6. Transports

The optional transports property declares one or more communication mechanisms for the MCP server. When present, it MUST contain at least one Transport Object. Omission means that the document declares no connection mechanism; it MUST NOT be interpreted as proof that the server has no transport.

6.1 Overview

MCP servers can be accessed through different transport mechanisms. The transports array allows a single MCP Description document to declare all supported transports, enabling clients to select the most appropriate one.

6.2 Transport Types

Each transport object MUST include a type property. The following transport types are defined:

Every Transport Object MAY contain protocolVersions and security. protocolVersions follows Section 4.5. A transport’s security value is a Security Requirement Array and follows Sections 6.4 and 7.

6.2.1 Streamable HTTP Transport

Property	Type	Required	Description
`type`	`"streamable-http"`	Yes	Transport type identifier
`url`	string (URI)	Yes	MCP endpoint URL

The streamable HTTP transport connects to an MCP server over HTTP with streaming response support. It is defined for MCP 2025-03-26 and later and is the RECOMMENDED transport for remote MCP servers in those revisions.

{
  "type": "streamable-http",
  "url": "https://chess-coach.example.com/mcp"
}

6.2.2 stdio Transport

Property	Type	Required	Description
`type`	`"stdio"`	Yes	Transport type identifier
`command`	string	Yes	Command to launch the server
`args`	array<string>	No	Command arguments
`env`	object	No	Environment variables (string values)

The stdio transport launches the MCP server as a subprocess and communicates over standard input/output.

{
  "type": "stdio",
  "command": "chess-coach",
  "args": ["mcp", "--level", "advanced"],
  "env": {
    "CHESS_DB_PATH": "/data/games.db"
  }
}

6.2.3 SSE Transport (Legacy)

Property	Type	Required	Description
`type`	`"sse"`	Yes	Transport type identifier
`url`	string (URI)	Yes	SSE endpoint URL

The Server-Sent Events transport is a legacy transport type retained for backward compatibility. It is the remote transport defined by MCP 2024-11-05. Validators SHOULD warn when it is associated with MCP 2025-03-26 or later, where Streamable HTTP is the standard remote transport. New implementations SHOULD use streamable-http instead.

{
  "type": "sse",
  "url": "https://chess-coach.example.com/sse"
}

6.3 Multiple Transports

A server MAY support multiple transports. Clients SHOULD select the most appropriate transport based on their environment and capabilities.

{
  "transports": [
    { "type": "streamable-http", "url": "https://chess-coach.example.com/mcp" },
    { "type": "stdio", "command": "chess-coach", "args": ["mcp"] }
  ]
}

6.4 Transport-Scoped Security

Each transport object MAY include a security property containing a Security Requirement Array (see Section 7). When present, transport security replaces root security for that transport.

Scenario	Effective security
Root `security` defined, transport `security` omitted	Inherits root security
Root `security` defined, transport `security` is `[]` (empty)	Explicitly no authentication
Root `security` defined, transport `security` defined	Transport’s own security
Root `security` omitted, transport `security` omitted	No authentication

This mechanism allows a single MCP Description document to declare different security requirements for different transports. For example, an HTTP transport typically requires bearer authentication while a stdio transport relies on OS-level process isolation:

{
  "transports": [
    {
      "type": "streamable-http",
      "url": "https://chess-coach.example.com/mcp",
      "security": [
        { "bearer": [] }
      ]
    },
    {
      "type": "stdio",
      "command": "chess-coach",
      "args": ["mcp"],
      "security": []
    }
  ]
}

The bearer name in this example MUST identify a root securitySchemes entry.

6.5 Protocol Coverage

When transports is present, the union of all effective transport protocol scopes MUST equal root protocolVersions. Transport scopes MAY overlap because a server can expose multiple transports for one revision.

A document with transports is invalid when any root revision has no applicable transport or when a transport scope contains a revision outside root coverage. When transports is omitted, transport coverage validation does not apply: validators MUST NOT infer a transport or report uncovered root revisions.

6.6 Extensibility

Transport Objects MAY carry x-* specification extensions. They MUST NOT contain other additional properties beyond those defined for their type plus the common optional protocolVersions and security properties.