[awf] Copilot engine: validate model alias resolution before AWF provider startup to surface retired-model errors clearly

[lpcox]

Problem

When a workflow specifies a retired model (e.g., gpt-5-codex), gh-aw accepts it, compiles the workflow, and passes the literal string to the Copilot CLI. The resulting startup failure looks like a provider auth error, not a model configuration error, confusing workflow authors.

Context

Upstream report: github/gh-aw#35032

Root Cause

gh-aw's model validation happens after AWF provider initialization. When the model alias resolves to a retired model, the Copilot CLI returns an auth-style error because it cannot load the model endpoint. The error message does not mention the model name.

Proposed Solution

1. In the gh-aw compiler/engine initialization, resolve model aliases to canonical names before starting the AWF provider.
2. Query the Copilot model catalogue at compile time; if the resolved model is absent or retired, emit a clear error: model 'gpt-5-codex' is retired — use one of: [list].
3. Fail fast before AWF containers are started, so users see the actionable error immediately.
4. Add a test asserting that a retired model name produces the new error message.

Generated by Firewall Issue Dispatcher · sonnet46 1.2M · ◷

[github-actions]

👋 Hello! I noticed this issue might be related to an existing issue:

• [awf] api-proxy: modelFallback rewrites Copilot model names, obscuring retired/restricted model errors #3987 — [awf] api-proxy: modelFallback rewrites Copilot model names, obscuring retired/restricted model errors (closed)

Both issues reference the same upstream report and describe the same user-visible symptom: a retired Copilot model produces a confusing auth-style error instead of a clear model-availability message. The key difference is in scope — #3987 focused on the api-proxy modelFallback rewriting as the AWF-side root cause, while this issue focuses on earlier validation in the gh-aw compiler/engine before AWF containers start.

If #3987's fix (disabling modelFallback for the Copilot engine) resolved the upstream symptom, this issue may represent a complementary improvement (fail-fast compile-time validation) rather than a duplicate. If the problem is still occurring after #3987, this issue likely captures the remaining root cause.

This is an automated message from the issue duplication detector.

Generated by Issue Duplication Detector for issue #4021 · sonnet46 569.2K · ◷