vercel/ai · error · HarnessBridgeCapabilityUnsupportedError

The ACP agent does not advertise support for ${value.type.to

Error message

The ACP agent does not advertise support for ${value.type.toUpperCase()} MCP servers.

What it means

Error "The ACP agent does not advertise support for ${value.type.toUpperCase()} MCP servers." thrown in vercel/ai.

Source

Thrown at packages/harness-acp/src/v1/bridge/index.ts:656

  return Object.entries(mcpServers).map(([name, value]) => {
    if (!isRecord(value)) {
      throw new Error(
        `ACP MCP server ${JSON.stringify(name)} must be configured with an object value.`,
      );
    }
    if (value.type === 'acp') {
      throw new HarnessBridgeCapabilityUnsupportedError({
        harnessId: bridgeType,
        message:
          'ACP-transport MCP servers require client-side mcp/connect handling, which this harness does not provide.',
      });
    }
    const mcpCapabilities = initialization.agentCapabilities?.mcpCapabilities;
    if (
      (value.type === 'http' && mcpCapabilities?.http !== true) ||
      (value.type === 'sse' && mcpCapabilities?.sse !== true)
    ) {
      throw new HarnessBridgeCapabilityUnsupportedError({
        harnessId: bridgeType,
        message: `The ACP agent does not advertise support for ${value.type.toUpperCase()} MCP servers.`,
      });
    }
    return { ...value, name } as acp.McpServer;
  });
}

function unknownUsage() {
  return {
    inputTokens: {
      total: undefined,
      noCache: undefined,
      cacheRead: undefined,
      cacheWrite: undefined,
    },
    outputTokens: {
      total: undefined,

View on GitHub (pinned to 69428b1f8b)

When it happens

Trigger: Thrown at packages/harness-acp/src/v1/bridge/index.ts:656 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of vercel/ai@69428b1f8b (2026-08-30). Data as JSON: /api/errors/797532f781a4ca47. Report an issue: GitHub.