different-ai/openwork · error · ProbeFailure

MCP_VERSION

MCP_VERSION

Error message

Server selected an unexpected MCP protocol version

What it means

Thrown when the server's initialize result selects a protocolVersion different from the version the probe requested in scenario.protocol.version. MCP requires the server to respond with the exact version the client requested if supported; any other value is treated as a failed negotiation and surfaces as an MCP_VERSION failure.

Source

Thrown at packages/enterprise-mcp-mock-server/src/testing/probe.ts:765

    const initializeEnvelope = await parseRpc(initializeResponse, "MCP_INITIALIZE")
    if (initializeEnvelope.error) {
      const versionEvidence = z.object({ supportedVersions: z.array(z.string()).min(1) }).safeParse(initializeEnvelope.error.data)
      throw versionEvidence.success || initializeEnvelope.error.message === "Unsupported MCP protocol version"
        ? new ProbeFailure("MCP_VERSION", "mcp_version", initializeEnvelope.error.message)
        : new ProbeFailure("MCP_INITIALIZE", "mcp_initialize", initializeEnvelope.error.message)
    }
    if (initializeEnvelope.id !== 1) {
      throw new ProbeFailure("MCP_INITIALIZE", "mcp_initialize", "Initialize response JSON-RPC id did not match the request")
    }
    const initialize = parseAt(
      initializeResultSchema,
      initializeEnvelope.result,
      "MCP_INITIALIZE",
      "mcp_initialize",
      "Initialize result did not match the required shape",
    )
    if (initialize.protocolVersion !== scenario.protocol.version) {
      throw new ProbeFailure("MCP_VERSION", "mcp_version", "Server selected an unexpected MCP protocol version")
    }
    mutable.negotiatedProtocolVersion = initialize.protocolVersion
    if (scenario.protocol.requireSession && !sessionId) {
      throw new ProbeFailure("MCP_INITIALIZE", "mcp_initialize", "Initialize response omitted required MCP-Session-Id")
    }
    recordPassed(phases, "MCP_INITIALIZE", startedAt, "MCP version, capabilities, and session negotiated")

    const sessionHeaders = {
      ...rpcHeaders,
      "mcp-session-id": sessionId,
      "mcp-protocol-version": initialize.protocolVersion,
    }
    startedAt = Date.now()
    const initializedResponse = await fetchStep(mcpUrl, {
      method: "POST",
      headers: sessionHeaders,
      body: JSON.stringify({ jsonrpc: "2.0", method: "notifications/initialized" }),
    }, "MCP_INITIALIZED", overallDeadline)

View on GitHub (pinned to 2b7df46e8a)

Solutions

1. Update the server to echo the client's requested protocolVersion when it supports it
2. Align scenario.protocol.version in the probe scenario with a version the server actually implements
3. Pin or upgrade the server's MCP SDK so its supported versions include the scenario's version

Example fix

// before (server)
return { protocolVersion: "2024-11-05", ... }
// after (server)
return { protocolVersion: request.params.protocolVersion, ... }

if (initialize.protocolVersion !== scenario.protocol.version) throw new Error(`server negotiated ${initialize.protocolVersion}, expected ${scenario.protocol.version}`)

function supportsVersion(serverVersions: string[], wanted: string): boolean {
  return serverVersions.includes(wanted)
}

try {
  await probeEnterpriseMcpMockServer(scenario)
} catch (e) {
  if (e instanceof ProbeFailure && e.code === "MCP_VERSION") {
    console.error("Protocol negotiation failed:", e.message)
  }
}

When it happens

Trigger: The scenario is configured with scenario.protocol.version (e.g. "2025-06-18") but the server's initialize result returns a different protocolVersion string, such as an older date-versioned spec.

Common situations: Server SDK updated (or not updated) relative to the scenario's expected spec version; scenario config points at a newer protocol version than the mock/real server implements; server ignores the client's requested version and always returns its default.

Related errors

• unsupported_protocol_version
• MCP_INITIALIZE
• MCP_UNSUPPORTED_VERSION
• invalid_mcp_token_payload
• invalid_mcp_connection_payload

AI-assisted analysis of different-ai/openwork@2b7df46e8a (2026-09-01). Data as JSON: /api/errors/f2363a4659ca7df3. Report an issue: GitHub.