Vision MCP (built-in server)

Why we implemented it this way

The upstream Vision MCP package (@z_ai/mcp-server) is designed as a local stdio server. In a desktop app + embedded proxy, requiring users (or the app) to manage a separate Node runtime/process increases operational complexity.

Instead, we implement a built-in Vision MCP server directly in the proxy:

• No extra runtime dependency.
• Single place to store the z.ai key (proxy config).
• Apps can talk to the local proxy using standard MCP over HTTP.

Local endpoint

• /mcp/zai-mcp-server/mcp

Wired in:

• src-tauri/src/proxy/server.rs (router)

Protocol surface (minimal Streamable HTTP MCP)

Handler:

• src-tauri/src/proxy/handlers/mcp.rs (handle_zai_mcp_server)

Implemented methods:

• POST /mcp:
  • initialize
  • tools/list
  • tools/call
• GET /mcp:
  • returns an SSE stream (keepalive) for an existing session
• DELETE /mcp:
  • terminates a session

Session storage:

• src-tauri/src/proxy/zai_vision_mcp.rs

Notes:

• This is intentionally minimal to support tool calls.
• Prompts/resources, resumability, and streamed tool output can be added later if needed.

Tool set

Tool registry:

• tool_specs() in src-tauri/src/proxy/zai_vision_tools.rs

Tool execution:

• call_tool(...) in src-tauri/src/proxy/zai_vision_tools.rs

Supported tools (mirrors the upstream package at a high level):

• ui_to_artifact
• extract_text_from_screenshot
• diagnose_error_screenshot
• understand_technical_diagram
• analyze_data_visualization
• ui_diff_check
• analyze_image
• analyze_video

Upstream calls

Vision tools call the z.ai vision chat completions endpoint:

• https://api.z.ai/api/paas/v4/chat/completions

Implementation:

• vision_chat_completion(...) in src-tauri/src/proxy/zai_vision_tools.rs

Auth:

• Uses Authorization: Bearer <proxy.zai.api_key>

Payload:

• model: glm-4.6v (currently hardcoded)
• messages: system prompt + a multimodal user message containing images/videos + text prompt
• stream: false (currently returns a single tool result)

Local file handling

To support local file paths passed by MCP clients:

• Images (.png, .jpg, .jpeg) are read and encoded as data:<mime>;base64,... (5 MB max)
• Videos (.mp4, .mov, .m4v) are read and encoded as data:<mime>;base64,... (8 MB max)

Implementation:

• image_source_to_content(...) in src-tauri/src/proxy/zai_vision_tools.rs
• video_source_to_content(...) in src-tauri/src/proxy/zai_vision_tools.rs

Quick validation (raw JSON-RPC)

1. Initialize:
   • POST /mcp/zai-mcp-server/mcp with {\"jsonrpc\":\"2.0\",\"id\":1,\"method\":\"initialize\",\"params\":{\"protocolVersion\":\"2024-11-05\",\"capabilities\":{}}}
   • capture Mcp-Session-Id response header
2. List tools:
   • POST /mcp/zai-mcp-server/mcp with Mcp-Session-Id: <id> and {\"jsonrpc\":\"2.0\",\"id\":2,\"method\":\"tools/list\"}
3. Call tool:
   • POST /mcp/zai-mcp-server/mcp with Mcp-Session-Id: <id> and {\"jsonrpc\":\"2.0\",\"id\":3,\"method\":\"tools/call\",\"params\":{\"name\":\"analyze_image\",\"arguments\":{\"image_source\":\"/path/to/file.png\",\"prompt\":\"Describe this image\"}}}