[Bug]: ACP registry drops an agent's empty-valued default choice, so GitHub Copilot always runs a custom agent #16898

Before submitting

• I searched existing issues and did not find a duplicate.
• I included enough detail to reproduce or investigate the problem.

Area

apps/server

Steps to reproduce

1. Create any GitHub Copilot CLI custom agent, for example ~/.copilot/agents/example.agent.md:

   ---
   name: example
   description: Example agent.
   ---
   An example agent.

2. Add GitHub Copilot through the ACP registry (acpRegistry, agent github-copilot-cli, CLI 1.0.93).
3. Start a new Copilot thread and open the Agent menu in the composer.

Happens with Settings → Plan mode on or off.

Expected behavior

The Agent menu offers Copilot's default agent ("Copilot") next to the custom agents and selects it by default, because that is what Copilot reports. New threads run the default Copilot agent unless the user picks a custom one.

Actual behavior

The Agent menu lists only example, and it is already selected. The default agent can't be picked. Once example is selected, there is no option to switch back to the default Copilot agent. Every new thread sends session/set_config_option agent=example, so the custom agent's instructions and its tools: allowlist apply to every Copilot thread. With a restrictive tools: list, the session loses bash, view, edit and similar tools.

Root cause

Copilot reports the default agent as a choice whose value is an empty string, and says that choice is selected (raw session/new response from copilot --acp --stdio):

{
  "type": "select",
  "id": "agent",
  "name": "Agent",
  "category": "_agent",
  "description": "Select a custom agent persona, or use the default Copilot agent.",
  "currentValue": "",
  "options": [
    { "value": "", "name": "Copilot", "description": "Default Copilot agent" },
    { "value": "example", "name": "example", "description": "Example agent." }
  ]
}

T3 handles it like this:

1. apps/server/src/provider/acp/AcpSessionConfig.ts: selectChoices passes every choice value through boundedOpaqueValue, which returns undefined for an empty string (value.length > 0). That drops the "" / "Copilot" choice. currentValue: "" is falsy, so the descriptor ends up with no currentValue either.
   Even without that check, the choice couldn't be stored: ProviderOptionChoice.id and SelectProviderOptionDescriptor.currentValue in packages/contracts/src/model.ts are both TrimmedNonEmptyString.
2. Contributing factor: apps/web/src/providerModels.ts: withoutPlanAgentOption, added in fix(web): hide opencode's plan agent when legacy plan mode is off #6420 for opencode's plan agent, runs on every provider's agent descriptor whenever planModeEnabled is false, which is the default. When there is no currentValue, it falls back to options[0]?.id, which is example here. With plan mode on, this fallback is skipped, but the default choice is still missing and any earlier example selection persists (see 3).
3. The composer then dispatches agent=example. The ACP adapter calls setConfigOption("agent", "example"), and the live configuration is published back into the provider snapshot (~/.t3/caches/acpRegistry_github_copilot.json now shows "currentValue": "example" with a single option). The composer also keeps agent=example as the last-used selection. This keeps the wrong default for future threads.

Suggested fix

• Map an empty choice value to a reserved ID (for example __acp_default__) in acpProviderOptionDescriptors, keep a currentValue of "" by mapping it the same way, and turn the reserved ID back into "" before setConfigOption.
• Limit withoutPlanAgentOption to the opencode driver, or at least don't fall back to options[0] for drivers other than opencode.

Impact

Minor bug or occasional failure

Version or commit

Desktop 0.0.46-nightly.20261007.2774; source checked at main @ a8c4802

Environment

Linux (WSL2, kernel 6.18.40.1-microsoft-standard-WSL2), T3 Code nightly desktop, GitHub Copilot CLI 1.0.93 through the ACP registry (acpRegistry_github_copilot), model claude-opus-5.5

Logs or stack traces

# Provider snapshot after the bug (~/.t3/caches/acpRegistry_github_copilot.json, agent descriptor)
{
  "id": "agent",
  "label": "Agent",
  "description": "Select a custom agent persona, or use the default Copilot agent.",
  "type": "select",
  "options": [
    { "id": "example", "label": "example", "description": "Example agent." }
  ],
  "currentValue": "example"
}

# Persisted thread modelSelection
{"instanceId":"acpRegistry_github_copilot","model":"claude-opus-5.5","options":[
  {"id":"mode","value":"https://agentclientprotocol.com/protocol/session-modes#agent"},
  {"id":"reasoning_effort","value":"high"},
  {"id":"agent","value":"example"},
  {"id":"allow_all","value":"on"}]}

Screenshots, recordings, or supporting files

No response

Workaround

Move every custom agent out of ~/.copilot/agents/ (and out of the repo's .github/agents/), then restart T3 Code. With no custom agents, Copilot reports only the "" choice. T3 drops it, which leaves the descriptor with no choices, so T3 hides the menu entirely and Copilot uses its default agent.