diff --git a/app/api/inbox/[address]/[messageId]/__tests__/d1-reads-flip.test.ts b/app/api/inbox/[address]/[messageId]/__tests__/d1-reads-flip.test.ts
new file mode 100644
index 00000000..e71a14d6
--- /dev/null
+++ b/app/api/inbox/[address]/[messageId]/__tests__/d1-reads-flip.test.ts
@@ -0,0 +1,269 @@
+/**
+ * Phase 2.5 Step 3.2 — GET /api/inbox/[address]/[messageId] D1 read flip.
+ *
+ * Covers:
+ *  1. 200 — message exists, returned correctly
+ *  2. 404 — message not found in D1
+ *  3. 404 — address-match guard (block-on-merge per #725 / secret-mars v167):
+ *       msg_X has to_btc_address=addr_A; GET .../addr_B/msg_X → 404
+ *       (not 200 with body — prevents non-auth disclosure regression)
+ *  4. 503 — D1 throws → structured fallback, not unhandled 500
+ *  5. Reply attachment — message with reply → response includes .reply object
+ *
+ * See: https://github.com/aibtcdev/landing-page/issues/725 (Step 3.2 spec)
+ */
+
+import { describe, it, expect, vi, beforeEach, type Mock } from "vitest";
+import { NextRequest } from "next/server";
+
+// ---- module mocks (must be declared before route imports) -------------------
+
+vi.mock("@opennextjs/cloudflare", () => ({
+  getCloudflareContext: vi.fn(),
+}));
+
+vi.mock("@/lib/agent-lookup", () => ({
+  lookupAgent: vi.fn(),
+}));
+
+vi.mock("@/lib/inbox/d1-reads", () => ({
+  getInboxMessageFromD1: vi.fn(),
+  fetchRepliesForMessages: vi.fn(),
+}));
+
+vi.mock("@/lib/inbox", () => ({
+  getMessage: vi.fn(),
+  updateMessage: vi.fn(),
+  validateMarkRead: vi.fn(),
+  buildMarkReadMessage: vi.fn(() => "Mark as Read | msg_test"),
+  decrementUnreadCount: vi.fn(),
+}));
+
+vi.mock("@/lib/inbox/d1-dual-write", () => ({
+  updateMessageStateD1: vi.fn().mockResolvedValue(undefined),
+}));
+
+vi.mock("@/lib/bitcoin-verify", () => ({
+  verifyBitcoinSignature: vi.fn(),
+}));
+
+vi.mock("@/lib/logging", () => ({
+  createLogger: () => ({ info: vi.fn(), warn: vi.fn(), error: vi.fn() }),
+  createConsoleLogger: () => ({ info: vi.fn(), warn: vi.fn(), error: vi.fn() }),
+  isLogsRPC: () => false,
+}));
+
+vi.mock("@/lib/env", () => ({
+  shouldFailClosed: vi.fn(() => false),
+}));
+
+// ---- imports after mocks ----------------------------------------------------
+
+import { GET } from "../route";
+import { getCloudflareContext } from "@opennextjs/cloudflare";
+import { lookupAgent } from "@/lib/agent-lookup";
+import {
+  getInboxMessageFromD1,
+  fetchRepliesForMessages,
+} from "@/lib/inbox/d1-reads";
+
+// ---- shared fixtures --------------------------------------------------------
+
+const ADDR_A = "bc1qxj5jtv8jwm7zv2nczn2xfq9agjgj0sqpsxn43h";
+const ADDR_B = "bc1qw0y4ant38zykzjqssgnujqmszruvhkwupvp6dn";
+const MSG_ID = "msg_1778221238475_test_address_match_guard";
+
+const AGENT_A = {
+  btcAddress: ADDR_A,
+  stxAddress: "SP3JR7JXFT7ZM9JKSQPBQG1HPT0D365MA5TN0P12E",
+  displayName: "Frosty Narwhal",
+};
+
+const AGENT_B = {
+  btcAddress: ADDR_B,
+  stxAddress: "SP3GXCKM4AB5EB1KJ8V5QSTR1XMTW3R142VQS2NVW",
+  displayName: "Amber Otter",
+};
+
+const INBOX_MESSAGE = {
+  messageId: MSG_ID,
+  fromAddress: "SP3GXCKM4AB5EB1KJ8V5QSTR1XMTW3R142VQS2NVW",
+  toBtcAddress: ADDR_A,
+  toStxAddress: "SP3JR7JXFT7ZM9JKSQPBQG1HPT0D365MA5TN0P12E",
+  content: "Hello from agent B to agent A",
+  paymentTxid: "abc123deadbeef",
+  paymentSatoshis: 100,
+  sentAt: "2026-05-08T06:20:38.665Z",
+  authenticated: false,
+  paymentStatus: "confirmed" as const,
+};
+
+const REPLY = {
+  messageId: MSG_ID,
+  fromAddress: ADDR_A,
+  toBtcAddress: ADDR_B,
+  reply: "Thanks for the message!",
+  signature: "sig_abc123",
+  repliedAt: "2026-05-08T07:00:00.000Z",
+};
+
+function makeMockDB() {
+  const stmt = {
+    bind: vi.fn().mockReturnThis(),
+    first: vi.fn(),
+    all: vi.fn(),
+    run: vi.fn(),
+    raw: vi.fn(),
+  };
+  return {
+    prepare: vi.fn().mockReturnValue(stmt),
+    batch: vi.fn(),
+    dump: vi.fn(),
+    exec: vi.fn(),
+  } as unknown as D1Database;
+}
+
+function buildGetRequest(address: string, messageId: string): NextRequest {
+  return new NextRequest(
+    `https://aibtc.com/api/inbox/${address}/${messageId}`,
+    { method: "GET" }
+  );
+}
+
+function buildContext(address: string, messageId: string) {
+  return { params: Promise.resolve({ address, messageId }) };
+}
+
+beforeEach(() => {
+  vi.clearAllMocks();
+
+  (getCloudflareContext as Mock).mockReturnValue({
+    env: {
+      DB: makeMockDB(),
+      VERIFIED_AGENTS: {} as KVNamespace,
+    },
+    ctx: { waitUntil: vi.fn() },
+  });
+});
+
+// ---- tests ------------------------------------------------------------------
+
+describe("Phase 2.5 Step 3.2 — GET /api/inbox/[address]/[messageId] D1 flip", () => {
+  it("returns 200 with message shape when message exists", async () => {
+    (lookupAgent as Mock).mockResolvedValue(AGENT_A);
+    (getInboxMessageFromD1 as Mock).mockResolvedValue(INBOX_MESSAGE);
+    (fetchRepliesForMessages as Mock).mockResolvedValue(new Map());
+
+    const res = await GET(
+      buildGetRequest(ADDR_A, MSG_ID),
+      buildContext(ADDR_A, MSG_ID)
+    );
+
+    expect(res.status).toBe(200);
+    const body = await res.json();
+    expect(body.message).toMatchObject({ messageId: MSG_ID, toBtcAddress: ADDR_A });
+    expect(body.reply).toBeNull();
+    expect(body.recipient).toMatchObject({ btcAddress: ADDR_A });
+  });
+
+  it("returns 404 when message_id not found in D1", async () => {
+    (lookupAgent as Mock).mockResolvedValue(AGENT_A);
+    (getInboxMessageFromD1 as Mock).mockResolvedValue(null);
+    (fetchRepliesForMessages as Mock).mockResolvedValue(new Map());
+
+    const res = await GET(
+      buildGetRequest(ADDR_A, "msg_nonexistent"),
+      buildContext(ADDR_A, "msg_nonexistent")
+    );
+
+    expect(res.status).toBe(404);
+    const body = await res.json();
+    expect(body.error).toBe("Message not found");
+  });
+
+  it("address-match guard: msg_X exists for addr_A but GET with addr_B returns 404 (not 200)", async () => {
+    // BLOCK-ON-MERGE per #725 / secret-mars v167. SQL security gate in `getInboxMessageFromD1` (lib/inbox/d1-reads.ts).
+    (lookupAgent as Mock).mockResolvedValue(AGENT_B);
+    // D1 returns null because ADDR_B does not match ADDR_A in the WHERE clause
+    (getInboxMessageFromD1 as Mock).mockResolvedValue(null);
+
+    const res = await GET(
+      buildGetRequest(ADDR_B, MSG_ID),
+      buildContext(ADDR_B, MSG_ID)
+    );
+
+    // MUST be 404, not 200 — a 200 would be a non-auth disclosure
+    expect(res.status).toBe(404);
+    const body = await res.json();
+    expect(body.error).toBe("Message not found");
+
+    // Verify the D1 query was called with ADDR_B (not ADDR_A), confirming
+    // the AND clause will never match ADDR_A's message for ADDR_B's query
+    expect(getInboxMessageFromD1).toHaveBeenCalledOnce();
+    const [, calledAddress, calledMessageId] = (getInboxMessageFromD1 as Mock).mock.calls[0];
+    expect(calledAddress).toBe(ADDR_B);
+    expect(calledMessageId).toBe(MSG_ID);
+  });
+
+  it("returns 503 with structured body when D1 throws — not unhandled 500", async () => {
+    (lookupAgent as Mock).mockResolvedValue(AGENT_A);
+    (getInboxMessageFromD1 as Mock).mockRejectedValue(
+      new Error("D1_ERROR: connection reset")
+    );
+
+    const res = await GET(
+      buildGetRequest(ADDR_A, MSG_ID),
+      buildContext(ADDR_A, MSG_ID)
+    );
+
+    expect(res.status).toBe(503);
+    expect(res.status).not.toBe(500);
+    const body = await res.json();
+    expect(body).toMatchObject({
+      error: "transient_d1_unavailable",
+      retry_after: 5,
+    });
+    expect(body.message).toMatch(/temporarily unavailable/i);
+    expect(res.headers.get("Retry-After")).toBe("5");
+  });
+
+  it("returns 503 when fetchRepliesForMessages throws after message found", async () => {
+    (lookupAgent as Mock).mockResolvedValue(AGENT_A);
+    (getInboxMessageFromD1 as Mock).mockResolvedValue(INBOX_MESSAGE);
+    (fetchRepliesForMessages as Mock).mockRejectedValue(
+      new Error("D1_ERROR: schema mismatch")
+    );
+
+    const res = await GET(
+      buildGetRequest(ADDR_A, MSG_ID),
+      buildContext(ADDR_A, MSG_ID)
+    );
+
+    expect(res.status).toBe(503);
+    const body = await res.json();
+    expect(body.error).toBe("transient_d1_unavailable");
+  });
+
+  it("reply attachment: message with reply returns .reply object in response", async () => {
+    (lookupAgent as Mock).mockResolvedValue(AGENT_A);
+    (getInboxMessageFromD1 as Mock).mockResolvedValue({
+      ...INBOX_MESSAGE,
+      repliedAt: "2026-05-08T07:00:00.000Z",
+    });
+    const repliesMap = new Map([[MSG_ID, REPLY]]);
+    (fetchRepliesForMessages as Mock).mockResolvedValue(repliesMap);
+
+    const res = await GET(
+      buildGetRequest(ADDR_A, MSG_ID),
+      buildContext(ADDR_A, MSG_ID)
+    );
+
+    expect(res.status).toBe(200);
+    const body = await res.json();
+    expect(body.reply).not.toBeNull();
+    expect(body.reply).toMatchObject({
+      messageId: MSG_ID,
+      reply: "Thanks for the message!",
+    });
+  });
+});
diff --git a/app/api/inbox/[address]/[messageId]/route.ts b/app/api/inbox/[address]/[messageId]/route.ts
index 5b110235..7c758c70 100644
--- a/app/api/inbox/[address]/[messageId]/route.ts
+++ b/app/api/inbox/[address]/[messageId]/route.ts
@@ -9,15 +9,18 @@ import { verifyBitcoinSignature } from "@/lib/bitcoin-verify";
 import { lookupAgent } from "@/lib/agent-lookup";
 import {
   getMessage,
-  getReply,
   updateMessage,
-  getAgentInbox,
   validateMarkRead,
   buildMarkReadMessage,
   decrementUnreadCount,
 } from "@/lib/inbox";
 import { shouldFailClosed } from "@/lib/env";
 import { updateMessageStateD1 } from "@/lib/inbox/d1-dual-write";
+import {
+  getInboxMessageFromD1,
+  fetchRepliesForMessages,
+} from "@/lib/inbox/d1-reads";
+import type { InboxMessage, OutboxReply } from "@/lib/inbox/types";
 
 /** Retry-After value (seconds) to return on 429s — matches the 60s binding window. */
 const RATE_LIMIT_RETRY_AFTER = 60;
@@ -44,35 +47,35 @@ export async function GET(
     );
   }
 
-  // Fetch message and reply in parallel
-  const [message, reply] = await Promise.all([
-    getMessage(kv, messageId),
-    getReply(kv, messageId),
-  ]);
-
-  if (!message) {
+  // Phase 2.5 Step 3.2 — D1 read flip. Security gate (composite WHERE message_id = ? AND to_btc_address = ?) in `getInboxMessageFromD1` (lib/inbox/d1-reads.ts); see #725 block-on-merge.
+  let message: InboxMessage | null;
+  let repliesMap: Map<string, OutboxReply>;
+  try {
+    const db = env.DB as D1Database;
+    message = await getInboxMessageFromD1(db, agent.btcAddress, messageId);
+    if (!message) {
+      return NextResponse.json(
+        {
+          error: "Message not found",
+          messageId,
+          hint: "Check GET /api/inbox/[address] to see available messages",
+        },
+        { status: 404 }
+      );
+    }
+    repliesMap = await fetchRepliesForMessages(db, [messageId]);
+  } catch (e) {
     return NextResponse.json(
       {
-        error: "Message not found",
-        messageId,
-        hint: "Check GET /api/inbox/[address] to see available messages",
+        error: "transient_d1_unavailable",
+        message: "Inbox database temporarily unavailable. Please retry shortly.",
+        retry_after: 5,
       },
-      { status: 404 }
+      { status: 503, headers: { "Retry-After": "5" } }
     );
   }
 
-  // Verify message belongs to this agent (compare resolved BTC address)
-  if (message.toBtcAddress !== agent.btcAddress) {
-    return NextResponse.json(
-      {
-        error: "Message does not belong to this address",
-        messageId,
-        expectedAddress: message.toBtcAddress,
-        providedAddress: address,
-      },
-      { status: 400 }
-    );
-  }
+  const reply = repliesMap.get(messageId) ?? null;
 
   // Resolve sender agent info (recipient is already `agent`)
   const senderAddr = message.senderBtcAddress || message.fromAddress;
@@ -83,7 +86,7 @@ export async function GET(
   return NextResponse.json(
     {
       message,
-      reply: reply || null,
+      reply,
       sender: senderAgent
         ? { btcAddress: senderAgent.btcAddress, stxAddress: senderAgent.stxAddress, displayName: senderAgent.displayName }
         : null,
@@ -101,6 +104,9 @@ export async function GET(
   );
 }
 
+// PATCH (mark-read) intentionally still writes to KV alongside the D1 dual-write
+// from #720. Phase 2.5 Step 4 (#730) removes the KV writes; until then, KV remains
+// authoritative for the write path so a Step 3.x rollback is bounded.
 export async function PATCH(
   request: NextRequest,
   { params }: { params: Promise<{ address: string; messageId: string }> }
diff --git a/lib/inbox/d1-reads.ts b/lib/inbox/d1-reads.ts
index 5db58cb4..3c32a549 100644
--- a/lib/inbox/d1-reads.ts
+++ b/lib/inbox/d1-reads.ts
@@ -2,6 +2,7 @@
  * D1 read helpers for GET /api/inbox/[address] and sibling routes.
  *
  * Phase 2.5 Step 3.1 — flip inbox-list GET from KV reads to D1 SELECTs.
+ * Phase 2.5 Step 3.2 — adds getInboxMessageFromD1 for the single-message GET.
  * KV writes are NOT removed here (that is Step 4).
  *
  * These helpers query the inbox_messages table that is being populated by
@@ -19,6 +20,7 @@
  *   (auth'd-branch separation / private no-store / pre-gate cache safety)
  *
  * See: https://github.com/aibtcdev/landing-page/issues/721 (Step 3.1 spec)
+ * See: https://github.com/aibtcdev/landing-page/issues/725 (Step 3.2 spec)
  * See: https://github.com/aibtcdev/landing-page/issues/697 (Phase 2.5 umbrella)
  * See: https://github.com/aibtcdev/landing-page/issues/723 (cache-invariant extraction)
  */
@@ -223,5 +225,45 @@ export async function fetchRepliesForMessages(
   return map;
 }
 
+/**
+ * Fetch a single inbound message by messageId AND btcAddress from D1.
+ *
+ * The AND clause on to_btc_address is the security gate: it prevents a caller
+ * from fetching a message that belongs to a different address (address-match
+ * guard — Step 3.2 block-on-merge per issue #725 / secret-mars v167 elevation).
+ *
+ * SQL shape:
+ *   SELECT … FROM inbox_messages
+ *   WHERE message_id = ? AND to_btc_address = ? AND is_reply = 0
+ *
+ * Returns null when:
+ *   - message_id does not exist
+ *   - message_id exists but to_btc_address does not match (→ 404, not 400/200)
+ */
+export async function getInboxMessageFromD1(
+  db: D1Database,
+  btcAddress: string,
+  messageId: string
+): Promise<InboxMessage | null> {
+  const sql = `
+    SELECT
+      message_id, from_stx_address, to_btc_address, to_stx_address,
+      content, payment_txid, payment_satoshis, payment_status,
+      payment_id, receipt_id, recovered_via_txid, authenticated,
+      bitcoin_signature, sender_btc_address,
+      sent_at, read_at, replied_at, reply_to_message_id
+    FROM inbox_messages
+    WHERE message_id = ? AND to_btc_address = ? AND is_reply = 0
+  `;
+
+  const row = await db
+    .prepare(sql)
+    .bind(messageId, btcAddress)
+    .first<D1InboxRow>();
+
+  if (!row) return null;
+  return rowToInboxMessage(row);
+}
+
 // Re-export the prefix so tests can verify synthesized IDs
 export { REPLY_D1_PK_PREFIX };