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This release includes several security-oriented fixes, in addition to a couple quality of life improvements for multi-user and multi-session workspaces:
The server now chooses its R session at tool-call time rather
than always connecting to the first session: it prefers the session
whose working directory matches its own (clients like Claude Code and
Posit Assistant launch the server in the project directory), falling
back to the only running session. When several sessions are running and
none matches, tools execute in the server’s own R process until the
client calls select_r_session. Previously, a second client
+ session pair in another project window would silently execute tools in
the first project’s session (#114).
Sessions now use per-user filesystem IPC sockets in an owner-only
(0700) directory instead of a shared address. On multi-user Linux hosts
(e.g. Posit Workbench) the previous default let any local user connect
to another user’s mcp_session() and execute tools in it;
sessions are now isolated by Unix user. The socket directory follows
MCPTOOLS_SOCKET_DIR >
XDG_RUNTIME_DIR/mcptools/ >
$TMPDIR/mcptools-<user>/ >
/tmp/mcptools-<user>/ (@kwbyron-lilly, #114).
The session and server now authenticate every IPC message with a per-user secret stored alongside the sockets, so a session acts only on tool calls from the paired server and the server accepts only responses from a genuine session.
Tools returning ellmer::content_image_url() inline
the image by fetching it server-side; that fetch is now restricted to
http/https URLs, and any redirects are
followed by mcptools itself so each hop is re-checked rather than
trusted to curl. When the server is reached over the network (an HTTP
deployment), the fetch additionally refuses private, loopback, and
link-local addresses, so a tool argument cannot steer it at
cloud-metadata endpoints, internal hosts, or local files. Local (stdio)
servers keep fetching addresses on their own machine.
Socket files left behind by a crashed session are now reclaimed automatically: the next session that needs the slot detects the dead file and reuses it.
list_r_sessions() no longer returns spurious
"5" entries when a session is slow to respond to discovery
probes.
mcp_tools() now initiates the MCP OAuth
authorization-code flow automatically when a remote server configured
with only a url answers an unauthenticated request with a
401 challenge. Previously, connecting to such a server
required either a static Authorization header or an
explicit oauth block; the oauth block is now
needed only to override defaults.
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).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).
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:
id = NULL
value, ensuring parse-error replies conform to the MCP
specification.mcp_tools() now sends and receives a
"notifications/initialized" (#77 by @galachad).mcp_session() now invisibly returns the nanonext
socket used for communicating with the server.
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).
r-devel-linux-x86_64-fedora-clang.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().
These binaries (installable software) and packages are in development.
They may not be fully stable and should be used with caution. We make no claims about them.
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