fix(shared): keep the newest shared usage scan

packages/shared/src/usageMerge.test.ts

@@ -146,6 +146,26 @@ describe("mergeUsage", () => {
     ).toEqual({ claude: 1, codex: 1 });
   });
 
+  it("uses the newest scan when environments share the same transcript directory", () => {
+    const source = { provider: "claude" as const, hostId: "mac", homePath: "/home/theo/.claude" };
+    const environments = [
+      environment("env-a", summary([bucket({ costUsd: 4, records: 2 })], [source])),
+      environment("env-b", {
+        ...summary([bucket()], [source]),
+        readAt: "2026-08-07T01:00:00.000Z",
+      }),
+    ];
+
+    for (const ordered of [environments, environments.toReversed()]) {
+      const merged = mergeUsage(ordered, USAGE_CONTRACT_VERSION);
+      expect(merged.costUsd).toBe(10);
+      expect(merged.records).toBe(5);
+      expect(merged.sessions).toBe(1);
+      expect(merged.contributingEnvironments).toEqual(["env-b"]);
+      expect(merged.duplicateSources).toEqual(["env-a: /home/theo/.claude"]);
+    }
+  });
+
   it("excludes an environment reporting an older contract version", () => {
     const merged = mergeUsage(
       [

packages/shared/src/usageMerge.ts

@@ -105,9 +105,9 @@ function fingerprintKey(fingerprint: UsageSourceFingerprint): string {
  *
  * Several environments on one machine (worktree servers, for instance) resolve
  * the same provider home and would otherwise double count every token. The
- * first environment in a stable order claims a fingerprint; the rest have that
- * provider's buckets dropped. Environments are sorted by id so the winner does
- * not change between renders.
+ * most recently read summary claims a fingerprint; the rest have that provider's
+ * buckets dropped. Environment ids break ties so the winner is stable when
+ * summaries have the same read time.
  */
 function claimSources(environments: readonly EnvironmentUsage[]): {
   readonly ownerByFingerprint: ReadonlyMap<string, EnvironmentId>;
@@ -116,7 +116,11 @@ function claimSources(environments: readonly EnvironmentUsage[]): {
   const ownerByFingerprint = new Map<string, EnvironmentId>();
   const duplicates: string[] = [];
 
-  const ordered = [...environments].sort((a, b) => a.environmentId.localeCompare(b.environmentId));
+  const ordered = [...environments].sort(
+    (a, b) =>
+      (Date.parse(b.summary.readAt) || 0) - (Date.parse(a.summary.readAt) || 0) ||
+      a.environmentId.localeCompare(b.environmentId),
+  );
 
   for (const environment of ordered) {
     for (const source of environment.summary.sources) {