diff --git a/src/app/api/medications/[id]/intake/route.ts b/src/app/api/medications/[id]/intake/route.ts
index 706a2e76c..4cbe9afd1 100644
--- a/src/app/api/medications/[id]/intake/route.ts
+++ b/src/app/api/medications/[id]/intake/route.ts
@@ -13,6 +13,7 @@ import {
   listIntakeEventsSchema,
 } from "@/lib/validations/medication";
 import { withIdempotency } from "@/lib/idempotency";
+import { consumeOneDose } from "@/lib/medications/inventory/service";
 import { NextRequest } from "next/server";
 
 type RouteParams = { params: Promise<{ id: string }> };
@@ -94,10 +95,45 @@ async function postIntake(request: NextRequest, { params }: RouteParams) {
       : []),
   ]);
 
+  // v1.4.25 W19b — pen-inventory dose decrement. Only fires for
+  // non-skipped intakes; a skipped event is not a consumption event.
+  // No-op when the medication has no tracked pens (most non-GLP-1
+  // meds). Failures here must never block the intake write, so
+  // errors are swallowed and logged — the intake is the source of
+  // truth, the inventory is an opt-in companion.
+  let inventoryOutcome: Awaited<ReturnType<typeof consumeOneDose>> = null;
+  if (!skipped) {
+    try {
+      inventoryOutcome = await consumeOneDose({
+        userId: user.id,
+        medicationId: id,
+        intakeAt: event.takenAt ?? event.scheduledFor,
+      });
+    } catch (err) {
+      annotate({
+        action: { name: "medication.inventory.consume_error" },
+        meta: {
+          medication_id: id,
+          message: err instanceof Error ? err.message : String(err),
+        },
+      });
+    }
+  }
+
   await auditLog("medication.intake", {
     userId: user.id,
     ipAddress: getClientIp(request),
-    details: { medicationId: id, eventId: event.id, skipped },
+    details: {
+      medicationId: id,
+      eventId: event.id,
+      skipped,
+      ...(inventoryOutcome
+        ? {
+            inventoryItemId: inventoryOutcome.itemId,
+            inventoryChange: inventoryOutcome.change,
+          }
+        : {}),
+    },
   });
 
   annotate({
@@ -106,7 +142,16 @@ async function postIntake(request: NextRequest, { params }: RouteParams) {
       entity_type: "intake_event",
       entity_id: event.id,
     },
-    meta: { medication_id: id, skipped },
+    meta: {
+      medication_id: id,
+      skipped,
+      ...(inventoryOutcome
+        ? {
+            inventory_item_id: inventoryOutcome.itemId,
+            inventory_change: inventoryOutcome.change,
+          }
+        : {}),
+    },
   });
 
   return apiSuccess(event, 201);
diff --git a/src/app/api/medications/[id]/inventory/[itemId]/route.ts b/src/app/api/medications/[id]/inventory/[itemId]/route.ts
new file mode 100644
index 000000000..1d02714a6
--- /dev/null
+++ b/src/app/api/medications/[id]/inventory/[itemId]/route.ts
@@ -0,0 +1,159 @@
+/**
+ * v1.4.25 W19b — per-pen inventory item operations.
+ *
+ *   PATCH  /api/medications/[id]/inventory/[itemId]
+ *     - mark-as-first-use (starts the 30-day clock manually)
+ *     - mark-as-used-up (terminal — operator override when a pen is
+ *       physically discarded but the dose ledger hasn't reached zero)
+ *     - update printed expiry (e.g. carton label correction)
+ *     - update notes
+ *
+ *   DELETE /api/medications/[id]/inventory/[itemId]
+ *     - hard delete; admin / cleanup path. The audit log captures
+ *       the before-state so a row can be reconstructed if needed.
+ */
+
+import { NextRequest } from "next/server";
+
+import { prisma } from "@/lib/db";
+import { apiHandler, requireAuth } from "@/lib/api-handler";
+import { auditLog } from "@/lib/auth/audit";
+import { annotate } from "@/lib/logging/context";
+import {
+  apiError,
+  apiSuccess,
+  getClientIp,
+  safeJson,
+} from "@/lib/api-response";
+import { updateInventoryItemSchema } from "@/lib/validations/medication";
+import { computeExpiresAt } from "@/lib/medications/inventory/state-machine";
+
+type RouteParams = { params: Promise<{ id: string; itemId: string }> };
+
+async function loadOwnedItem(
+  medicationId: string,
+  itemId: string,
+  userId: string,
+) {
+  const item = await prisma.medicationInventoryItem.findUnique({
+    where: { id: itemId },
+  });
+  if (!item || item.userId !== userId || item.medicationId !== medicationId) {
+    return null;
+  }
+  return item;
+}
+
+export const PATCH = apiHandler(
+  async (request: NextRequest, { params }: RouteParams) => {
+    const { user } = await requireAuth();
+    const { id, itemId } = await params;
+
+    const existing = await loadOwnedItem(id, itemId, user.id);
+    if (!existing) return apiError("Inventory item not found", 404);
+
+    const { data: body, error: jsonError } = await safeJson(request);
+    if (jsonError) return jsonError;
+
+    const parsed = updateInventoryItemSchema.safeParse(body);
+    if (!parsed.success) {
+      return apiError(parsed.error.issues[0].message, 422);
+    }
+
+    const { markAsFirstUseAt, markAsUsedUp, printedExpiry, notes } =
+      parsed.data;
+
+    // Compose the next-row shape. Each mutation field is optional and
+    // commutative — applying them in any order produces the same row.
+    let nextFirstUseAt = existing.firstUseAt;
+    let nextState = existing.state;
+    let nextDosesRemaining = existing.dosesRemaining;
+    let nextPrintedExpiry = existing.printedExpiry;
+
+    if (markAsFirstUseAt) {
+      // The clock can be set manually if the user opened the pen
+      // without logging an intake event.
+      nextFirstUseAt = markAsFirstUseAt;
+      if (nextState === "ACTIVE") nextState = "IN_USE";
+    }
+
+    if (printedExpiry !== undefined) {
+      nextPrintedExpiry = printedExpiry;
+    }
+
+    if (markAsUsedUp === true) {
+      nextDosesRemaining = 0;
+      nextState = "USED_UP";
+    }
+
+    const nextExpiresAt = computeExpiresAt(nextFirstUseAt, nextPrintedExpiry);
+
+    const updated = await prisma.medicationInventoryItem.update({
+      where: { id: itemId },
+      data: {
+        state: nextState,
+        firstUseAt: nextFirstUseAt,
+        dosesRemaining: nextDosesRemaining,
+        printedExpiry: nextPrintedExpiry,
+        expiresAt: nextExpiresAt,
+        notes: notes === undefined ? existing.notes : notes,
+      },
+    });
+
+    await auditLog("medication.inventory.update", {
+      userId: user.id,
+      ipAddress: getClientIp(request),
+      details: {
+        medicationId: id,
+        itemId,
+        prevState: existing.state,
+        nextState: updated.state,
+      },
+    });
+
+    annotate({
+      action: {
+        name: "medication.inventory.update",
+        entity_type: "inventory_item",
+        entity_id: itemId,
+      },
+      meta: { medication_id: id, prev: existing.state, next: updated.state },
+    });
+
+    return apiSuccess(updated);
+  },
+);
+
+export const DELETE = apiHandler(
+  async (request: NextRequest, { params }: RouteParams) => {
+    const { user } = await requireAuth();
+    const { id, itemId } = await params;
+
+    const existing = await loadOwnedItem(id, itemId, user.id);
+    if (!existing) return apiError("Inventory item not found", 404);
+
+    await prisma.medicationInventoryItem.delete({ where: { id: itemId } });
+
+    await auditLog("medication.inventory.delete", {
+      userId: user.id,
+      ipAddress: getClientIp(request),
+      details: {
+        medicationId: id,
+        itemId,
+        finalState: existing.state,
+        dosesRemaining: existing.dosesRemaining,
+      },
+    });
+
+    annotate({
+      action: {
+        name: "medication.inventory.delete",
+        entity_type: "inventory_item",
+        entity_id: itemId,
+      },
+      meta: { medication_id: id },
+    });
+
+    return apiSuccess({ id: itemId, deleted: true });
+  },
+);
diff --git a/src/app/api/medications/[id]/inventory/__tests__/route.test.ts b/src/app/api/medications/[id]/inventory/__tests__/route.test.ts
new file mode 100644
index 000000000..6838b9f25
--- /dev/null
+++ b/src/app/api/medications/[id]/inventory/__tests__/route.test.ts
@@ -0,0 +1,315 @@
+import { describe, it, expect, vi, beforeEach } from "vitest";
+import { NextRequest } from "next/server";
+
+vi.mock("@/lib/db", () => ({
+  prisma: {
+    medication: { findUnique: vi.fn() },
+    medicationInventoryItem: {
+      findMany: vi.fn(),
+      findUnique: vi.fn(),
+      create: vi.fn(),
+      update: vi.fn(),
+      delete: vi.fn(),
+    },
+  },
+}));
+
+vi.mock("@/lib/auth/session", () => ({ getSession: vi.fn() }));
+
+vi.mock("@/lib/auth/audit", () => ({
+  auditLog: vi.fn().mockResolvedValue(undefined),
+}));
+
+vi.mock("@/lib/rate-limit", () => ({
+  checkRateLimit: vi.fn().mockResolvedValue({
+    allowed: true,
+    remaining: 29,
+    resetAt: Date.now() + 60_000,
+  }),
+  rateLimitHeaders: vi.fn(() => ({})),
+}));
+
+vi.mock("@/lib/logging/transports", () => ({ emitIfSampled: vi.fn() }));
+
+vi.mock("@/lib/db-compat", () => ({
+  ensureDbCompatibility: vi.fn().mockResolvedValue(undefined),
+}));
+
+vi.mock("next/headers", () => ({
+  headers: vi.fn(async () => ({ get: () => null })),
+  cookies: vi.fn(async () => ({
+    get: () => undefined,
+    set: () => {},
+    delete: () => {},
+  })),
+}));
+
+import { GET, POST } from "../route";
+import { PATCH, DELETE } from "../[itemId]/route";
+import { prisma } from "@/lib/db";
+import { getSession } from "@/lib/auth/session";
+import { checkRateLimit } from "@/lib/rate-limit";
+
+const SESSION_OK = {
+  session: { id: "sess-1", expiresAt: new Date(Date.now() + 3_600_000) },
+  user: { id: "user-1", username: "marc", role: "USER" as const },
+};
+
+const MED_OK = { id: "med-1", userId: "user-1" };
+
+function jsonReq(url: string, body: unknown, method = "POST"): NextRequest {
+  return new NextRequest(url, {
+    method,
+    headers: { "content-type": "application/json" },
+    body: JSON.stringify(body),
+  });
+}
+
+beforeEach(() => {
+  vi.resetAllMocks();
+  vi.mocked(checkRateLimit).mockResolvedValue({
+    allowed: true,
+    remaining: 29,
+    resetAt: Date.now() + 60_000,
+  });
+});
+
+describe("GET /api/medications/[id]/inventory", () => {
+  it("returns 401 when unauthenticated", async () => {
+    vi.mocked(getSession).mockResolvedValue(null);
+    const res = await GET(
+      new NextRequest("http://localhost/api/medications/med-1/inventory"),
+      { params: Promise.resolve({ id: "med-1" }) },
+    );
+    expect(res.status).toBe(401);
+  });
+
+  it("returns 404 when the medication is not owned by the caller", async () => {
+    vi.mocked(getSession).mockResolvedValue(SESSION_OK as never);
+    vi.mocked(prisma.medication.findUnique).mockResolvedValue({
+      id: "med-1",
+      userId: "OTHER",
+    } as never);
+
+    const res = await GET(
+      new NextRequest("http://localhost/api/medications/med-1/inventory"),
+      { params: Promise.resolve({ id: "med-1" }) },
+    );
+    expect(res.status).toBe(404);
+  });
+
+  it("returns the inventory list for the owned medication", async () => {
+    vi.mocked(getSession).mockResolvedValue(SESSION_OK as never);
+    vi.mocked(prisma.medication.findUnique).mockResolvedValue(MED_OK as never);
+    vi.mocked(prisma.medicationInventoryItem.findMany).mockResolvedValue([
+      { id: "inv-1", state: "ACTIVE" },
+      { id: "inv-2", state: "IN_USE" },
+    ] as never);
+
+    const res = await GET(
+      new NextRequest("http://localhost/api/medications/med-1/inventory"),
+      { params: Promise.resolve({ id: "med-1" }) },
+    );
+    expect(res.status).toBe(200);
+    const body = (await res.json()) as {
+      data: { items: unknown[]; meta: { total: number } };
+    };
+    expect(body.data.items).toHaveLength(2);
+    expect(body.data.meta.total).toBe(2);
+  });
+});
+
+describe("POST /api/medications/[id]/inventory", () => {
+  it("rejects unauthenticated", async () => {
+    vi.mocked(getSession).mockResolvedValue(null);
+    const res = await POST(
+      jsonReq("http://localhost/api/medications/med-1/inventory", {
+        dosesTotal: 4,
+      }),
+      { params: Promise.resolve({ id: "med-1" }) },
+    );
+    expect(res.status).toBe(401);
+  });
+
+  it("returns 422 when dosesTotal is missing or invalid", async () => {
+    vi.mocked(getSession).mockResolvedValue(SESSION_OK as never);
+    vi.mocked(prisma.medication.findUnique).mockResolvedValue(MED_OK as never);
+    const res = await POST(
+      jsonReq("http://localhost/api/medications/med-1/inventory", {
+        dosesTotal: 0,
+      }),
+      { params: Promise.resolve({ id: "med-1" }) },
+    );
+    expect(res.status).toBe(422);
+  });
+
+  it("creates an ACTIVE pen with computed expiresAt", async () => {
+    vi.mocked(getSession).mockResolvedValue(SESSION_OK as never);
+    vi.mocked(prisma.medication.findUnique).mockResolvedValue(MED_OK as never);
+    vi.mocked(prisma.medicationInventoryItem.create).mockResolvedValue({
+      id: "inv-new",
+      state: "ACTIVE",
+    } as never);
+
+    const printed = "2027-06-01T00:00:00Z";
+    const res = await POST(
+      jsonReq("http://localhost/api/medications/med-1/inventory", {
+        dosesTotal: 4,
+        printedExpiry: printed,
+      }),
+      { params: Promise.resolve({ id: "med-1" }) },
+    );
+    expect(res.status).toBe(201);
+    expect(prisma.medicationInventoryItem.create).toHaveBeenCalledWith({
+      data: expect.objectContaining({
+        userId: "user-1",
+        medicationId: "med-1",
+        state: "ACTIVE",
+        dosesTotal: 4,
+        dosesRemaining: 4,
+        firstUseAt: null,
+        printedExpiry: new Date(printed),
+        expiresAt: new Date(printed),
+      }),
+    });
+  });
+
+  it("returns 429 when the rate limit is exceeded", async () => {
+    vi.mocked(getSession).mockResolvedValue(SESSION_OK as never);
+    vi.mocked(prisma.medication.findUnique).mockResolvedValue(MED_OK as never);
+    vi.mocked(checkRateLimit).mockResolvedValueOnce({
+      allowed: false,
+      remaining: 0,
+      resetAt: Date.now() + 60_000,
+    });
+    const res = await POST(
+      jsonReq("http://localhost/api/medications/med-1/inventory", {
+        dosesTotal: 4,
+      }),
+      { params: Promise.resolve({ id: "med-1" }) },
+    );
+    expect(res.status).toBe(429);
+  });
+});
+
+describe("PATCH /api/medications/[id]/inventory/[itemId]", () => {
+  const existingActive = {
+    id: "inv-1",
+    userId: "user-1",
+    medicationId: "med-1",
+    state: "ACTIVE" as const,
+    dosesTotal: 4,
+    dosesRemaining: 4,
+    firstUseAt: null,
+    printedExpiry: null,
+    purchasedAt: null,
+    notes: null,
+    expiresAt: null,
+    createdAt: new Date(),
+    updatedAt: new Date(),
+  };
+
+  it("flips ACTIVE → IN_USE on markAsFirstUseAt", async () => {
+    vi.mocked(getSession).mockResolvedValue(SESSION_OK as never);
+    vi.mocked(prisma.medicationInventoryItem.findUnique).mockResolvedValue(
+      existingActive as never,
+    );
+    vi.mocked(prisma.medicationInventoryItem.update).mockResolvedValue({
+      ...existingActive,
+      state: "IN_USE",
+    } as never);
+
+    const intake = "2026-05-14T12:00:00Z";
+    const res = await PATCH(
+      jsonReq(
+        "http://localhost/api/medications/med-1/inventory/inv-1",
+        { markAsFirstUseAt: intake },
+        "PATCH",
+      ),
+      { params: Promise.resolve({ id: "med-1", itemId: "inv-1" }) },
+    );
+    expect(res.status).toBe(200);
+    const updateCall = vi.mocked(prisma.medicationInventoryItem.update).mock
+      .calls[0][0];
+    expect(updateCall.data).toMatchObject({
+      state: "IN_USE",
+      firstUseAt: new Date(intake),
+    });
+  });
+
+  it("flips state to USED_UP and zeros remaining on markAsUsedUp", async () => {
+    vi.mocked(getSession).mockResolvedValue(SESSION_OK as never);
+    vi.mocked(prisma.medicationInventoryItem.findUnique).mockResolvedValue({
+      ...existingActive,
+      state: "IN_USE",
+      firstUseAt: new Date("2026-05-01"),
+      dosesRemaining: 2,
+    } as never);
+    vi.mocked(prisma.medicationInventoryItem.update).mockResolvedValue({
+      ...existingActive,
+      state: "USED_UP",
+      dosesRemaining: 0,
+    } as never);
+
+    const res = await PATCH(
+      jsonReq(
+        "http://localhost/api/medications/med-1/inventory/inv-1",
+        { markAsUsedUp: true },
+        "PATCH",
+      ),
+      { params: Promise.resolve({ id: "med-1", itemId: "inv-1" }) },
+    );
+    expect(res.status).toBe(200);
+    const updateCall = vi.mocked(prisma.medicationInventoryItem.update).mock
+      .calls[0][0];
+    expect(updateCall.data).toMatchObject({
+      state: "USED_UP",
+      dosesRemaining: 0,
+    });
+  });
+
+  it("rejects 404 when the item belongs to another user", async () => {
+    vi.mocked(getSession).mockResolvedValue(SESSION_OK as never);
+    vi.mocked(prisma.medicationInventoryItem.findUnique).mockResolvedValue({
+      ...existingActive,
+      userId: "OTHER",
+    } as never);
+
+    const res = await PATCH(
+      jsonReq(
+        "http://localhost/api/medications/med-1/inventory/inv-1",
+        { markAsUsedUp: true },
+        "PATCH",
+      ),
+      { params: Promise.resolve({ id: "med-1", itemId: "inv-1" }) },
+    );
+    expect(res.status).toBe(404);
+  });
+});
+
+describe("DELETE /api/medications/[id]/inventory/[itemId]", () => {
+  it("hard-deletes the row and audit-logs the final state", async () => {
+    vi.mocked(getSession).mockResolvedValue(SESSION_OK as never);
+    vi.mocked(prisma.medicationInventoryItem.findUnique).mockResolvedValue({
+      id: "inv-1",
+      userId: "user-1",
+      medicationId: "med-1",
+      state: "EXPIRED",
+      dosesRemaining: 2,
+    } as never);
+    vi.mocked(prisma.medicationInventoryItem.delete).mockResolvedValue({
+      id: "inv-1",
+    } as never);
+
+    const res = await DELETE(
+      new NextRequest("http://localhost/api/medications/med-1/inventory/inv-1", {
+        method: "DELETE",
+      }),
+      { params: Promise.resolve({ id: "med-1", itemId: "inv-1" }) },
+    );
+    expect(res.status).toBe(200);
+    expect(prisma.medicationInventoryItem.delete).toHaveBeenCalledWith({
+      where: { id: "inv-1" },
+    });
+  });
+});
diff --git a/src/app/api/medications/[id]/inventory/route.ts b/src/app/api/medications/[id]/inventory/route.ts
new file mode 100644
index 000000000..f9c68da38
--- /dev/null
+++ b/src/app/api/medications/[id]/inventory/route.ts
@@ -0,0 +1,138 @@
+/**
+ * v1.4.25 W19b — pen / vial inventory CRUD.
+ *
+ *   GET  /api/medications/[id]/inventory  — list (user-scoped, all states)
+ *   POST /api/medications/[id]/inventory  — register a new pen / vial
+ *
+ * Per-item operations live on the `[itemId]` sub-route.
+ *
+ * Auth: cookie-session via requireAuth(); the medication is verified
+ * to belong to the caller before any read/write touches inventory.
+ * Rate-limit 30/min/user on the POST path (creation is the only
+ * surface that can spam writes; GET is read-only and benign).
+ * Audit-log every mutation with the affected itemId.
+ */
+
+import { NextRequest } from "next/server";
+
+import { prisma } from "@/lib/db";
+import { apiHandler, requireAuth } from "@/lib/api-handler";
+import { auditLog } from "@/lib/auth/audit";
+import { annotate } from "@/lib/logging/context";
+import {
+  apiError,
+  apiSuccess,
+  getClientIp,
+  safeJson,
+} from "@/lib/api-response";
+import { checkRateLimit, rateLimitHeaders } from "@/lib/rate-limit";
+import { createInventoryItemSchema } from "@/lib/validations/medication";
+import { buildCreateInventoryInput } from "@/lib/medications/inventory/service";
+
+type RouteParams = { params: Promise<{ id: string }> };
+
+const POST_RATE_LIMIT = 30;
+const POST_WINDOW_MS = 60_000;
+
+async function assertMedicationOwnership(
+  medicationId: string,
+  userId: string,
+) {
+  const med = await prisma.medication.findUnique({
+    where: { id: medicationId },
+    select: { id: true, userId: true },
+  });
+  if (!med || med.userId !== userId) {
+    return apiError("Medication not found", 404);
+  }
+  return null;
+}
+
+export const GET = apiHandler(
+  async (_request: NextRequest, { params }: RouteParams) => {
+    const { user } = await requireAuth();
+    const { id } = await params;
+
+    const guard = await assertMedicationOwnership(id, user.id);
+    if (guard) return guard;
+
+    const items = await prisma.medicationInventoryItem.findMany({
+      where: { userId: user.id, medicationId: id },
+      orderBy: [{ state: "asc" }, { expiresAt: "asc" }, { createdAt: "asc" }],
+    });
+
+    annotate({
+      action: { name: "medication.inventory.list" },
+      meta: { medication_id: id, total: items.length },
+    });
+
+    return apiSuccess({ items, meta: { total: items.length } });
+  },
+);
+
+export const POST = apiHandler(
+  async (request: NextRequest, { params }: RouteParams) => {
+    const { user } = await requireAuth();
+    const { id } = await params;
+
+    const guard = await assertMedicationOwnership(id, user.id);
+    if (guard) return guard;
+
+    // Per-user POST rate-limit — 30/min is generous for normal pen
+    // registrations but cuts off the spam case.
+    const rl = await checkRateLimit(
+      `medication-inventory:post:${user.id}`,
+      POST_RATE_LIMIT,
+      POST_WINDOW_MS,
+    );
+    if (!rl.allowed) {
+      const response = apiError("Too many requests", 429);
+      for (const [k, v] of Object.entries(rateLimitHeaders(rl))) {
+        response.headers.set(k, v);
+      }
+      return response;
+    }
+
+    const { data: body, error: jsonError } = await safeJson(request);
+    if (jsonError) return jsonError;
+
+    const parsed = createInventoryItemSchema.safeParse(body);
+    if (!parsed.success) {
+      return apiError(parsed.error.issues[0].message, 422);
+    }
+
+    const { dosesTotal, printedExpiry, purchasedAt, notes } = parsed.data;
+
+    const created = await prisma.medicationInventoryItem.create({
+      data: buildCreateInventoryInput({
+        userId: user.id,
+        medicationId: id,
+        dosesTotal,
+        printedExpiry: printedExpiry ?? null,
+        purchasedAt: purchasedAt ?? null,
+        notes: notes ?? null,
+      }),
+    });
+
+    await auditLog("medication.inventory.create", {
+      userId: user.id,
+      ipAddress: getClientIp(request),
+      details: {
+        medicationId: id,
+        itemId: created.id,
+        dosesTotal,
+      },
+    });
+
+    annotate({
+      action: {
+        name: "medication.inventory.create",
+        entity_type: "inventory_item",
+        entity_id: created.id,
+      },
+      meta: { medication_id: id },
+    });
+
+    return apiSuccess(created, 201);
+  },
+);
diff --git a/src/lib/medications/inventory/__tests__/service.test.ts b/src/lib/medications/inventory/__tests__/service.test.ts
new file mode 100644
index 000000000..053a73e0e
--- /dev/null
+++ b/src/lib/medications/inventory/__tests__/service.test.ts
@@ -0,0 +1,189 @@
+import { describe, it, expect, vi, beforeEach } from "vitest";
+
+vi.mock("@/lib/db", () => ({
+  prisma: {
+    medicationInventoryItem: {
+      findFirst: vi.fn(),
+      findMany: vi.fn(),
+      update: vi.fn(),
+    },
+  },
+}));
+
+import { prisma } from "@/lib/db";
+import {
+  buildCreateInventoryInput,
+  consumeOneDose,
+  expireStaleInUseItems,
+} from "../service";
+
+const NOW = new Date("2026-05-14T12:00:00Z");
+const MS_PER_DAY = 24 * 60 * 60 * 1000;
+
+beforeEach(() => {
+  vi.resetAllMocks();
+});
+
+describe("consumeOneDose", () => {
+  it("returns null when no eligible pen exists for the medication", async () => {
+    vi.mocked(prisma.medicationInventoryItem.findFirst).mockResolvedValueOnce(
+      null,
+    );
+    vi.mocked(prisma.medicationInventoryItem.findFirst).mockResolvedValueOnce(
+      null,
+    );
+
+    const result = await consumeOneDose({
+      userId: "u",
+      medicationId: "m",
+      intakeAt: NOW,
+    });
+    expect(result).toBeNull();
+    expect(prisma.medicationInventoryItem.update).not.toHaveBeenCalled();
+  });
+
+  it("flips an ACTIVE pen to IN_USE on first use", async () => {
+    // No IN_USE pen, fall back to ACTIVE.
+    vi.mocked(prisma.medicationInventoryItem.findFirst).mockResolvedValueOnce(
+      null,
+    );
+    vi.mocked(prisma.medicationInventoryItem.findFirst).mockResolvedValueOnce({
+      id: "inv-1",
+      state: "ACTIVE",
+      dosesTotal: 4,
+      dosesRemaining: 4,
+      firstUseAt: null,
+      printedExpiry: null,
+    } as never);
+    vi.mocked(prisma.medicationInventoryItem.update).mockResolvedValue({} as never);
+
+    const result = await consumeOneDose({
+      userId: "u",
+      medicationId: "m",
+      intakeAt: NOW,
+    });
+    expect(result).toEqual({ itemId: "inv-1", change: "first_use" });
+
+    const updateCall = vi.mocked(prisma.medicationInventoryItem.update).mock
+      .calls[0][0];
+    expect(updateCall.data).toMatchObject({
+      state: "IN_USE",
+      dosesRemaining: 3,
+      firstUseAt: NOW,
+    });
+  });
+
+  it("prefers the IN_USE pen with the earliest expiry", async () => {
+    const firstUse = new Date(NOW.getTime() - 5 * MS_PER_DAY);
+    vi.mocked(prisma.medicationInventoryItem.findFirst).mockResolvedValueOnce({
+      id: "inv-active",
+      state: "IN_USE",
+      dosesTotal: 4,
+      dosesRemaining: 3,
+      firstUseAt: firstUse,
+      printedExpiry: null,
+    } as never);
+    vi.mocked(prisma.medicationInventoryItem.update).mockResolvedValue({} as never);
+
+    const result = await consumeOneDose({
+      userId: "u",
+      medicationId: "m",
+      intakeAt: NOW,
+    });
+    expect(result).toEqual({ itemId: "inv-active", change: "consumed" });
+    expect(prisma.medicationInventoryItem.findFirst).toHaveBeenCalledTimes(1);
+  });
+
+  it("returns 'depleted' when the final dose is consumed", async () => {
+    const firstUse = new Date(NOW.getTime() - 5 * MS_PER_DAY);
+    vi.mocked(prisma.medicationInventoryItem.findFirst).mockResolvedValueOnce({
+      id: "inv-last",
+      state: "IN_USE",
+      dosesTotal: 4,
+      dosesRemaining: 1,
+      firstUseAt: firstUse,
+      printedExpiry: null,
+    } as never);
+    vi.mocked(prisma.medicationInventoryItem.update).mockResolvedValue({} as never);
+
+    const result = await consumeOneDose({
+      userId: "u",
+      medicationId: "m",
+      intakeAt: NOW,
+    });
+    expect(result).toEqual({ itemId: "inv-last", change: "depleted" });
+    const updateCall = vi.mocked(prisma.medicationInventoryItem.update).mock
+      .calls[0][0];
+    expect(updateCall.data).toMatchObject({
+      state: "USED_UP",
+      dosesRemaining: 0,
+    });
+  });
+});
+
+describe("expireStaleInUseItems", () => {
+  it("transitions IN_USE rows whose 30-day window has lapsed to EXPIRED", async () => {
+    const firstUseAgo = new Date(NOW.getTime() - 31 * MS_PER_DAY);
+    vi.mocked(prisma.medicationInventoryItem.findMany).mockResolvedValue([
+      {
+        id: "inv-stale",
+        state: "IN_USE",
+        dosesTotal: 4,
+        dosesRemaining: 2,
+        firstUseAt: firstUseAgo,
+        printedExpiry: null,
+      },
+    ] as never);
+    vi.mocked(prisma.medicationInventoryItem.update).mockResolvedValue({} as never);
+
+    const count = await expireStaleInUseItems({ nowMs: NOW.getTime() });
+    expect(count).toBe(1);
+    expect(prisma.medicationInventoryItem.update).toHaveBeenCalledWith({
+      where: { id: "inv-stale" },
+      data: { state: "EXPIRED" },
+    });
+  });
+
+  it("scopes to a single user when userId is given", async () => {
+    vi.mocked(prisma.medicationInventoryItem.findMany).mockResolvedValue([] as never);
+
+    await expireStaleInUseItems({ userId: "user-1", nowMs: NOW.getTime() });
+    expect(prisma.medicationInventoryItem.findMany).toHaveBeenCalledWith({
+      where: expect.objectContaining({ userId: "user-1" }),
+      select: expect.anything(),
+    });
+  });
+});
+
+describe("buildCreateInventoryInput", () => {
+  it("computes expiresAt = printedExpiry when firstUseAt is null", () => {
+    const printed = new Date("2027-06-01T00:00:00Z");
+    const input = buildCreateInventoryInput({
+      userId: "u",
+      medicationId: "m",
+      dosesTotal: 4,
+      printedExpiry: printed,
+      purchasedAt: null,
+      notes: null,
+    });
+    expect(input).toMatchObject({
+      state: "ACTIVE",
+      dosesTotal: 4,
+      dosesRemaining: 4,
+      firstUseAt: null,
+      expiresAt: printed,
+    });
+  });
+
+  it("computes expiresAt = null when neither firstUseAt nor printedExpiry is set", () => {
+    const input = buildCreateInventoryInput({
+      userId: "u",
+      medicationId: "m",
+      dosesTotal: 4,
+      printedExpiry: null,
+      purchasedAt: null,
+      notes: null,
+    });
+    expect(input.expiresAt).toBeNull();
+  });
+});
diff --git a/src/lib/medications/inventory/service.ts b/src/lib/medications/inventory/service.ts
new file mode 100644
index 000000000..5ea29193e
--- /dev/null
+++ b/src/lib/medications/inventory/service.ts
@@ -0,0 +1,167 @@
+/**
+ * v1.4.25 W19b — inventory persistence helpers.
+ *
+ * Thin layer on top of Prisma that the route handlers and the
+ * intake hook compose. Keeps the state-machine math (pure) separate
+ * from the I/O (here), so unit tests on the math don't need a DB.
+ */
+
+import { prisma } from "@/lib/db";
+import {
+  computeExpiresAt,
+  computeInventoryState,
+  decrementDose,
+  type DecrementOutcome,
+} from "./state-machine";
+
+/**
+ * Consume one dose from the user's active in-use pen for the given
+ * medication. Called from the intake POST handler after the intake
+ * event has been created.
+ *
+ * Selection policy:
+ *   1. Prefer the IN_USE pen with the earliest `expiresAt` — that's
+ *      the pen the user is currently working through.
+ *   2. Fall back to the oldest ACTIVE pen (FIFO consumption — the
+ *      pen that was purchased first gets opened first). The intake
+ *      flips it ACTIVE → IN_USE and sets firstUseAt.
+ *   3. If no eligible pen exists, this is a no-op. The user just
+ *      logged an intake without a tracked pen — perfectly valid
+ *      since inventory is opt-in per the W19a doses-per-unit
+ *      design.
+ *
+ * The function intentionally only updates one pen per intake — if a
+ * user double-doses (rare), they need to record two intake events
+ * and the second one will pick up the same or a new pen.
+ */
+export async function consumeOneDose(input: {
+  userId: string;
+  medicationId: string;
+  intakeAt: Date;
+}): Promise<{
+  itemId: string;
+  change: DecrementOutcome;
+} | null> {
+  const { userId, medicationId, intakeAt } = input;
+
+  // Prefer the IN_USE pen with the earliest expiry; tie-break by
+  // createdAt ascending.
+  const inUsePen = await prisma.medicationInventoryItem.findFirst({
+    where: {
+      userId,
+      medicationId,
+      state: "IN_USE",
+      dosesRemaining: { gt: 0 },
+    },
+    orderBy: [{ expiresAt: "asc" }, { createdAt: "asc" }],
+  });
+
+  const target =
+    inUsePen ??
+    (await prisma.medicationInventoryItem.findFirst({
+      where: {
+        userId,
+        medicationId,
+        state: "ACTIVE",
+        dosesRemaining: { gt: 0 },
+      },
+      orderBy: [{ purchasedAt: "asc" }, { createdAt: "asc" }],
+    }));
+
+  if (!target) return null;
+
+  const { item: nextValues, change } = decrementDose(
+    {
+      state: target.state,
+      dosesTotal: target.dosesTotal,
+      dosesRemaining: target.dosesRemaining,
+      firstUseAt: target.firstUseAt,
+      printedExpiry: target.printedExpiry,
+    },
+    intakeAt,
+  );
+
+  await prisma.medicationInventoryItem.update({
+    where: { id: target.id },
+    data: {
+      state: nextValues.state,
+      dosesRemaining: nextValues.dosesRemaining,
+      firstUseAt: nextValues.firstUseAt,
+      expiresAt: nextValues.expiresAt,
+    },
+  });
+
+  return { itemId: target.id, change };
+}
+
+/**
+ * Daily cron pass — flip any IN_USE rows whose 30-day window has
+ * elapsed into EXPIRED. Returns the count of transitions for logging.
+ *
+ * Optionally scoped to a single user — useful for the per-user
+ * trigger path. Cron with `userId: null` sweeps every user.
+ */
+export async function expireStaleInUseItems(input: {
+  userId?: string;
+  nowMs: number;
+}): Promise<number> {
+  const { userId, nowMs } = input;
+  const now = new Date(nowMs);
+
+  // Candidates: IN_USE rows whose expiresAt has lapsed. EXPIRED rows
+  // are already terminal; USED_UP rows are off-window by definition.
+  const candidates = await prisma.medicationInventoryItem.findMany({
+    where: {
+      state: "IN_USE",
+      expiresAt: { lt: now },
+      ...(userId ? { userId } : {}),
+    },
+    select: {
+      id: true,
+      state: true,
+      dosesTotal: true,
+      dosesRemaining: true,
+      firstUseAt: true,
+      printedExpiry: true,
+    },
+  });
+
+  let transitions = 0;
+  for (const row of candidates) {
+    const nextState = computeInventoryState(row, nowMs);
+    if (nextState === row.state) continue;
+    await prisma.medicationInventoryItem.update({
+      where: { id: row.id },
+      data: { state: nextState },
+    });
+    transitions += 1;
+  }
+  return transitions;
+}
+
+/**
+ * Helper for the POST /[medId]/inventory route. Composes the
+ * Prisma-create with the computed `expiresAt` so the caller doesn't
+ * have to re-derive it.
+ */
+export function buildCreateInventoryInput(input: {
+  userId: string;
+  medicationId: string;
+  dosesTotal: number;
+  printedExpiry: Date | null;
+  purchasedAt: Date | null;
+  notes: string | null;
+}) {
+  return {
+    userId: input.userId,
+    medicationId: input.medicationId,
+    state: "ACTIVE" as const,
+    dosesTotal: input.dosesTotal,
+    dosesRemaining: input.dosesTotal,
+    firstUseAt: null,
+    printedExpiry: input.printedExpiry,
+    purchasedAt: input.purchasedAt,
+    expiresAt: computeExpiresAt(null, input.printedExpiry),
+    notes: input.notes,
+  };
+}
diff --git a/src/lib/validations/medication.ts b/src/lib/validations/medication.ts
index 76ebad0c5..6d37f64af 100644
--- a/src/lib/validations/medication.ts
+++ b/src/lib/validations/medication.ts
@@ -104,6 +104,47 @@ export const updateIntakeEventSchema = z.object({
     .optional(),
 });
 
+/**
+ * v1.4.25 W19b — inventory (pen / vial) CRUD validators.
+ *
+ * The Prisma model carries a 4-state enum
+ * (ACTIVE | IN_USE | EXPIRED | USED_UP); the API surface only lets
+ * the user explicitly transition into IN_USE (mark-as-first-use) or
+ * USED_UP (manual override). EXPIRED is owned by the daily cron and
+ * the intake hook, so the PATCH schema deliberately omits it.
+ */
+export const createInventoryItemSchema = z.object({
+  dosesTotal: z.number().int().min(1).max(100),
+  printedExpiry: z.iso
+    .datetime({ offset: true })
+    .transform((s) => new Date(s))
+    .nullable()
+    .optional(),
+  purchasedAt: z.iso
+    .datetime({ offset: true })
+    .transform((s) => new Date(s))
+    .nullable()
+    .optional(),
+  notes: z.string().max(200).nullable().optional(),
+});
+
+export const updateInventoryItemSchema = z.object({
+  markAsFirstUseAt: z.iso
+    .datetime({ offset: true })
+    .transform((s) => new Date(s))
+    .optional(),
+  markAsUsedUp: z.boolean().optional(),
+  printedExpiry: z.iso
+    .datetime({ offset: true })
+    .transform((s) => new Date(s))
+    .nullable()
+    .optional(),
+  notes: z.string().max(200).nullable().optional(),
+});
+
+export type CreateInventoryItemInput = z.infer<typeof createInventoryItemSchema>;
+export type UpdateInventoryItemInput = z.infer<typeof updateInventoryItemSchema>;
+
 export type CreateMedicationInput = z.infer<typeof createMedicationSchema>;
 export type IntakeInput = z.infer<typeof intakeSchema>;
 export type ListIntakeEventsInput = z.infer<typeof listIntakeEventsSchema>;