3.9.1

Changed

• The optional-argument-allowlist startup warning now names the remedy. It
  previously said only "optional argument allowlist may fail open", which
  identified the role, tool and argument but gave an operator nothing to act on.
  It now explains the failure mode — the allowed-value list is enforced only
  when the caller supplies the argument, so a tool substituting its own default
  bypasses it — and names the fix: required = true in TOML, or
  ArgumentAllowlist::new_required in code. Warning text only; no behaviour
  change. This remains the only mitigation available in 3.x, because flipping
  the required default is a breaking change reserved for 4.0.

Documentation

• The README Cargo-features table listed only two of the four declared
  features. oauth-mtls-client and test-helpers were both absent, so a
  reader arriving from crates.io had no signal that test-helpers exists — let
  alone that it must never be enabled in a production build, since some of its
  helpers deliberately bypass SSRF screening, the JWKS refresh cooldown, the CDP
  discovery rate limiter, and CRL verifier publication. That hazard was already
  documented in src/lib.rs, Cargo.toml and docs/GUIDE.md, but not on the
  landing page most consumers read first.
• Operator guidance for key_eviction_policy. The guide now explains the
  trade-off rather than only listing the accepted values: evict_lru (the
  default) favours admitting new clients at the cost of letting an evicted quiet
  tenant return with a fresh quota, while reject_new favours isolating
  established tenants at the cost of turning away genuinely new ones. It records
  why evict_lru is the default — under a high-cardinality spray attack
  reject_new would let an attacker fill the table and deny every new
  legitimate client — and states that a reject_new rejection returns 503,
  not 429, because it signals server-side admission capacity rather than a
  per-client quota breach.