HydraFusion: accepting an Astra plan can stall without starting implementation

[gfortaine]

Describe the bug

With GPT-6 Astra configured as the plan-mode model and HydraFusion as the session
model, accepting a plan can leave the session apparently stuck: no visible
implementation starts after leaving plan mode.

This has occurred repeatedly in interactive use, including when selecting
Accept plan and build on default permissions. Multiple exit choices have been
tried. This report is specifically about an action that should start implementation,
not the expected inactivity of Exit plan mode and I will prompt myself.

There is no error message available to include. The visible symptom is that nothing
happens after accepting/leaving the plan. It is not yet established whether this is
a UI hang, a stalled model transition, an unstarted continuation, or hidden
HydraFusion work.

Affected version

GitHub Copilot CLI 1.0.84-1, macOS, Apple Silicon / arm64.

Steps to reproduce the behavior

The following describes the interactive workflow where the problem occurs; it has
not yet been reduced to an isolated minimal repository.

1. Enable experimental features and select HydraFusion using /model.

2. Configure GPT-6 Astra as the plan model. The relevant existing settings are:

   {
     "experimental": true,
     "model": "hydrafusion",
     "planModel": "gpt-6-astra",
     "planEffortLevel": "xhigh"
   }

3. Enter plan mode and request an implementation plan for a coding task.

4. Let Astra produce the plan and present the plan-approval menu.

5. Select Accept plan and build on default permissions.

6. Observe that implementation does not visibly begin and the session appears
   stuck after the transition.

Expected behavior

Accepting the plan should leave plan mode, restore the HydraFusion session
selection, and start implementation of the approved plan, subject to the normal
interactive permission prompts.

If the transition or HydraFusion initialization cannot proceed, the CLI should
surface the failure or an actionable pending state rather than appear stalled.

Additional context

• The separate plan/session-model workflow is documented in
  v1.0.74, associated
  with Automatic switching between model for planning and execution #2792.
• Entry into plan mode is working in the inspected session: the model changes from
  hydrafusion to gpt-6-astra, with source: plan_mode and reasoning effort
  xhigh. This does not establish which part of the reverse transition stalls.
• No global effortLevel, contextTier, or planContextTier is configured in the
  user settings. Experimental features are already enabled.
• Post-plan-approval system message contradicts itself ("edits require manual approval" + "Proceed with implementing"), causing the model to stop instead of starting work #3322 is adjacent, but describes a text-only acknowledgement followed by stopping.
  Here the reported symptom is an apparent silent stall; a shared cause is not
  established.
• The HydraFusion plan-mode selection report
  concerns selecting HydraFusion while already in plan mode, rather than accepting
  an Astra-authored plan and returning to HydraFusion.
• There is also a separate
  HydraFusion startup/loading report.
  Startup fallback has been observed locally too, but is not assumed to cause this
  post-approval stall.

No private repository content, session identifiers, or full diagnostic logs are
attached. This report records the observed workflow and symptom, not a confirmed
deadlock or root-cause diagnosis.

[gfortaine]

Opened prematurely before isolating a distinct failure. This may overlap #3322 or the existing HydraFusion discussion. Closing pending a minimal reproduction; this can be reopened if a separate defect is established.