mcp: binary file attachment in tool result permanently breaks the session #52448

Description

An MCP tool result with a type: "file" content item whose media type the provider can't inline (e.g. application/vnd.apache.parquet) is persisted into session history. The next model request fails:

AI.Error: OpenAI Chat does not support media type application/vnd.apache.parquet
  at OpenAIChat.lowerMedia
  at OpenAIChat.lowerToolMessages

Every later turn in that session fails the same way while replaying history — the session becomes permanently unusable, even for unrelated messages. Expected: unsupported file items dropped or converted to text at persistence time (byte materialization to a local file would be a bonus), or tolerated during lowering.

OpenCode version

2.0.21

Steps to reproduce

1. Configure an MCP server whose tool returns a binary attachment (a file read returning { type: "file", mime: "application/vnd.apache.parquet", uri: "data:...;base64,..." } works).
2. Call it in a session on an OpenAI Chat provider.
3. Tool completes, then every request — including follow-ups about something else — fails with the error above.

Operating System

macOS 15.6 (Darwin 24.6.0, arm64)

Terminal

xterm-256color