mcptools 1.0.0

mcp_server()

New features:

• mcptools can now run as a Posit Connect R API engine. Add a _server.yml with engine: mcptools, point tools to an .R file returning ellmer::tool() objects, and deploy with rsconnect::deployAPI(".", contentCategory = "mcp").

• mcp_server() can now return inline image content from tools that produce ellmer::ContentImageInline results, including mixed text and image content (#96, #102).

• mcp_server() now returns structuredContent alongside serialized JSON text for successful tool results that are naturally represented as JSON objects, when using MCP protocol version 2025-06-18 or later (#104).

• mcp_server() now includes ellmer tool annotations in tools/list responses, preserving MCP safety hints such as title, readOnlyHint, destructiveHint, idempotentHint, and openWorldHint (#100, #105).

Bug fixes:

• HTTP mcp_server() requests now honor the MCP-Protocol-Version header, return 400 Bad Request for unsupported protocol versions, and no longer let protocol negotiation from one HTTP client shape responses for another.

• JSON output now serializes R NULL values as JSON null, fixing JSON-RPC responses with null request IDs.

• mcp_server() no longer falls through after reporting Invalid Request for invalid stdio client messages.

• Forwarded mcp_session() tool calls now return JSON-RPC errors when the selected R session does not respond within two minutes, rather than hanging indefinitely. Configure the timeout with the mcptools.session_response_timeout_seconds option or the MCPTOOLS_SESSION_RESPONSE_TIMEOUT_SECONDS environment variable. Session receive errors are also logged instead of silently discarded (#98).

mcp_tools()

New features:

• mcp_tools() can now connect directly to remote Streamable HTTP MCP servers, configured with url instead of command. Static headers are supported for token auth, and full OAuth 2.1 (authorization-server discovery, Dynamic Client Registration, PKCE, and automatic token refresh) is handled via httr2, which also caches tokens across sessions (#88).

• mcp_tools() now converts MCP tool-result content blocks into ellmer-native text and image content, allowing ellmer chats to receive image results from MCP tools.

Bug fixes:

• mcp_tools() now launches MCP server processes with an allowlisted environment plus configured env variables. Previously, servers without configured env inherited the full R process environment, while servers with configured env received only those variables. The new behavior more closely matches reference MCP SDKs, reduces accidental credential exposure, and fixes Windows startup failures when env is configured. Servers that need additional non-allowlisted variables should list them in env (#62).

mcptools 0.2.1

• mcp_server() now ensures that inputSchema always includes a properties field, even for tools with no arguments (#91 by @itkonen).

• mcp_server() now negotiates the protocol version with clients, supporting versions 2024-11-05 through 2025-11-25 (#92 by @itkonen).

mcptools 0.2.0

Server

• mcp_server() now supports HTTP transport in addition to stdio. Use type = "http" to start an HTTP server, with optional host and port arguments. For now, the implementation is authless.

• mcp_server() now formats tool results in the same way as ellmer (#78 by @gadenbuie).

• mcp_server() gains logical argument session_tools, allowing users to opt-out of presenting R session tools (that make it possible to communicate with mcp_session()s) to clients.

• Several tightenings-up of the implementation:

  • JSON-RPC responses now retain an explicit id = NULL value, ensuring parse-error replies conform to the MCP specification.
  • mcp_tools() now sends and receives a "notifications/initialized" (#77 by @galachad).
  • The implementation now supports the 2025-06-18 protocol version, updated from 2024-11-05.

• mcp_session() now invisibly returns the nanonext socket used for communicating with the server.

Client

• Notably, mcp_tools() did not gain an implementation of the HTTP transport. Instead, we now recommend the @npx mcp-remote tool for serving local MCP servers via the HTTP transport in the documentation.

• mcp_tools() now errors more informatively when an MCP server process exits unexpectedly (#82).

mcptools 0.1.1

• Addressed an issue in tests on r-devel-linux-x86_64-fedora-clang.

mcptools 0.1.0

• Initial CRAN submission.

Before the initial release of the package, mcptools was called acquaint and supplied a default set of tools from btw, currently a GitHub-only package, when R was used as an MCP server. The direction of the dependency has been reversed; to use the same functionality from before, transition acquaint::mcp_server() to btw::btw_mcp_server() and acquaint::mcp_session() to btw::btw_mcp_session().

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