[Bug]: ACP provider session fails to open - acpRegistry_cline throws "Provider session failed to open" mid-session#17072

Before submitting

• I searched existing issues and did not find a duplicate.
• I included enough detail to reproduce or investigate the problem.

Area

apps/server

Steps to reproduce

1. Launch T3 Code desktop app on Windows.
2. Open thread Greeting under No project (Worktree 2026-10-08-hello-2b6de38c).
3. Use ACP provider backed by acpRegistry_cline (composer shows Muse Spark 1.3 Contributor / Act - Auto-approve tools On / Full access).
4. Send a prompt / continue the session.
5. Mid-session the provider session open fails repeatedly.

Expected behavior

The ACP provider session should open/resume normally and accept the new turn.

Actual behavior

Red banner + thread error:

Failed to open acpRegistry provider session provider-session:provider-instance:acpRegistry_cline:thread:1a5ba3cc-f08b-4bac-8079-81ff2b6de38c:37b09b3a-0b6c-41a2-85a9-f80c0beffb1a. Provider session failed to open Failed to open acpRegistry provider session provider-session:provider-instance:acpRegistry_cline:thread:1a5ba3cc-f08b-4bac-8079-81ff2b6de38c:37b09b3a-0b6c-41a2-85a9-f80c0beffb1a.

Sidebar shows Failed for the thread. Session stays failed, blocks further turns.

Impact

Blocks work completely

Version or commit

TODO - fill from app About / t3 --version / Release, e.g. main @ abc1234

Environment

TODO - e.g. Windows 11, T3 Code desktop vX.Y.Z, acpRegistry_cline provider Composer selection in screenshot: Muse Spark 1.3 Contributor

Logs or stack traces

Screenshots, recordings, or supporting files

img_18dc73a1f1e2cccc-bafd-4dc6-341eda777.png

Workaround

No workaround found yet.