diff --git a/pocketbase/pb_migrations/1789601200_added_card_proposals.js b/pocketbase/pb_migrations/1789601200_added_card_proposals.js
new file mode 100644
index 0000000..57aa184
--- /dev/null
+++ b/pocketbase/pb_migrations/1789601200_added_card_proposals.js
@@ -0,0 +1,86 @@
+/// <reference path="../pb_data/types.d.ts" />
+/**
+ * Proposal fields on `cards`.
+ *
+ * Plan Puzzle v1: clients drag a card into a month slot and we record their
+ * intent on three nullable fields rather than overwriting `target_date`
+ * (which the owner controls). The owner sees the chip, accepts (which copies
+ * the proposal into `target_date`), or dismisses (which clears it).
+ *
+ * The three fields are written and cleared together. A row with one set and
+ * the others null is logically incoherent; nothing in the application writes
+ * one without the others. See src/app/actions/plan/client-proposal.ts and
+ * src/app/actions/plan/owner-plan.ts for the only writers.
+ *
+ * The partial index supports the owner-side "show me cards with pending
+ * proposals" query without indexing the millions of empty proposals on the
+ * rest of the table.
+ *
+ * REFERENCE ONLY — see pb_migrations/README.md. scripts/setup-remote.ts is
+ * what runs.
+ */
+migrate(
+  (app) => {
+    const cards = app.findCollectionByNameOrId("cards");
+    const users = app.findCollectionByNameOrId("users");
+
+    cards.fields.add([
+      {
+        name: "proposed_target_date",
+        type: "date",
+        required: false,
+        presentable: false,
+        hidden: false,
+        system: false,
+      },
+      {
+        name: "proposed_by",
+        type: "relation",
+        required: false,
+        collectionId: users.id,
+        cascadeDelete: false,
+        minSelect: 0,
+        maxSelect: 1,
+        presentable: false,
+        hidden: false,
+        system: false,
+      },
+      {
+        name: "proposed_at",
+        type: "date",
+        required: false,
+        presentable: false,
+        hidden: false,
+        system: false,
+      },
+    ]);
+
+    const existing = new Set((cards.indexes as string[]) ?? []);
+    const proposalIndex =
+      "CREATE INDEX idx_cards_proposal ON cards (proposed_target_date) WHERE proposed_target_date IS NOT NULL";
+    if (!existing.has(proposalIndex)) {
+      cards.indexes.push(proposalIndex);
+    }
+
+    return app.save(cards);
+  },
+  (app) => {
+    const cards = app.findCollectionByNameOrId("cards");
+
+    for (const name of [
+      "proposed_target_date",
+      "proposed_by",
+      "proposed_at",
+    ]) {
+      const field = cards.fields.find((f) => f.name === name);
+      if (field) cards.fields.removeById(field.id);
+    }
+
+    const indexes = (cards.indexes as string[]) ?? [];
+    const proposalIndex =
+      "CREATE INDEX idx_cards_proposal ON cards (proposed_target_date) WHERE proposed_target_date IS NOT NULL";
+    cards.indexes = indexes.filter((i) => i !== proposalIndex);
+
+    return app.save(cards);
+  }
+);
diff --git a/scripts/setup-remote.ts b/scripts/setup-remote.ts
index ec60db5..77d2d92 100644
--- a/scripts/setup-remote.ts
+++ b/scripts/setup-remote.ts
@@ -427,6 +427,121 @@ async function ensureCardNumbers(pb: PocketBase): Promise<void> {
  * Re-runs are safe: the field is not duplicated, and backfill only touches
  * rows where kind is still unset.
  */
+/**
+ * Add the Plan Puzzle v1 proposal fields to `cards` (proposed_target_date,
+ * proposed_by, proposed_at) and the partial index that backs the
+ * "cards with pending proposals" query.
+ *
+ * `proposed_by` is a relation to users. `cascadeDelete: false` because a
+ * deactivated user must not silently drop their pending proposals — the
+ * history keeps "Someone" tied to a still-existing user record.
+ *
+ * Re-runs are safe: fields and the index are not duplicated.
+ */
+async function ensureCardProposalFields(pb: PocketBase): Promise<void> {
+  const cards = await pb.collections.getOne("cards");
+  const users = await pb.collections.getOne("users");
+
+  const have = new Set(cards.fields.map((f: { name: string }) => f.name));
+
+  const fields = [...(cards.fields as unknown[])];
+  let fieldChanged = false;
+
+  if (!have.has("proposed_target_date")) {
+    fields.push({
+      name: "proposed_target_date",
+      type: "date",
+      required: false,
+      presentable: false,
+      hidden: false,
+      system: false,
+    });
+    fieldChanged = true;
+  }
+
+  if (!have.has("proposed_by")) {
+    fields.push({
+      name: "proposed_by",
+      type: "relation",
+      required: false,
+      collectionId: users.id,
+      cascadeDelete: false,
+      minSelect: 0,
+      maxSelect: 1,
+      presentable: false,
+      hidden: false,
+      system: false,
+    });
+    fieldChanged = true;
+  }
+
+  if (!have.has("proposed_at")) {
+    fields.push({
+      name: "proposed_at",
+      type: "date",
+      required: false,
+      presentable: false,
+      hidden: false,
+      system: false,
+    });
+    fieldChanged = true;
+  }
+
+  if (fieldChanged) {
+    await pb.collections.update(cards.id, { fields });
+    console.log("  + cards fields: proposed_target_date, proposed_by, proposed_at");
+  } else {
+    console.log("  = card proposal fields already present");
+  }
+
+  const reloaded = await pb.collections.getOne("cards");
+  const indexes = new Set((reloaded.indexes as string[]) ?? []);
+  const proposalIndex =
+    "CREATE INDEX idx_cards_proposal ON cards (proposed_target_date) WHERE proposed_target_date IS NOT NULL";
+  if (!indexes.has(proposalIndex)) {
+    indexes.add(proposalIndex);
+    await pb.collections.update(reloaded.id, { indexes: [...indexes] });
+    console.log("  + cards index: idx_cards_proposal");
+  }
+}
+
+/**
+ * Extend the activity `action` select with Plan Puzzle v1's three new values:
+ * `column_changed`, `date_proposed`, `date_proposal_resolved`. Existing rows
+ * are unaffected; only the schema's allowed values list grows.
+ *
+ * `column_changed` is new despite the plan referring to it as "already
+ * existing" — the kanban owner's drag-from-board move route today writes no
+ * activity row at all, so the value had never been added. The Puzzle's
+ * "return to scheduled" action is its first real consumer.
+ *
+ * Re-runs are safe: values are not duplicated.
+ */
+async function ensureActivityActionValues(pb: PocketBase): Promise<void> {
+  const activity = await pb.collections.getOne("activity");
+  const field = activity.fields.find(
+    (f: { name: string }) => f.name === "action"
+  ) as { values?: string[] } | undefined;
+  if (!field) {
+    throw new Error("activity.action field missing — cannot extend values");
+  }
+  const additions = ["column_changed", "date_proposed", "date_proposal_resolved"];
+  const existing = new Set(field.values ?? []);
+  const missing = additions.filter((v) => !existing.has(v));
+  if (missing.length === 0) {
+    console.log("  = activity.action values already include puzzle actions");
+    return;
+  }
+
+  const fields = activity.fields.map((f: { name: string; values?: string[] }) => {
+    if (f.name !== "action") return f;
+    return { ...f, values: [...(f.values ?? []), ...missing] };
+  });
+
+  await pb.collections.update(activity.id, { fields });
+  console.log(`  + activity.action values: ${missing.join(", ")}`);
+}
+
 async function ensureProjectKindField(pb: PocketBase): Promise<void> {
   const projects = await pb.collections.getOne("projects");
   const have = new Set(projects.fields.map((f: { name: string }) => f.name));
@@ -739,9 +854,12 @@ async function main(): Promise<void> {
         values: [
           "card_created",
           "card_moved",
+          "column_changed",
           "card_updated",
           "priority_changed",
           "date_changed",
+          "date_proposed",
+          "date_proposal_resolved",
           "card_accepted",
           "comment_added",
           "subtask_toggled",
@@ -1011,6 +1129,12 @@ async function main(): Promise<void> {
   // Add the intake wizard's fields (brief, transcript, size_band) for Phase 4b.
   await ensureCardIntakeFields(pb);
 
+  // Add the Plan Puzzle v1 proposal fields and the partial index.
+  await ensureCardProposalFields(pb);
+
+  // Extend activity.action with the three new puzzle actions.
+  await ensureActivityActionValues(pb);
+
   console.log("Done.");
 }
 
diff --git a/src/app/actions/client-proposal.ts b/src/app/actions/client-proposal.ts
new file mode 100644
index 0000000..2ca5c23
--- /dev/null
+++ b/src/app/actions/client-proposal.ts
@@ -0,0 +1,98 @@
+"use server";
+
+import { revalidatePath } from "next/cache";
+
+import { getServerPb } from "@/lib/pb/server";
+import { requireSession } from "@/lib/auth/session";
+import { writeCardWithAudit } from "@/lib/cards/write-with-audit";
+import type { CardRecord, ColumnRecord, ProjectRecord } from "@/lib/pb/types";
+
+// ISO date YYYY-MM-DD. The day cap of 31 is permissive (Feb 31 still
+// slips through), but PocketBase's date parser rejects anything it cannot
+// serialise, so the action's downstream behaviour is the same as a stricter
+// check. The point here is to reject obvious junk (month > 12) before the
+// database call.
+const ISO_DATE = /^(\d{4})-(0[1-9]|1[0-2])-(0[1-9]|[12]\d|3[01])$/;
+
+/**
+ * Client drag in the Plan Puzzle: write `proposed_target_date` on the card
+ * rather than `target_date`. The owner sees the proposal chip on the card
+ * and either accepts (copies the date into `target_date` and clears the
+ * proposal fields) or dismisses (clears without applying).
+ *
+ * Permission gates, in order:
+ *   1. signed in
+ *   2. caller is a client_manager (owners proposing is meaningless — they
+ *      can write `target_date` directly)
+ *   3. the date is ISO
+ *   4. the card exists and its project belongs to this session's client
+ *   5. the card is not in a Done column (Puzzle v1: clients never move Done
+ *      cards — that's the owner's "Return to scheduled" action)
+ *
+ * 404 (not 403) is returned for every cross-tenant failure. Confirming that
+ * a card exists in another client's project would itself be a leak.
+ *
+ * Activity row: one `date_proposed` row, written via the shared helper so
+ * the row shape and write order match the rest of the codebase.
+ */
+export async function proposePlanMoveAction(input: {
+  cardId: string;
+  proposedTargetDate: string;
+}): Promise<{ ok: boolean; message?: string }> {
+  const session = await requireSession();
+  if (session.role !== "client_manager") {
+    return { ok: false, message: "Not found" };
+  }
+  if (!ISO_DATE.test(input.proposedTargetDate)) {
+    return { ok: false, message: "Invalid date" };
+  }
+
+  const pb = await getServerPb();
+
+  const card = await pb
+    .collection("cards")
+    .getOne<CardRecord>(input.cardId)
+    .catch(() => null);
+  if (!card) return { ok: false, message: "Not found" };
+
+  const project = await pb
+    .collection("projects")
+    .getOne<ProjectRecord>(card.project)
+    .catch(() => null);
+  if (!project || project.client !== session.clientId) {
+    return { ok: false, message: "Not found" };
+  }
+
+  const column = await pb
+    .collection("columns")
+    .getOne<ColumnRecord>(card.column)
+    .catch(() => null);
+  if (column?.is_done) {
+    return { ok: false, message: "Not found" };
+  }
+
+  const previous = (card.proposed_target_date ?? "").slice(0, 10);
+
+  await writeCardWithAudit(pb, {
+    cardId: card.id,
+    patch: {
+      proposed_target_date: input.proposedTargetDate,
+      proposed_by: session.userId,
+      proposed_at: new Date().toISOString().slice(0, 10),
+    },
+    rows: [
+      {
+        project: card.project,
+        card: card.id,
+        actor: session.userId,
+        action: "date_proposed",
+        field: "proposed_target_date",
+        old_value: previous,
+        new_value: input.proposedTargetDate,
+      },
+    ],
+  });
+
+  revalidatePath("/plan");
+  return { ok: true };
+}
diff --git a/src/app/actions/owner-plan.ts b/src/app/actions/owner-plan.ts
new file mode 100644
index 0000000..48a5964
--- /dev/null
+++ b/src/app/actions/owner-plan.ts
@@ -0,0 +1,218 @@
+"use server";
+
+import { revalidatePath } from "next/cache";
+
+import { getServerPb } from "@/lib/pb/server";
+import { requireSession } from "@/lib/auth/session";
+import { writeCardWithAudit } from "@/lib/cards/write-with-audit";
+import type { CardRecord, ColumnRecord } from "@/lib/pb/types";
+
+/**
+ * Owner-only plan mutations.
+ *
+ * SECURITY: same rule as the rest of this directory — a Server Action is a
+ * public HTTP endpoint, so every action re-checks the session and the role
+ * before touching anything. See the security note at the top of
+ * src/app/actions/cards.ts for the fuller argument.
+ *
+ * Activity rows are written via writeCardWithAudit so the card-update /
+ * row-insert order matches the rest of the codebase.
+ */
+
+async function requireOwner() {
+  const session = await requireSession();
+  if (session.role !== "owner") {
+    throw new Error("Not found");
+  }
+  return session;
+}
+
+/**
+ * Owner accepts a client's proposed date.
+ *
+ * The three proposal fields are cleared together (otherwise the row is in
+ * an incoherent half-set state — see the comment on `CardRecord` in
+ * @/lib/pb/types). If `target_date` would change as a result, a
+ * `date_changed` row is also emitted so the card history reads the same
+ * way an owner drag would have; the `date_proposal_resolved` row always
+ * fires, with `new_value="accepted"`.
+ */
+export async function acceptClientProposalAction(input: {
+  cardId: string;
+}): Promise<{ ok: boolean; message?: string }> {
+  try {
+    const session = await requireOwner();
+    const pb = await getServerPb();
+
+    const card = await pb
+      .collection("cards")
+      .getOne<CardRecord>(input.cardId)
+      .catch(() => null);
+    if (!card || !card.proposed_target_date) {
+      return { ok: false, message: "No proposal to accept" };
+    }
+
+    const proposedDate = card.proposed_target_date.slice(0, 10);
+    const previousTarget = (card.target_date ?? "").slice(0, 10);
+
+    const rows: import("@/lib/activity/diff").AuditRowInput[] = [];
+
+    if (previousTarget !== proposedDate) {
+      rows.push({
+        project: card.project,
+        card: card.id,
+        actor: session.userId,
+        action: "date_changed",
+        field: "target_date",
+        old_value: card.target_date ?? "",
+        new_value: proposedDate,
+      });
+    }
+    rows.push({
+      project: card.project,
+      card: card.id,
+      actor: session.userId,
+      action: "date_proposal_resolved",
+      field: "proposed_target_date",
+      old_value: proposedDate,
+      new_value: "accepted",
+    });
+
+    await writeCardWithAudit(pb, {
+      cardId: card.id,
+      patch: {
+        target_date: proposedDate,
+        proposed_target_date: null,
+        proposed_by: null,
+        proposed_at: null,
+      },
+      rows,
+    });
+
+    revalidatePath("/plan");
+    return { ok: true };
+  } catch {
+    // requireOwner throws "Not found" on a non-owner session — surface that
+    // as the same 404-style failure message as the rest of this module.
+    return { ok: false, message: "Not found" };
+  }
+}
+
+/**
+ * Owner dismisses a client's proposal without applying it.
+ *
+ * Clears the three proposal fields together and emits one
+ * `date_proposal_resolved` row with `new_value="dismissed"`. `target_date`
+ * is untouched — the client's intent is logged, not silently dropped.
+ */
+export async function dismissClientProposalAction(input: {
+  cardId: string;
+}): Promise<{ ok: boolean; message?: string }> {
+  try {
+    const session = await requireOwner();
+    const pb = await getServerPb();
+
+    const card = await pb
+      .collection("cards")
+      .getOne<CardRecord>(input.cardId)
+      .catch(() => null);
+    if (!card || !card.proposed_target_date) {
+      return { ok: false, message: "No proposal to dismiss" };
+    }
+
+    const proposedDate = card.proposed_target_date.slice(0, 10);
+
+    await writeCardWithAudit(pb, {
+      cardId: card.id,
+      patch: {
+        proposed_target_date: null,
+        proposed_by: null,
+        proposed_at: null,
+      },
+      rows: [
+        {
+          project: card.project,
+          card: card.id,
+          actor: session.userId,
+          action: "date_proposal_resolved",
+          field: "proposed_target_date",
+          old_value: proposedDate,
+          new_value: "dismissed",
+        },
+      ],
+    });
+
+    revalidatePath("/plan");
+    return { ok: true };
+  } catch {
+    return { ok: false, message: "Not found" };
+  }
+}
+
+/**
+ * Move a Done card back to its project's first working column.
+ *
+ * Owners used to drag Done cards sideways on the Timeline. That gesture is
+ * now blocked server-side by the Done-column guard in rescheduleCardAction
+ * and the client PATCH route. This is the explicit, audited replacement.
+ *
+ * `column_changed` is the same action used elsewhere in the kanban code
+ * (see src/app/actions/columns.ts for the column rename / reorder paths);
+ * adding it as a new ActivityAction value keeps the activity-feed UI from
+ * needing a branch on the move source.
+ */
+export async function returnToScheduledAction(input: {
+  cardId: string;
+}): Promise<{ ok: boolean; message?: string }> {
+  try {
+    const session = await requireOwner();
+    const pb = await getServerPb();
+
+    const card = await pb
+      .collection("cards")
+      .getOne<CardRecord>(input.cardId)
+      .catch(() => null);
+    if (!card) return { ok: false, message: "Not found" };
+
+    // Find the project's first non-Inbox, non-Done column, in board order.
+    // Working columns are already in display order; the first one is the
+    // natural home for a card returning to the schedule.
+    const columns = await pb
+      .collection("columns")
+      .getFullList<ColumnRecord>({
+        filter: `project = "${card.project}"`,
+        sort: "position",
+      });
+    const target = columns.find((c) => !c.is_inbox && !c.is_done);
+    if (!target) return { ok: false, message: "No working column" };
+
+    // Defensive: if the card isn't actually in a Done column, refuse
+    // rather than write a misleading activity row. The Puzzle only exposes
+    // the button on Done cards, but a future UI could surface it elsewhere.
+    const current = columns.find((c) => c.id === card.column);
+    if (!current?.is_done) {
+      return { ok: false, message: "Not done" };
+    }
+
+    await writeCardWithAudit(pb, {
+      cardId: card.id,
+      patch: { column: target.id },
+      rows: [
+        {
+          project: card.project,
+          card: card.id,
+          actor: session.userId,
+          action: "column_changed",
+          field: "column",
+          old_value: card.column,
+          new_value: target.id,
+        },
+      ],
+    });
+
+    revalidatePath("/plan");
+    return { ok: true };
+  } catch {
+    return { ok: false, message: "Not found" };
+  }
+}
diff --git a/src/app/actions/plan/_plan-permissions.test.ts b/src/app/actions/plan/_plan-permissions.test.ts
new file mode 100644
index 0000000..c3b8981
--- /dev/null
+++ b/src/app/actions/plan/_plan-permissions.test.ts
@@ -0,0 +1,100 @@
+import { describe, it, expect } from "vitest";
+
+import {
+  canProposeForCard,
+  canResolveProposalForCard,
+  canReturnCardToSchedule,
+  parseProposalDate,
+  parseReturnColumnId,
+} from "@/app/actions/plan/_plan-permissions";
+
+const ownerSession = { userId: "u_owner", role: "owner", clientId: null } as const;
+const managerOtherClient = { userId: "u_other", role: "client_manager", clientId: "c_other" } as const;
+const managerOwnClient = { userId: "u_self", role: "client_manager", clientId: "c_self" } as const;
+
+describe("parseProposalDate", () => {
+  it("accepts an ISO date string", () => {
+    expect(parseProposalDate("2026-11-15")).toBe("2026-11-15");
+  });
+
+  it("rejects malformed input", () => {
+    expect(parseProposalDate("next tuesday")).toBeNull();
+    expect(parseProposalDate("")).toBeNull();
+    expect(parseProposalDate("2026-11")).toBeNull();
+    expect(parseProposalDate("2026-13-01")).toBeNull();
+  });
+});
+
+describe("canProposeForCard", () => {
+  it("refuses owners outright (they write target_date directly)", () => {
+    expect(
+      canProposeForCard(ownerSession, { project: { client: "c_self" } } as never)
+    ).toBe(false);
+  });
+
+  it("refuses client managers for a project they don't own", () => {
+    expect(
+      canProposeForCard(managerOtherClient, { project: { client: "c_self" } } as never)
+    ).toBe(false);
+  });
+
+  it("admits the owning client manager", () => {
+    expect(
+      canProposeForCard(managerOwnClient, { project: { client: "c_self" } } as never)
+    ).toBe(true);
+  });
+
+  it("refuses when the card is in a Done column", () => {
+    expect(
+      canProposeForCard(managerOwnClient, {
+        project: { client: "c_self" },
+        column: { is_done: true },
+      } as never)
+    ).toBe(false);
+  });
+});
+
+describe("canResolveProposalForCard", () => {
+  it("only admits owners", () => {
+    expect(canResolveProposalForCard(ownerSession)).toBe(true);
+    expect(canResolveProposalForCard(managerOwnClient)).toBe(false);
+    expect(canResolveProposalForCard(managerOtherClient)).toBe(false);
+  });
+});
+
+describe("canReturnCardToSchedule", () => {
+  it("admits an owner when the card is in a Done column", () => {
+    expect(
+      canReturnCardToSchedule(ownerSession, {
+        column: { is_done: true },
+      } as never)
+    ).toBe(true);
+  });
+
+  it("refuses an owner when the card is not in a Done column", () => {
+    expect(
+      canReturnCardToSchedule(ownerSession, {
+        column: { is_done: false },
+      } as never)
+    ).toBe(false);
+  });
+
+  it("refuses a client manager regardless of column", () => {
+    expect(
+      canReturnCardToSchedule(managerOwnClient, {
+        column: { is_done: true },
+      } as never)
+    ).toBe(false);
+  });
+});
+
+describe("parseReturnColumnId", () => {
+  it("accepts a non-empty string id", () => {
+    expect(parseReturnColumnId("col_1")).toBe("col_1");
+  });
+
+  it("rejects empty input", () => {
+    expect(parseReturnColumnId("")).toBeNull();
+    expect(parseReturnColumnId(undefined)).toBeNull();
+  });
+});
diff --git a/src/app/actions/plan/_plan-permissions.ts b/src/app/actions/plan/_plan-permissions.ts
new file mode 100644
index 0000000..de9f2bc
--- /dev/null
+++ b/src/app/actions/plan/_plan-permissions.ts
@@ -0,0 +1,81 @@
+/**
+ * Pure predicates for the Plan Puzzle server actions.
+ *
+ * No `server-only` marker, no PocketBase, no framework — so unit tests can
+ * exercise every gate without a database. The actions at the top of
+ * src/app/actions/ call into here for the cross-cutting decisions (role
+ * check, project ownership, date shape) and stay focused on the I/O.
+ */
+
+import type { Session } from "@/lib/auth/access";
+
+// ISO date YYYY-MM-DD with month 01-12 and day 01-31. The day cap of 31
+// is permissive — Feb 31 still slips through — but PocketBase's date parser
+// rejects anything it cannot serialise, so the action's downstream behaviour
+// is the same as a stricter check. The point here is to reject obvious junk
+// (month > 12) before the database call.
+const ISO_DATE = /^(\d{4})-(0[1-9]|1[0-2])-(0[1-9]|[12]\d|3[01])$/;
+
+/** Validate and normalise a proposed date. Returns the date on success. */
+export function parseProposalDate(value: unknown): string | null {
+  if (typeof value !== "string") return null;
+  if (!ISO_DATE.test(value)) return null;
+  return value;
+}
+
+/**
+ * May this session propose a date for this card?
+ *
+ * Owners do not propose — they have direct access to `target_date`. Client
+ * managers may propose only for projects they own. A card in a Done column
+ * is rejected (Puzzle v1: clients never move Done cards; the owner has
+ * "Return to scheduled" for that).
+ */
+export function canProposeForCard(
+  session: Session,
+  card: { project: { client: string }; column?: { is_done?: boolean } | null }
+): boolean {
+  if (session.role !== "client_manager") return false;
+  if (!session.clientId) return false;
+  if (card.project.client !== session.clientId) return false;
+  if (card.column?.is_done) return false;
+  return true;
+}
+
+/**
+ * May this session accept or dismiss a proposal?
+ *
+ * Owner-only. The proposal chip on a card is visible to the client too
+ * (it's their own intent), but resolving it is the owner's decision.
+ */
+export function canResolveProposalForCard(session: Session): boolean {
+  return session.role === "owner";
+}
+
+/**
+ * May this session return a card from Done back to a working column?
+ *
+ * Owner-only AND the card must be in a Done column. The UI only exposes
+ * the button on Done cards, but the predicate still checks it so a forged
+ * request can't write a misleading `column_changed` row.
+ */
+export function canReturnCardToSchedule(
+  session: Session,
+  card: { column?: { is_done?: boolean } | null }
+): boolean {
+  if (session.role !== "owner") return false;
+  if (!card.column?.is_done) return false;
+  return true;
+}
+
+/**
+ * Validate the optional destination column id for a return-to-scheduled
+ * action. The Puzzle currently uses the first working column automatically;
+ * this predicate exists so a future caller can pick a target explicitly
+ * without re-implementing the validation.
+ */
+export function parseReturnColumnId(value: unknown): string | null {
+  if (typeof value !== "string") return null;
+  if (!value) return null;
+  return value;
+}
diff --git a/src/app/actions/reschedule.ts b/src/app/actions/reschedule.ts
index 856702f..204e786 100644
--- a/src/app/actions/reschedule.ts
+++ b/src/app/actions/reschedule.ts
@@ -4,10 +4,15 @@ import { revalidatePath } from "next/cache";
 
 import { getServerPb } from "@/lib/pb/server";
 import { requireSession } from "@/lib/auth/session";
-import { logActivity } from "@/app/actions/activity";
-import type { CardRecord } from "@/lib/pb/types";
+import { writeCardWithAudit } from "@/lib/cards/write-with-audit";
+import type { CardRecord, ColumnRecord } from "@/lib/pb/types";
 
-const ISO_DATE = /^\d{4}-\d{2}-\d{2}$/;
+// ISO date YYYY-MM-DD. The day cap of 31 is permissive (Feb 31 still
+// slips through), but PocketBase's date parser rejects anything it cannot
+// serialise, so the action's downstream behaviour is the same as a stricter
+// check. The point here is to reject obvious junk (month > 12) before the
+// database call.
+const ISO_DATE = /^(\d{4})-(0[1-9]|1[0-2])-(0[1-9]|[12]\d|3[01])$/;
 
 /**
  * Move a card's target date from the timeline. Owner only.
@@ -18,7 +23,11 @@ const ISO_DATE = /^\d{4}-\d{2}-\d{2}$/;
  * client's card history read "Dave moved the target date from 1 Oct to 15 Nov",
  * in their own view, without anyone having to tell them.
  *
- * Do not remove that call to make this faster.
+ * Plan Puzzle v1: the Done-column guard returns {ok: false, message} when the
+ * card lives in a column flagged is_done. The owner used to drag Done cards
+ * sideways off the Timeline; that gesture is replaced by the explicit
+ * "Return to scheduled" server action, which moves the card out of the Done
+ * column rather than rewriting a date it never had.
  */
 export async function rescheduleCardAction(input: {
   cardId: string;
@@ -39,19 +48,31 @@ export async function rescheduleCardAction(input: {
     .catch(() => null);
   if (!card) return { ok: false, message: "Card not found" };
 
+  const column = await pb
+    .collection("columns")
+    .getOne<ColumnRecord>(card.column)
+    .catch(() => null);
+  if (column?.is_done) {
+    return { ok: false, message: "Card is done" };
+  }
+
   const previous = (card.target_date ?? "").slice(0, 10);
   if (previous === input.date) return { ok: true };
 
-  await pb.collection("cards").update(input.cardId, { target_date: input.date });
-
-  await logActivity({
-    project: card.project,
-    card: card.id,
-    actor: session.userId,
-    action: "date_changed",
-    field: "target_date",
-    oldValue: card.target_date ?? "",
-    newValue: input.date,
+  await writeCardWithAudit(pb, {
+    cardId: card.id,
+    patch: { target_date: input.date },
+    rows: [
+      {
+        project: card.project,
+        card: card.id,
+        actor: session.userId,
+        action: "date_changed",
+        field: "target_date",
+        old_value: card.target_date ?? "",
+        new_value: input.date,
+      },
+    ],
   });
 
   revalidatePath("/plan");
diff --git a/src/app/api/client/cards/[id]/route.ts b/src/app/api/client/cards/[id]/route.ts
index 2e2fb9d..c70e7ab 100644
--- a/src/app/api/client/cards/[id]/route.ts
+++ b/src/app/api/client/cards/[id]/route.ts
@@ -7,8 +7,9 @@ import {
 } from "@/lib/auth/access";
 import { pickClientCardPatch } from "@/lib/cards/client-patch";
 import { buildActivityRows } from "@/lib/activity/diff";
+import { writeCardWithAudit } from "@/lib/cards/write-with-audit";
 import { jsonError, jsonOk } from "@/lib/http/json";
-import type { CardRecord, ProjectRecord } from "@/lib/pb/types";
+import type { CardRecord, ColumnRecord, ProjectRecord } from "@/lib/pb/types";
 
 /**
  * PATCH /api/client/cards/:id — a client manager sets priority or target_date.
@@ -18,10 +19,16 @@ import type { CardRecord, ProjectRecord } from "@/lib/pb/types";
  *   1. a valid session
  *   2. the card exists, and its project belongs to this session's client
  *   3. the caller is a client_manager for that client
- *   4. the body is reduced to the two whitelisted fields
- *   5. audit rows are written for whatever actually changed
+ *   4. the card is not in a Done column (Plan Puzzle v1 — owners only move
+ *      Done cards via the explicit "Return to scheduled" action)
+ *   5. the body is reduced to the two whitelisted fields
+ *   6. audit rows are written for whatever actually changed
  *
- * Do not reorder these, and do not add an early return that skips step 5.
+ * Do not reorder these, and do not add an early return that skips step 6.
+ *
+ * The Done-column check uses 404 rather than 409 for the same anti-
+ * enumeration reason as the project check: a 4xx that confirms the card is
+ * in Done leaks the project's structure to a foreign session.
  */
 export async function PATCH(
   request: Request,
@@ -59,6 +66,19 @@ export async function PATCH(
       return jsonError("Card not found", 404);
     }
 
+    // Plan Puzzle v1: clients do not move Done cards. Owners do, via the
+    // dedicated "Return to scheduled" action. A client manager reaching this
+    // route on a Done card is either confused or hostile — refuse with the
+    // same 404 as the project-not-found path so the response is
+    // indistinguishable.
+    const column = await pb
+      .collection("columns")
+      .getOne<ColumnRecord>(card.column)
+      .catch(() => null);
+    if (column?.is_done) {
+      return jsonError("Card not found", 404);
+    }
+
     const { patch, dropped } = pickClientCardPatch(body);
 
     if (dropped.length > 0) {
@@ -73,19 +93,17 @@ export async function PATCH(
 
     const rows = buildActivityRows(card, patch, session.userId);
 
-    const updated = await pb
-      .collection("cards")
-      .update<CardRecord>(id, patch);
-
-    for (const row of rows) {
-      await pb.collection("activity").create(row);
-    }
+    const updated = await writeCardWithAudit(pb, {
+      cardId: id,
+      patch: { ...patch },
+      rows,
+    });
 
     return jsonOk({
       card: {
-        id: updated.id,
-        priority: updated.priority,
-        target_date: updated.target_date,
+        id: updated?.id ?? id,
+        priority: updated?.priority ?? card.priority,
+        target_date: updated?.target_date ?? card.target_date,
       },
       changes: rows.length,
     });
@@ -96,4 +114,4 @@ export async function PATCH(
     console.error("[client-patch] unexpected error", error);
     return jsonError("Something went wrong", 500);
   }
-}
\ No newline at end of file
+}
diff --git a/src/lib/activity/diff.ts b/src/lib/activity/diff.ts
index 1b12148..2b51442 100644
--- a/src/lib/activity/diff.ts
+++ b/src/lib/activity/diff.ts
@@ -4,12 +4,24 @@
  * Only genuine changes produce rows. A client saving a form without altering
  * anything must not generate log noise, or the log stops being readable and
  * therefore stops being used.
+ *
+ * The `AuditRowInput` shape is also what the rest of the codebase builds
+ * directly when it knows the field it is writing (the Plan Puzzle proposal
+ * actions in src/app/actions/plan/, for instance). It is shared with
+ * src/lib/cards/write-with-audit.ts so every writer produces rows of the
+ * same shape.
  */
 
 import type { ActivityAction, CardRecord } from "@/lib/pb/types";
 import type { ClientCardPatch } from "@/lib/cards/client-patch";
 
-export interface ActivityRowInput {
+/**
+ * One row in the `activity` collection, in the wire shape PocketBase writes.
+ *
+ * `project` is denormalised onto every row so a "history of this card" read
+ * does not need a join to `cards` first.
+ */
+export interface AuditRowInput {
   project: string;
   card: string;
   actor: string;
@@ -38,8 +50,8 @@ export function buildActivityRows(
   card: CardRecord,
   patch: ClientCardPatch,
   actorId: string
-): ActivityRowInput[] {
-  const rows: ActivityRowInput[] = [];
+): AuditRowInput[] {
+  const rows: AuditRowInput[] = [];
 
   for (const key of Object.keys(patch) as Array<keyof ClientCardPatch>) {
     const next = patch[key];
@@ -60,4 +72,4 @@ export function buildActivityRows(
   }
 
   return rows;
-}
\ No newline at end of file
+}
diff --git a/src/lib/activity/format.test.ts b/src/lib/activity/format.test.ts
index e895193..4fbda47 100644
--- a/src/lib/activity/format.test.ts
+++ b/src/lib/activity/format.test.ts
@@ -82,6 +82,74 @@ describe("describeActivity", () => {
     ).toBe("reopened “Wire up the till”");
   });
 
+  it("describes a column move with the new column name", () => {
+    expect(
+      describeActivity(
+        row({ action: "column_changed", field: "column", old_value: "col_done", new_value: "Backlog" })
+      )
+    ).toBe("moved this card to Backlog");
+  });
+
+  it("describes a column move out of a column when only the old name is known", () => {
+    expect(
+      describeActivity(
+        row({ action: "column_changed", field: "column", old_value: "Done", new_value: "" })
+      )
+    ).toBe("moved this card out of Done");
+  });
+
+  it("describes a client proposal in human terms", () => {
+    expect(
+      describeActivity(
+        row({
+          action: "date_proposed",
+          field: "proposed_target_date",
+          old_value: "",
+          new_value: "2026-11-15",
+        })
+      )
+    ).toBe("proposed moving this card to 15 Nov 2026");
+  });
+
+  it("describes an accepted proposal with its date", () => {
+    expect(
+      describeActivity(
+        row({
+          action: "date_proposal_resolved",
+          field: "proposed_target_date",
+          old_value: "2026-11-15",
+          new_value: "accepted",
+        })
+      )
+    ).toBe("accepted the proposal for 15 Nov 2026");
+  });
+
+  it("describes a dismissed proposal with its date", () => {
+    expect(
+      describeActivity(
+        row({
+          action: "date_proposal_resolved",
+          field: "proposed_target_date",
+          old_value: "2026-11-15",
+          new_value: "dismissed",
+        })
+      )
+    ).toBe("dismissed the proposal for 15 Nov 2026");
+  });
+
+  it("falls back gracefully when a resolved proposal has no date", () => {
+    expect(
+      describeActivity(
+        row({
+          action: "date_proposal_resolved",
+          field: "proposed_target_date",
+          old_value: "",
+          new_value: "accepted",
+        })
+      )
+    ).toBe("accepted a proposal");
+  });
+
   it("describes a generic update", () => {
     expect(describeActivity(row({ action: "card_updated", field: "description" })))
       .toBe("updated the description");
diff --git a/src/lib/activity/format.ts b/src/lib/activity/format.ts
index ba47b01..693e9de 100644
--- a/src/lib/activity/format.ts
+++ b/src/lib/activity/format.ts
@@ -65,6 +65,28 @@ export function describeActivity(row: ActivityRecord): string {
         ? `moved this card to ${row.new_value}`
         : "moved this card";
 
+    case "column_changed":
+      // Both directions (working → Done, Done → working) are written with
+      // the same action. The value pairs name the columns; the surrounding
+      // UI is expected to show a name, not a raw id. We render the new
+      // value as-is because the writer (the move route) has already
+      // resolved it to a label.
+      return row.new_value
+        ? `moved this card to ${row.new_value}`
+        : `moved this card out of ${row.old_value}`;
+
+    case "date_proposed":
+      return row.new_value
+        ? `proposed moving this card to ${longDate(row.new_value)}`
+        : "proposed a date for this card";
+
+    case "date_proposal_resolved": {
+      const proposed = longDate(row.old_value);
+      const verdict = row.new_value === "accepted" ? "accepted" : "dismissed";
+      if (!proposed) return `${verdict} a proposal`;
+      return `${verdict} the proposal for ${proposed}`;
+    }
+
     case "comment_added":
       return "commented";
 
diff --git a/src/lib/cards/write-with-audit.test.ts b/src/lib/cards/write-with-audit.test.ts
new file mode 100644
index 0000000..da68afe
--- /dev/null
+++ b/src/lib/cards/write-with-audit.test.ts
@@ -0,0 +1,213 @@
+import { describe, it, expect, vi } from "vitest";
+
+import {
+  auditRow,
+  writeCardWithAudit,
+} from "@/lib/cards/write-with-audit";
+import type { CardRecord } from "@/lib/pb/types";
+
+/**
+ * A mock PocketBase that records every collection call so we can assert on
+ * the order and payload without a running database.
+ */
+function mockPb(card: CardRecord) {
+  const cards = {
+    update: vi.fn(async () => card),
+    getOne: vi.fn(async () => card),
+  };
+  const activity = {
+    create: vi.fn(async () => ({ id: "row_1" })),
+  };
+  return {
+    collection: vi.fn((name: string) => {
+      if (name === "cards") return cards;
+      if (name === "activity") return activity;
+      throw new Error(`unexpected collection ${name}`);
+    }),
+    _cards: cards,
+    _activity: activity,
+  };
+}
+
+describe("writeCardWithAudit", () => {
+  it("applies a card patch then writes each activity row in order", async () => {
+    const card = {
+      id: "card_1",
+      project: "proj_1",
+      target_date: "2026-10-01",
+    } as CardRecord;
+    const pb = mockPb(card);
+
+    const result = await writeCardWithAudit(pb as never, {
+      cardId: "card_1",
+      patch: { target_date: "2026-11-15" },
+      rows: [
+        auditRow({
+          project: "proj_1",
+          card: "card_1",
+          actor: "u_owner",
+          action: "date_changed",
+          field: "target_date",
+          oldValue: "2026-10-01",
+          newValue: "2026-11-15",
+        }),
+      ],
+    });
+
+    expect(pb._cards.update).toHaveBeenCalledWith("card_1", {
+      target_date: "2026-11-15",
+    });
+    expect(pb._activity.create).toHaveBeenCalledTimes(1);
+    expect(pb._activity.create).toHaveBeenCalledWith({
+      project: "proj_1",
+      card: "card_1",
+      actor: "u_owner",
+      action: "date_changed",
+      field: "target_date",
+      old_value: "2026-10-01",
+      new_value: "2026-11-15",
+    });
+    expect(pb._cards.getOne).toHaveBeenCalledWith("card_1");
+    expect(result?.id).toBe("card_1");
+  });
+
+  it("writes the card update before the activity rows", async () => {
+    // The order matters: if the card update fails, no rows should be
+    // emitted; if the activity emit fails, the card change should still
+    // land. This test pins the "card first, rows after" contract.
+    const card = { id: "c", project: "p" } as CardRecord;
+    const pb = mockPb(card);
+
+    const calls: string[] = [];
+    pb._cards.update.mockImplementation(async () => {
+      calls.push("cards.update");
+      return card;
+    });
+    pb._activity.create.mockImplementation(async () => {
+      calls.push("activity.create");
+      return { id: "r" };
+    });
+
+    await writeCardWithAudit(pb as never, {
+      cardId: "c",
+      patch: { priority: "high" },
+      rows: [
+        auditRow({
+          project: "p",
+          card: "c",
+          actor: "u",
+          action: "priority_changed",
+          field: "priority",
+          oldValue: "medium",
+          newValue: "high",
+        }),
+      ],
+    });
+
+    expect(calls).toEqual(["cards.update", "activity.create"]);
+  });
+
+  it("skips the card update when the patch is empty (log-only actions)", async () => {
+    // Accept/dismiss proposals only need to clear the three proposal
+    // fields — the server action accepts that an empty patch is a no-op
+    // write and proceeds to emit rows. This is what `returnToScheduled`
+    // and the proposal-resolution actions rely on.
+    const card = { id: "c", project: "p" } as CardRecord;
+    const pb = mockPb(card);
+
+    await writeCardWithAudit(pb as never, {
+      cardId: "c",
+      patch: {},
+      rows: [
+        auditRow({
+          project: "p",
+          card: "c",
+          actor: "u",
+          action: "date_proposal_resolved",
+          field: "proposed_target_date",
+          oldValue: "2026-11-15",
+          newValue: "dismissed",
+        }),
+      ],
+    });
+
+    expect(pb._cards.update).not.toHaveBeenCalled();
+    expect(pb._activity.create).toHaveBeenCalledTimes(1);
+  });
+
+  it("returns null when the post-update read fails (write did not)", async () => {
+    const card = { id: "c", project: "p" } as CardRecord;
+    const pb = mockPb(card);
+    pb._cards.getOne.mockRejectedValue(new Error("not found"));
+
+    const result = await writeCardWithAudit(pb as never, {
+      cardId: "c",
+      patch: { priority: "low" },
+      rows: [],
+    });
+
+    expect(pb._cards.update).toHaveBeenCalledWith("c", { priority: "low" });
+    expect(result).toBeNull();
+  });
+
+  it("emits multiple rows in the order given", async () => {
+    const card = { id: "c", project: "p" } as CardRecord;
+    const pb = mockPb(card);
+
+    await writeCardWithAudit(pb as never, {
+      cardId: "c",
+      patch: { target_date: "2026-11-15" },
+      rows: [
+        auditRow({
+          project: "p",
+          card: "c",
+          actor: "u",
+          action: "date_changed",
+          field: "target_date",
+          oldValue: "2026-10-01",
+          newValue: "2026-11-15",
+        }),
+        auditRow({
+          project: "p",
+          card: "c",
+          actor: "u",
+          action: "date_proposal_resolved",
+          field: "proposed_target_date",
+          oldValue: "2026-11-15",
+          newValue: "accepted",
+        }),
+      ],
+    });
+
+    const actions: string[] = [];
+    for (const call of pb._activity.create.mock.calls as unknown as Array<[unknown]>) {
+      const arg = call[0] as { action: string };
+      actions.push(arg.action);
+    }
+    expect(actions).toEqual(["date_changed", "date_proposal_resolved"]);
+  });
+});
+
+describe("auditRow", () => {
+  it("maps camelCase args onto the wire shape (snake_case)", () => {
+    const row = auditRow({
+      project: "p",
+      card: "c",
+      actor: "u",
+      action: "date_proposed",
+      field: "proposed_target_date",
+      oldValue: "",
+      newValue: "2026-11-15",
+    });
+
+    expect(row).toEqual({
+      project: "p",
+      card: "c",
+      actor: "u",
+      action: "date_proposed",
+      field: "proposed_target_date",
+      old_value: "",
+      new_value: "2026-11-15",
+    });
+  });
+});
diff --git a/src/lib/cards/write-with-audit.ts b/src/lib/cards/write-with-audit.ts
new file mode 100644
index 0000000..04b2f7e
--- /dev/null
+++ b/src/lib/cards/write-with-audit.ts
@@ -0,0 +1,107 @@
+/**
+ * Single source of truth for "patch a card and emit activity rows that
+ * describe the change".
+ *
+ * Three writers used to do this inline:
+ *   - the client PATCH /api/client/cards/[id] route handler
+ *   - src/app/actions/reschedule.ts (owner drag-from-timeline)
+ *   - the new Plan Puzzle server actions in src/app/actions/plan/
+ *
+ * Each one's inline copy drifted subtly. The PATCH route used the caller's
+ * superuser admin client and called `pb.collection("activity").create(row)`
+ * in a loop; rescheduleCardAction called `logActivity` (which is itself an
+ * admin helper). Today they disagree about who emits and how; a future
+ * audit-row bug would be fixed in one and not the other. This module
+ * collapses all of that into one place.
+ *
+ * Every caller still composes its own `patch` and `rows` — the helper just
+ * applies both atomically with respect to the caller. The order is
+ * intentional: card first, then activity. If the activity insert blows up,
+ * the card change already landed, which is the same order every previous
+ * writer used and matches the "history is allowed to be partial" trade-off
+ * the rest of the codebase already makes (see logActivity's doc comment).
+ *
+ * The `AuditRowInput` type lives in @/lib/activity/diff so unit tests can
+ * import it without the `server-only` guard here firing.
+ */
+
+import "server-only";
+
+import type PocketBase from "pocketbase";
+
+import type { AuditRowInput } from "@/lib/activity/diff";
+import type { CardRecord } from "@/lib/pb/types";
+
+export type { AuditRowInput };
+
+export interface CardPatchWithAudit {
+  cardId: string;
+  /**
+   * PocketBase update payload. Empty object means "no card change" — useful
+   * for actions that only need to log activity (accept/dismiss resolutions,
+   * today). The helper still returns the freshly-read card.
+   */
+  patch: Record<string, unknown>;
+  /**
+   * Activity rows that should be emitted after the card update. Each is
+   * written in order; a row that throws aborts the helper but does not
+   * roll back the card change. Callers must build these before calling.
+   */
+  rows: AuditRowInput[];
+}
+
+/**
+ * Apply a card patch and emit activity rows.
+ *
+ * The PocketBase instance is injected so this module is callable from both
+ * superuser (admin) contexts — `/api/client/cards/[id]` — and request-
+ * scoped server-action contexts — `rescheduleCardAction`, the puzzle
+ * actions. Both need exactly the same code path; what differs is the
+ * token.
+ *
+ * Returns the freshly-read card so callers can chain `getOne`-style reads
+ * without an extra round-trip. Returns null when the post-update read
+ * fails — the write itself did succeed, so callers should treat null as
+ * "trust the patch you sent in" rather than as a failure.
+ */
+export async function writeCardWithAudit(
+  pb: PocketBase,
+  input: CardPatchWithAudit
+): Promise<CardRecord | null> {
+  if (Object.keys(input.patch).length > 0) {
+    await pb.collection("cards").update(input.cardId, input.patch);
+  }
+
+  for (const row of input.rows) {
+    await pb.collection("activity").create(row);
+  }
+
+  return pb.collection("cards").getOne<CardRecord>(input.cardId).catch(() => null);
+}
+
+/**
+ * Build one activity row for a card-field change.
+ *
+ * Caller chooses the action, the field name, and the before/after values.
+ * A helper rather than a class so the input shape stays JSON-serialisable
+ * and easy to compose in server-action return paths.
+ */
+export function auditRow(args: {
+  project: string;
+  card: string;
+  actor: string;
+  action: import("@/lib/pb/types").ActivityAction;
+  field: string;
+  oldValue: string;
+  newValue: string;
+}): AuditRowInput {
+  return {
+    project: args.project,
+    card: args.card,
+    actor: args.actor,
+    action: args.action,
+    field: args.field,
+    old_value: args.oldValue,
+    new_value: args.newValue,
+  };
+}
diff --git a/src/lib/pb/types.ts b/src/lib/pb/types.ts
index f1c9dce..d86446f 100644
--- a/src/lib/pb/types.ts
+++ b/src/lib/pb/types.ts
@@ -9,9 +9,12 @@ export type CreatedByRole = "owner" | "client";
 export type ActivityAction =
   | "card_created"
   | "card_moved"
+  | "column_changed"
   | "card_updated"
   | "priority_changed"
   | "date_changed"
+  | "date_proposed"
+  | "date_proposal_resolved"
   | "card_accepted"
   | "comment_added"
   | "subtask_toggled"
@@ -94,6 +97,17 @@ export interface CardRecord extends BaseRecord {
   /** The interview question-and-answer pairs. */
   transcript: unknown;
   size_band: "small" | "medium" | "large" | "";
+  /**
+   * Client-proposed target date. Owners do not write this field through the
+   * Puzzle — they write `target_date` directly. The three proposal fields
+   * are written together by `clientProposePlanAction` and cleared together
+   * by `acceptClientProposalAction` / `dismissClientProposalAction`. A row
+   * with one set and the others unset is logically incoherent; the server
+   * actions never produce it.
+   */
+  proposed_target_date?: string;
+  proposed_by?: string;
+  proposed_at?: string;
 }
 
 export interface SubtaskRecord extends BaseRecord {
diff --git a/src/lib/roadmap/slot-date.test.ts b/src/lib/roadmap/slot-date.test.ts
new file mode 100644
index 0000000..f356f9e
--- /dev/null
+++ b/src/lib/roadmap/slot-date.test.ts
@@ -0,0 +1,141 @@
+import { describe, it, expect } from "vitest";
+
+import {
+  SLOTS,
+  isSlotId,
+  monthLabel,
+  slotFromDate,
+  slotToDate,
+} from "@/lib/roadmap/slot-date";
+
+const today = new Date("2026-09-17T12:00:00Z");
+
+describe("SLOTS", () => {
+  it("lists the three slots in left-to-right order", () => {
+    expect([...SLOTS]).toEqual(["this_month", "next_month", "month_after"]);
+  });
+});
+
+describe("isSlotId", () => {
+  it("accepts each slot id", () => {
+    expect(isSlotId("this_month")).toBe(true);
+    expect(isSlotId("next_month")).toBe(true);
+    expect(isSlotId("month_after")).toBe(true);
+  });
+
+  it("rejects everything else", () => {
+    expect(isSlotId("")).toBe(false);
+    expect(isSlotId("now")).toBe(false);
+    expect(isSlotId("later")).toBe(false);
+    expect(isSlotId(null)).toBe(false);
+    expect(isSlotId(undefined)).toBe(false);
+    expect(isSlotId(2)).toBe(false);
+  });
+});
+
+describe("slotToDate", () => {
+  it("writes the 15th of each slot's month, UTC", () => {
+    expect(slotToDate("this_month", today)).toBe("2026-09-15");
+    expect(slotToDate("next_month", today)).toBe("2026-10-15");
+    expect(slotToDate("month_after", today)).toBe("2026-11-15");
+  });
+
+  it("rolls over the year boundary for month-after in December", () => {
+    const december = new Date("2026-12-04T00:00:00Z");
+    expect(slotToDate("this_month", december)).toBe("2026-12-15");
+    expect(slotToDate("next_month", december)).toBe("2027-01-15");
+    expect(slotToDate("month_after", december)).toBe("2027-02-15");
+  });
+
+  it("is symmetric with slotFromDate for every slot id", () => {
+    for (const slot of SLOTS) {
+      const date = slotToDate(slot, today);
+      expect(slotFromDate(date, today)).toBe(slot);
+    }
+  });
+
+  it("is deterministic regardless of clock-time within the same day", () => {
+    const morning = new Date("2026-09-17T01:00:00Z");
+    const evening = new Date("2026-09-17T23:00:00Z");
+    expect(slotToDate("this_month", morning)).toBe(
+      slotToDate("this_month", evening)
+    );
+  });
+});
+
+describe("slotFromDate", () => {
+  it("places the 15th of each window into the matching slot", () => {
+    expect(slotFromDate("2026-09-15", today)).toBe("this_month");
+    expect(slotFromDate("2026-10-15", today)).toBe("next_month");
+    expect(slotFromDate("2026-11-15", today)).toBe("month_after");
+  });
+
+  it("is forgiving about the day-of-month within the slot", () => {
+    // The puzzle always writes the 15th, but a card whose target_date was
+    // hand-set on a different day should still resolve into its slot — the
+    // 30-day approximation is exact for mid-month dates and close enough
+    // for any date that lands within the slot's window.
+    expect(slotFromDate("2026-09-01", today)).toBe("this_month");
+    expect(slotFromDate("2026-09-30", today)).toBe("this_month");
+    expect(slotFromDate("2026-10-02", today)).toBe("next_month");
+    expect(slotFromDate("2026-10-31", today)).toBe("next_month");
+  });
+
+  it("returns null for a date in the past", () => {
+    expect(slotFromDate("2026-08-15", today)).toBeNull();
+    expect(slotFromDate("2025-12-15", today)).toBeNull();
+  });
+
+  it("returns null for a date beyond the month-after window", () => {
+    expect(slotFromDate("2026-12-15", today)).toBeNull();
+    expect(slotFromDate("2027-01-15", today)).toBeNull();
+  });
+
+  it("returns null for an empty string", () => {
+    expect(slotFromDate("", today)).toBeNull();
+  });
+
+  it("returns null for a date the puzzle could not have written", () => {
+    // The 31st is a 30-day-month edge case. Returning null (and the caller
+    // falls back to the tray) is the safe choice.
+    expect(slotFromDate("2026-01-31", today)).toBeNull();
+  });
+
+  it("returns null for malformed input rather than throwing", () => {
+    expect(slotFromDate("not-a-date", today)).toBeNull();
+    expect(slotFromDate("2026-13-01", today)).toBeNull();
+  });
+
+  it("survives a PocketBase-style datetime string", () => {
+    // load.ts reads card.target_date directly; if a row has a datetime
+    // instead of a date, the slice-and-parse path must still land in the
+    // right slot. The function operates on a substring (slice(0,10)) so
+    // callers are expected to pre-trim — verify the contract holds.
+    expect(slotFromDate("2026-10-15 00:00:00.000Z".slice(0, 10), today)).toBe(
+      "next_month"
+    );
+  });
+});
+
+describe("monthLabel", () => {
+  it("returns full English month names", () => {
+    expect(monthLabel("this_month", today)).toBe("September");
+    expect(monthLabel("next_month", today)).toBe("October");
+    expect(monthLabel("month_after", today)).toBe("November");
+  });
+
+  it("rolls the year forward across the December boundary", () => {
+    const december = new Date("2026-12-04T00:00:00Z");
+    expect(monthLabel("this_month", december)).toBe("December");
+    expect(monthLabel("next_month", december)).toBe("January");
+    expect(monthLabel("month_after", december)).toBe("February");
+  });
+});
+
+describe("round-trip — slot → date → slot", () => {
+  it("returns to the same slot for every slot id", () => {
+    for (const slot of SLOTS) {
+      expect(slotFromDate(slotToDate(slot, today), today)).toBe(slot);
+    }
+  });
+});
diff --git a/src/lib/roadmap/slot-date.ts b/src/lib/roadmap/slot-date.ts
new file mode 100644
index 0000000..428ccc8
--- /dev/null
+++ b/src/lib/roadmap/slot-date.ts
@@ -0,0 +1,88 @@
+/**
+ * Slot ↔ target_date mapping for the Plan Puzzle.
+ *
+ * The Puzzle places cards into one of three slots — This Month, Next Month,
+ * Month After — and writes the mid-month (15th) of the slot's month as the
+ * card's `target_date`. Reads do the reverse: given an ISO date, return the
+ * slot it lands in, or `null` if the date falls outside the three-month
+ * window.
+ *
+ * Every function takes `now` as an optional argument so unit tests can pin
+ * the calendar to a fixed instant. Without that the tests would flake
+ * every time a real-world clock crossed a month boundary mid-run.
+ *
+ * Pixel's `plan-puzzle-v1-frontend` branch published a draft of this file
+ * with the same signatures; this is the canonical implementation on the
+ * `plan-puzzle-v1-backend` branch and the one the production code imports.
+ */
+
+export type SlotId = "this_month" | "next_month" | "month_after";
+
+/** The puzzle's three slots, in left-to-right order. */
+export const SLOTS: readonly SlotId[] = [
+  "this_month",
+  "next_month",
+  "month_after",
+] as const;
+
+const DAYS_PER_MONTH_MS = 30 * 86_400_000;
+
+/** True if the value is one of the three slot ids. Narrows `string` for callers. */
+export function isSlotId(value: unknown): value is SlotId {
+  return (
+    value === "this_month" || value === "next_month" || value === "month_after"
+  );
+}
+
+/**
+ * UTC midnight on the 15th of the month `slotIdx` months after `now`'s month.
+ *
+ * Mid-month is deliberate: a card whose target_date is the 1st reads as
+ * "starting now", and the 30th reads as "running out of time". The 15th is
+ * the only day that doesn't bias either way.
+ */
+export function slotToDate(slot: SlotId, now: Date = new Date()): string {
+  const offset = SLOTS.indexOf(slot);
+  // SlotId is always one of SLOTS; indexOf never returns -1 for valid input.
+  const time = Date.UTC(now.getUTCFullYear(), now.getUTCMonth() + offset, 15);
+  return new Date(time).toISOString().slice(0, 10);
+}
+
+/**
+ * Which slot a date lands in. Returns `null` when the date is before this
+ * month, more than two months out, or unparseable — the caller treats `null`
+ * as "this card lives in the tray, not a slot".
+ *
+ * Uses a 30-day month approximation. For mid-month dates (always the 15th,
+ * written by the puzzle itself) the arithmetic is exact; for dates written
+ * by hand, like `2026-01-31`, the result can drift by one slot. The
+ * mitigation is the explicit "no slot → tray" fallback in the caller.
+ */
+export function slotFromDate(iso: string, now: Date = new Date()): SlotId | null {
+  if (!iso) return null;
+  const year = Number(iso.slice(0, 4));
+  const month = Number(iso.slice(5, 7));
+  if (!Number.isFinite(year) || !Number.isFinite(month)) return null;
+
+  const target = Date.UTC(year, month - 1, 15);
+  const base = Date.UTC(now.getUTCFullYear(), now.getUTCMonth(), 15);
+  const months = Math.round((target - base) / DAYS_PER_MONTH_MS);
+
+  if (months === 0) return "this_month";
+  if (months === 1) return "next_month";
+  if (months === 2) return "month_after";
+  return null;
+}
+
+/**
+ * "October", "November", "December" — the column header text.
+ *
+ * Rendered in en-US for now. The spec says "month names, updates annually"
+ * — which it does, because the month is computed from `now` each render —
+ * so no yearly refresh job is needed.
+ */
+export function monthLabel(slot: SlotId, now: Date = new Date()): string {
+  const offset = SLOTS.indexOf(slot);
+  const d = new Date(Date.UTC(now.getUTCFullYear(), now.getUTCMonth() + offset, 1));
+  return d.toLocaleString("en-US", { month: "long", timeZone: "UTC" });
+}
diff --git a/vitest.config.ts b/vitest.config.ts
index 523bf30..3041e51 100644
--- a/vitest.config.ts
+++ b/vitest.config.ts
@@ -2,6 +2,10 @@ import { defineConfig } from "vitest/config";
 import path from "node:path";
 import { fileURLToPath } from "node:url";
 
+const serverOnlyStub = fileURLToPath(
+  new URL("./src/lib/server-only-stub.ts", import.meta.url)
+);
+
 export default defineConfig({
   resolve: {
     alias: { "@": path.resolve(__dirname, "./src") },
@@ -9,7 +13,15 @@ export default defineConfig({
   test: {
     projects: [
       {
-        resolve: { alias: { "@": path.resolve(__dirname, "./src") } },
+        // Unit tests can also import server-only modules (e.g.
+        // writeCardWithAudit) and replace the guard with the empty stub,
+        // matching the integration project's alias.
+        resolve: {
+          alias: {
+            "@": path.resolve(__dirname, "./src"),
+            "server-only": serverOnlyStub,
+          },
+        },
         test: {
           name: "unit",
           environment: "node",
@@ -23,9 +35,7 @@ export default defineConfig({
             // Replace server-only with the empty stub so integration tests
             // can import server modules without "imported from a Client
             // Component module" blowing up at prepare time.
-            "server-only": fileURLToPath(
-              new URL("./src/lib/server-only-stub.ts", import.meta.url)
-            ),
+            "server-only": serverOnlyStub,
           },
           conditions: ["react-server", "node", "import", "default"],
         },
@@ -40,4 +50,4 @@ export default defineConfig({
       },
     ],
   },
-});
\ No newline at end of file
+});