Plugin agents ignore `model:` frontmatter

[brian-kelley-intel]

Describe the bug

Agents loaded via copilot plugin install ignore the model: field in YAML frontmatter. The CLI logs definitionModel="(none)" and falls back to the session's default model.

Repo-level agents (.github/agents/) read the same field correctly.

Affected version

GitHub Copilot CLI 1.0.14 (and GitHub Copilot CLI 1.0.11)

Steps to reproduce the behavior

1. Create a minimal plugin

my-plugin/
├── plugin.json
└── agents/
    ├── orchestrator.agent.md
    └── fast-helper.agent.md

plugin.json

{
  "name": "my-plugin",
  "version": "1.0.0",
  "agents": "agents/"
}

agents/orchestrator.agent.md

---
name: orchestrator
description: Delegates to sub-agents.
model: claude-sonnet-4.5
tools: ["agent"]
agents: ["fast-helper"]
---

Delegate all tasks to `fast-helper` via the task tool. Report back what it said.

agents/fast-helper.agent.md

---
name: fast-helper
description: Fast helper on Haiku.
model: claude-haiku-4.5
tools: []
user-invocable: false
---

State which model you are running on, then answer the question.

2. Install and run

copilot plugin install /path/to/my-plugin

copilot --log-level=all --model gpt-5.4 \
  --agent my-plugin/orchestrator \
  -p "Delegate to fast-helper: what is 2+2?"

3. Check the log

In ~/.copilot/logs/process-*.log:

Using model: gpt-5.4              # parent: ignores model: claude-sonnet-4.5

Agent "my-plugin:fast-helper":
  definitionModel="(none)"         # bug: model: claude-haiku-4.5 not read
  sessionModel="gpt-5.4"

Using model: gpt-5.4              # sub-agent: falls back to session default

The usage summary confirms only one model ran:

Breakdown by AI model:
 gpt-5.4    21.9k in, 338 out

Expected behavior

The plugin agent's model: claude-haiku-4.5 should appear as definitionModel="claude-haiku-4.5" and the CLI should use that model, the same way repo-level agents work.

Control: repo-level agents work

Place identical agent files in .github/agents/ instead of the plugin. Same command, same CLI version. The log now reads:

Using model "claude-sonnet-4.5" from custom agent "orchestrator"

Agent "fast-helper":
  definitionModel="claude-haiku-4.5"
  sessionModel="claude-sonnet-4.5"

Using model: claude-haiku-4.5

Breakdown by AI model:
 claude-sonnet-4.5   23.1k in, 402 out
 claude-haiku-4.5    3.7k in, 154 out

Both agents use their declared models.

Additional context

Field	Value
Logs (redacted)	process-redacted.log
OS	SUSE Linux Enterprise Server 15 SP4
CPU	x86_64
Shell	bash 4.4.23
Terminal	bash & tmux (TERM=xterm-color)