ACP native-process proof for https://github.com/pingdotgg/t3code/pull/19567
Head: 5b07a62439d9d5b0d36664a3275e27aecc5d19b0
Base: ab6be13d438abeace7617e8d789d59cf2e880f42
Linux, Node 24. Independent local NDJSON ACP peer, real registry probe and adapter/session runtime.
Catalog resolver and provider settings point to the peer; runtime/process transport is not injected.
No external fx/Cline/Grok service, authenticated provider account, browser or native client used.
No UI layout changes; protocol output is the relevant evidence.

Before: six production files loaded from base through external Vite aliases; same peer and scenario.
Both scenarios fail during actual adapter.openSession for the final advertised catalog choice, later-model:
selects: AcpRequestError -32602, invalid later-model for config provider; allowed gateway,codex.
mixed (preceding model-category boolean): AcpRequestError -32602, invalid later-model for config fast; expected boolean.
Before result: 2 failed tests, 0 passed. Correct cause confirmed in before.log.

After: unmodified committed head. Both scenarios pass: catalog gateway,codex, current gateway;
select final advertised model codex; real wire write session/set_config_option configId provider value codex.
Explicit later-model choice fails with typed -32602 before a write; total config writes remains one.
After result: 2 passed tests. All captured before/after child processes exited after scope cleanup.

After receipts (synthetic fixture values only):
[
  {
    "mode": "selects",
    "catalog": [
      {
        "id": "gateway",
        "name": "Gateway",
        "description": null
      },
      {
        "id": "codex",
        "name": "Codex",
        "description": null
      }
    ],
    "currentModelId": "gateway",
    "actualWrites": [
      {
        "sessionId": "live-option-session",
        "configId": "provider",
        "value": "codex"
      }
    ],
    "laterChoiceRejected": true,
    "pids": [
      311906,
      311913,
      311920
    ]
  },
  {
    "mode": "mixed",
    "catalog": [
      {
        "id": "gateway",
        "name": "Gateway",
        "description": null
      },
      {
        "id": "codex",
        "name": "Codex",
        "description": null
      }
    ],
    "currentModelId": "gateway",
    "actualWrites": [
      {
        "sessionId": "live-option-session",
        "configId": "provider",
        "value": "codex"
      }
    ],
    "laterChoiceRejected": true,
    "pids": [
      311927,
      311934,
      311941
    ]
  }
]