fix: prune start checkpoint on eviction

AGENTS.md

@@ -23,8 +23,8 @@ ESM-only, Node ≥22. Toolchain is Vite+ (`vp`), not plain npm:
 - **Notify only on real change.** A failed `commit`, an empty `transaction`, and
   a no-op `undo`/`redo`/`jump` notify _nobody_.
 - **Checkpoints anchor to entries.** Pruned when their entry is evicted (`limit`
-  overflow) or dropped (redo branch truncated by a fresh commit); `revertTo`
-  throws on an unknown/pruned name.
+  overflow) or dropped (redo branch truncated by a fresh commit); eviction also
+  prunes position-0 checkpoints. `revertTo` throws on an unknown/pruned name.
 - **Rollback is reverse-order and best-effort.** A throwing inverse stops
   rollback and surfaces.
 

CLAUDE.md

@@ -1,6 +1 @@
-# @b2m9/reversible
-
-The agent guide for this project is maintained in a single source of truth,
-`AGENTS.md` (the cross-tool standard). Claude Code reads it via this import:
-
 @AGENTS.md

README.md

@@ -143,9 +143,11 @@ history.subscribe((meta) => { /* ... */ }); // returns unsubscribe
 ```
 
 With `limit`, the oldest entries are evicted past the cap. Checkpoints anchor to
-the entry they sit after, so they stay correct as eviction renumbers positions; a
-checkpoint whose anchor is evicted (or dropped when a commit clears a redo branch)
-is pruned, and `revertTo` on it throws.
+the entry they sit after, so they stay correct as eviction renumbers positions.
+A checkpoint is pruned when its anchor is destroyed: the entry it sits after is
+evicted or dropped by a commit that clears the redo branch. A position-0
+checkpoint is pruned by any eviction, since the undos back to it are gone.
+`revertTo` on a pruned name throws.
 
 ## Non-goals
 

src/core.ts

@@ -65,13 +65,10 @@ export function createHistory(options: HistoryOptions = {}): History {
     for (const listener of listeners) listener(meta);
   };
 
-  const pruneCheckpoints = (removed: Entry[]): void => {
-    if (checkpoints.size === 0 || removed.length === 0) return;
-    const removedSet = new Set<Entry>(removed);
+  /** Drop every checkpoint whose anchor was destroyed. */
+  const pruneCheckpoints = (dead: readonly Anchor[]): void => {
     for (const [name, anchor] of checkpoints) {
-      if (anchor !== START && removedSet.has(anchor)) {
-        checkpoints.delete(name);
-      }
+      if (dead.includes(anchor)) checkpoints.delete(name);
     }
   };
 
@@ -82,10 +79,12 @@ export function createHistory(options: HistoryOptions = {}): History {
     }
     entries.push(entry);
     cursor += 1;
-    // Evict the oldest entries past the cap.
+    // Evict the oldest entries past the cap. Eviction also kills START: the
+    // undos that reached position 0 are gone — unlike a redo-branch drop,
+    // which leaves it intact.
     if (limit !== undefined && entries.length > limit) {
       const overflow = entries.length - limit;
-      pruneCheckpoints(entries.splice(0, overflow));
+      pruneCheckpoints([...entries.splice(0, overflow), START]);
       cursor -= overflow;
     }
   };
@@ -179,6 +178,7 @@ export function createHistory(options: HistoryOptions = {}): History {
     if (anchor === undefined) {
       throw new Error(`reversible: unknown checkpoint "${name}"`);
     }
+    // Pruning keeps the map free of stale anchors, so indexOf never returns -1.
     const target = anchor === START ? 0 : entries.indexOf(anchor) + 1;
     move(target - cursor);
   };

tests/checkpoint.test.ts

@@ -89,6 +89,29 @@ describe("checkpoints", () => {
     expect(() => history.revertTo("first")).toThrow();
   });
 
+  test("a position-0 checkpoint is pruned when eviction makes its state unreachable", () => {
+    const history = createHistory({ limit: 2 });
+    history.checkpoint("pristine"); // position 0, before any entry
+    history.commit(noop);
+    history.commit(noop);
+    history.commit(noop); // evicts entry 0 -> position 0 no longer names the pristine state
+
+    expect(history.hasCheckpoint("pristine")).toBe(false);
+    expect(() => history.revertTo("pristine")).toThrow();
+  });
+
+  test("a position-0 checkpoint survives a dropped redo branch", () => {
+    const history = createHistory();
+    history.checkpoint("pristine");
+    history.commit(noop);
+    history.undo();
+    history.commit(noop); // drops the redo branch; position 0 is still reachable
+
+    expect(history.hasCheckpoint("pristine")).toBe(true);
+    history.revertTo("pristine");
+    expect(history.position).toBe(0);
+  });
+
   test("a checkpoint in a dropped redo branch is pruned", () => {
     const history = createHistory();
     history.commit(noop); // e1