diff --git a/src/asap/server.c b/src/asap/server.c
index d681f9b..fa39b0c 100644
--- a/src/asap/server.c
+++ b/src/asap/server.c
@@ -282,11 +282,14 @@ static int handle_mcp_tool_call(const asap_envelope_t *in, asap_envelope_t *out,
 		return -32603;
 	}
 	result_buf[0] = '\0';
+	/* Same mutex discipline as task.request: tools touch shared session/memory. */
+	agent_lock();
 	if (ctx->tool_call_hook) {
 		int hr = ctx->tool_call_hook(ctx, tool_name, args_json, result_buf, RESULT_CAP);
 		exec_rc = hr == 0 ? 0 : 2;
 	} else
 		exec_rc = dispatch_tool_by_name(ctx, tool_name, args_json, result_buf, RESULT_CAP);
+	agent_unlock();
 	free(args_json);
 	if (exec_rc == 1) {
 		free(result_buf);