TurboMCP Protocol

Model Context Protocol (MCP) specification implementation with JSON-RPC 2.0 and runtime schema validation.

Table of Contents

• Overview
  • MCP Version Support
• Key Features
• Version Selection Guide
  • Choosing the Right MCP Version
  • Feature-by-Feature Selection
  • Runtime Version Negotiation
  • Migration Path
• Architecture
• MCP Message Types
• Usage
• Message Flow
• Feature Flags
  • MCP 2025-11-25 Draft Features
• Supported MCP Methods
• Integration

Overview

turbomcp-protocol provides a specification-compliant implementation of the Model Context Protocol (MCP). This crate handles protocol-level concerns including message formatting, capability negotiation, and runtime validation.

MCP Version Support

TurboMCP v3.0 fully implements MCP 2025-11-25 with all specification features enabled by default. No feature flags needed for core protocol capabilities.

Spec Version	Status	Notes
MCP 2025-11-25	✅ Full Support	Icons, URL elicitation, sampling tools, enum improvements
MCP 2025-06-18	✅ Compatible	Negotiated at runtime via protocol version

Quick Start:

[dependencies]
turbomcp-protocol = "3.0.0-exp"

Only the experimental Tasks API (SEP-1686) requires a feature flag:

[dependencies]
turbomcp-protocol = { version = "3.0.0-exp", features = ["experimental-tasks"] }

Key Features

MCP Specification Support

• MCP specification implementation with current message types
• Tools, resources, prompts, and capabilities support
• Capability negotiation with feature detection and handshake
• Version compatibility support

JSON-RPC 2.0 Implementation

• Compliant message format with request, response, and notification handling
• ID correlation for automatic request/response matching
• Standard JSON-RPC error codes and extensions
• Support for batch request/response operations

Runtime Schema Validation

• JSON Schema validation using jsonschema crate
• Rust type definitions for MCP message types
• Tool and resource parameter validation
• Schema generation from Rust types

Capability Management

• Type-State Capability Builders - Compile-time validated capability configuration
• Server capabilities for tools, resources, prompts declarations
• Client capabilities including sampling, roots, progress reporting
• Feature negotiation with capability matching
• Support for custom capability extensions

MCP Enhanced Features

• Bidirectional communication for server-initiated requests to clients
• Elicitation support for server-requested structured input from users
• Completion context with references and metadata
• Resource templates for dynamic resource generation with parameters
• Ping/keepalive for connection health monitoring

Architecture

┌─────────────────────────────────────────────┐
│            TurboMCP Protocol                │
├─────────────────────────────────────────────┤
│ MCP Message Types                          │
│ ├── InitializeRequest/InitializeResult     │
│ ├── Tool/Resource/Prompt messages          │
│ ├── Capability negotiation               │
│ └── Notification handling                 │
├─────────────────────────────────────────────┤
│ JSON-RPC 2.0 Layer                        │
│ ├── Request/Response correlation          │
│ ├── ID generation and tracking           │
│ ├── Error code standardization           │
│ └── Batch message processing             │
├─────────────────────────────────────────────┤
│ Schema Validation                          │
│ ├── Runtime JSON schema validation       │
│ ├── Parameter type checking              │
│ ├── Response format validation           │
│ └── Custom schema extension support      │
└─────────────────────────────────────────────┘

MCP Message Types

Core Message Types

use turbomcp_protocol::{
    // Re-exported for convenience (most common types)
    InitializeRequest, InitializeResult,
    CallToolRequest, CallToolResult,
    ReadResourceRequest, ReadResourceResult,
    GetPromptRequest, GetPromptResult,
    // List types available via types module
};

use turbomcp_protocol::types::{
    ListToolsRequest, ListToolsResult,
    ListResourcesRequest, ListResourcesResult,
    ListPromptsRequest, ListPromptsResult,
    Tool, Resource, Prompt,
};

MCP Enhanced Types

use turbomcp_protocol::{
    // Bidirectional communication support (trait)
    ServerToClientRequests,
};

use turbomcp_protocol::types::{
    // Elicitation - Server requests user input
    ElicitRequest, ElicitResult,

    // Completion - Intelligent autocompletion
    CompleteRequest, CompletionResponse,

    // Resource Templates - Dynamic resources
    ListResourceTemplatesRequest, ListResourceTemplatesResult,

    // Ping - Bidirectional health monitoring
    PingRequest, PingResult,
};

JSON-RPC Infrastructure

use turbomcp_protocol::{
    JsonRpcRequest, JsonRpcResponse, JsonRpcNotification,
    JsonRpcError, JsonRpcErrorCode, RequestId,
};

Usage

Basic Protocol Handling

use turbomcp_protocol::{
    JsonRpcRequest, JsonRpcResponse, InitializeRequest,
    ListToolsRequest, Error as McpError
};

// Parse incoming JSON-RPC request
let json_data = r#"{
    "jsonrpc": "2.0",
    "id": 1,
    "method": "initialize",
    "params": {
        "protocolVersion": "2025-06-18",
        "capabilities": {},
        "clientInfo": {"name": "test-client", "version": "2.0.0"}
    }
}"#;

let request: JsonRpcRequest = serde_json::from_str(json_data)?;

// Handle specific message types
match request.method.as_str() {
    "initialize" => {
        let params = request.params.unwrap_or_default();
        let init_req: InitializeRequest = serde_json::from_value(params)?;
        // Process initialization
    },
    "tools/list" => {
        let params = request.params.unwrap_or_default();
        let tools_req: ListToolsRequest = serde_json::from_value(params)?;
        // Process tools list request
    },
    _ => {
        // Handle unknown method
    }
}

Message Validation

use turbomcp_protocol::{
    JsonRpcRequest,
    validation::{ProtocolValidator, ValidationResult}
};

// Create a validator with default rules
let validator = ProtocolValidator::default();

// Parse and validate a JSON-RPC request
let json_data = r#"{
    "jsonrpc": "2.0",
    "id": 1,
    "method": "tools/call",
    "params": {"name": "add", "arguments": {"a": 5, "b": 3}}
}"#;

let request: JsonRpcRequest = serde_json::from_str(json_data)?;
let result = validator.validate_request(&request);

match result {
    ValidationResult::Valid => {
        println!("Request is valid");
    },
    ValidationResult::ValidWithWarnings(warnings) => {
        println!("Request valid with {} warnings", warnings.len());
    },
    ValidationResult::Invalid(errors) => {
        eprintln!("Request invalid: {} errors", errors.len());
    }
}

Type-State Capability Builders

use turbomcp_protocol::capabilities::builders::{
    ServerCapabilitiesBuilder, ClientCapabilitiesBuilder
};

// Compile-time validated server capabilities
let server_caps = ServerCapabilitiesBuilder::new()
    .enable_tools()                    // Enable tools capability
    .enable_resources()                // Enable resources capability
    .enable_prompts()                  // Enable prompts capability
    .enable_tool_list_changed()        // ✅ Only available when tools enabled
    .enable_resources_subscribe()      // ✅ Only available when resources enabled
    .enable_resources_list_changed()   // ✅ Only available when resources enabled
    .build();

// Opt-out client capabilities (all enabled by default)
let client_caps = ClientCapabilitiesBuilder::new()
    .enable_roots_list_changed()       // Configure sub-capabilities
    .build();                          // All capabilities enabled!

// Opt-in pattern for restrictive clients
let minimal_client = ClientCapabilitiesBuilder::minimal()
    .enable_sampling()                 // Only enable what you need
    .enable_roots()
    .build();

Traditional Capability Negotiation

use turbomcp_protocol::{
    ServerCapabilities, ClientCapabilities,
    types::{ToolsCapabilities, ResourcesCapabilities, PromptsCapabilities, RootsCapabilities}
};

// Traditional approach (still supported)
let server_caps = ServerCapabilities {
    tools: Some(ToolsCapabilities {
        list_changed: Some(true),
    }),
    resources: Some(ResourcesCapabilities {
        subscribe: Some(true),
        list_changed: Some(true),
    }),
    prompts: Some(PromptsCapabilities {
        list_changed: Some(false),
    }),
    experimental: None,
    ..Default::default()
};

// Define client capabilities
let client_caps = ClientCapabilities {
    sampling: None,
    roots: Some(RootsCapabilities {
        list_changed: Some(true),
    }),
    experimental: None,
    ..Default::default()
};

Error Handling

The protocol crate provides Error, a rich MCP 2025-06-18 specification-compliant error type with comprehensive context and observability support.

Understanding Box<Error>

All error constructors return Box<Error> for important architectural reasons:

use turbomcp_protocol::Error;

// Constructors return Box<Error>, not Error
let err: Box<Error> = Error::tool_not_found("calculator");
let err: Box<Error> = Error::invalid_params("Email required");

Why Box<Error>?

1. Cheap Cloning: Errors clone efficiently across async boundaries
2. Rich Context Preservation: Full error chain, metadata, and backtrace
3. Observability Integration: Seamless tracing and metrics
4. Memory Efficiency: Error type is large (contains UUID, context, backtrace) - boxing keeps it off the stack

Creating Errors

use turbomcp_protocol::{Error, ErrorKind};

// MCP specification errors (map to standard error codes)
let err = Error::tool_not_found("calculator");              // -32001
let err = Error::tool_execution_failed("calc", "div by 0"); // -32002
let err = Error::prompt_not_found("code_review");           // -32003
let err = Error::resource_not_found("file:///missing");     // -32004
let err = Error::resource_access_denied("file:///etc/passwd", "forbidden"); // -32005
let err = Error::invalid_params("Email must be valid");     // -32602
let err = Error::user_rejected("User declined sampling");   // -1

// Add rich context with builder pattern
let err = Error::internal("Database error")
    .with_operation("user_create")
    .with_component("postgres_repository")
    .with_request_id("req-123")
    .with_context("user_id", user_id)
    .with_context("table", "users");

// Error chaining for root cause analysis
let database_error = Error::internal("Connection pool exhausted");
let app_error = Error::unavailable("Service temporarily unavailable")
    .with_source(database_error);

Working with JSON-RPC Errors

use turbomcp_protocol::{JsonRpcError, JsonRpcErrorCode, Error};

// Create JSON-RPC errors directly
fn handle_tool_error(error: &str) -> JsonRpcError {
    JsonRpcError {
        code: JsonRpcErrorCode::InvalidParams,
        message: format!("Tool validation failed: {}", error),
        data: None,
    }
}

// Convert protocol Error to JSON-RPC error code
let err = Error::tool_not_found("calculator");
let code = err.jsonrpc_error_code();  // -32001
let http = err.http_status_code();    // 404

// Create Error from JSON-RPC error code (preserves semantics)
let err = Error::rpc(-32001, "Tool 'calculator' not found");
assert_eq!(err.kind, ErrorKind::ToolNotFound);

Error Properties

use turbomcp_protocol::Error;

let err = Error::timeout("Request took too long");

// Check error characteristics
if err.is_retryable() {
    // Retry the operation
}

if err.is_temporary() {
    // Wait and retry
}

// Get HTTP status code for REST APIs
let status = err.http_status_code();  // 408

// Get MCP-compliant JSON-RPC error code
let code = err.jsonrpc_error_code();  // -32012

Integration with Application Layer

If you're using the main turbomcp crate, you typically use McpError in your tool handlers. The server layer automatically converts to Box<Error>:

// In your tool handler (turbomcp crate)
use turbomcp::{McpError, McpResult};

#[tool("My tool")]
async fn my_tool(&self) -> McpResult<String> {
    Err(McpError::tool("Something failed".into()))  // Simple error
}

// Server layer converts to:
// ServerError::Protocol(Error::tool_execution_failed("my_tool", "Something failed"))

See the turbomcp crate error handling docs for the complete error architecture.

Custom Message Types

use turbomcp_protocol::{JsonRpcRequest, JsonRpcResponse, RequestId};
use serde::{Serialize, Deserialize};

// Define custom message types
#[derive(Serialize, Deserialize)]
struct CustomRequest {
    custom_field: String,
    optional_data: Option<serde_json::Value>,
}

#[derive(Serialize, Deserialize)]
struct CustomResponse {
    result: String,
    metadata: serde_json::Value,
}

// Create custom JSON-RPC messages
fn create_custom_request(id: RequestId, params: CustomRequest) -> JsonRpcRequest {
    JsonRpcRequest {
        jsonrpc: "2.0".to_string(),
        id,
        method: "custom/method".to_string(),
        params: serde_json::to_value(params).unwrap(),
    }
}

Message Flow

sequenceDiagram
    participant Client
    participant Protocol as turbomcp-protocol
    participant Server
    
    Client->>Protocol: Raw JSON message
    Protocol->>Protocol: Parse JSON-RPC
    Protocol->>Protocol: Validate message format
    Protocol->>Protocol: Extract MCP message
    Protocol->>Protocol: Validate against schema
    Protocol->>Server: Typed MCP message
    Server->>Protocol: Typed MCP response
    Protocol->>Protocol: Serialize response
    Protocol->>Protocol: Wrap in JSON-RPC
    Protocol->>Client: JSON response

Version Selection Guide

Choosing the Right MCP Version

Use MCP 2025-06-18 (Stable) when:

• Building production systems
• Need maximum interoperability with existing MCP clients/servers
• Require stable, well-tested protocol features
• Want long-term API stability guarantees

Use MCP 2025-11-25 (Draft) when:

• Experimenting with cutting-edge features
• Building systems that need advanced capabilities (multi-select forms, tasks API, etc.)
• Contributing to MCP specification development
• Willing to accept potential breaking changes in future releases

Core Features (Always Enabled)

All core MCP 2025-11-25 features are now always available - no feature flags needed:

• URL Elicitation (SEP-1036) - URL mode for OAuth/sensitive data
• Sampling Tools (SEP-1577) - Tool calling in LLM sampling
• Icons (SEP-973) - Icon metadata for tools, resources, prompts
• Enum Improvements (SEP-1330) - Standards-compliant enum schemas

[dependencies]
turbomcp-protocol = "3.0.0-exp"  # All core features included

Runtime Version Negotiation

Client-side:

use turbomcp_protocol::{InitializeRequest, ClientCapabilities};

// Request draft features (server may downgrade)
let init = InitializeRequest {
    protocol_version: "2025-11-25".to_string(),
    capabilities: ClientCapabilities::default(),
    client_info: /* ... */,
    _meta: None,
};

Server-side:

use turbomcp_protocol::{InitializeResult, ServerCapabilities};

// Respond with actual supported version
let result = InitializeResult {
    protocol_version: "2025-06-18".to_string(),  // Or 2025-11-25
    capabilities: ServerCapabilities::default(),
    server_info: /* ... */,
    instructions: None,
    _meta: None,
};

Key Principle: Clients request, servers decide. The server's response version is the negotiated protocol version for the session.

Migration from v2.x

TurboMCP v3.0 simplifies feature flags - all MCP 2025-11-25 features are now core:

Before (v2.x):

turbomcp-protocol = { version = "2.x", features = ["mcp-url-elicitation", "mcp-icons"] }

After (v3.0):

turbomcp-protocol = "3.0.0-exp"  # All features included by default

Example:

// No feature guards needed - URLElicitRequestParams is always available
use turbomcp_protocol::types::URLElicitRequestParams;

fn handle_sensitive_input(params: URLElicitRequestParams) {
    // URL mode is always available
}

Feature Flags

Default Features

Feature	Description	Default
std	Standard library support	✅
simd	SIMD-accelerated JSON parsing (simd-json, sonic-rs)	✅

Performance Features

Feature	Description	Default
zero-copy	Zero-copy message handling with serde serialization	❌
messagepack	MessagePack serialization support	❌
mmap	Memory-mapped file support	❌
lock-free	Lock-free data structures (experimental, requires unsafe)	❌

Observability Features

Feature	Description	Default
tracing	OpenTelemetry tracing integration	❌
metrics	Prometheus metrics collection	❌
fancy-errors	Rich error reporting with miette	❌

Platform Features

Feature	Description	Default
wasm	WebAssembly support (no_std compatible)	❌

MCP 2025-11-25 Features

Core Features (Always Enabled)

These MCP 2025-11-25 features are now always available - no feature flag needed:

Feature	SEP	Description
URL Elicitation	SEP-1036	URL mode for OAuth/sensitive data collection
Sampling Tools	SEP-1577	Tool calling support in LLM sampling
Icons	SEP-973	Icon metadata for tools/resources/prompts
Enum Improvements	SEP-1330	Standards-based enum schemas (oneOf, anyOf)

Experimental Features (Require Feature Flag)

Feature	SEP	Description	Enabled by Default
experimental-tasks	SEP-1686	Experimental Tasks API for durable long-running requests	❌

Legend:

• ✅ : Always enabled (core feature)
• ❌ : Disabled by default (requires explicit feature flag)

Authentication & Security Features (Auth-Related SEPs):

Feature	Description	Specification
SSRF Protection	Built-in SSRF guards for URL validation	Always enabled
CIMD	Client ID Metadata Documents (OAuth 2.1)	Always enabled
OpenID Discovery	RFC 8414 + OIDC 1.0 discovery support	Always enabled
Incremental Consent	WWW-Authenticate with scope expansion (SEP-835)	Always enabled

Note: Auth features are always enabled for maximum security - no feature flag required.

Feature Flag Examples

Minimal build (stable spec only):

[dependencies]
turbomcp-protocol = { version = "3.0.0-exp", default-features = false, features = ["std"] }

High-performance build:

[dependencies]
turbomcp-protocol = { version = "3.0.0-exp", features = ["simd", "zero-copy", "lock-free"] }

Observable production build:

[dependencies]
turbomcp-protocol = { version = "3.0.0-exp", features = ["simd", "tracing", "metrics"] }

Full MCP 2025-11-25 support (default):

[dependencies]
turbomcp-protocol = "3.0.0-exp"  # All core features included

With experimental Tasks API:

[dependencies]
turbomcp-protocol = { version = "3.0.0-exp", features = ["experimental-tasks"] }

Supported MCP Methods

Core Methods

• initialize - Protocol initialization and capability negotiation
• initialized - Initialization completion notification

Tool Methods

• tools/list - List available tools
• tools/call - Execute a tool with parameters

Resource Methods

• resources/list - List available resources
• resources/read - Read resource content
• resources/updated - Resource change notification

Prompt Methods

• prompts/list - List available prompts
• prompts/get - Get prompt content

Capability Methods

• capabilities/changed - Capability change notification

Integration

With TurboMCP Framework

Protocol handling is automatic when using the main framework:

use turbomcp::prelude::*;

#[server]
impl MyServer {
    #[tool("Add numbers")]
    async fn add(&self, a: f64, b: f64) -> McpResult<f64> {
        // Protocol parsing and validation handled automatically
        Ok(a + b)
    }
}

Direct Protocol Usage

For custom implementations or integrations:

use turbomcp_protocol::{JsonRpcRequest, JsonRpcResponse};

struct CustomProtocolHandler;

impl CustomProtocolHandler {
    async fn handle_message(&self, raw_json: &str) -> Result<String, Box<dyn std::error::Error>> {
        // Parse JSON-RPC message
        let request: JsonRpcRequest = serde_json::from_str(raw_json)?;
        
        // Handle based on method
        let response = match request.method.as_str() {
            "tools/list" => self.handle_tools_list(request).await?,
            "tools/call" => self.handle_tools_call(request).await?,
            _ => return Err("Unknown method".into()),
        };
        
        // Serialize response
        Ok(serde_json::to_string(&response)?)
    }
}

Development

Building

# Build with all features
cargo build --all-features

# Build minimal (std only)
cargo build --no-default-features --features std

# Build with specific features
cargo build --features simd,messagepack,tracing,metrics

Testing

# Run protocol compliance tests
cargo test

# Test with all features enabled
cargo test --all-features

# Validate against MCP specification
cargo test mcp_compliance

Schema Validation

# Run validation tests
cargo test validation

# Run message validation tests
cargo test message_validation

Related Crates

• turbomcp - Main framework (uses this crate)
• turbomcp-transport - Transport layer
• turbomcp-server - Server framework

Note: In v2.0.0, turbomcp-core was merged into this crate to eliminate circular dependencies and improve cohesion.

External Resources

• MCP Specification - Official protocol specification
• JSON-RPC 2.0 - JSON-RPC specification
• JSON Schema - Schema validation specification

License

Licensed under the MIT License.

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Part of the TurboMCP Rust SDK for the Model Context Protocol.