// protocol.fbs
// Core Model Context Protocol (MCP) message definitions
// Version: 1.0.0
include "base_types.fbs";
namespace mcp.fbs.v1.protocol;
// ============================================================================
// MCP Protocol Version
// ============================================================================
enum ProtocolVersion: int {
V2024_11_05 = 20241105,
V2025_01_01 = 20250101,
V2025_06_18 = 20250618
}
// ============================================================================
// Content Types
// ============================================================================
// Text content
table TextContent {
text: string (required);
}
// Image content
table ImageContent {
data: string (required); // Base64 encoded or URL
mime_type: string (required);
}
// Resource content reference
table ResourceContent {
uri: string (required);
mime_type: string;
text: string;
}
// Union of content types
union Content {
TextContent,
ImageContent,
ResourceContent
}
// ============================================================================
// Tool Definitions
// ============================================================================
// Tool parameter definition
table ToolParameter {
name: string (required);
description: string;
type: string; // JSON schema type
required: bool;
default_value: string;
}
// Tool definition
table Tool {
name: string (required, key);
description: string (required);
parameters: [ToolParameter];
input_schema: string; // JSON schema
metadata: [mcp.fbs.v1.KeyValue];
}
// Tool call request
table ToolCall {
id: mcp.fbs.v1.UUID (required);
tool_name: string (required);
arguments: [mcp.fbs.v1.KeyValue] (required);
timestamp: mcp.fbs.v1.Timestamp;
}
// Tool execution result
table ToolResult {
tool_call_id: mcp.fbs.v1.UUID (required);
success: bool;
content: [Content];
error: mcp.fbs.v1.Error;
metadata: [mcp.fbs.v1.KeyValue];
execution_time_ms: uint32;
}
// ============================================================================
// Resource Definitions
// ============================================================================
// Resource definition
table Resource {
uri: string (required, key);
name: string (required);
description: string;
mime_type: string;
size_bytes: uint64;
metadata: [mcp.fbs.v1.KeyValue];
}
// Resource content with data
table ResourceData {
resource: Resource (required);
content: Content (required);
last_modified: mcp.fbs.v1.Timestamp;
etag: string;
}
// ============================================================================
// Prompt Definitions
// ============================================================================
// Prompt argument definition
table PromptArgument {
name: string (required);
description: string;
required: bool;
default_value: string;
}
// Prompt message (for multi-turn prompts)
table PromptMessage {
role: string (required); // "user", "assistant", "system"
content: Content (required);
}
// Prompt template definition
table Prompt {
name: string (required, key);
description: string;
content: Content (required);
arguments: [PromptArgument];
messages: [PromptMessage];
metadata: [mcp.fbs.v1.KeyValue];
}
// Applied prompt with filled arguments
table AppliedPrompt {
prompt_name: string (required);
filled_content: Content (required);
argument_values: [mcp.fbs.v1.KeyValue];
}
// ============================================================================
// MCP Messages
// ============================================================================
// Message header
table MessageHeader {
version: ProtocolVersion = V2025_01_01;
message_id: mcp.fbs.v1.UUID (required);
timestamp: mcp.fbs.v1.Timestamp (required);
session_id: string;
correlation_id: string;
}
// Request types
union RequestPayload {
ToolCall,
ResourceRequest,
PromptRequest
}
// Response types
union ResponsePayload {
ToolResult,
ResourceData,
AppliedPrompt,
ErrorResponse
}
// Generic request message
table Request {
header: MessageHeader (required);
payload: RequestPayload (required);
}
// Generic response message
table Response {
header: MessageHeader (required);
payload: ResponsePayload (required);
success: bool;
}
root_type Request;
// ============================================================================
// Resource and Prompt Request Types
// ============================================================================
table ResourceRequest {
uri: string (required);
range_start: uint64;
range_end: uint64;
}
table PromptRequest {
name: string (required);
arguments: [mcp.fbs.v1.KeyValue];
}
table ErrorResponse {
error: mcp.fbs.v1.Error (required);
}
// ============================================================================
// Server and Client Capabilities
// ============================================================================
// Server capabilities
table ServerCapabilities {
supports_tools: bool;
supports_resources: bool;
supports_prompts: bool;
supports_sampling: bool;
supports_logging: bool;
protocol_version: ProtocolVersion;
server_name: string;
server_version: string;
}
// Client capabilities
table ClientCapabilities {
supports_tools: bool;
supports_resources: bool;
supports_prompts: bool;
protocol_version: ProtocolVersion;
client_name: string;
client_version: string;
}
// Initialization handshake
table InitializeRequest {
client_capabilities: ClientCapabilities (required);
client_info: [mcp.fbs.v1.KeyValue];
}
table InitializeResponse {
server_capabilities: ServerCapabilities (required);
server_info: [mcp.fbs.v1.KeyValue];
}
// ============================================================================
// Progress and Notifications
// ============================================================================
// Progress notification
table ProgressNotification {
operation_id: mcp.fbs.v1.UUID (required);
progress: float; // 0.0 to 1.0
message: string;
current_step: uint32;
total_steps: uint32;
}
// Log message
enum LogLevel: byte {
Debug = 0,
Info = 1,
Warning = 2,
Error = 3,
Critical = 4
}
table LogMessage {
level: LogLevel;
message: string (required);
timestamp: mcp.fbs.v1.Timestamp;
source: string;
metadata: [mcp.fbs.v1.KeyValue];
}
