Module protocol 

mcpr-protocol

MCP specification layer: JSON-RPC 2.0 parsing, MCP method classification, and session lifecycle management.

This crate is the foundation of the mcpr workspace. It contains everything related to understanding the MCP protocol itself, with zero coupling to HTTP frameworks or proxy logic.

Responsibilities

• JSON-RPC 2.0 parsing (lib.rs): Parse and classify JSON-RPC 2.0 messages (requests, notifications, responses). Provides ParsedBody for batch-aware parsing and McpMethod for typed MCP method discrimination.

• MCP method constants: Single source of truth for MCP method strings (initialize, tools/call, resources/read, etc.).

• Error handling: JSON-RPC error codes, error response builders, and error extraction from response bodies.

• Session management (session module): MCP session state machine (Created -> Initialized -> Active -> Closed), SessionStore trait for pluggable storage backends, and MemorySessionStore for in-memory use.

• Schema primitives (schema module): Pagination detection/merging, schema diffing (detect added/removed/modified tools, resources, prompts), and schema method classification. Pure helpers, no state.

• Schema manager (schema_manager module): Top-level per-upstream view of an MCP server. Owns versioned snapshots built from ingested discovery responses, exposes query APIs (list_tools, get_tool, …), tracks stale flags, and defines the SchemaScanner trait for active discovery.

Module Structure

mcpr-protocol/src/
+-- lib.rs              # JSON-RPC 2.0 types, parsing, MCP method classification
+-- schema.rs           # Pagination merge + diff primitives
+-- schema_manager/     # SchemaManager, SchemaStore, SchemaScanner, SchemaVersion
+-- session.rs          # Session state, SessionStore trait, MemorySessionStore

Dependencies

Minimal: serde, serde_json, chrono, dashmap. No HTTP framework deps.

Modules

error_code

schema

MCP schema capture: types, pagination merging, and diff logic.

schema_manager

Schema manager: the top-level per-upstream view of an MCP server.

session

Structs

JsonRpcError

JsonRpcNotification

JsonRpcRequest

JsonRpcResponse

ParsedBody

Result of parsing a POST body as JSON-RPC 2.0.

Enums

JsonRpcId

A parsed JSON-RPC 2.0 id (number or string).

JsonRpcMessage

A classified JSON-RPC 2.0 message.

McpMethod

Known MCP methods — lets the proxy know exactly what function is being called.

Constants

COMPLETION_COMPLETE

INITIALIZE

INITIALIZED

LOGGING_SET_LEVEL

NOTIFICATIONS_CANCELLED

NOTIFICATIONS_PROGRESS

NOTIFICATIONS_TOOLS_LIST_CHANGED

PING

PROMPTS_GET

PROMPTS_LIST

RESOURCES_LIST

RESOURCES_READ

RESOURCES_SUBSCRIBE

RESOURCES_TEMPLATES_LIST

RESOURCES_UNSUBSCRIBE

TOOLS_CALL

TOOLS_LIST

Functions

error_response

Build a JSON-RPC 2.0 error response as bytes. id can be a request id or Value::Null if the request id couldn’t be parsed.

extract_error_code

Extract the JSON-RPC error code from a response body (if it’s an error response).

parse_body

Parse a POST body as JSON-RPC 2.0 — single message or batch. Returns None if the body is not valid JSON-RPC 2.0.

parse_message

Parse a single JSON value as a JSON-RPC 2.0 message. Returns None if it doesn’t have "jsonrpc": "2.0".