An entry is a record:

Entry = { uuid        : UUID
          parentUuid  : UUID | nil      ;; nil = root
          type        : "user" | "assistant" | "summary" | "system"
          message     : { role: string, content: Content }
          version     : SemVer           ;; e.g. "2.1.170"
          timestamp   : ISO-8601
          ... }                          ;; ~19 additional fields (see version-drift)

uuid is opaque (not content-addressed); parentUuid links to the preceding entry. The pair forms a singly-linked list per session.

A session is a sequence of entries: e_0, e_1, ..., e_n where e_i.parentUuid = e_{i-1}.uuid (or nil for e_0).

A fork creates a second session sharing a prefix:

Session A:  e_0 → e_1 → e_2 → e_3 → e_4
Session B:  e_0 → e_1 → e_2 → f_3 → f_4     (forked at e_2)

The shared prefix (e_0, e_1, e_2) is file-level: session B copies the JSONL bytes of entries 0–2, then appends new entries. This is path-copying (Driscoll et al., 1989), not pointer sharing.

Each entry carries a version field. The version-drift analysis found:

• 38 distinct versions (2.1.2 through 2.1.170)
• Hard breakpoint at 2.1.78: pre-stabilization entries lack ~19 keys
• Convergence: 2.1.161 is -1 key, 2.1.168+ is 0 keys vs current

A fork crossing a version boundary must backfill the missing keys for the current client to accept the replayed prefix.

Legal fork points are tool-balanced turn boundaries: the prefix must end after a complete assistant turn, never inside a tool_use / tool_result pair.

∀ prefix P: |tool_use entries in P| = |tool_result entries in P|

A fork only adds entries after the resumption point. The prefix is immutable — the grandfather paradox is unrepresentable.

fork(session, uuid) =
  let prefix = entries(session)[0..indexOf(uuid)]
  in  newSession(prefix ++ [freshEntry(...)])

prefix is byte-identical to the original. The fork diverges only after the fork point.

When a fork from version v_old is resumed under client v_new:

∀ e in prefix:
  missingKeys(e, schema(v_new)) must be backfillable from defaults

The version-drift migration table quantifies backfill cost per era.