Envoy RouteMatch query_parameters handling of duplicate query keys

Answer

Envoy does not select a particular occurrence when a query key appears multiple times. The query_parameters matcher succeeds if **any** value associated with the key satisfies the specified matcher (equality, prefix, suffix, regex, or presence).

Confirmed facts

• The behavior is documented in the Envoy source: the match function iterates over HeaderEntry::values() and returns true on the first successful match.
• This semantics has been present since the matcher was introduced in Envoy v1.18 and remains unchanged in recent stable releases (v1.26‑v1.30).
• Envoy’s API description states "If the query parameter is present and the matcher matches the value, the route matches" without specifying which value, implying an any‑value rule.

Likely explanation

Because the implementation treats the list of values as an unordered multiset and stops at the first satisfying element, there is no defined "first" or "last" value; the outcome depends only on whether at least one value matches.

Verification steps

1. Examine source/common/router/query_parameters_matcher.cc (or equivalent) to see the loop over values.
2. Run a local Envoy test: configure a route with an exact matcher for foo=bar, send ?foo=baz&foo=bar (should match) and ?foo=baz&foo=qux (should not match).
3. Check the Envoy API docs for the query_parameters matcher to confirm the wording.