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Changelog

Source: NEWS.md

plumber2mcp 0.4.0

Breaking Changes

  • Protocol Version Update: Upgraded from MCP protocol version 2024-11-05 to 2025-06-18
    • This is a breaking change for clients still using the old protocol version
    • Clients must support protocol version 2025-06-18 to communicate with this version

Critical Bug Fixes

  • Fixed default parameter value serialization (#issue-tbd)
    • Default values using c(), list(), and other R expressions are now correctly evaluated
    • Previously, c("a", "b") was serialized as ["c", "a", "b"] (incorrect)
    • Now correctly serialized as ["a", "b"] (correct)
    • This fixes Claude Desktop compatibility issues where tools were disabled due to invalid schemas
    • Added comprehensive test suite (22 new tests) for default value handling
    • Fixes validation errors: “malformed default” in MCP schema validators
  • Fixed description field being array instead of string (#issue-tbd)
    • When endpoints had multiple documentation sources (plumber + roxygen), description was an array
    • Previously: {"description": ["First doc", "Second doc"]} (incorrect - violates MCP spec)
    • Now: {"description": "First doc"} (correct - single string as required by MCP spec)
    • Prioritizes plumber-specific documentation (#* comments) over roxygen docs (#’ comments)
    • Filters out NA values and empty strings before processing
    • This fixes Claude Desktop showing servers as disabled/grayed out
    • Added comprehensive test suite (17 new tests) for description handling
    • Ensures strict MCP schema compliance

New Features

MCP Protocol 2025-06-18 Support

  • HTTP Header Support: Added MCP-Protocol-Version header for HTTP transport (required by new spec)
  • Structured Tool Output: Re-enabled outputSchema support for tools (now standard feature in 2025-06-18)
  • Tool Titles: Added optional title fields to tools for human-friendly display names
    • Automatically extracted from roxygen comments or generated from method and path

Improvements

  • Better protocol negotiation during initialization
  • Enhanced tool definitions with both machine-readable names and human-friendly titles
  • Output schemas now provide AI assistants with structured format expectations
  • All 393 tests passing with new protocol version

Technical Details

  • Updated all protocol version strings throughout codebase
  • Modified HTTP handler to set and read MCP-Protocol-Version header
  • Updated both HTTP and stdio transports to include new fields
  • Enhanced handle_tools_list to include title and outputSchema when available
  • Package documentation updated to reflect new protocol version

Migration Guide

For users upgrading from 0.3.0: * No code changes required - the package handles the new protocol automatically * MCP clients must support protocol version 2025-06-18 * Tools now include optional title and outputSchema fields * HTTP responses include MCP-Protocol-Version header

plumber2mcp 0.3.0

New Features

Prompts Support

  • Added full MCP Prompts support allowing you to define reusable prompt templates that AI assistants can discover and use
  • New pr_mcp_prompt() function to register prompt templates with your MCP server
  • Prompts support optional arguments with required/optional parameters
  • Prompts can return simple strings, structured messages, or multi-turn conversations
  • Both HTTP and stdio transports fully support prompts
  • Added prompts/list and prompts/get JSON-RPC handlers
  • Updated initialize response to advertise prompts capability

Enhanced Testing

  • Added comprehensive edge case testing (24 new tests)
  • Added integration tests for combined features (10 new tests)
  • Added security and validation tests (18 new tests)
  • Total test count increased from ~69 to ~121 tests (75% increase)
  • Tests now cover error handling, malformed inputs, injection attacks, and real-world scenarios

Improvements

  • Enhanced schema generation now includes output schemas
  • Better error handling for malformed JSON-RPC requests
  • Improved handling of NULL, NA, and empty values
  • More robust handling of complex nested data structures
  • Added support for functions with req/res parameters

Documentation

  • Comprehensive README updates with prompts examples and use cases
  • Added example file demonstrating prompts usage (inst/examples/prompts_example.R)
  • Detailed documentation of prompt message formats
  • Added use cases for workflow guidance, code generation, and domain-specific assistance

Bug Fixes

  • Fixed parameter ordering in stdio transport handlers
  • Fixed JSON-RPC error responses for unknown methods
  • Improved type mapping for plumber parameters

plumber2mcp 0.2.0

New Features

Resources Support

  • Added MCP Resources support for exposing R content to AI assistants
  • New pr_mcp_resource() function to add custom resources
  • New pr_mcp_help_resources() convenience function for R help topics
  • Resources can provide documentation, data summaries, session info, and more
  • Added resources/list and resources/read handlers

Stdio Transport

  • Added native stdio transport support for standard MCP clients
  • New pr_mcp_stdio() function for stdio-based servers
  • Compatible with mcp-cli, Claude Desktop, and other MCP clients
  • Handles JSON-RPC over stdin/stdout

Improvements

  • Enhanced schema generation with rich descriptions from roxygen comments
  • Automatic detection of required vs optional parameters
  • Improved type inference for function parameters and return values
  • Better handling of empty properties (serialize as object, not array)

Documentation

  • Comprehensive README with examples for all transport types
  • Added resources examples and documentation
  • Added MCP Inspector testing instructions

plumber2mcp 0.1.0

Initial Release

Core Features

  • HTTP transport for MCP protocol
  • Automatic endpoint discovery from Plumber APIs
  • JSON-RPC 2.0 protocol implementation
  • Tool schema generation from function signatures
  • Support for include/exclude endpoint filters
  • Customizable server name, version, and mount path

MCP Protocol Support

  • initialize - Server initialization and capabilities
  • tools/list - List available API endpoints as tools
  • tools/call - Execute API endpoints through MCP
  • ping - Server health check

Documentation

  • Basic README with installation and usage
  • Example Plumber API files
  • GitHub Actions CI/CD setup