Why: The sandbox tier wrote the session directory directly, so a buggy
or runaway script could leave the workspace half-mutated and nothing
bounded what a timeout had already written. The plan's first landing
shape (section 4.4) swaps the session bind for a frozen-base overlay:
each exec freezes the session dir (hardlink clone + sha256 baseline),
runs on a private upper, and only a clean exit promotes the change set
back via CAS (baseline hash match -> atomic replace/remove; externally
changed targets double-store as *.conflict-* instead of clobbering).
Timeouts and kills discard the upper entirely - half-written state
never lands.
Evidence: go test ./... green including bwrap integration roundtrips
(TestSandboxWorkspaceOverlayRoundtrip: add/modify/delete through a real
overlayfs upper with whiteout handling and ws cleanup;
TestSandboxWorkspaceOverlayTimeoutDiscards: partial writes discarded on
timeout) and unit anchors for freeze manifest (.agentx excluded), CAS
add/modify, conflict double-store, and the change summary.
Impact: Opt-in via --exec-workspace overlay / AGENT_SYSTEM_EXEC_WORKSPACE
(only with --exec-backend sandbox; fail-closed on unknown modes). The
model sees a one-line change summary appended to the tool output, so
discards and conflict files are visible. Default behavior is unchanged.
Refs: proposal 2026-10-10-agent-delegation-routing-sandbox.md section
4.4; Dev task feature/agent-s0-workspace-overlay.
weichen