diff --git a/src/test/mesh-store-shared.test.ts b/src/test/mesh-store-shared.test.ts
new file mode 100644
index 0000000..50a3992
--- /dev/null
+++ b/src/test/mesh-store-shared.test.ts
@@ -0,0 +1,131 @@
+import { describe, expect, it } from "vitest";
+import {
+  mergeMessageHistories,
+  ROOM_TOKEN_LIFETIME_MS,
+} from "../core/mesh-store-shared.js";
+
+const DAYS = 30;
+const HOURS_PER_DAY = 24;
+const MINUTES_PER_HOUR = 60;
+const SECONDS_PER_MINUTE = 60;
+const MS_PER_SECOND = 1000;
+
+describe("ROOM_TOKEN_LIFETIME_MS", () => {
+  it("is exactly 30 days, in milliseconds", () => {
+    expect(ROOM_TOKEN_LIFETIME_MS).toBe(
+      DAYS *
+        HOURS_PER_DAY *
+        MINUTES_PER_HOUR *
+        SECONDS_PER_MINUTE *
+        MS_PER_SECOND,
+    );
+  });
+});
+
+interface TestMessage {
+  id: string;
+  readBy: string[];
+  text?: string;
+}
+
+function msg(id: string, readBy: readonly string[] = []): TestMessage {
+  return { id, readBy: [...readBy] };
+}
+
+describe("mergeMessageHistories", () => {
+  it("appends an incoming entry the local list does not have", () => {
+    const merged = mergeMessageHistories([msg("a")], [msg("b")]);
+    expect(merged.map((m) => m.id)).toEqual(["a", "b"]);
+  });
+
+  it("appends multiple new entries in incoming order", () => {
+    const merged = mergeMessageHistories([msg("a")], [msg("b"), msg("c")]);
+    expect(merged.map((m) => m.id)).toEqual(["a", "b", "c"]);
+  });
+
+  it("preserves local ordering -- new entries are appended, not inserted", () => {
+    const merged = mergeMessageHistories([msg("a"), msg("z")], [msg("m")]);
+    expect(merged.map((m) => m.id)).toEqual(["a", "z", "m"]);
+  });
+
+  it("unions a new reader onto an existing entry's readBy", () => {
+    const merged = mergeMessageHistories(
+      [msg("a", ["u1"])],
+      [msg("a", ["u2"])],
+    );
+    expect(merged[0]?.readBy.sort()).toEqual(["u1", "u2"]);
+  });
+
+  it("does not duplicate a reader already present locally", () => {
+    const merged = mergeMessageHistories(
+      [msg("a", ["u1"])],
+      [msg("a", ["u1"])],
+    );
+    expect(merged[0]?.readBy).toEqual(["u1"]);
+  });
+
+  it("unions several readers from one incoming entry", () => {
+    const merged = mergeMessageHistories(
+      [msg("a", ["u1"])],
+      [msg("a", ["u1", "u2", "u3"])],
+    );
+    expect(merged[0]?.readBy.sort()).toEqual(["u1", "u2", "u3"]);
+  });
+
+  it("leaves an existing entry's readBy untouched when incoming has none", () => {
+    const merged = mergeMessageHistories([msg("a", ["u1"])], [msg("a", [])]);
+    expect(merged[0]?.readBy).toEqual(["u1"]);
+  });
+
+  it("does not grow the merged length for an already-known entry", () => {
+    const merged = mergeMessageHistories([msg("a")], [msg("a")]);
+    expect(merged).toHaveLength(1);
+  });
+
+  it("handles an empty incoming list as a no-op, returning an equivalent copy", () => {
+    const merged = mergeMessageHistories([msg("a", ["u1"])], []);
+    expect(merged).toEqual([msg("a", ["u1"])]);
+  });
+
+  it("handles an empty local list, returning everything incoming", () => {
+    const merged = mergeMessageHistories([], [msg("a"), msg("b")]);
+    expect(merged.map((m) => m.id)).toEqual(["a", "b"]);
+  });
+
+  it("merges a mix of new and existing entries in one call", () => {
+    const merged = mergeMessageHistories(
+      [msg("a", ["u1"]), msg("b", [])],
+      [msg("a", ["u2"]), msg("c", ["u3"])],
+    );
+    expect(merged.map((m) => m.id)).toEqual(["a", "b", "c"]);
+    expect(merged[0]?.readBy.sort()).toEqual(["u1", "u2"]);
+    expect(merged[1]?.readBy).toEqual([]);
+    expect(merged[2]?.readBy).toEqual(["u3"]);
+  });
+
+  it("does not mutate either input array", () => {
+    const local = [msg("a", ["u1"])];
+    const incoming = [msg("a", ["u2"]), msg("b")];
+    mergeMessageHistories(local, incoming);
+    expect(local).toEqual([msg("a", ["u1"])]);
+    expect(incoming).toEqual([msg("a", ["u2"]), msg("b")]);
+  });
+
+  it("does not mutate an existing local entry's own readBy array in place", () => {
+    const originalEntry = msg("a", ["u1"]);
+    const local = [originalEntry];
+    mergeMessageHistories(local, [msg("a", ["u2"])]);
+    // The merged result's entry for "a" must be a distinct object/array from the one still referenced by the caller's own original local list -- otherwise a caller holding onto its own pre-merge array would see readers silently appear in it too.
+    expect(originalEntry.readBy).toEqual(["u1"]);
+  });
+
+  it("unions two same-id entries within one incoming batch into a single merged entry, not a duplicate", () => {
+    // A brand-new id appearing twice in the same incoming batch is the one case where the freshly-added entry must be looked up again within the same call, not just against local's own pre-existing entries -- proves the id-to-entry index is kept in sync as new entries are added mid-merge, not only seeded from local up front.
+    const merged = mergeMessageHistories(
+      [],
+      [msg("a", ["u1"]), msg("a", ["u2"])],
+    );
+    expect(merged.map((m) => m.id)).toEqual(["a"]);
+    expect(merged[0]?.readBy.sort()).toEqual(["u1", "u2"]);
+  });
+});