bug: byok-copilot feature generates empty COPILOT_MODEL fallback, breaking BYOK mode

[lpcox]

Problem

When using the byok-copilot feature flag, the compiler generates COPILOT_MODEL with an empty string fallback:

COPILOT_MODEL: ${{ vars.GH_AW_MODEL_AGENT_COPILOT || '' }}

Copilot CLI 1.0.28 in BYOK mode requires an explicit model and fails immediately with:

BYOK providers require an explicit model. Run `copilot help providers` for configuration details.

This means every recompile of a byok-copilot workflow requires a manual patch to the .lock.yml to set a model fallback (e.g. claude-opus-4.6), which is fragile and easy to forget.

Reproduction

1. Create a workflow with features: byok-copilot: true
2. gh aw compile .github/workflows/my-workflow.md
3. Inspect the generated .lock.yml — COPILOT_MODEL fallback is ''
4. Run the workflow — Copilot CLI exits immediately with exit code 1

Expected behavior

The compiler should generate a non-empty default model for BYOK workflows, or allow specifying a default model in the .md frontmatter. For example:

# Option A: compiler picks a sensible default for byok-copilot
COPILOT_MODEL: ${{ vars.GH_AW_MODEL_AGENT_COPILOT || 'claude-sonnet-4.6' }}

# Option B: allow model to be specified in frontmatter
# smoke-copilot.md
features:
  byok-copilot: true
model: claude-opus-4.6

Workaround

After every gh aw compile, manually patch the lock file:

# Replace empty fallback with explicit model
sed -i "s/GH_AW_MODEL_AGENT_COPILOT || '' /GH_AW_MODEL_AGENT_COPILOT || 'claude-opus-4.6' /" .github/workflows/smoke-copilot.lock.yml

Or set the GH_AW_MODEL_AGENT_COPILOT repository variable (but this affects all copilot workflows, not just BYOK ones).

Impact

This affects both smoke-copilot and smoke-copilot-byok workflows in github/gh-aw-firewall. Every recompile (including gh aw upgrade) resets the fallback to empty, requiring manual intervention.

References

• Failed CI run: https://github.com/github/gh-aw-firewall/actions/runs/24492526114
• Error: BYOK providers require an explicit model. Run \copilot help providers` for configuration details.`
• Copilot CLI version: 1.0.28
• gh-aw version: v0.68.4