diff --git a/packages/boulder-state/test/normalize-session-id.test.ts b/packages/boulder-state/test/normalize-session-id.test.ts
new file mode 100644
index 000000000..05ecd6769
--- /dev/null
+++ b/packages/boulder-state/test/normalize-session-id.test.ts
@@ -0,0 +1,63 @@
+/// <reference path="../../../bun-test.d.ts" />
+
+import { describe, expect, test } from "bun:test"
+import * as boulderState from "../src"
+
+describe("normalizeSessionId", () => {
+  test("#given a bare id #when normalized without a platform #then opencode is used by default", () => {
+    // given
+    expect(typeof boulderState.normalizeSessionId).toBe("function")
+
+    // when
+    const normalized = boulderState.normalizeSessionId("sess_abc")
+
+    // then
+    expect(normalized).toBe("opencode:sess_abc")
+  })
+
+  test("#given a bare id and codex platform #when normalized #then codex is used", () => {
+    // given
+    expect(typeof boulderState.normalizeSessionId).toBe("function")
+
+    // when
+    const normalized = boulderState.normalizeSessionId("sess_abc", "codex")
+
+    // then
+    expect(normalized).toBe("codex:sess_abc")
+  })
+
+  test("#given an opencode-prefixed id #when normalized #then the id is unchanged", () => {
+    // given
+    const input = "opencode:sess_abc"
+    expect(typeof boulderState.normalizeSessionId).toBe("function")
+
+    // when
+    const normalized = boulderState.normalizeSessionId(input)
+
+    // then
+    expect(normalized).toBe(input)
+  })
+
+  test("#given a codex-prefixed id and opencode platform #when normalized #then the existing prefix wins", () => {
+    // given
+    expect(typeof boulderState.normalizeSessionId).toBe("function")
+
+
+    // when
+    const normalized = boulderState.normalizeSessionId("codex:sess_abc", "opencode")
+
+    // then
+    expect(normalized).toBe("codex:sess_abc")
+  })
+
+  test("#given an empty id #when normalized #then opencode empty id is preserved", () => {
+    // given
+    expect(typeof boulderState.normalizeSessionId).toBe("function")
+
+    // when
+    const normalized = boulderState.normalizeSessionId("")
+
+    // then
+    expect(normalized).toBe("opencode:")
+  })
+})