--model fusion fails with "requires dynamic Auto tiers" unless undocumented DYNAMIC_AUTO_TIERS flag is set (1.0.105) #5298

Describe the bug

copilot --model fusion fails in 1.0.105 even though Fusion is documented as GA in the CLI and enabled for the org:

Failed to initialize session: Error: model resolution leaf failed: The Copilot Fusion model alias requires dynamic Auto tiers.

Reproduces locally and in GitHub Actions (GitHub Enterprise Cloud with data residency tenant, workflow GITHUB_TOKEN with copilot-requests: write, GH_HOST set). --model auto and explicit models work with the same token. Also reported for the JetBrains plugin / CLI: microsoft/copilot-intellij-feedback#2137.

Workaround found

COPILOT_CLI_ENABLED_FEATURE_FLAGS=DYNAMIC_AUTO_TIERS (undocumented, see #5290) makes it work, locally and in Actions:

• --auto-tier balance and --auto-tier intelligence: OK
• --auto-tier standard / high (the tier names the docs use): Auto preference "standard" is unavailable for this account
• --auto-tier efficiency in Actions: Auto routing is unavailable; server-discovered preferences require the /auto endpoint (works locally)

Expected

• Fusion works without an undocumented feature flag, or the error says how to enable it.
• Docs use --fusion-tier standard|high|max, but 1.0.105 only has --auto-tier (see missing --fusion-tier argument #5290); tier names should match the docs.

Affected version

1.0.105 (latest on npm), Linux