mcp-protocol-fundamentals
MCP is built on JSON-RPC 2.0 for bidirectional communication between Claude and servers. PROACTIVELY activate for: (1) understanding MCP protocol structure, (2) JSON-RPC message formats, (3) tool vs resource distinction. Triggers: "mcp protocol", "json-rpc", "model context protocol"
What this skill does
# MCP Protocol Fundamentals Skill
## Metadata (Tier 1)
**Keywords**: mcp, model context protocol, json-rpc, protocol, specification
**File Patterns**: **/mcp_server.py, **/protocol.py
**Modes**: backend_python, api_development
---
## Instructions (Tier 2)
### JSON-RPC 2.0 Foundation
MCP is built on **JSON-RPC 2.0** for bidirectional communication between Claude and servers.
**Request Structure**:
```json
{
"jsonrpc": "2.0",
"method": "tools/call",
"params": {
"name": "search",
"arguments": {"query": "test"}
},
"id": 1
}
```
**Response Structure**:
```json
{
"jsonrpc": "2.0",
"result": {"data": "..."},
"id": 1
}
```
**Error Response**:
```json
{
"jsonrpc": "2.0",
"error": {
"code": -32601,
"message": "Method not found"
},
"id": 1
}
```
### Core Concepts
**Tools** (Model-Controlled):
- Exposed by server, invoked by Claude
- Have strict input/output schemas (Pydantic V2)
- Can have side effects (create, update, delete)
- Examples: search_code, create_file, execute_query
**Resources** (App-Controlled):
- Provide context to Claude (read-only)
- URI-based identification (file://, db://, api://)
- Returned in list_resources, read via read_resource
- Examples: project docs, database schemas, logs
**Prompts** (Optional):
- Pre-defined prompt templates
- Can include dynamic arguments
- Help standardize common workflows
### Protocol Methods
**Server → Client** (Server exposes):
```
tools/list → List available tools
tools/call → Execute a tool
resources/list → List available resources
resources/read → Read resource content
prompts/list → List prompt templates
prompts/get → Get prompt with arguments
```
**Client → Server** (Server can request):
```
sampling/createMessage → Request Claude to generate text
roots/list → Get workspace roots
```
### Message Flow
```
Claude Desktop MCP Server
| |
|---tools/list-------->|
|<--[tool schemas]-----|
| |
|---tools/call-------->|
| {name, arguments} |
|<--result/error-------|
```
### Pydantic V2 Integration
**CRITICAL**: Use Pydantic strict mode for all schemas.
```python
from pydantic import BaseModel, ConfigDict
class ToolInput(BaseModel):
model_config = ConfigDict(strict=True)
param: str
# Auto-generates inputSchema:
# {
# "type": "object",
# "properties": {"param": {"type": "string"}},
# "required": ["param"]
# }
```
### Transport Layer
MCP supports two transports:
1. **stdio** (Standard I/O):
```python
async def main():
async with stdio_server() as (read, write):
await server.run(read, write)
```
2. **HTTP/SSE** (Server-Sent Events):
```python
@app.post("/mcp")
async def handle_mcp(request: JsonRpcRequest):
return await server.handle_request(request)
```
### Error Codes (JSON-RPC 2.0)
```
-32700 → Parse error (invalid JSON)
-32600 → Invalid request
-32601 → Method not found
-32602 → Invalid params
-32603 → Internal error
-32000 to -32099 → Server-defined errors
```
### Anti-Patterns
❌ Manual JSON schema writing (use Pydantic)
❌ Blocking operations in async handlers
❌ Missing error handling
❌ Invalid JSON-RPC 2.0 structure
---
## Resources (Tier 3)
**Official Specification**: https://spec.modelcontextprotocol.io/
**JSON-RPC 2.0 Spec**: https://www.jsonrpc.org/specification
**Python SDK**: https://github.com/modelcontextprotocol/python-sdk
**Example Servers**:
- Filesystem MCP: Read/write local files
- GitHub MCP: Repository operations
- Database MCP: SQL query execution
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