All GPT-5.6 Sol turns fail because MultiAgentV2 uses reserved collaboration.spawn_agent

What issue are you seeing?

For affected GPT-5.6 Sol sessions, every request fails before the model processes the prompt with:

Invalid Value: 'tools'. Function 'collaboration.spawn_agent' is reserved for use by this model and must match the configured schema.

This happens even for prompts that do not request a subagent, making GPT-5.6 Sol unusable with the default configuration for affected users.

The failure appears to be a collision between the model-reserved collaboration.spawn_agent tool contract and Codex's independently generated MultiAgentV2 spawn_agent schema:

• models-manager/models.json selects tool_mode = "code_mode_only" and multi_agent_version = "v2" for gpt-5.6-sol.
• core/src/config/mod.rs defaults features.multi_agent_v2.tool_namespace to collaboration.
• core/src/tools/spec_plan.rs wraps the locally generated MultiAgentV2 functions in that namespace.
• The Responses API recognizes collaboration.spawn_agent as model-reserved and rejects the request because the submitted schema does not exactly match the configured model schema.

This default namespace was introduced by #29067.

What steps can reproduce the bug?

1. Use GPT-5.6 Sol in a fresh Codex task with the default MultiAgentV2 configuration.
2. Send any prompt; it does not need to request delegation or call spawn_agent.
3. Observe that the request fails immediately with the reserved-schema error above.
4. Add this configuration and start a fresh task:

[features.multi_agent_v2]
tool_namespace = "agents"

1. Send the same prompt again.
2. Observe that it succeeds.

The A/B result changes the fully qualified tool identity from functions.collaboration.spawn_agent to functions.agents.spawn_agent without changing the underlying MultiAgentV2 implementation. This strongly isolates the failure to the reserved collaboration namespace/schema collision.

Observed with the current GPT-5.6 Sol rollout. The checked local Codex runtime reports codex-cli 0.144.0; current repository main at the time of investigation was a6b99ee5c45e35165c9bac9da4cec65f2ca55bc9.

What is the expected behavior?

GPT-5.6 Sol should accept ordinary requests with the default Codex configuration.

Codex should either:

• submit the exact canonical schema configured for Sol's reserved collaboration tools;
• use a non-reserved namespace for its independently generated MultiAgentV2 schema; or
• negotiate the model's supported collaboration tool contract rather than assuming the namespace is available.

A tool incompatibility should not make every turn fail before inference.

Fixes

Workaround:

[features.multi_agent_v2]
tool_namespace = "agents"

Ideal fix:
Either make that the default, or change the Responses API backend to no longer make collaboration special.

Additional information

This appears distinct from #26753, #27331, and #27589. Those issues report request-wide failures caused by encrypted parameters not being enabled for the selected model. This report concerns the separate validator error that collaboration.spawn_agent is reserved and does not match the model-configured schema.