Sub-agent `model:` frontmatter is ignored — sub-agent always runs on parent model

[tsm-harmoney]

Bug: Sub-agent model: frontmatter is ignored — sub-agent always runs on parent model

Summary

When a workflow defines an inline sub-agent with a model: override, the Copilot CLI ignores it at runtime. The sub-agent runs on the parent session model instead of its own configured model.

Environment

• gh-aw CLI version: v0.79.8
• Copilot CLI version: 1.0.60
• AWF firewall version: v0.27.2
• Engine: copilot

Test workflow

---
name: "Agentic Workflow Test"
engine:
  id: copilot
  model: claude-sonnet-4.6
on:
  workflow_dispatch:
---
This is a test workflow. Invoke the `test-sub-agent` sub-agent and call `noop` with the message from the sub-agent output.

## agent: `test-sub-agent`
---
model: claude-haiku-4.5
description: Tests a sub-agent that is called from a parent agent, it doesn't do anything useful, just returns a string to the parent agent.
---
This is a test workflow. Output "Hello from the sub-agent!".

Expected behaviour

The sub-agent (test-sub-agent) runs using claude-haiku-4.5 as specified in its frontmatter.

Actual behaviour

The sub-agent runs on claude-sonnet-4.6 — the parent session model — ignoring the model: field entirely.

Agent terminal output (from workflow-logs/2_agent.txt)

● Test-sub-agent(claude-sonnet-4.6) Invoking test sub-agent
  └ ● Test-sub-agent(claude-sonnet-4.6) Running test sub-agent
      └ Hello from the sub-agent!

The sub-agent display name shows claude-sonnet-4.6, not claude-haiku-4.5.

API proxy token usage (from sandbox/firewall/logs/api-proxy-logs/token-usage.jsonl)

All 7 completions used claude-sonnet-4.6. Zero calls to any haiku-class model:

#	model	input_tokens	output_tokens	AIC
1	claude-sonnet-4.6	22 926	248	7.25
2	claude-sonnet-4.6	19 157	221	6.08
3	claude-sonnet-4.6	19 165	97	0.81
4	claude-sonnet-4.6	19 276	10	0.71
5	claude-sonnet-4.6	19 447	12	0.77
6	claude-sonnet-4.6	23 245	85	1.41
7	claude-sonnet-4.6	23 347	33	0.78

Total AIC: 17.81. Sub-agent turns (roughly calls 4–6) should have appeared as claude-haiku-4.5 with significantly lower AIC if the model override had been respected.

[pelikhan]

@copilot add tests to ensure that the model field is properly extracted for inlined agents.