Sub-agent model selection is ignored and the parent session model is used instead

[tore-unumed]

Summary

When a workflow session is pinned to one model, sub-agent model selection does not appear to be honored.

I was able to reproduce this in two ways:

1. An inline sub-agent with its own model field.
2. A file-defined custom sub-agent with either its own model field or an explicit Task model override.

In both cases, the sub-agent is recognized and invoked correctly, but execution still uses the parent session model.

Expected behavior

If a sub-agent is configured with model: gpt-5-mini, or if the Task invocation explicitly requests gpt-5-mini, the sub-agent should run on that model.

Actual behavior

The runtime appears to resolve or acknowledge the requested sub-agent model, but then still executes on the parent session model.

Examples of the kind of log sequence I observed:

• definitionModel="gpt-5-mini"
• effectiveModel="gpt-5-mini"
• later: Using model: gpt-5.4

I also observed a case where the runtime explicitly logged that it was using the session model instead of the sub-agent definition model.

Minimal reproduction shape

Parent workflow/session:

• engine model: gpt-5.4

Sub-agent case A:

• inline sub-agent definition includes model: gpt-5-mini
• parent invokes that sub-agent once with a trivial prompt

Sub-agent case B:

• file-defined custom sub-agent includes model: gpt-5-mini
• parent invokes it via Task with an explicit model override of gpt-5-mini

What reproduces

In both cases:

• the sub-agent is available
• the sub-agent is invoked
• the prompt is delivered correctly
• the sub-agent returns a valid response
• the actual execution model remains the parent session model

Why this seems like a framework/runtime bug

This looks like a model-routing bug rather than a prompting or config parse issue, because:

• the sub-agent is loaded successfully
• the requested model is visible in runtime/debug logs
• execution still falls back to the session model

Impact

This makes it hard to use smaller/cheaper sub-agents for routing or narrow subtasks while keeping a stronger parent model for the main workflow.

Question

Is per-sub-agent model selection currently expected to work under all runtime/provider configurations, or is there a known limitation where sub-agents inherit the parent session model even when a different model is specified?