diff --git a/src/lib/documents/__tests__/document-summary-serialisation.test.ts b/src/lib/documents/__tests__/document-summary-serialisation.test.ts
new file mode 100644
index 000000000..a4be9e6b1
--- /dev/null
+++ b/src/lib/documents/__tests__/document-summary-serialisation.test.ts
@@ -0,0 +1,105 @@
+import { beforeEach, describe, expect, it, vi } from "vitest";
+
+import { _resetCryptoCacheForTests } from "@/lib/crypto";
+import {
+  encryptDocumentSummary,
+  serialiseDocumentDetail,
+  type SerialisableDocument,
+} from "@/lib/documents/store";
+
+/**
+ * What the detail DTO says about a summary has to match what the user can
+ * actually see. The state column is the source of truth for the four outcomes,
+ * with one exception the view depends on: a READY row whose ciphertext will not
+ * open has no summary to render, so reporting READY would leave a heading over
+ * nothing.
+ */
+
+const KEY_V1 =
+  "0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef";
+
+beforeEach(() => {
+  vi.unstubAllEnvs();
+  vi.stubEnv("ENCRYPTION_KEYS", "");
+  vi.stubEnv("ENCRYPTION_ACTIVE_KEY_ID", "");
+  vi.stubEnv("ENCRYPTION_KEY", KEY_V1);
+  _resetCryptoCacheForTests();
+});
+
+function row(overrides: Partial<SerialisableDocument> = {}) {
+  return {
+    id: "doc-1",
+    userId: "user-1",
+    kind: "OTHER",
+    title: "Discharge letter",
+    filename: "letter.pdf",
+    mimeType: "application/pdf",
+    byteSize: 1024,
+    contentCodec: "binary2",
+    contentSha256: null,
+    status: "STORED",
+    providerType: null,
+    reportDate: null,
+    documentDate: null,
+    errorReason: null,
+    summaryEncrypted: null,
+    summaryGeneratedAt: null,
+    summaryState: "NONE",
+    createdAt: new Date("2026-02-14T00:00:00Z"),
+    updatedAt: new Date("2026-02-14T00:00:00Z"),
+    deletedAt: null,
+    ...overrides,
+  } as unknown as SerialisableDocument;
+}
+
+describe("serialiseDocumentDetail — summary state", () => {
+  it("returns the stored summary as READY", () => {
+    const dto = serialiseDocumentDetail(
+      row({
+        summaryEncrypted: encryptDocumentSummary("A discharge letter."),
+        summaryGeneratedAt: new Date("2026-02-15T09:00:00Z"),
+        summaryState: "READY",
+      }),
+      [],
+    );
+
+    expect(dto.summary).toBe("A discharge letter.");
+    expect(dto.summaryState).toBe("READY");
+    expect(dto.summaryGeneratedAt).toBe("2026-02-15T09:00:00.000Z");
+  });
+
+  it.each(["NONE", "PENDING", "WITHHELD", "UNAVAILABLE"] as const)(
+    "passes %s through with no summary",
+    (summaryState) => {
+      const dto = serialiseDocumentDetail(row({ summaryState }), []);
+
+      expect(dto.summary).toBeNull();
+      expect(dto.summaryState).toBe(summaryState);
+    },
+  );
+
+  it("degrades an undecryptable READY row to UNAVAILABLE", () => {
+    // A rotated-away key. There is no summary to show, so the DTO must not
+    // keep promising one — and the ciphertext is never returned either.
+    const dto = serialiseDocumentDetail(
+      row({
+        summaryEncrypted: new TextEncoder().encode("not-ciphertext"),
+        summaryState: "READY",
+      }),
+      [],
+    );
+
+    expect(dto.summary).toBeNull();
+    expect(dto.summaryState).toBe("UNAVAILABLE");
+  });
+
+  it("degrades a READY row with no ciphertext at all to UNAVAILABLE", () => {
+    const dto = serialiseDocumentDetail(
+      row({ summaryEncrypted: null, summaryState: "READY" }),
+      [],
+    );
+
+    expect(dto.summary).toBeNull();
+    expect(dto.summaryState).toBe("UNAVAILABLE");
+  });
+});