Changelog

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All notable changes to this project are documented in this file.

The format is based on Keep a Changelog, and this project adheres to Semantic Versioning.

v0.5.0

Added

  • Tool output schemas via deftool's output_schema: option, advertised as outputSchema in tools/list.
  • Structured tool results in the structuredContent field of tools/call, with the serialized JSON still returned in a text content block for backwards compatibility.

Changed

Declaring an output_schema now constrains what a tool may return. MCP 2025-11-25 requires that a server providing an output schema MUST return structured results conforming to it, and restricts those schemas to type: "object" at the root. Two rules are now enforced:

  • The schema's root type must be "object". Declaring an array or scalar root type raises ArgumentError where the tool is defined, rather than advertising a shape structuredContent cannot carry. To return a list, wrap it:

    # Before — no longer valid
output_schema: %{type: "array", items: %{type: "object"}}

# After
output_schema: %{
  type: "object",
  properties: %{entries: %{type: "array", items: %{type: "object"}}},
  required: ["entries"]
}

  • A tool declaring a schema must return a map. A list, a scalar, or pre-formatted content blocks cannot conform, so they now produce a tool execution error (isError: true) instead of a successful response silently missing the structuredContent the tool advertised. Unstructured content may accompany a structured result, but cannot replace it.

    Tools that return content blocks directly should omit output_schema.

Tools that declare no output schema are unaffected — every return shape that worked in 0.4.x still works.

Fixed

  • The README installation snippet recommended ~> 0.4.0, which excludes 0.5.x.

v0.4.0

Added

  • Idle sessions hibernate after hibernate_after (default 60s), substantially reducing memory held by long-lived idle sessions. Set to :infinity to disable.

v0.3.1

Added

  • CI workflow covering the oldest and current supported toolchains.

Fixed

  • Session RPC no longer crashes the request when it reaches a downed node in a distributed registry.

v0.3.0

Added

  • Prompts primitive: defprompt, prompts/list, prompts/get.
  • Resources primitive: defresource, defresource_template, resources/list, resources/templates/list, resources/read.

v0.2.0

Added

  • Tool annotations support in deftool.

Fixed

  • MCP spec compliance: origin validation, protocol version negotiation, and HTTP status codes.