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fix: Correctly propose protocol version vs. error #60

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Merged
jpmcb merged 1 commit into from
Dec 29, 2025
Merged

fix: Correctly propose protocol version vs. error #60

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56 changes: 21 additions & 35 deletions src/server/index.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -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";
Expand Down Expand Up @@ -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(
Expand Down
27 changes: 8 additions & 19 deletions src/server/initialization.test.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -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({
Expand All @@ -179,7 +177,7 @@ describe("MCPServer", () => {
id: 0,
method: "initialize",
params: {
protocolVersion: "2001-01-01",
protocolVersion: "2099-01-01",
capabilities: {
sampling: {},
roots: {
Expand All @@ -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");
}
});
});
Expand Down