Ultra multi-agent / Responses multi_agent does not work with regular API-key model providers

What version of Codex CLI is running?

0.147.0 (also reproduces on recent Desktop / app-server builds)

What subscription do you have?

ChatGPT Pro (and separately OpenAI API key / custom model_providers for inference)

Which model were you using?

gpt-5.6-sol / gpt-5.6-terra / gpt-5.6-luna (and equivalent IDs via a custom Responses-compatible provider)

What platform is your computer?

macOS (also relevant to any platform using custom model_providers)

What issue are you seeing?

Full “Ultra” multi-agent / subagent orchestration does not work when inference goes through a regular API provider (custom model_providers.* with wire_api = "responses" + env_key API-key auth).

This is distinct from:

• setting model_reasoning_effort = "ultra" (reasoning effort alone can work on API providers), and
• ChatGPT-signed-in / first-party OpenAI provider sessions, where native Ultra multi-agent coordination works.

Observed failure modes with regular API providers:

1. Responses API multi_agent is rejected or non-functional
   When a Responses request includes the multi-agent surface (e.g. multi_agent.enabled = true / multi-agent beta), OpenAI-compatible or non–ChatGPT-backend endpoints commonly respond with:

   • 400 / Unsupported parameter: multi_agent
   • or auth / routing errors that never produce a multi-agent tree

2. Native Codex Ultra subagent product path is ChatGPT-backend-centric
   With a custom API-key model_provider, Ultra-level parallel OpenAI multi-agent coordination (server-side subagent tree, multi_agent_v2 product behavior tied to first-party models) does not behave like a ChatGPT-auth session. Client-side spawn_agent / local multi-agent tooling may partially work, but that is not the same as first-party Ultra multi-agent.

3. Related hardcoded ChatGPT-backend assumptions
   GPT-5.6 Sol/Terra/Luna paths that assume ChatGPT-backend fields (e.g. responses-lite / multi_agent version headers) already cause 400s on non–ChatGPT backends — see related issues below. That further blocks “just use Ultra models + multi-agent on any Responses-compatible base_url”.

Net: users who correctly configure a regular API provider for Sol/Terra/Luna get coding + reasoning effort, but not the full Ultra multi-agent / subagent product experience.

What steps can reproduce the bug?

1. Configure a working API-key custom provider (not ChatGPT OAuth), for example:

[model_providers.my_api]
name = "OpenAI-compatible Responses API"
base_url = "https://api.example.com/v1"
wire_api = "responses"
env_key = "MY_API_KEY"
requires_openai_auth = false

model_provider = "my_api"
model = "gpt-5.6-sol"
model_reasoning_effort = "ultra"

[features]
multi_agent_v2 = true

2. Confirm plain inference works:

codex exec --skip-git-repo-check "Reply with exactly: API_OK"

3. Attempt Ultra multi-agent / subagent work, e.g.:

   • ask the agent to spawn multiple specialized subagents in parallel and synthesize, or
   • send a Responses request through the same provider with multi-agent enabled.

4. Compare with the same prompt on the default ChatGPT-authenticated OpenAI provider / profile.

What is the expected behavior?

Either:

A (preferred): Full Ultra multi-agent / native subagent orchestration works for regular API-key Responses providers when the upstream model supports it — including transparent multi-agent parameters and no ChatGPT-only internal headers required for Sol/Terra/Luna, or

B: Codex clearly documents and surfaces in product UI/CLI that Ultra multi-agent / server multi-agent is ChatGPT-backend-only, while API-key custom providers only support single-agent inference (and optionally client-local spawn), so users are not left debugging silent no-spawn / Unsupported parameter: multi_agent failures.

Today neither A nor B is complete: config accepts Ultra + multi_agent_v2 + custom providers, but the Ultra multi-agent experience is incomplete or broken on regular API providers.

Additional information

Works on API providers (in our testing):

• Single-agent Responses inference
• Reasoning effort including ultra (as an effort level)
• Many built-in tools that the upstream Responses API actually implements (e.g. web_search when the endpoint supports it)

Does not work / incomplete on API providers:

• Responses multi_agent / server-side multi-agent tree
• Full native Ultra multi-agent product behavior comparable to ChatGPT-auth sessions
• Paths that inject ChatGPT-backend-only multi_agent / responses-lite assumptions against a plain API provider

Related issues:

• #17598 — native subagent orchestration broken / incomplete with non-OpenAI custom providers
• #31882 — gpt-5.6-sol/terra/luna hardcode use_responses_lite / multi_agent_version → 400s on non–ChatGPT backends
• #31870 — X-OpenAI-Internal-Codex-Responses-Lite failures on Azure / non-ChatGPT backends
• #13204 — subagent ignores active profile/provider
• #14039 — per-subagent model/provider/profile selection

Please treat this as a product + protocol gap for regular API providers, not an Azure-only issue: any model_providers.* entry that is “just” Responses + API key hits the same Ultra multi-agent wall.

Happy to provide redacted request/response pairs (no secrets) if useful.

Start by reproducing the configuration in the issue, run the supplied codex exec command, and compare the custom API-key provider with a ChatGPT-authenticated provider. Trace the Responses request when multi_agent_v2 is enabled and inspect the resulting errors. Done means either supported providers work without ChatGPT-only assumptions or the CLI clearly reports the provider limitation.