MCP stdio transport sends batched requests without proper flush, breaking local servers #21516

Description

OpenCode's MCP stdio transport is broken for local MCP servers that expect proper sequential JSON-RPC messages with flush. Requests appear to be batched or not properly flushed, causing authentication to never reach the API.

Environment

• OpenCode version: v1.4.0
• Platform: macOS (Apple Silicon)
• Node/Bun runtime: Bun

Repro Steps

1. Set up a local MCP server using stdio transport (e.g., minimax-coding-plan-mcp)
2. Configure it in opencode as a local MCP server
3. Call any tool via the MCP server
4. Tool returns "login fail" error from the MCP server

Observed Behavior

• MCP server starts, initialize handshake completes successfully
• tools/list returns tools correctly
• Calling any tool returns "login fail" from the MCP server
• No useful error logs in OpenCode

Expected Behavior

• Tools should work via OpenCode's MCP client the same way they work via direct subprocess call

Root Cause

Verified working via direct subprocess:

uvx minimax-coding-plan-mcp
# sequential writes with flush() → works
# understand_image returns correct analysis

Verified broken via OpenCode:

• Same MCP server + OpenCode's MCP tool dispatch → "login fail"
• Token Plan quota: 174/1500 (plenty of room)

The bug is in OpenCode's MCP client stdio transport layer. OpenCode likely:

• Batches messages without flushing
• Sends malformed JSON-RPC requests
• Or fails to parse responses properly

Workaround

Using uvx or uv tool run to invoke the MCP server directly works.

Logs

MCP stderr shows nothing unusual - the authentication request from OpenCode simply never arrives properly formatted at the MCP server.

See Also

• ACP stdio transport crashes on oversized tool_call_update payloads (truncate large text fields) #17019 (ACP stdio transport crashes on oversized payloads - related)
• PR feat: handle MCP notifications/message from servers #19928 (handle MCP notifications/message from servers)