MCP client serializes nested object params as JSON string instead of object

Description

When an MCP tool declares a parameter with a nested object type (e.g. metadata: Option<StoreMetadata> where StoreMetadata is a struct with tags and type fields), OpenCode's MCP client sends the parameter value as a JSON string instead of a JSON object.

The MCP server (Rust, using serde deserialization) receives:
"metadata": "{"tags": ["tag1", "tag2"], "type": "decision"}"
instead of:
"metadata": {"tags": ["tag1", "tag2"], "type": "decision"}

This causes -32602: failed to deserialize parameters: invalid type: string "...", expected struct StoreMetadata.

Steps to reproduce

1. Configure an MCP server with a tool that accepts a nested object parameter (e.g. opencode-memory with memory_store which takes metadata: {tags, type})
2. Ask the agent to store a memory with metadata
3. The tool call fails with the deserialization error

Expected behavior

OpenCode should send nested object parameters as JSON objects, not as stringified JSON. The MCP schema correctly declares these as object types — the client should honor that.

Root cause analysis

The MCP tool schema for memory_store defines:

"metadata": {
  "anyOf": [
    {"$ref": "#/$defs/StoreMetadata"},
    {"type": "null"}
  ],
  "description": "Optional metadata object; tags + type live here."
}

And StoreMetadata is:

"StoreMetadata": {
  "type": "object",
  "properties": {
    "tags": {...},
    "type": {"type": "string"}
  }
}

OpenCode appears to stringify the object value before passing it to the MCP server via JSON-RPC, likely in the parameter serialization layer.
Environment

    OpenCode version: latest (opencode/big-pickle via Ollama)
    MCP server: opencode-memory v0.1.4 (Rust, rajarshighoshal/opencode-memory)
    Transport: stdio (local)
    OS: Linux

Suggested fix

In the MCP client parameter serialization path, detect when a parameter value is a string that parses as valid JSON matching the expected schema type (object), and deserialize it before sending to the MCP server. Alternatively, ensure the LLM-generated tool call arguments preserve proper JSON object nesting.
Related

    This affects any MCP server with nested object parameters (not just opencode-memory)
    The same MCP server works correctly with Claude Code and Codex CLI


### Plugins

_No response_

### OpenCode version

_No response_

### Steps to reproduce

1. Configure an MCP server with a tool that accepts a nested object parameter (e.g. `opencode-memory` with `memory_store` which takes `metadata: {tags, type}`)
2. Ask the agent to store a memory with metadata
3. The tool call fails with the deserialization error

### Screenshot and/or share link

_No response_

### Operating System

Ubuntu 24.06 LTS

### Terminal

_No response_