diff --git a/packages/opencode/test/id/id.test.ts b/packages/opencode/test/id/id.test.ts
new file mode 100644
index 000000000..9f97ff98c
--- /dev/null
+++ b/packages/opencode/test/id/id.test.ts
@@ -0,0 +1,91 @@
+import { describe, test, expect } from "bun:test"
+import { Identifier } from "../../src/id/id"
+
+describe("Identifier", () => {
+  describe("ascending", () => {
+    test("generates ID with correct prefix", () => {
+      const id = Identifier.ascending("session")
+      expect(id).toStartWith("ses_")
+      expect(id.length).toBeGreaterThan(10)
+    })
+
+    test("generates monotonically increasing IDs", () => {
+      const ids = Array.from({ length: 100 }, () => Identifier.ascending("message"))
+      for (let i = 1; i < ids.length; i++) {
+        expect(ids[i] > ids[i - 1]).toBe(true)
+      }
+    })
+
+    test("returns given ID when prefix matches", () => {
+      const given = "ses_abc123"
+      const id = Identifier.ascending("session", given)
+      expect(id).toBe(given)
+    })
+
+    test("accepts any string that starts with the prefix — prefix check is permissive", () => {
+      // "session_xyz" starts with "ses", so generateID accepts it
+      const given = "ses_sion_xyz"
+      const id = Identifier.ascending("session", given)
+      expect(id).toBe(given)
+    })
+
+    test("throws on mismatched prefix", () => {
+      expect(() => Identifier.ascending("session", "msg_abc123")).toThrow(
+        "ID msg_abc123 does not start with ses",
+      )
+    })
+  })
+
+  describe("descending", () => {
+    test("generates ID with correct prefix", () => {
+      const id = Identifier.descending("session")
+      expect(id).toStartWith("ses_")
+    })
+
+    test("generates monotonically decreasing IDs", () => {
+      const ids = Array.from({ length: 100 }, () => Identifier.descending("session"))
+      for (let i = 1; i < ids.length; i++) {
+        expect(ids[i] < ids[i - 1]).toBe(true)
+      }
+    })
+  })
+
+  describe("timestamp", () => {
+    test("relative comparison works between IDs created close together", () => {
+      // timestamp() is used in tool/truncation.ts for relative age comparisons.
+      // It only stores 48 bits, so absolute values overflow for modern timestamps,
+      // but relative comparisons between IDs in the same era are consistent.
+      const earlier = Identifier.create("tool", false, Date.now() - 60_000)
+      const later = Identifier.create("tool", false, Date.now())
+      expect(Identifier.timestamp(later)).toBeGreaterThan(Identifier.timestamp(earlier))
+    })
+
+    test("extracts consistent value from same ID", () => {
+      const id = Identifier.ascending("session")
+      const ts1 = Identifier.timestamp(id)
+      const ts2 = Identifier.timestamp(id)
+      expect(ts1).toBe(ts2)
+    })
+  })
+
+  describe("all prefixes", () => {
+    const prefixes = [
+      ["session", "ses_"],
+      ["message", "msg_"],
+      ["permission", "per_"],
+      ["question", "que_"],
+      ["user", "usr_"],
+      ["part", "prt_"],
+      ["pty", "pty_"],
+      ["tool", "tool_"],
+      ["workspace", "wrk_"],
+    ] as const
+
+    for (const [key, prefix] of prefixes) {
+      test(`${key} generates IDs starting with ${prefix}`, () => {
+        const id = Identifier.ascending(key as any)
+        expect(id).toStartWith(prefix)
+      })
+    }
+  })
+})