diff --git a/apps/api/src/services/messaging/__tests__/consent.test.ts b/apps/api/src/services/messaging/__tests__/consent.test.ts
new file mode 100644
index 0000000..d810c9e
--- /dev/null
+++ b/apps/api/src/services/messaging/__tests__/consent.test.ts
@@ -0,0 +1,214 @@
+import { describe, it, expect, vi, beforeEach } from "vitest";
+import { detectKeyword } from "../consent.js";
+
+const mockDb = {
+  insert: vi.fn(),
+  update: vi.fn(),
+  select: vi.fn(),
+};
+
+vi.mock("@groombook/db", () => ({
+  getDb: () => mockDb,
+  clients: {},
+  messageConsentEvents: {},
+  businessSettings: {},
+  eq: vi.fn(),
+}));
+
+const { handleConsentKeyword } = await import("../consent.js");
+
+describe("detectKeyword", () => {
+  it.each([
+    ["STOP", "opt_out"],
+    ["STOPALL", "opt_out"],
+    ["UNSUBSCRIBE", "opt_out"],
+    ["CANCEL", "opt_out"],
+    ["END", "opt_out"],
+    ["QUIT", "opt_out"],
+  ])("opt-out keyword %s → opt_out", (keyword, expected) => {
+    expect(detectKeyword(keyword)).toEqual({ kind: expected });
+  });
+
+  it.each([
+    ["START", "opt_in"],
+    ["UNSTOP", "opt_in"],
+    ["YES", "opt_in"],
+    ["SUBSCRIBE", "opt_in"],
+  ])("opt-in keyword %s → opt_in", (keyword, expected) => {
+    expect(detectKeyword(keyword)).toEqual({ kind: expected });
+  });
+
+  it.each([
+    ["HELP", "help"],
+    ["INFO", "help"],
+  ])("help keyword %s → help", (keyword, expected) => {
+    expect(detectKeyword(keyword)).toEqual({ kind: expected });
+  });
+
+  it("is case insensitive", () => {
+    expect(detectKeyword("stop")).toEqual({ kind: "opt_out" });
+    expect(detectKeyword("Stop")).toEqual({ kind: "opt_out" });
+    expect(detectKeyword("sToP")).toEqual({ kind: "opt_out" });
+  });
+
+  it("trims whitespace", () => {
+    expect(detectKeyword("  STOP  ")).toEqual({ kind: "opt_out" });
+    expect(detectKeyword("\tSTART\n")).toEqual({ kind: "opt_in" });
+  });
+
+  it("returns null for non-keyword messages", () => {
+    expect(detectKeyword("hello")).toBeNull();
+    expect(detectKeyword("STOP IT")).toBeNull();
+    expect(detectKeyword("help me")).toBeNull();
+  });
+});
+
+describe("handleConsentKeyword", () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+    mockDb.insert.mockReturnValue({
+      values: vi.fn().mockResolvedValue([{ id: "event-1" }]),
+    } as any);
+    mockDb.update.mockReturnValue({
+      set: vi.fn().mockReturnValue({
+        where: vi.fn().mockResolvedValue([]),
+      }),
+    } as any);
+  });
+
+  const baseOpts = {
+    clientId: "client-1",
+    businessId: "biz-1",
+    db: mockDb as unknown as ReturnType<typeof import("@groombook/db").getDb>,
+  };
+
+  describe("opt_out", () => {
+    it("inserts consent event with sms_keyword source", async () => {
+      mockDb.select.mockReturnValue({
+        from: vi.fn().mockReturnValue({
+          where: vi.fn().mockReturnValue({
+            limit: vi.fn().mockResolvedValue([{ smsOptIn: true }]),
+          }),
+        }),
+      } as any);
+
+      await handleConsentKeyword({ ...baseOpts, kind: "opt_out" });
+
+      expect(mockDb.insert).toHaveBeenCalledOnce();
+    });
+
+    it("sets smsOptIn=false and smsOptOutDate when currently opted in", async () => {
+      mockDb.select.mockReturnValue({
+        from: vi.fn().mockReturnValue({
+          where: vi.fn().mockReturnValue({
+            limit: vi.fn().mockResolvedValue([{ smsOptIn: true }]),
+          }),
+        }),
+      } as any);
+
+      await handleConsentKeyword({ ...baseOpts, kind: "opt_out" });
+
+      expect(mockDb.update).toHaveBeenCalled();
+    });
+
+    it("is idempotent — second opt-out logs event but skips client update", async () => {
+      mockDb.select.mockReturnValue({
+        from: vi.fn().mockReturnValue({
+          where: vi.fn().mockReturnValue({
+            limit: vi.fn().mockResolvedValue([{ smsOptIn: false }]),
+          }),
+        }),
+      } as any);
+
+      await handleConsentKeyword({ ...baseOpts, kind: "opt_out" });
+
+      expect(mockDb.update).not.toHaveBeenCalled();
+    });
+
+    it("returns unsubscribe reply text", async () => {
+      mockDb.select.mockReturnValue({
+        from: vi.fn().mockReturnValue({
+          where: vi.fn().mockReturnValue({
+            limit: vi.fn().mockResolvedValue([{ smsOptIn: true }]),
+          }),
+        }),
+      } as any);
+
+      const result = await handleConsentKeyword({ ...baseOpts, kind: "opt_out" });
+      expect(result.replyText).toBe(
+        "You have been unsubscribed and will no longer receive messages. Reply START to resubscribe."
+      );
+    });
+  });
+
+  describe("opt_in", () => {
+    it("sets smsOptIn=true and smsConsentDate when currently opted out", async () => {
+      mockDb.select.mockReturnValue({
+        from: vi.fn().mockReturnValue({
+          where: vi.fn().mockReturnValue({
+            limit: vi.fn().mockResolvedValue([{ smsOptIn: false, smsConsentDate: null }]),
+          }),
+        }),
+      } as any);
+
+      await handleConsentKeyword({ ...baseOpts, kind: "opt_in" });
+
+      expect(mockDb.update).toHaveBeenCalled();
+    });
+
+    it("clears smsOptOutDate on opt-in after opt-out", async () => {
+      mockDb.select.mockReturnValue({
+        from: vi.fn().mockReturnValue({
+          where: vi.fn().mockReturnValue({
+            limit: vi.fn().mockResolvedValue([{ smsOptIn: false }]),
+          }),
+        }),
+      } as any);
+
+      await handleConsentKeyword({ ...baseOpts, kind: "opt_in" });
+
+      expect(mockDb.update).toHaveBeenCalled();
+    });
+
+    it("is idempotent — second opt-in skips client update", async () => {
+      mockDb.select.mockReturnValue({
+        from: vi.fn().mockReturnValue({
+          where: vi.fn().mockReturnValue({
+            limit: vi.fn().mockResolvedValue([{ smsOptIn: true }]),
+          }),
+        }),
+      } as any);
+
+      await handleConsentKeyword({ ...baseOpts, kind: "opt_in" });
+
+      expect(mockDb.update).not.toHaveBeenCalled();
+    });
+
+    it("returns resubscribe reply text", async () => {
+      mockDb.select.mockReturnValue({
+        from: vi.fn().mockReturnValue({
+          where: vi.fn().mockReturnValue({
+            limit: vi.fn().mockResolvedValue([{ smsOptIn: false }]),
+          }),
+        }),
+      } as any);
+
+      const result = await handleConsentKeyword({ ...baseOpts, kind: "opt_in" });
+      expect(result.replyText).toBe(
+        "You have been resubscribed to messages. Reply STOP to unsubscribe. Msg & data rates may apply."
+      );
+    });
+  });
+
+  describe("help", () => {
+    it("returns default help reply without querying businessSettings", async () => {
+      const result = await handleConsentKeyword({ ...baseOpts, kind: "help" });
+
+      expect(mockDb.update).not.toHaveBeenCalled();
+      expect(mockDb.select).not.toHaveBeenCalled();
+      expect(result.replyText).toBe(
+        "Reply STOP to unsubscribe or START to resubscribe. For help, contact your groomer directly."
+      );
+    });
+  });
+});
\ No newline at end of file
diff --git a/apps/api/src/services/messaging/consent.ts b/apps/api/src/services/messaging/consent.ts
new file mode 100644
index 0000000..0d28985
--- /dev/null
+++ b/apps/api/src/services/messaging/consent.ts
@@ -0,0 +1,77 @@
+import { getDb, clients, messageConsentEvents, eq } from "@groombook/db";
+import type { Db } from "@groombook/db";
+
+export type KeywordKind = "opt_in" | "opt_out" | "help";
+
+const OPT_OUT_KEYWORDS = new Set(["STOP", "STOPALL", "UNSUBSCRIBE", "CANCEL", "END", "QUIT"]);
+const OPT_IN_KEYWORDS = new Set(["START", "UNSTOP", "YES", "SUBSCRIBE"]);
+const HELP_KEYWORDS = new Set(["HELP", "INFO"]);
+
+export function detectKeyword(body: string): { kind: KeywordKind } | null {
+  const normalized = body.trim().toUpperCase();
+  if (OPT_OUT_KEYWORDS.has(normalized)) return { kind: "opt_out" };
+  if (OPT_IN_KEYWORDS.has(normalized)) return { kind: "opt_in" };
+  if (HELP_KEYWORDS.has(normalized)) return { kind: "help" };
+  return null;
+}
+
+export async function handleConsentKeyword(opts: {
+  clientId: string;
+  businessId: string;
+  kind: KeywordKind;
+  db: Db;
+}): Promise<{ replyText: string }> {
+  const { clientId, businessId, kind, db: database } = opts;
+
+  await database.insert(messageConsentEvents).values({
+    clientId,
+    businessId,
+    kind,
+    source: "sms_keyword",
+  });
+
+  if (kind === "opt_out") {
+    const [existing] = await database
+      .select({ smsOptIn: clients.smsOptIn })
+      .from(clients)
+      .where(eq(clients.id, clientId))
+      .limit(1);
+
+    if (existing?.smsOptIn !== false) {
+      await database
+        .update(clients)
+        .set({ smsOptIn: false, smsOptOutDate: new Date() })
+        .where(eq(clients.id, clientId));
+    }
+
+    return {
+      replyText: "You have been unsubscribed and will no longer receive messages. Reply START to resubscribe.",
+    };
+  }
+
+  if (kind === "opt_in") {
+    const [existing] = await database
+      .select({ smsOptIn: clients.smsOptIn, smsConsentDate: clients.smsConsentDate })
+      .from(clients)
+      .where(eq(clients.id, clientId))
+      .limit(1);
+
+    if (existing?.smsOptIn !== true) {
+      await database
+        .update(clients)
+        .set({ smsOptIn: true, smsConsentDate: new Date(), smsOptOutDate: null })
+        .where(eq(clients.id, clientId));
+    }
+
+    return {
+      replyText:
+        "You have been resubscribed to messages. Reply STOP to unsubscribe. Msg & data rates may apply.",
+    };
+  }
+
+  // kind === "help"
+  const replyText =
+    "Reply STOP to unsubscribe or START to resubscribe. For help, contact your groomer directly.";
+
+  return { replyText };
+}
\ No newline at end of file
diff --git a/apps/api/src/services/messaging/inbound.ts b/apps/api/src/services/messaging/inbound.ts
index de9d0e4..4b41576 100644
--- a/apps/api/src/services/messaging/inbound.ts
+++ b/apps/api/src/services/messaging/inbound.ts
@@ -1,5 +1,7 @@
 import { getDb, conversations, messages, businessSettings, clients, eq, and } from "@groombook/db";
 import { v4 as uuidv4 } from "uuid";
+import { detectKeyword, handleConsentKeyword } from "./consent.js";
+import { sendMessage } from "./outbound.js";
 
 export interface TelnyxMessageReceivedPayload {
   data: {
@@ -152,7 +154,7 @@ export async function handleMessageReceived(payload: TelnyxMessageReceivedPayloa
     throw new Error(`No business owns messaging number: ${toPhone}`);
   }
 
-  const { id: conversationId } = await findOrCreateConversation(businessId, fromPhone, toPhone);
+  const { id: conversationId, clientId } = await findOrCreateConversation(businessId, fromPhone, toPhone);
 
   await getDb()
     .update(conversations)
@@ -167,6 +169,22 @@ export async function handleMessageReceived(payload: TelnyxMessageReceivedPayloa
     "received"
   );
 
+  const keyword = detectKeyword(message.body ?? "");
+  if (keyword) {
+    const { replyText } = await handleConsentKeyword({
+      clientId,
+      businessId,
+      kind: keyword.kind,
+      db: getDb(),
+    });
+    await sendMessage({
+      businessId,
+      clientId,
+      body: replyText,
+      sentByStaffId: undefined,
+    });
+  }
+
   return { conversationId, messageId };
 }