Under a multiplexed gateway, `_build_safe_env` forwarded `os.environ[name]` for
every name tagged in the process-global `_SECRET_SOURCES` map. That map is
filled by EVERY served profile's secret-source hydration, while `os.environ`
only ever holds the LAUNCH (default) profile's values — so once any profile's
1Password/Bitwarden source supplied e.g. GITHUB_TOKEN, every profile's stdio
MCP server was started with the default profile's token.
Resolve those names through the active profile's secret scope (`get_secret`)
instead: the routed profile's value, or omitted when that profile has none.
Under multiplex `get_secret` never falls through to environ; single-profile
runs keep the .env overlay + environ behaviour, so the existing "vault vars
reach MCP subprocesses" contract still holds there. `secret_source_names()`
exposes the tagged NAMES only — values are never read from the shared map.
Docs: the multi-profile guide's "MCP subprocesses only see their own profile's
secrets" claim is now true for source-injected names too; say so explicitly.
Teknium
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