fix: parse JSON-string MCP tool arguments before type check

src/core/tools/UseMcpToolTool.ts

@@ -111,7 +111,14 @@ export class UseMcpToolTool extends BaseTool<"use_mcp_tool"> {
 			return { isValid: false }
 		}
 
-		// Native-only: arguments are already a structured object.
+		// Some LLMs emit arguments as JSON-encoded strings rather than objects.
+		// Parse them early so the type check below sees the unwrapped object.
+		if (typeof params.arguments === "string") {
+			try {
+				params.arguments = JSON.parse(params.arguments)
+			} catch {}
+		}
+
 		let parsedArguments: Record<string, unknown> | undefined
 		if (params.arguments !== undefined) {
 			if (typeof params.arguments !== "object" || params.arguments === null || Array.isArray(params.arguments)) {

src/core/tools/__tests__/useMcpToolTool.spec.ts

@@ -254,6 +254,53 @@ describe("useMcpToolTool", () => {
 			expect(mockPushToolResult).toHaveBeenCalledWith("Tool result: Tool executed successfully")
 		})
 
+		it("should parse JSON-string arguments and pass parsed object to callTool", async () => {
+			const callToolMock = vi.fn().mockResolvedValue({
+				content: [{ type: "text", text: "Browser session started" }],
+				isError: false,
+			})
+
+			mockProviderRef.deref.mockReturnValue({
+				getMcpHub: () => ({
+					callTool: callToolMock,
+					getAllServers: vi.fn().mockReturnValue([
+						{ name: "test_server", tools: [{ name: "test_tool", description: "Test Tool" }] },
+					]),
+				}),
+				postMessageToWebview: vi.fn(),
+			})
+
+			const block: ToolUse = {
+				type: "tool_use",
+				name: "use_mcp_tool",
+				params: {
+					server_name: "test_server",
+					tool_name: "test_tool",
+					arguments: '{"headless": true}',
+				},
+				nativeArgs: {
+					server_name: "test_server",
+					tool_name: "test_tool",
+					arguments: '{"headless": true}' as unknown as Record<string, unknown>,
+				},
+				partial: false,
+			}
+
+			mockAskApproval.mockResolvedValue(true)
+
+			await useMcpToolTool.handle(mockTask as Task, block as any, {
+				askApproval: mockAskApproval,
+				handleError: mockHandleError,
+				pushToolResult: mockPushToolResult,
+			})
+
+			expect(mockTask.consecutiveMistakeCount).toBe(0)
+			expect(mockTask.recordToolError).not.toHaveBeenCalled()
+			expect(callToolMock).toHaveBeenCalledWith("test_server", "test_tool", { headless: true })
+			expect(mockTask.say).toHaveBeenCalledWith("mcp_server_request_started")
+			expect(mockTask.say).toHaveBeenCalledWith("mcp_server_response", "Browser session started", [])
+		})
+
 		it("should handle user rejection", async () => {
 			const block: ToolUse = {
 				type: "tool_use",