Load the MCP client SDK lazily on first outbound connection (#1747)

Author: RhysSullivan

.changeset/lazy-mcp-client-module.md

@@ -0,0 +1,5 @@
+---
+"@executor-js/plugin-mcp": patch
+---
+
+Load the MCP client SDK lazily on first outbound connection instead of at module evaluation. Runtimes that bundle the plugin (notably Cloudflare Workers) no longer pay the client package's module-eval memory and CPU on startup or on code paths that never dial an MCP server.

packages/plugins/mcp/package.json

@@ -58,7 +58,8 @@
     "typecheck": "tsgo --noEmit",
     "test": "vitest run",
     "test:watch": "vitest",
-    "typecheck:slow": "bunx tsc --noEmit -p tsconfig.json"
+    "typecheck:slow": "bunx tsc --noEmit -p tsconfig.json",
+    "gen:call-tool-result-schema": "bun scripts/gen-call-tool-result-schema.ts"
   },
   "dependencies": {
     "@cfworker/json-schema": "^4.1.1",

packages/plugins/mcp/scripts/gen-call-tool-result-schema.ts

@@ -0,0 +1,26 @@
+// Regenerates src/sdk/call-tool-result-schema.gen.ts from the installed
+// @modelcontextprotocol/core. The schema is baked into a generated file so
+// the runtime never imports the core package (importing it costs ~8.6MB of
+// heap per Cloudflare isolate; see client-module.ts). Run after bumping the
+// core dependency:
+//   bun scripts/gen-call-tool-result-schema.ts
+// call-tool-result-schema.test.ts fails if the generated copy drifts from the
+// installed package.
+import { writeFileSync } from "node:fs";
+import { resolve } from "node:path";
+
+import { CallToolResultSchema } from "@modelcontextprotocol/core";
+
+const schema = CallToolResultSchema.toJSONSchema();
+const target = resolve(import.meta.dir, "../src/sdk/call-tool-result-schema.gen.ts");
+
+writeFileSync(
+  target,
+  `// Generated by scripts/gen-call-tool-result-schema.ts — do not edit.
+// JSON Schema of @modelcontextprotocol/core's CallToolResultSchema, baked in
+// so the runtime never pays the core package's module-eval cost (see
+// client-module.ts). call-tool-result-schema.test.ts guards drift.
+export const callToolResultJsonSchema = ${JSON.stringify(schema, null, 2)} as const;
+`,
+);
+console.log(`wrote ${target}`);