diff --git a/dat-gateway/src/main/resources/static/tester.html b/dat-gateway/src/main/resources/static/tester.html
index fbb6eade..fa469e2f 100644
--- a/dat-gateway/src/main/resources/static/tester.html
+++ b/dat-gateway/src/main/resources/static/tester.html
@@ -513,8 +513,24 @@
             }
         }
         
-        async function executeRawCall(name, payload) {
-            const reqPayload = { jsonrpc: "2.0", method: "tools/call", params: { name: name, arguments: payload }, id: Date.now() };
+async function executeRawCall(name, payload) {
+    // Auto-coerce payload types based on schema to prevent JSON validation errors
+    const tool = allTools.find(t => t.name === name);
+    if (tool && tool.inputSchema && tool.inputSchema.properties) {
+        const props = tool.inputSchema.properties;
+        for (const key in payload) {
+            if (props[key]) {
+                const type = props[key].type;
+                if ((type === 'integer' || type === 'number') && typeof payload[key] === 'string') {
+                    const parsed = Number(payload[key]);
+                    if (!isNaN(parsed)) payload[key] = parsed;
+                } else if (type === 'boolean' && typeof payload[key] === 'string') {
+                    payload[key] = payload[key].toLowerCase() === 'true';
+                }
+            }
+        }
+    }
+    const reqPayload = { jsonrpc: "2.0", method: "tools/call", params: { name: name, arguments: payload }, id: Date.now() };
             const startTime = Date.now();
             try {
                 const response = await fetch('/mcp/api/v1/tools/call', {