Crate fastmcp_protocol 

MCP protocol types and JSON-RPC implementation.

This crate provides:

• JSON-RPC 2.0 message types
• MCP-specific method types (tools, resources, prompts)
• Protocol version negotiation
• Message serialization/deserialization

MCP Protocol Overview

MCP (Model Context Protocol) uses JSON-RPC 2.0 over various transports. The protocol defines:

• Tools: Executable functions the client can invoke
• Resources: Data sources the client can read
• Prompts: Template prompts for the client to use

Wire Format

All messages are newline-delimited JSON (NDJSON).

Role in the System

fastmcp-protocol is the shared vocabulary for FastMCP:

• The server uses these types to validate and serialize responses.
• The client uses the same types to construct requests and parse replies.
• Transports carry these messages without needing to know business logic.

If you are integrating FastMCP with a custom runtime or embedding it into another system, depend on this crate to get the canonical JSON-RPC and MCP data models.

Re-exports

pub use schema::ValidationError;

pub use schema::ValidationResult;

pub use schema::validate;

pub use schema::validate_strict;

Modules

schema

JSON Schema validation for MCP tool inputs.

Structs

CallToolParams

tools/call request params.

CallToolResult

tools/call response result.

CancelTaskParams

tasks/cancel request params.

CancelTaskResult

tasks/cancel response result.

CancelledParams

Cancelled notification params.

ClientCapabilities

Client capabilities.

ClientInfo

Client information.

CreateMessageParams

sampling/createMessage request params.

CreateMessageResult

sampling/createMessage response result.

ElicitCompleteNotificationParams

Elicitation complete notification params.

ElicitRequestFormParams

Parameters for form mode elicitation requests.

ElicitRequestUrlParams

Parameters for URL mode elicitation requests.

ElicitResult

elicitation/create response result.

ElicitationCapability

Elicitation capability.

ElicitationRequiredErrorData

Error data for URL elicitation required errors.

FormElicitationCapability

Capability for form mode elicitation.

GetPromptParams

prompts/get request params.

GetPromptResult

prompts/get response result.

GetTaskParams

tasks/get request params.

GetTaskResult

tasks/get response result.

Icon

Icon metadata for visual representation of components.

InitializeParams

Initialize request params.

InitializeResult

Initialize response result.

JsonRpcError

JSON-RPC 2.0 error object.

JsonRpcRequest

JSON-RPC 2.0 request.

JsonRpcResponse

JSON-RPC 2.0 response.

ListPromptsParams

prompts/list request params.

ListPromptsResult

prompts/list response result.

ListResourceTemplatesParams

resources/templates/list request params.

ListResourceTemplatesResult

resources/templates/list response result.

ListResourcesParams

resources/list request params.

ListResourcesResult

resources/list response result.

ListRootsParams

roots/list request params.

ListRootsResult

roots/list response result.

ListTasksParams

tasks/list request params.

ListTasksResult

tasks/list response result.

ListToolsParams

tools/list request params.

ListToolsResult

tools/list response result.

LogMessageParams

Log message notification params.

LoggingCapability

Logging capability.

ModelHint

A hint for model selection.

ModelPreferences

Model preferences for sampling requests.

ProgressParams

Progress notification params.

Prompt

Prompt definition.

PromptArgument

Prompt argument definition.

PromptMessage

A message in a prompt.

PromptsCapability

Prompt capabilities.

ReadResourceParams

resources/read request params.

ReadResourceResult

resources/read response result.

RequestMeta

Request metadata containing optional progress marker.

Resource

Resource definition.

ResourceContent

Resource content in a message.

ResourceTemplate

Resource template definition.

ResourceUpdatedNotificationParams

Resource updated notification params.

ResourcesCapability

Resource capabilities.

Root

A root definition representing a filesystem location.

RootsCapability

Roots capability.

RootsListChangedNotificationParams

Notification params for roots/list_changed.

SamplingCapability

Sampling capability.

SamplingMessage

A message in a sampling conversation.

ServerCapabilities

Server capabilities advertised during initialization.

ServerInfo

Server information.

SetLogLevelParams

logging/setLevel request params.

SubmitTaskParams

tasks/submit request params.

SubmitTaskResult

tasks/submit response result.

SubscribeResourceParams

resources/subscribe request params.

TaskId

Task identifier.

TaskInfo

Information about a background task.

TaskResult

Task result payload.

TaskStatusNotificationParams

Task status change notification params.

TasksCapability

Task capability for server capabilities.

Tool

Tool definition.

ToolAnnotations

Tool annotations for additional metadata.

ToolsCapability

Tool capabilities.

UnsubscribeResourceParams

resources/unsubscribe request params.

UrlElicitationCapability

Capability for URL mode elicitation.

Enums

Content

Content types in MCP messages.

ElicitAction

User action in response to an elicitation request.

ElicitContentValue

Content type for elicitation responses.

ElicitMode

Elicitation mode.

ElicitRequestParams

Parameters for elicitation requests (either form or URL mode).

IncludeContext

Context inclusion mode for sampling.

JsonRpcMessage

A JSON-RPC message (request, response, or notification).

LogLevel

Log level.

ProgressMarker

Progress marker used to correlate progress notifications with requests.

RequestId

JSON-RPC request ID.

Role

Role in prompt messages.

SamplingContent

Message content for sampling requests.

StopReason

Stop reason for sampling responses.

TaskStatus

Status of a background task.

Constants

JSONRPC_VERSION

The JSON-RPC version string. Used as a static reference to avoid allocations.

PROTOCOL_VERSION

MCP protocol version.

Type Aliases

ElicitRequestedSchema

JSON Schema for elicitation requests.