# MCP Protocol Contract

**Purpose**: Define the JSON-RPC communication contract between AI assistants and the Obsidian MCP server

## Transport

**Protocol**: JSON-RPC 2.0 over stdio  
**Direction**: Bidirectional (client ↔ server)

### Stdio Requirements

- **stdin**: Server reads JSON-RPC requests line-by-line
- **stdout**: Server writes JSON-RPC responses line-by-line (one message per line)
- **stderr**: Server logs debugging information (never JSON-RPC messages)

### Message Format

All messages are single-line JSON objects ending with `\n`:

```json
{"jsonrpc":"2.0","id":1,"method":"tools/call","params":{...}}\n
```

## Initialization Handshake

### Request: initialize

Client sends capabilities and version information:

```json
{
  "jsonrpc": "2.0",
  "id": 1,
  "method": "initialize",
  "params": {
    "protocolVersion": "2024-11-05",
    "capabilities": {},
    "clientInfo": {
      "name": "Claude Desktop",
      "version": "1.0.0"
    }
  }
}
```

### Response: initialize

Server responds with its capabilities:

```json
{
  "jsonrpc": "2.0",
  "id": 1,
  "result": {
    "protocolVersion": "2024-11-05",
    "capabilities": {
      "tools": {}
    },
    "serverInfo": {
      "name": "obsidian-mcp",
      "version": "1.0.0"
    }
  }
}
```

## Tool Discovery

### Request: tools/list

Client requests available tools:

```json
{
  "jsonrpc": "2.0",
  "id": 2,
  "method": "tools/list",
  "params": {}
}
```

### Response: tools/list

Server returns tool definitions with schemas:

```json
{
  "jsonrpc": "2.0",
  "id": 2,
  "result": {
    "tools": [
      {
        "name": "obsidian_create_note",
        "description": "Create a new note in the vault",
        "inputSchema": {
          "type": "object",
          "properties": {
            "name": {
              "type": "string",
              "description": "Note name (without .md extension) or exact path"
            },
            "content": {
              "type": "string",
              "description": "Initial note content (markdown)"
            },
            "overwrite": {
              "type": "boolean",
              "description": "Overwrite if file exists"
            }
          },
          "required": ["name"]
        }
      }
      // ... more tools
    ]
  }
}
```

## Tool Invocation

### Request: tools/call

Client invokes a tool:

```json
{
  "jsonrpc": "2.0",
  "id": 3,
  "method": "tools/call",
  "params": {
    "name": "obsidian_create_note",
    "arguments": {
      "name": "Meeting Notes",
      "content": "# Meeting Notes\n\n- Agenda item 1"
    }
  }
}
```

### Response: Success

Tool execution succeeded:

```json
{
  "jsonrpc": "2.0",
  "id": 3,
  "result": {
    "content": [
      {
        "type": "text",
        "text": "{\"success\":true,\"data\":{\"path\":\"Meeting Notes.md\",\"created\":true}}"
      }
    ]
  }
}
```

### Response: Error

Tool execution failed:

```json
{
  "jsonrpc": "2.0",
  "id": 3,
  "error": {
    "code": -32000,
    "message": "Obsidian application is not running. Please start Obsidian and try again.",
    "data": {
      "errorCode": "OBSIDIAN_NOT_RUNNING"
    }
  }
}
```

## Error Codes

### JSON-RPC Standard Errors

| Code | Message | Meaning |
|------|---------|---------|
| -32700 | Parse error | Invalid JSON |
| -32600 | Invalid Request | Invalid JSON-RPC structure |
| -32601 | Method not found | Unknown method |
| -32602 | Invalid params | Parameter validation failed |
| -32603 | Internal error | Server internal error |

### Application Errors

| Code | Message | When |
|------|---------|------|
| -32000 | Server error | Generic application error |
| -32001 | Obsidian not running | Obsidian app not detected |
| -32002 | Vault not found | Invalid vault name |
| -32003 | File not found | Note doesn't exist |
| -32004 | Ambiguous name | Multiple files match name |
| -32005 | Permission denied | Filesystem permission issue |
| -32006 | Timeout | Operation exceeded 30s |

## Shutdown

### Notification: exit

Client notifies server of shutdown (no response expected):

```json
{
  "jsonrpc": "2.0",
  "method": "exit"
}
```

Server should:
1. Complete any in-flight operations
2. Close file handles
3. Flush logs
4. Exit process

### EOF on stdin

If stdin closes without `exit` notification, server should gracefully shutdown.

## Performance Requirements

- **initialize response**: <100ms
- **tools/list response**: <200ms
- **tools/call response**: <3s for file ops, <5s for search (per SC-001, SC-002)

## Compliance Checklist

- [x] JSON-RPC 2.0 format for all messages
- [x] Stdio transport (stdin for requests, stdout for responses)
- [x] Line-by-line message format (newline-delimited)
- [x] Initialize handshake before tool calls
- [x] Tool schemas in tools/list response
- [x] Structured error responses with codes
- [x] Graceful shutdown on exit notification
- [x] No stdout pollution (logs to stderr only)
- [x] Immediate stdout flushing after responses
- [x] EOF handling (graceful shutdown)