mcp: output schema mismatch hard-fails tool calls instead of degrading gracefully #56116

Summary

When a remote MCP server returns structured content that violates its own declared outputSchema, opencode hard-fails the entire tool call. A single mistyped field makes an otherwise-working tool 100% unusable and discards the (valid) content. opencode should degrade gracefully — normalize the value, or surface the raw text plus a warning — instead of treating the mismatch as fatal.

Environment

• opencode version: 2.0.29
• OS: Darwin 27.0.0 (arm64)
• Terminal: Apple Terminal (TERM_PROGRAM=Apple_Terminal, TERM=xterm-256color, COLORTERM=truecolor)
• Shell: /bin/zsh
• Install/channel: latest
• Active plugins: none found (~/.config/opencode/opencode.json defines only an MCP server)

Reproduction

1. Configure a remote MCP server (example: Robinhood trading, https://agent.robinhood.com/mcp/trading) and authenticate via /mcps.

2. Call review_advanced_order with any valid arguments, e.g.:

   {
     "account_number": "••••1971",
     "symbol": "SOFI",
     "side": "sell",
     "quantity": "6",
     "take_profit_limit_price": "15.64",
     "stop_loss_stop_price": "13.76",
     "time_in_force": "gtc",
     "market_hours": "regular_hours"
   }

3. Observe the call fail with a schema-validation error.

Expected Behavior

The tool call succeeds or degrades gracefully. A response whose fields are valid JSON but do not exactly match the declared schema should not be a fatal error. opencode should normalize the value, or return the raw content with a warning, so the tool remains usable.

Actual Behavior

The call fails outright and no content is returned:

MCP tool "review_advanced_order" on server "robinhood" failed: Structured content does not match the tool's output schema: data/data/quote_data/previous_close_date must be null,object

Reproducible consistently — it failed on two consecutive attempts with identical parameters.

Additional Context

• Root cause is server-side: review_advanced_order's declared output schema types quote_data.previous_close_date as null | object, but the server returns a string (e.g. "2026-10-14"). Sibling tools on the same server (place_equity_order, review_equity_order, get_equity_quotes) correctly declare the same field as null | string, so the server schema is inconsistent for this one tool.
• Impact: because opencode treats the mismatch as fatal, the tool is completely unusable and the otherwise-valid response is discarded. A graceful path would keep users working while the server fixes its schema, since not every third-party server can be patched on the client's timeline.
• Workaround: skip review_advanced_order and call place_advanced_order directly (different response schema; works).