deepseek-ai/deepseek-harness · error

mcp-client: serverName "${config.serverName}" is already in

Error message

mcp-client: serverName "${config.serverName}" is already in use by another mcp-client instance — pick a unique serverName in cordis.yml

What it means

Error "mcp-client: serverName "${config.serverName}" is already in use by another mcp-client instance — pick a unique serverName in cordis.yml" thrown in deepseek-ai/deepseek-harness.

Source

Thrown at packages/mcp/mcp-client/src/index.ts:155

 * @param config - resolved transport and server namespace configuration.
 * @returns startup readiness after connection and initial tool discovery settle.
 */
export async function apply(ctx: Context, config: Config): Promise<void> {
  // Fail loud at load: reconnect misconfiguration (including programmatic
  // construction that bypassed Schemastery) rejects THIS instance before any
  // effect registers.
  const reconnect = resolveReconnectPolicy(config.reconnect, `mcp-client(${config.serverName}): reconnect`)

  // Reserve the namespace next: a duplicate `serverName` fails THIS instance
  // at load with an actionable error and leaves the earlier instance intact.
  ctx.effect(() => {
    let names = activeServerNames.get(ctx.root)
    if (!names) {
      names = new Set()
      activeServerNames.set(ctx.root, names)
    }
    if (names.has(config.serverName)) {
      throw new Error(
        `mcp-client: serverName "${config.serverName}" is already in use by another mcp-client instance — pick a unique serverName in cordis.yml`,
      )
    }
    names.add(config.serverName)
    return () => void names.delete(config.serverName)
  }, 'mcp-client.serverName')

  // The supervisor owns the client/transport generations, the reconnect
  // loop, and the live tool registrations; disposal stops reconnection,
  // quiesces in-flight work, and unregisters the current generation.
  const connection = startConnection(ctx, config, reconnect)

  ctx.effect(() => {
    return () => connection.dispose()
  }, 'mcp-client.connection')

  // Block plugin activation on the initial connection + tool discovery so
  // Cordis consumers observe the tools immediately after the fiber activates.

View on GitHub (pinned to b150a551b8)

Solutions

1. Pick a unique serverName in cordis.yml for each mcp-client instance.

When it happens

Trigger: Thrown at packages/mcp/mcp-client/src/index.ts:155 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of deepseek-ai/deepseek-harness@b150a551b8 (2026-08-24). Data as JSON: /api/errors/c33b95aa9ab2b87e. Report an issue: GitHub.