diff --git a/package-lock.json b/package-lock.json
index b04e4b6..f96825d 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -50,7 +50,7 @@
         "vitest": "^4.0.18"
       },
       "engines": {
-        "node": ">=18.0.0"
+        "node": ">=18.0.0 <26.0.0"
       }
     },
     "node_modules/@ast-grep/lang-bash": {
diff --git a/package.json b/package.json
index 2cf13b1..5d7d91d 100644
--- a/package.json
+++ b/package.json
@@ -101,6 +101,6 @@
     "vitest": "^4.0.18"
   },
   "engines": {
-    "node": ">=18.0.0"
+    "node": ">=18.0.0 <26.0.0"
   }
 }
diff --git a/src/index.ts b/src/index.ts
index ebfdb90..953bf85 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -2,6 +2,34 @@
 // SPDX-License-Identifier: AGPL-3.0-only
 // Copyright (C) 2026 Giancarlo Erra - Altaire Limited
 
+// Pre-flight: refuse to start on Node versions known to break @qdrant/js-client-rest.
+// The qdrant client pins undici ^6 and constructs an undici.Agent it passes to Node's
+// built-in fetch() as a dispatcher. Node 26+ ships a stricter undici whose dispatcher
+// hook validation rejects the v6 Agent's contract — surfaces as
+// `UND_ERR_INVALID_ARG: invalid onError method` on the first qdrant request.
+// (The imports below are evaluated before this check at runtime per ESM semantics,
+// but qdrant-js's module-init is side-effect-light — only an actual request triggers
+// the undici path — so exiting here is enough to spare users the opaque error later.)
+// Tracked upstream: https://github.com/qdrant/qdrant-js/issues/134
+// Upstream PRs under discussion: qdrant/qdrant-js#123 (undici major upgrade) and
+// qdrant/qdrant-js#128 (inject fetch into REST transport). If either lands — or any
+// other fix supersedes them — raise the upper bound in package.json's `engines.node`
+// and remove this check.
+const nodeMajor = Number.parseInt(process.versions.node.split(".")[0], 10);
+if (Number.isFinite(nodeMajor) && nodeMajor >= 26) {
+  // fs.writeSync(2, …) is the canonical Node idiom for "print fatal error then die":
+  // blocking (no truncation when stderr is piped — every MCP host pipes stderr) and
+  // synchronous (so process.exit(1) runs before any further top-level code).
+  const msg =
+    `socraticode: Node ${process.versions.node} is not supported.\n` +
+    "  @qdrant/js-client-rest is incompatible with the undici bundled in Node 26+.\n" +
+    "  Use Node 22.x (via nvm: `nvm install 22 && nvm use 22`, or `brew install node@22` on macOS).\n" +
+    "  See https://github.com/qdrant/qdrant-js/issues/134.\n";
+  writeSync(2, msg);
+  process.exit(1);
+}
+
+import { writeSync } from "node:fs";
 import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
 import { StdioServerTransport } from "@modelcontextprotocol/sdk/server/stdio.js";
 import { z } from "zod";