Mocking and response modification

Mock patterns use deterministic URL globs. ** crosses path separators; * matches within one segment. The first matching rule wins. Shorthand mock rules have precedence, followed by ordered intercept rules, then upstream modify_response rules.

{
  "mock": [
    {
      "url": "**/api/payment",
      "delay_ms": 500,
      "response": { "status": 500, "json": { "error": "provider_down" } }
    }
  ]
}

Response JSON mutation uses RFC 7396 merge-patch semantics: objects merge and null removes a field. Header additions and removals are applied after the upstream response.