Summary

Under runner.topology: arc-dind with the copilot engine, gh-aw exports
XDG_CONFIG_HOME="$HOME", but at the point that export runs $HOME is the unwritable
passwd home /home/runner (root-owned) — not the writable arc-dind home
${RUNNER_TEMP}/gh-aw/home that gh-aw itself sets HOME to. Any tool that resolves its config
dir from XDG_CONFIG_HOME (Flutter's Config/tool_state, and others) then tries to write into
/home/runner and fails with a permission error:

Flutter failed to write to a file at "/home/runner/tool_state".
The flutter tool cannot access the file or directory.
Please ensure that the SDK and/or project is installed in a location that has read/write permissions for the current user.

Impact

• Breaks any XDG-respecting tool run by the agent under arc-dind (observed with the Flutter SDK,
  but it affects anything using XDG_CONFIG_HOME).
• HOME itself is correctly the writable ${RUNNER_TEMP}/gh-aw/home, so this is subtle: only
  the XDG-derived config path is poisoned, which masks the root cause.
• Setting XDG_CONFIG_HOME via the workflow engine.env does not help — this
  export XDG_CONFIG_HOME="$HOME" runs later and overwrites it. The only reliable consumer-side
  workaround is to override HOME to a writable literal in engine.env so $HOME is writable
  when the export evaluates.

Environment

• Engine: copilot. Topology: runner.topology: arc-dind (self-hosted ARC + AWF firewall).
• Observed on v0.82.14; the offending line is unchanged on main.
• Introduced in v0.79.8 by commit 6171a09696
  ("fix(copilot): support arbitrary HOME directory, not just /home/runner (fix(copilot): support arbitrary HOME directory, not just /home/runner #38725)").

Diagnostic (from inside the sandbox)

A trivial agent step dumping the environment shows the split:

HOME=/home/runner/_work/_temp/gh-aw/home          # correct, writable
XDG_CONFIG_HOME=/home/runner                       # WRONG — unwritable
id -> uid=1001(runner) gid=1001(runner)
getent passwd 1001 -> runner:x:1001:1001:...:/home/runner:/bin/bash   # passwd home = /home/runner
ls -ld /home/runner                       -> drwxr-xr-x root root ...   writable: NO
ls -ld /home/runner/_work/_temp/gh-aw/home-> drwxr-xr-x runner runner   writable: yes

So HOME was correctly overridden to the writable arc-dind home, but XDG_CONFIG_HOME captured
the unwritable passwd home /home/runner.

Root cause

Permalinks against main:

• pkg/workflow/copilot_engine_execution.go#L115 — buildCopilotMCPConfigExport always emits:

  b.WriteString("export XDG_CONFIG_HOME=\"$HOME\"\n")

  (doc comment: "XDG_CONFIG_HOME=$HOME (Copilot CLI resolves its config dir from this)").
• Under arc-dind, gh-aw already knows the writable home path — it's the compiler constant
  awfArcDindHomePathExpr = "${RUNNER_TEMP}/gh-aw/home" (pkg/workflow/awf_helpers.go#L47), and
  rewriteArcDindEngineCommand (awf_helpers.go#L142) prepends export HOME=${RUNNER_TEMP}/gh-aw/home
  to the engine command.
• But XDG_CONFIG_HOME is indirected through $HOME, and at exec time inside the chroot $HOME
  resolves to the synthesized passwd home /home/runner (root-owned, unwritable) rather than the
  writable arc-dind home. Result: XDG_CONFIG_HOME=/home/runner.

Net: HOME is fixed up for arc-dind, but the XDG export that "follows" it captures the wrong,
unwritable home.

Suggested fix

When compiling for runner.topology: arc-dind, export XDG_CONFIG_HOME to the same writable
constant used for HOME, instead of indirecting through $HOME:

// arc-dind: XDG must point at the writable arc-dind home, not the (unwritable) passwd $HOME
b.WriteString(fmt.Sprintf("export XDG_CONFIG_HOME=%q\n", awfArcDindHomePathExpr))

(Non-arc-dind keeps export XDG_CONFIG_HOME="$HOME".) Alternatively, ensure the HOME override
is in effect in the same shell before the XDG export evaluates.

Notes

• Related but distinct from [ARC/DinD] safe-outputs: dynamic base-branch unresolved — GH_AW_INPUT_* not passed to MCP container #47795 (dynamic base-branch / GH_AW_INPUT_* not passed to the MCP
  container). Both surfaced upgrading a Flutter workflow to native arc-dind.
• Consumer workaround meanwhile: set HOME to a writable literal in engine.env so $HOME is
  writable when export XDG_CONFIG_HOME="$HOME" runs.