ACP does not advertise Copilot Fusion or its tier configuration when Fusion is available interactively #5354

Describe the bug

Copilot Fusion is available in the interactive Copilot CLI, but ACP advertises the routing model as Auto and exposes no Fusion/Auto-tier configuration.

This prevents ACP clients such as T3 Code from displaying Fusion and offering its tier selector. The behavior reproduces directly with copilot --acp, without T3 Code.

Enabling COPILOT_CLI_ENABLED_FEATURE_FLAGS=DYNAMIC_AUTO_TIERS for the ACP subprocess did not change the observed advertisement.

This report concerns model discovery and configuration parity. It does not establish that auto cannot route to Fusion internally.

Affected version

GitHub Copilot CLI 1.0.105.

Steps to reproduce the behavior

1. Use an account for which Fusion is available in the interactive Copilot CLI's model picker.

2. Launch the same CLI version as an ACP server:

   copilot --acp

3. Send newline-delimited JSON-RPC requests through stdin. Initialize the connection:

   {"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":1,"clientCapabilities":{},"clientInfo":{"name":"fusion-acp-repro","version":"1.0.0"}}}

4. Create a session, replacing /absolute/path/to/workspace with an existing workspace:

   {"jsonrpc":"2.0","id":2,"method":"session/new","params":{"cwd":"/absolute/path/to/workspace","mcpServers":[]}}

5. Inspect the returned model options and session configuration. Observed:

   • The routing model has ID auto and display name Auto, not Copilot Fusion.
   • Duplicate auto options were present.
   • No Fusion/Auto-tier configuration option was advertised.
   • With the initial Claude selection, configuration IDs were mode, model, reasoning_effort, agent, and allow_all.

6. Select Auto, substituting the returned session ID:

   {"jsonrpc":"2.0","id":3,"method":"session/set_config_option","params":{"sessionId":"<session-id>","configId":"model","value":"auto"}}

   The resulting configuration IDs were mode, model, agent, and allow_all. reasoning_effort disappeared, but no Fusion/Auto-tier option appeared.

7. Restart the server with the dynamic-tier feature flag and repeat:

   COPILOT_CLI_ENABLED_FEATURE_FLAGS=DYNAMIC_AUTO_TIERS copilot --acp

   Fusion naming and tier configuration were still absent.

The advertised /model command was also tested through ACP. It returned:

The model-picker dialog is only available in the interactive CLI.

Expected behavior

When Fusion is available for the authenticated account, ACP should expose enough metadata and configuration for clients to:

• Identify the routing option as Copilot Fusion where appropriate.
• Discover supported tiers and the current selection.
• Change tiers through the supported ACP configuration interface.

Keeping auto as the stable underlying model ID is fine; a separate model ID is not required.

If Fusion tier selection is intentionally unsupported over ACP, please document that limitation and the supported alternative.

Additional context

Environment

• macOS, Apple Silicon (arm64).
• Copilot CLI: 1.0.105.
• Affected ACP client: T3 Code desktop nightly 0.1.8-nightly.20261104.4179.
• Direct ACP reproduction bypassed T3 entirely.

Cross-surface observations

• Interactive Copilot CLI: Fusion available.
• VS Code Agents Window: "Copilot Fusion" with "High" tier visible.
• Regular VS Code Chat: Fusion not visible.
• T3 Code via Copilot ACP: Auto available, no Fusion tier selector.

The regular VS Code Chat observation may have a separate cause and is included for context, not as evidence of an ACP defect.

Investigation findings

• T3 reads advertised ACP model/configuration options and preserves their names. The direct reproduction demonstrates that the missing metadata exists upstream of T3's picker.
• Repeating the probe with T3's Copilot-specific client capability metadata also did not expose Fusion or tier configuration.
• The inspected VS Code Agent Host implementation explicitly enables dynamic Auto tiers and uses discovered Fusion-tier metadata to present auto as Fusion.
• That VS Code source checkout references Copilot runtime 1.0.106-6, so its behavior is not a same-runtime comparison with CLI 1.0.105.
• No model-generation request was used to verify which backend auto ultimately executes.

Related reports

• --model fusion fails with "requires dynamic Auto tiers" unless undocumented DYNAMIC_AUTO_TIERS flag is set (1.0.105) #5298 - Fusion alias requires the dynamic Auto tiers flag in 1.0.105.
• missing --fusion-tier argument #5290 - documented --fusion-tier argument is missing in 1.0.105.
• Hydrafusion not available on experimental mode #4975 - HydraFusion unavailable in experimental mode.

These appear related, but none was confirmed to cover ACP model/configuration advertisement specifically.

Documentation: https://docs.github.com/en/copilot/concepts/models/copilot-fusion

Prepared with GitHub Copilot in T3 Code using Auto model selection. The specific underlying model was not exposed by the session.