fix: Correctly propose protocol version vs. error

Signed-off-by: John McBride <john@zuplo.com>

Author: jpmcb

src/server/index.ts

@@ -42,10 +42,6 @@ import type {
 } from "../mcp/20251125/types.js";
 import {
   LATEST_PROTOCOL_VERSION,
-  PROTOCOL_VERSION_2024_10_07,
-  PROTOCOL_VERSION_2024_11_05,
-  PROTOCOL_VERSION_2025_03_26,
-  PROTOCOL_VERSION_2025_06_18,
   SUPPORTED_PROTOCOL_VERSIONS,
 } from "../mcp/versions.js";
 import type { PromptConfig, RegisteredPrompt } from "../prompts/types.js";
@@ -432,37 +428,27 @@ export class MCPServer {
       });
     }
 
-    const protocolVersion = parseResult.data.params.protocolVersion;
-
-    switch (protocolVersion) {
-      case LATEST_PROTOCOL_VERSION:
-      case PROTOCOL_VERSION_2025_06_18:
-      case PROTOCOL_VERSION_2025_03_26:
-      case PROTOCOL_VERSION_2024_11_05:
-      case PROTOCOL_VERSION_2024_10_07: {
-        const initResponse: InitializeResult = {
-          protocolVersion: protocolVersion,
-          capabilities: this.getCapabilities(),
-          serverInfo: {
-            name: this.name,
-            version: this.version,
-          },
-          ...(this.instructions ? { instructions: this.instructions } : {}),
-        };
-
-        return newJSONRPCReponse({ id: request.id, result: initResponse });
-      }
-      default: {
-        return newJSONRPCError({
-          id: request.id,
-          code: ErrorCode.InvalidParams,
-          message: `Unsupported protocol version: ${protocolVersion} - supported versions: ${SUPPORTED_PROTOCOL_VERSIONS}`,
-          data: {
-            supportedVersions: SUPPORTED_PROTOCOL_VERSIONS,
-          },
-        });
-      }
-    }
+    // The MCP spec defines that a server should attempt to negotiate
+    // the highest protocol version it supports:
+    // https://modelcontextprotocol.io/specification/2025-11-25/basic/lifecycle#version-negotiation
+    const requestedVersion = parseResult.data.params.protocolVersion;
+    const negotiatedVersion = SUPPORTED_PROTOCOL_VERSIONS.includes(
+      requestedVersion
+    )
+      ? requestedVersion
+      : LATEST_PROTOCOL_VERSION;
+
+    const initResponse: InitializeResult = {
+      protocolVersion: negotiatedVersion,
+      capabilities: this.getCapabilities(),
+      serverInfo: {
+        name: this.name,
+        version: this.version,
+      },
+      ...(this.instructions ? { instructions: this.instructions } : {}),
+    };
+
+    return newJSONRPCReponse({ id: request.id, result: initResponse });
   }
 
   private async handleToolListRequest(

src/server/initialization.test.ts

@@ -165,9 +165,7 @@ describe("MCPServer", () => {
       }
     });
 
-    it("handles ill-supported MCP protocolVersion", async () => {
-      // should get an error JSON RPC 2 message if protocolVersion is something
-      // totally untenable like "2001-01-01"
+    it("negotiates unsupported protocolVersion to server's latest", async () => {
       const serverName = "example ping server";
       const serverVersion = "0.0.0";
       const server = new MCPServer({
@@ -179,7 +177,7 @@ describe("MCPServer", () => {
         id: 0,
         method: "initialize",
         params: {
-          protocolVersion: "2001-01-01",
+          protocolVersion: "2099-01-01",
           capabilities: {
             sampling: {},
             roots: {
@@ -199,22 +197,13 @@ describe("MCPServer", () => {
       expect(response.jsonrpc).toBe("2.0");
       expect(response.id).toBe(0);
 
-      if ("error" in response) {
-        expect(response.error.code).toBe(-32602);
-        expect(response.error.message).toContain(
-          "Unsupported protocol version"
-        );
-        expect(response.error.data).toEqual({
-          supportedVersions: [
-            "2025-11-25",
-            "2025-06-18",
-            "2025-03-26",
-            "2024-11-05",
-            "2024-10-07",
-          ],
-        });
+      if ("result" in response) {
+        const result = response.result as InitializeResult;
+        expect(result.protocolVersion).toBe(LATEST_PROTOCOL_VERSION);
+        expect(result.serverInfo.name).toBe(serverName);
+        expect(result.serverInfo.version).toBe(serverVersion);
       } else {
-        fail("Expected an error response, got a result");
+        fail("Expected a result response, got an error");
       }
     });
   });