fix(opencode): normalize MCP structured arguments - #32812

Issue for this PR

Related to #28472

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Some setups hand MCP servers an object or array argument as a JSON string, which the server then rejects with expected object, received string. When the advertised schema says a field is an object or array, this PR parses that JSON string back into a structured value before calling client.callTool. Empty strings become {}/[]; invalid JSON is left untouched so the server still returns its own error. It is a no-op when the argument is already structured.

How did you verify your code works?

• bun test test/mcp/catalog.test.ts and bun run typecheck from packages/opencode; bun run lint from the repo root (0 errors).
• Instrumented the forwarding boundary and ran real tool calls through OpenCode against a real MCP server, across 5 models / 3 providers (OpenAI gpt-5.4, gpt-5.4-mini; Copilot gpt-4.1, gpt-5.4; Gemini 2.5 Flash) and 3 object-schema shapes (shallow object, free-form object, deeply nested object like ms-365 send-mail body). With current models the argument always arrives as a proper object and the unfixed path already accepts it. When a JSON string is forced, the unfixed path returns the exact -32602 ... expected object, received string and this change accepts it.

Scope and limitations

This is a tolerance shim. It only does anything when an upstream component (an older model, a proxy, or a re-serializing SDK) emits a JSON string for an object/array field; otherwise it is a no-op. I could not reproduce the original failure with current models in clean OpenCode, so this is defensive rather than a guaranteed fix for a specific server. Note that next-devtools' nextjs_call advertises args as type: "string", so this change does not affect it. Schema detection is intentionally minimal: anyOf/oneOf/$ref and string | object unions are not handled. Happy to extend, or to drop this if you'd rather not carry it.

Screenshots / recordings

Not a UI change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR