MCP tool results: structuredContent is dropped — only content[].text reaches the model

Summary

When an MCP server returns a tool result carrying both content (text) and structuredContent (structured JSON), opencode forwards only content[].text to the model and discards structuredContent. Any tool that puts its real payload in structuredContent — with a terse human summary in content[].text — becomes unusable from the agent side: the model sees only the summary string, not the data.

Observed

Local stdio MCP server (LaunchWeld mcp-dev, Go, using the Go MCP SDK's idiomatic NewToolResultStructured(data, summary) helper). Its tool responses look like:

{
  "content": [{"type":"text","text":"4 items"}],
  "structuredContent": {"items":[ {"id":"…","title":"…","status":"…", …}, … ]}
}

Calling list_items from an opencode agent returns only 4 items to the model — the entire items array in structuredContent is dropped. Same for single-item reads (get_item_details → 1 item(s), 0 missing, with no fields), list_sprints, list_projects, etc. The model cannot see any of the actual data these tools return, so it cannot act on ticket/project/sprint state.

Confirming it's a client-side drop and not a server omission: invoking the same server binary directly over stdio JSON-RPC returns the full structuredContent payload; the truncation appears only when the result passes through opencode.

Expected

Per the MCP spec, structuredContent carries the structured result and content carries a human-readable representation. Clients should surface the structured data to the model (e.g. by serializing structuredContent as JSON into the tool-result text the model receives), not discard it. The spec explicitly allows servers to put the machine-readable result in structuredContent and a summary in content.

Repro

1. Configure any MCP server that returns structuredContent (e.g. a Go MCP SDK server using NewToolResultStructured(data, summary)), type: "local" stdio.
2. Call a tool whose real data lives in structuredContent.
3. Observe the model receives only the content[].text summary.

Environment

• opencode CLI; MCP server configured in ~/.config/opencode/opencode.jsonc as type: "local".
• tool_output.max_bytes / max_lines are not the cause — the data is dropped before those limits apply, at the content-vs-structuredContent selection step.

Suggested fix

When an MCP CallToolResult includes structuredContent, serialize it (JSON) and include it in the tool-result text delivered to the model — appended to content[].text or as an additional content block. At minimum, expose a config flag to surface structuredContent, since the current default makes a large class of spec-compliant MCP servers unusable from agents.