fix(mcp): strip null values from tool call arguments before sending

krissding · claude · krissding · commit df236047039b · 2026-06-25T19:24:51.000+08:00
Some MCP servers (e.g. awslabs eks-mcp-server) distinguish between
null and absent for optional parameters. When the LLM passes null for
an optional field, treat it as "not provided" by stripping top-level
null values from arguments before calling the MCP server.

Co-Authored-By: Claude Sonnet 4.6 &lt;noreply@anthropic.com&gt;
diff --git a/src/agents/agent-bundle-mcp-runtime.ts b/src/agents/agent-bundle-mcp-runtime.ts
@@ -37,6 +37,16 @@ import { loadEmbeddedAgentMcpConfig } from "./embedded-agent-mcp.js";
 import { isMcpConfigRecord } from "./mcp-config-shared.js";
 import { resolveMcpTransport } from "./mcp-transport.js";
 
+function stripNullValues(args: Record<string, unknown>): Record<string, unknown> {
+  const result: Record<string, unknown> = {};
+  for (const [key, value] of Object.entries(args)) {
+    if (value !== null) {
+      result[key] = value;
+    }
+  }
+  return result;
+}
+
 type BundleMcpSession = {
   serverName: string;
   client: Client;
@@ -902,7 +912,7 @@ export function createSessionMcpRuntime(params: {
           (await session.client.callTool(
             {
               name: toolName,
-              arguments: isMcpConfigRecord(input) ? input : {},
+              arguments: isMcpConfigRecord(input) ? stripNullValues(input) : {},
             },
             undefined,
             { timeout: session.requestTimeoutMs },
