ADR 0124: Bind Temporary Sessions to Their Own Scratch Workspace

• Status: Accepted
• Date: 2026-08-26
• Related: Issue #16, ADR 0059, ADR 0118

Context

Temporary sessions intentionally keep projectPath empty so they do not inherit or mutate the currently visible project. That left their native tools without a root, however, even though each session already owns a deterministic <data_dir>/scratch/<sessionId> directory. The empty home also used the same hero copy for a temporary chat and for having no active session.

Decision

1. Host-core resolves a persisted path-less session's tool workspace to its own <data_dir>/scratch/<sessionId> directory and creates that directory when the binding is first resolved. The session schema remains unchanged and projectPath stays absent.
2. Read, Glob, Grep, Write, Edit, and Bash use that scratch directory as the temporary session's workspace root. The root is never taken from the mutable global workspace, and the existing containment and permission checks still apply. A missing-session compatibility call may retain the legacy global-workspace fallback.
3. Plan and Goal workspace validation continues to require a persisted project binding, so a scratch workspace does not broaden contract-mode execution.
4. The renderer derives the empty-home hero state from the selected session: project sessions retain the project-underlined welcome, temporary sessions show dedicated temporary-chat copy without a project action, and no active session shows the generic welcome.

Consequences

• Temporary chats can safely use native tools without touching a project or inheriting the most recently active project.
• Relative tool paths in temporary chats are useful immediately; scratch data remains ephemeral and follows the existing session deletion/startup sweep.
• Temporary sessions remain visibly and structurally distinct from project sessions without introducing a new session type or persistence migration.
• Plan and Goal keep their existing project-root boundary.

Alternatives considered

• Inherit the visible project: rejected because it violates the temporary session isolation invariant and can mutate unrelated project files.
• Add a new persisted session type: rejected because the existing path-less session model already provides the correct lifecycle and grouping.
• Leave tools unavailable: rejected because it makes temporary chats unable to perform otherwise isolated inspection and scratch work.