diff --git a/src/lib/ai/narrative.test.ts b/src/lib/ai/narrative.test.ts
new file mode 100644
index 0000000..6f42c0a
--- /dev/null
+++ b/src/lib/ai/narrative.test.ts
@@ -0,0 +1,123 @@
+import { describe, it, expect } from "vitest";
+import {
+  buildNarrativePrompt,
+  narrativeIsStale,
+  shaFingerprint,
+  type NarrativeCard,
+  type NarrativeCommit,
+} from "@/lib/ai/narrative";
+
+const card: NarrativeCard = {
+  title: "Add a POS system",
+  description: "Card payments at the counter.",
+};
+
+function commit(sha: string, subject: string, date = "2026-06-01"): NarrativeCommit {
+  return { sha, subject, summary: "", authoredAt: `${date}T10:00:00Z` };
+}
+
+describe("shaFingerprint", () => {
+  it("is stable regardless of order", () => {
+    expect(shaFingerprint(["b", "a", "c"])).toBe(shaFingerprint(["a", "c", "b"]));
+  });
+
+  it("changes when a sha is added or removed", () => {
+    expect(shaFingerprint(["a", "b"])).not.toBe(shaFingerprint(["a", "b", "c"]));
+    expect(shaFingerprint(["a", "b"])).not.toBe(shaFingerprint(["a"]));
+  });
+
+  it("handles an empty set", () => {
+    expect(shaFingerprint([])).toBe("");
+  });
+});
+
+describe("narrativeIsStale", () => {
+  it("is stale when there is no narrative yet", () => {
+    expect(narrativeIsStale({ narrative: "", version: 1, shas: "" }, ["a"], 1)).toBe(true);
+  });
+
+  it("is not stale when the commit set and version match", () => {
+    const fp = shaFingerprint(["a", "b"]);
+    expect(narrativeIsStale({ narrative: "text", version: 1, shas: fp }, ["b", "a"], 1)).toBe(false);
+  });
+
+  it("is stale when a commit is added", () => {
+    const fp = shaFingerprint(["a"]);
+    expect(narrativeIsStale({ narrative: "text", version: 1, shas: fp }, ["a", "b"], 1)).toBe(true);
+  });
+
+  it("is stale when the prompt version moves on", () => {
+    // The text was written under rules that no longer exist.
+    const fp = shaFingerprint(["a"]);
+    expect(narrativeIsStale({ narrative: "text", version: 1, shas: fp }, ["a"], 2)).toBe(true);
+  });
+
+  it("is NOT stale merely because the linker ran again", () => {
+    // Re-linking happens on every push. If an unchanged commit set counted as
+    // stale, every push would regenerate every narrative on the project and
+    // rewrite text the client has already read.
+    const fp = shaFingerprint(["a", "b", "c"]);
+    const stored = { narrative: "text", version: 1, shas: fp };
+    expect(narrativeIsStale(stored, ["c", "b", "a"], 1)).toBe(false);
+  });
+
+  it("is not stale for a card with no commits and no narrative", () => {
+    // Nothing to write about. Generating anything here would be invention.
+    expect(narrativeIsStale({ narrative: "", version: 1, shas: "" }, [], 1)).toBe(false);
+  });
+});
+
+describe("buildNarrativePrompt", () => {
+  const commits = [
+    commit("aaa", "feat(pos): card reader integration", "2026-06-01"),
+    commit("bbb", "fix(pos): handle declined payments", "2026-06-05"),
+  ];
+
+  it("includes the card title and description", () => {
+    const prompt = buildNarrativePrompt(card, commits);
+    expect(prompt).toContain("Add a POS system");
+    expect(prompt).toContain("Card payments at the counter.");
+  });
+
+  it("lists commits oldest first", () => {
+    const prompt = buildNarrativePrompt(card, [commits[1], commits[0]]);
+    expect(prompt.indexOf("card reader")).toBeLessThan(prompt.indexOf("declined payments"));
+  });
+
+  it("prefers an existing one-liner over the raw subject", () => {
+    const withSummary: NarrativeCommit = {
+      ...commits[0],
+      summary: "Connected the card reader.",
+    };
+    const prompt = buildNarrativePrompt(card, [withSummary]);
+    expect(prompt).toContain("Connected the card reader.");
+    expect(prompt).not.toContain("feat(pos): card reader integration");
+  });
+
+  it("falls back to the raw subject when there is no one-liner", () => {
+    const prompt = buildNarrativePrompt(card, commits);
+    expect(prompt).toContain("card reader integration");
+  });
+
+  it("instructs the model not to claim completion", () => {
+    const prompt = buildNarrativePrompt(card, commits).toLowerCase();
+    expect(prompt).toContain("complete");
+  });
+
+  it("includes dates so the narrative can describe a sequence", () => {
+    const prompt = buildNarrativePrompt(card, commits);
+    expect(prompt).toContain("2026-06-01");
+  });
+
+  it("flattens newlines so one commit is one line", () => {
+    const prompt = buildNarrativePrompt(card, [commit("ccc", "one\ntwo")]);
+    expect(prompt).toContain("one two");
+  });
+
+  it("caps a very long commit list", () => {
+    const many = Array.from({ length: 500 }, (_, i) => commit(`s${i}`, `subject ${i}`));
+    const prompt = buildNarrativePrompt(card, many);
+    // A card with 500 commits must not blow the context window or the bill.
+    expect(prompt.length).toBeLessThan(40_000);
+  });
+});
diff --git a/src/lib/ai/narrative.ts b/src/lib/ai/narrative.ts
new file mode 100644
index 0000000..5ed5762
--- /dev/null
+++ b/src/lib/ai/narrative.ts
@@ -0,0 +1,95 @@
+/**
+ * Card progress narratives.
+ *
+ * Keyed on the SET of linked commit SHAs rather than a timestamp, because
+ * re-linking runs on every push. A time-based check would regenerate every
+ * narrative on every push — rewriting text a client has already read, and
+ * paying for it each time.
+ */
+
+/** Most recent commits kept when a card has more than this. */
+const MAX_COMMITS_IN_PROMPT = 120;
+
+export interface NarrativeCard {
+  title: string;
+  description: string;
+}
+
+export interface NarrativeCommit {
+  sha: string;
+  subject: string;
+  /** The Plan 3b one-liner, or "" when not generated. */
+  summary: string;
+  authoredAt: string;
+}
+
+export interface StoredNarrative {
+  narrative: string;
+  version: number;
+  /** The fingerprint the stored text was written from. */
+  shas: string;
+}
+
+/** Order-independent identity for a set of SHAs. */
+export function shaFingerprint(shas: string[]): string {
+  if (shas.length === 0) return "";
+  return [...shas].sort().join(",");
+}
+
+export function narrativeIsStale(
+  stored: StoredNarrative,
+  currentShas: string[],
+  promptVersion: number
+): boolean {
+  // Nothing to write about. Generating here would be invention.
+  if (currentShas.length === 0 && !stored.narrative) return false;
+
+  if (!stored.narrative) return true;
+  if (stored.version !== promptVersion) return true;
+
+  return stored.shas !== shaFingerprint(currentShas);
+}
+
+function flatten(value: string): string {
+  return (value ?? "").replace(/\s+/g, " ").trim();
+}
+
+export function buildNarrativePrompt(
+  card: NarrativeCard,
+  commits: NarrativeCommit[]
+): string {
+  const ordered = [...commits].sort((a, b) =>
+    a.authoredAt.localeCompare(b.authoredAt)
+  );
+
+  // Keep the most recent when a card is enormous: the oldest commits on a
+  // long-running card tell the client least about where it stands now.
+  const kept =
+    ordered.length > MAX_COMMITS_IN_PROMPT
+      ? ordered.slice(ordered.length - MAX_COMMITS_IN_PROMPT)
+      : ordered;
+
+  const lines = kept.map((commit) => {
+    const date = commit.authoredAt.replace(" ", "T").slice(0, 10);
+    // The one-liner is already in the client's language; prefer it.
+    const text = flatten(commit.summary || commit.subject);
+    return `- ${date}: ${text}`;
+  });
+
+  const omitted = ordered.length - kept.length;
+  const note = omitted > 0 ? `\n(${omitted} earlier entries omitted.)` : "";
+
+  return `Write a short progress update for the client about this piece of work.
+
+Work item: ${flatten(card.title)}
+${card.description ? `What was asked for: ${flatten(card.description)}` : ""}
+
+What has happened, oldest first:
+${lines.join("\n")}${note}
+
+Write two or three short paragraphs covering what was built, anything that changed along the way, and where the work stands according to these entries.
+
+Do not say the work is complete, finished or done — whether it is finished is recorded separately on the project board, not in this history. Do not mention dates for future work. Do not invent anything that is not above.
+
+Reply with the paragraphs only.`;
+}