MetaObject typedef

MetaObject = Map<String, Object?>

Represents the contents of a _meta field, which clients and servers use to attach additional metadata to their interactions.

Certain key names are reserved by MCP for protocol-level metadata; implementations MUST NOT make assumptions about values at these keys. Additionally, specific schema definitions may reserve particular names for purpose-specific metadata, as declared in those definitions.

Valid keys have two segments:

Prefix:

  • Optional — if specified, MUST be a series of labels separated by dots (.), followed by a slash (/).
  • Labels MUST start with a letter and end with a letter or digit. Interior characters may be letters, digits, or hyphens (-).
  • Implementations SHOULD use reverse DNS notation (e.g., com.example/ rather than example.com/).
  • Any prefix where the second label is modelcontextprotocol or mcp is reserved for MCP use. For example: io.modelcontextprotocol/, dev.mcp/, org.modelcontextprotocol.api/, and com.mcp.tools/ are all reserved. However, com.example.mcp/ is NOT reserved, as the second label is example.

Name:

  • Unless empty, MUST start and end with an alphanumeric character ([a-z0-9A-Z]).
  • Interior characters may be alphanumeric, hyphens (-), underscores (_), or dots (.).

Revisions: 2026-07-28, ext-tasks/2026-07-28.

Implementation

typedef MetaObject = Map<String, Object?>;