(bug) Runner silently drops agent files when the durable workspace mount is skipped

[mmabrouk]

When a Daytona sandbox can't reach the object store (no public endpoint, no tunnel), the runner logs tunnel discovery failed, the mount degrades, and the session continues silently. Every file the agent writes during that session is lost, with no user-visible signal that anything went wrong.

Context

The common cause of this is fixed in v0.105 (#5320, opt-in store exposure), but the fail-open path is still there for misconfigured deployments: any deployment where the mount is required but the store isn't reachable will still lose files silently.

Wanted behavior

When the durable mount is required and unavailable, fail loudly with a clear user-facing error, the same way #5318 made the permission-extension failure fail closed (PI_PERMISSION_EXTENSION_UNAVAILABLE_MESSAGE), and following the existing TOOL_MCP_UNAVAILABLE_MESSAGE precedent in tool-mcp-assets.ts. Right now this path fails open instead.

Relevant code

• Reachability check: services/runner/src/engines/sandbox_agent/mount.ts (~line 186)
• Tunnel discovery: services/runner/src/engines/sandbox_agent/mount.ts (~line 512)

Evidence

docs/design/agent-workflows/projects/qa/STATUS.md on branch qa-prerelease-agent-runtime (v0.105.0 release-night QA, 2026-07-14).

[linear-code]

AGE-3956

[shivamm2606]

Hi! I'd like to work on this issue if it's still available. I'll investigate the mount failure path, reproduce the behavior locally and submit a PR if I can identify a clean fix that aligns with the existing error-handling patterns