ACP session initialization fails with third-party models (DashScope/bailian) — "Invalid input: expected object, received undefined"

[wattshen]

Bug Description

ACP session initialization fails when using third-party models like DashScope (bailian) via ANTHROPIC_BASE_URL. The SDK Zod schema validation throws Invalid input: expected object, received undefined during session initialization.

Steps to Reproduce

1. Set environment variables for DashScope:

   export ANTHROPIC_API_KEY="your-api-key"
   export ANTHROPIC_BASE_URL="https://coding.dashscope.aliyuncs.com/apps/anthropic/v1"

2. Run: acpx claude exec "echo hello"
3. Error occurs during session initialization

Expected Behavior

The ACP agent should work with any Anthropic-compatible API provider, including DashScope/bailian models (qwen3.5-plus, glm-5, kimi-k2.5, etc.).

Actual Error

Invalid input: expected object, received undefined
{
  sessionUpdate: { _errors: [Array] },
  title: { _errors: [Array] },
  toolCallId: { _errors: [Array] },
  entries: { _errors: [Array] },
  availableCommands: { _errors: [Array] },
  currentModeId: { _errors: [Array] },
  configOptions: { _errors: [Array] },
  used: { _errors: [Array] }
}

The error repeats many times in a loop during SDK initialization.

Environment

• @agentclientprotocol/claude-agent-acp: 0.24.2
• @agentclientprotocol/sdk: 0.17.0
• @anthropic-ai/claude-agent-sdk: 0.2.84
• Node.js: v22.22.1
• OS: Linux (WSL)
• acpx: 0.3.1

Additional Context

The old package @zed-industries/claude-agent-acp (v0.23.1) had Bailian/DashScope workaround code in getAvailableModels() with a BAILIAN_MODELS array. This code is no longer present in the new @agentclientprotocol package. When ANTHROPIC_BASE_URL contains dashscope, isBailian was previously set to true to handle DashScope-specific model behavior. The new package removed this workaround, causing initialization to go through the standard (Anthropic-only) code path which expects fields that DashScope responses do not provide.

Possible Fix

1. Restore the Bailian/DashScope detection and workaround code in getAvailableModels()
2. Or add a more robust error handling layer that gracefully handles missing/undefined fields in ACP messages for non-Anthropic providers
3. Or document officially supported providers and ensure compatibility