[atesgoral]

Motivation and Context

Client::Stdio passed the whole parent environment to the server process, because Open3.popen3 merges env: into it. That includes variables the server has no use for. The server process now starts with a small base environment (an OS-specific allow-list based on the Python SDK lists, with GEM_HOME and GEM_PATH added for Ruby), with env: merged over it. Explicit values override inherited ones, and nil removes one. BUNDLE_GEMFILE is excluded from the defaults because it selects an application's Gemfile. A caller can pass it through env:. inherit_env: true restores full inheritance. I used Ruby's unsetenv_others: true so that Ruby enforces the boundary when it spawns the process. README and the transport docs describe the new default.

Spec and reference SDK checks:

• MCP 2025-11-25, Transports, stdio: "The client launches the MCP server as a subprocess."
• MCP 2026-07-28, stdio: "In the stdio transport, the client launches the MCP server as a subprocess." Neither section prescribes an inherited-environment list. This changes an SDK default, not message validation or lifecycle behavior.
• TypeScript DEFAULT_INHERITED_ENV_VARS and getDefaultEnvironment: "Environment variables to inherit by default, if an environment is not explicitly given." Its spawn code merges ...getDefaultEnvironment(), ...this._serverParams.env, so an explicit caller value remains accepted.
• Python's current platform lists match the v1.26.0 lists. Its env documentation says: "Extra environment variables, merged over get_default_environment()."

Neither reference SDK includes BUNDLE_GEMFILE in its defaults. Our platform base follows Python's; TypeScript's Windows list is broader. Ruby adds GEM_HOME and GEM_PATH so RubyGems can find nondefault gem locations. Both platform arrays are now named frozen constants.

Fixes #662.

How Has This Been Tested?

test/mcp/client/stdio_environment_test.rb (new) starts real subprocesses. It covers omitted, empty and explicit env:, the base values, RubyGems paths, default omission and explicit selection of BUNDLE_GEMFILE, case-insensitive lookup as on Windows, override and removal, and inherit_env: true. The default-isolation cases fail on main. The Gemfile-omission regression fails on the previous published implementation. Explicit Gemfile selection and RubyGems-path inheritance pass there and remain covered. test/mcp/client/stdio_test.rb has a small update.

The stdio tests pass (69 runs, 290 assertions), and so does the full suite (2,513 runs, 8,280 assertions). RuboCop is clean on the changed files. I did not run it on Windows.

Breaking Changes

Yes. The default changes: the server process no longer inherits the full parent environment. The inherited base includes GEM_HOME and GEM_PATH on Unix and Windows. Pass other variables a server needs through env:, or set inherit_env: true to keep the old behavior.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

[koic]

BUNDLE_GEMFILE should not be inherited by default. Under bundle exec it names the client application's Gemfile, so a server from another project started with bundle exec resolves its gems against the client's Gemfile and loads the wrong versions or fails to start; with RUBYOPT, RUBYLIB and BUNDLE_BIN_PATH not inherited, the Bundler state is partial as well. GEM_HOME and GEM_PATH are fine to keep. A caller that wants the same Gemfile can pass it through env:.

[koic]

And, It might be better to extract this into a constant.

[atesgoral]

BUNDLE_GEMFILE is no longer in either default list. GEM_HOME and GEM_PATH stay inherited, and a caller can supply a Gemfile through env:. I also extracted the Windows list into the frozen WINDOWS_INHERITED_ENV_VARS constant.

[koic]

Drop BUNDLE_GEMFILE here, in the README sentence and in the test expectation as well.

[atesgoral]

Removed BUNDLE_GEMFILE from the README, both documented default lists and the default-inheritance expectation. Added regression tests for default omission and explicit selection through env:.

[koic]

It might be better to extract this into a constant.

Ditto.

[atesgoral]

The Unix list is now the frozen UNIX_INHERITED_ENV_VARS constant. DEFAULT_INHERITED_ENV_VARS selects the appropriate named platform list.