0202: Detect forks at their own session start, and guard the parent

Status: accepted (2026-10-07) · Amends: ADR 0057 · Scope: product tier (hook naming, name history, session-start and prompt context, agents status, agents end, agents lifecycle)

Context

ADR 0057 recorded fork lineage from the tool.pre_use heal because, in August 2026, a Claude Code fork never fired SessionStart under its own id. The V2 and V3 ledger migrations later removed that heal path, so no code path stamped forked_from for a Claude Code fork at all.

On October 7, 2026 a session forked with the desktop app’s “Fork from here” saw its parent in the peer list, read it as its own earlier run, and offered to close it. The parent was a live session in the middle of its own work.

Probes against Claude Code 2.1.283 on October 7, 2026 established three facts:

Both fork flows now fire SessionStart under the fork’s own id. The CLI --resume <id> --fork-session flow sends source: "fork".
At that SessionStart the CLI fork’s transcript does not exist yet, and the payload names no parent. The transcript appears before the first tool call, with every copied row’s sessionId rewritten and message uuids preserved.
The desktop fork copies the parent’s rows with their original sessionId, so the parent’s id is readable from the fork’s own transcript.

Decision

Every main-session naming path goes through one helper. For a brand-new Claude Code instance it runs fork detection and writes the result on the first history row: forked_from when the parent is known, fork: true when the instance is a fork, and fork_checked: true when detection had the evidence it needs.
Detection uses two signals. Rows stamped with another session id name the parent directly (desktop). A declared fork (source: "fork") whose rows carry no other id falls back to the ADR 0057 uuid containment scan (CLI).
Inherited rows alone are not proof. If the session they name still has an unended generation, the new instance is a fork. If it has ended, the new instance is a continuation and gets no lineage.
When session start lacked evidence (a CLI fork’s transcript is not on disk yet), the first tool call finishes detection and appends one source: "fork.detected" row that copies the name and adds the lineage. fork_checked stops later tool calls from scanning again.
A later row without lineage, such as an identity assumption, no longer erases forked_from.
Session-start context for a fork whose parent is live names the parent, its task, and the files it holds, says they belong to the parent, and says not to end, release, or finalize that session without an operator instruction. A fork whose parent is not yet known gets the same warning without the name.
The per-prompt peer table and agents status label the parent as “your fork parent, live” and a fork as “your fork”, and list them first.
agents end --session-id/--instance-id and agents lifecycle --session-id refuse a target that is the caller’s recorded fork ancestor with session_is_fork_ancestor. --force-ancestor overrides it when the operator asked for it, as with identity assume.

Alternatives considered

Rely on source: "fork" alone. It identifies the CLI flow but not its parent, and the desktop fork’s source is not recorded anywhere we can check.
Treat any inherited row as a fork. A continuation of a finished session also inherits rows. Labeling it a fork would attach a warning about a parent that no longer exists.
Scan sibling transcripts for every new session. The uuid scan reads every transcript in the project directory. It runs only for declared forks; the inherited-id scan is a bounded tail read of the session’s own file.

Consequences

Sessions named before this change carry no fork_checked, so each runs the inherited-id scan once on its next tool call and gains one history row.
The transcript layout is Claude Code’s internal format. Every read fails open: an unknown layout records no lineage and keeps the generic notice.
Codex and Cursor forks still have no detection; this decision covers Claude Code only.