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McpServer.registerTool rejects tool names inherited from Object.prototype (constructor, toString, __proto__) #3019
Description
McpServer stores its tools in a plain object (private _registeredTools: { [name: string]: RegisteredTool } = {}, mcp.ts:124). The duplicate check in registerTool (mcp.ts:1102) and the lookup in tools/call (mcp.ts:297) index that object by the tool name. A name that is inherited from Object.prototype therefore finds the inherited member, and registerTool throws Tool <name> is already registered on an empty server.
This matters for servers that proxy another server's tools: one upstream tool named constructor makes the proxy fail its whole tool registration.
Repro (@modelcontextprotocol/server 2.0.0, Node 24; the same code is in 2.3.1 at line 1096 and on main at 9bc40e9):
import { McpServer } from "@modelcontextprotocol/server";
const s = new McpServer({ name: "r", version: "0" });
for (const n of ["constructor", "toString", "__proto__", "hasOwnProperty"]) {
try { s.registerTool(n, { description: "x" }, async () => ({ content: [] })); console.log(n, "ok"); }
catch (e) { console.log(n, "->", e.message); }
}constructor -> Tool constructor is already registered
toString -> Tool toString is already registered
__proto__ -> Tool __proto__ is already registered
hasOwnProperty -> Tool hasOwnProperty is already registered
Expected: each name registers, lists and calls like any other. These are valid tool names under the spec's naming rules.
Suggested fix: make the registries Maps, or create them with Object.create(null), or check Object.hasOwn in both registerTool and the tools/call lookup. _registeredPrompts, _registeredResources and _registeredResourceTemplates use the same pattern (mcp.ts:120-125), so they probably have the same problem.
Our workaround is to skip any upstream tool whose name is an own property of Object.prototype, and log a warning.
Activity
Ben-Home commentedon Oct 14, 2026
Worth flagging before someone picks option three:
Object.hasOwnguards in the duplicate check and the lookup are not sufficient on their own, because of the assignment path.this._registeredTools[name] = toolon an object literal runs the inherited__proto__setter when the name is__proto__, so instead of creating an own property the write attempts to replace the prototype (and silently does nothing when the value is not an object). A fix that only swaps the truthiness test forObject.hasOwnturns today's loud throw into a silently corrupt registry for exactly that name.That is why
Object.create(null)or aMapis the right primitive here: with a null-prototype registry,__proto__is an ordinary key and own-property semantics come for free on both the read and write paths. Same for the sibling registries (_registeredPrompts,_registeredResources,_registeredResourceTemplates) that share the pattern; a shared registry helper keeps them from drifting apart in future fixes.A test matrix built on your repro set: register
constructor,toString,__proto__, andhasOwnProperty; asserttools/listreturns all four andtools/calldispatches each, which also covers the lookup half that the current throw hides; plus one proxy-registration case, since a single upstream tool namedconstructortaking down the whole proxy listing is the practical failure mode.Your skip-and-warn workaround is the correct local mitigation until the registry is swapped.
bunnysayzz commentedon Oct 17, 2026
this looks like a real bug. the problem is in packages/server/src/server/mcp.ts:
_registeredToolsis a plain object literal (line 124), so the duplicate check at line 1102 trips on inherited members likeconstructorandtoString, and registering__proto__at line 1024 mutates the prototype instead of setting an own property. initializing it withObject.create(null)fixes registration and all the lookup sites. happy to put up a PR if you can assign the issue to me.bunnysayzz commentedon Oct 17, 2026
Withdrawing my assignment request, #3040 already covers this with the same fix. Will leave it to that PR.
fix(server): use null-prototype maps for McpServer registries
fix(server): use null-prototype maps for McpServer registries
fix(server): use null-prototype maps for McpServer registries