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The blocker on this issue was whether BCC or the client does the expanding.
Answer, from Anthropic's docs: Claude Code expands ${VAR} and
${VAR:-default} itself, in command, args, env, url and headers, for both
project .mcp.json and user-scope ~/.claude.json. Claude Desktop has no
documented support.
So this is a per-client capability, not a global one, and BCC does NOT
expand on write: resolving a reference into the file would put the secret
back on disk -- the whole thing the user is avoiding -- and would defeat a
feature the client already implements correctly. BCC authors, validates and
warns; expand_env_refs exists to preview what the client will do.
Semantics mirror the documented ones exactly, including the unusual bit:
an unset variable with no default is left as literal ${VAR} text rather
than blanked, because that is what Claude Code passes through.
Gating uses the existing profile_targets_claude_desktop(), so a config that
is correct under Claude Code and broken under Desktop is reported against
whichever profile is actually loaded. The two warnings are worded
differently on purpose -- 'this client will never expand these' is a
different problem from 'this variable looks unset here'.
Two existing behaviours were backwards for this feature and are fixed:
- Secret masking hid placeholders. is_secret_key('API_KEY') is true, so
${API_KEY} rendered as dots -- making a reference indistinguishable from
a stored credential, which is the one distinction that makes the feature
worth adopting. should_mask_value() now skips references, in the table
delegate, _redact_server_data and redact_args alike.
- args_secret_warning fired on placeholders. Moving a token into ${VAR} is
the recommended fix for that warning; continuing to warn punished the
fix. It now skips references while still flagging a real secret that
follows one.
Real secrets are still masked everywhere they were before -- asserted, not
assumed.
Refs #76