diff --git a/src/resources/extensions/mcp-client/index.ts b/src/resources/extensions/mcp-client/index.ts
index 8d4eae552..bdb60c2ed 100644
--- a/src/resources/extensions/mcp-client/index.ts
+++ b/src/resources/extensions/mcp-client/index.ts
@@ -354,7 +354,8 @@ export default function (pi: ExtensionAPI) {
 				description: "Tool name on that server, e.g. 'railway_list_projects'",
 			}),
 			args: Type.Optional(
-				Type.Record(Type.String(), Type.Unknown(), {
+				Type.Object({}, {
+					additionalProperties: true,
 					description:
 						"Tool arguments as key-value pairs matching the tool's input schema",
 				}),
diff --git a/src/tests/mcp-client-schema.test.ts b/src/tests/mcp-client-schema.test.ts
new file mode 100644
index 000000000..a28f07a75
--- /dev/null
+++ b/src/tests/mcp-client-schema.test.ts
@@ -0,0 +1,21 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+import { Type } from "@sinclair/typebox";
+
+test("mcp_call args schema uses additionalProperties instead of patternProperties", () => {
+  const schema = Type.Object({
+    server: Type.String(),
+    tool: Type.String(),
+    args: Type.Optional(
+      Type.Object({}, {
+        additionalProperties: true,
+        description: "Tool arguments as key-value pairs matching the tool's input schema",
+      }),
+    ),
+  });
+
+  const argsSchema = (schema.properties as any).args;
+  assert.equal(argsSchema.type, "object");
+  assert.equal(argsSchema.additionalProperties, true);
+  assert.ok(!("patternProperties" in argsSchema));
+});