSchema,
+      { type: Type.OBJECT }, // this is a dummy Schema for MCP, will be not be used to construct the FunctionDeclaration
       true, // isOutputMarkdown
       false, // canUpdateOutput
     );
   }
 
+  /**
+   * Overrides the base schema to use parametersJsonSchema when building
+   * FunctionDeclaration
+   */
+  override get schema(): FunctionDeclaration {
+    return {
+      name: this.name,
+      description: this.description,
+      parametersJsonSchema: this.parameterSchemaJson,
+    };
+  }
+
   async shouldConfirmExecute(
     _params: ToolParams,
     _abortSignal: AbortSignal,
diff --git a/packages/core/src/tools/tools.ts b/packages/core/src/tools/tools.ts
index 68739d0e..6f6d3f58 100644
--- a/packages/core/src/tools/tools.ts
+++ b/packages/core/src/tools/tools.ts
@@ -97,7 +97,7 @@ export abstract class BaseTool<
    * @param description Description of what the tool does
    * @param isOutputMarkdown Whether the tool's output should be rendered as markdown
    * @param canUpdateOutput Whether the tool supports live (streaming) output
-   * @param parameterSchema JSON Schema defining the parameters
+   * @param parameterSchema Open API 3.0 Schema defining the parameters
    */
   constructor(
     readonly name: string,