diff --git a/deploy/fly/Dockerfile b/deploy/fly/Dockerfile
index 89257f93..22cf0aa2 100644
--- a/deploy/fly/Dockerfile
+++ b/deploy/fly/Dockerfile
@@ -26,6 +26,7 @@ RUN printf '{"name":"agentmemory-deploy","version":"1.0.0","private":true,"overr
  && ln -s /opt/agentmemory/node_modules/.bin/agentmemory /usr/local/bin/agentmemory
 
 ENV AGENTMEMORY_III_VERSION=${III_VERSION} \
+    AGENTMEMORY_VIEWER_HOST=:: \
     TINI_SUBREAPER=1
 
 COPY --chmod=0755 entrypoint.sh /usr/local/bin/agentmemory-entrypoint.sh
diff --git a/deploy/fly/README.md b/deploy/fly/README.md
index 020d9751..9f6194d0 100644
--- a/deploy/fly/README.md
+++ b/deploy/fly/README.md
@@ -75,6 +75,11 @@ fly proxy 3113:3113 --app "$APP"
 viewer's bearer token still has to ride a loopback connection on your
 laptop — the v0.9.12 plaintext-bearer guard stays satisfied.
 
+The deploy image sets `AGENTMEMORY_VIEWER_HOST=::` so the viewer listens
+on the machine's `fly-local-6pn` WireGuard interface as well as loopback.
+Without that override the viewer would bind to `127.0.0.1` only and
+`fly proxy` would reset the connection.
+
 ## Rotate the HMAC secret
 
 ```bash
diff --git a/src/viewer/server.ts b/src/viewer/server.ts
index 533d4bb4..9674d143 100644
--- a/src/viewer/server.ts
+++ b/src/viewer/server.ts
@@ -13,6 +13,10 @@ const ALLOWED_ORIGINS = (
   .split(",")
   .map((o) => o.trim());
 
+export function resolveViewerHost(): string {
+  return process.env.AGENTMEMORY_VIEWER_HOST?.trim() || "127.0.0.1";
+}
+
 function corsHeaders(req: IncomingMessage): Record<string, string> {
   const origin = req.headers.origin || "";
   const allowed = ALLOWED_ORIGINS.includes(origin)
@@ -117,9 +121,10 @@ export function startViewerServer(
 
   let attempt = 0;
   let currentPort = requestedPort;
+  const host = resolveViewerHost();
 
   const tryListen = (): void => {
-    server.listen(currentPort, "127.0.0.1");
+    server.listen(currentPort, host);
   };
 
   server.on("listening", () => {
diff --git a/test/viewer-host.test.ts b/test/viewer-host.test.ts
new file mode 100644
index 00000000..0a1fad38
--- /dev/null
+++ b/test/viewer-host.test.ts
@@ -0,0 +1,80 @@
+import { describe, it, expect, beforeEach, afterEach, vi } from "vitest";
+import type { Server } from "node:http";
+import type { AddressInfo } from "node:net";
+import { resolveViewerHost, startViewerServer } from "../src/viewer/server.js";
+
+describe("resolveViewerHost", () => {
+  const originalEnv = process.env.AGENTMEMORY_VIEWER_HOST;
+
+  afterEach(() => {
+    if (originalEnv === undefined) {
+      delete process.env.AGENTMEMORY_VIEWER_HOST;
+    } else {
+      process.env.AGENTMEMORY_VIEWER_HOST = originalEnv;
+    }
+  });
+
+  it("defaults to 127.0.0.1 when AGENTMEMORY_VIEWER_HOST is unset", () => {
+    delete process.env.AGENTMEMORY_VIEWER_HOST;
+    expect(resolveViewerHost()).toBe("127.0.0.1");
+  });
+
+  it("defaults to 127.0.0.1 when AGENTMEMORY_VIEWER_HOST is empty", () => {
+    process.env.AGENTMEMORY_VIEWER_HOST = "";
+    expect(resolveViewerHost()).toBe("127.0.0.1");
+  });
+
+  it("returns the configured value when AGENTMEMORY_VIEWER_HOST is set", () => {
+    process.env.AGENTMEMORY_VIEWER_HOST = "::";
+    expect(resolveViewerHost()).toBe("::");
+  });
+
+  it("trims surrounding whitespace", () => {
+    process.env.AGENTMEMORY_VIEWER_HOST = "  ::1  ";
+    expect(resolveViewerHost()).toBe("::1");
+  });
+});
+
+describe("startViewerServer host binding", () => {
+  const originalEnv = process.env.AGENTMEMORY_VIEWER_HOST;
+  let server: Server | undefined;
+  let logSpy: ReturnType<typeof vi.spyOn>;
+
+  beforeEach(() => {
+    logSpy = vi.spyOn(console, "log").mockImplementation(() => {});
+  });
+
+  afterEach(async () => {
+    if (server) {
+      await new Promise<void>((resolve) => server!.close(() => resolve()));
+      server = undefined;
+    }
+    logSpy.mockRestore();
+    if (originalEnv === undefined) {
+      delete process.env.AGENTMEMORY_VIEWER_HOST;
+    } else {
+      process.env.AGENTMEMORY_VIEWER_HOST = originalEnv;
+    }
+  });
+
+  async function waitForListening(s: Server): Promise<void> {
+    if (s.listening) return;
+    await new Promise<void>((resolve) => s.once("listening", () => resolve()));
+  }
+
+  it("binds to 127.0.0.1 by default — preserves loopback-only security", async () => {
+    delete process.env.AGENTMEMORY_VIEWER_HOST;
+    server = startViewerServer(0, null, null);
+    await waitForListening(server);
+    const addr = server.address() as AddressInfo;
+    expect(addr.address).toBe("127.0.0.1");
+  });
+
+  it("binds to AGENTMEMORY_VIEWER_HOST when set — covers the deploy/fly fix for #434", async () => {
+    process.env.AGENTMEMORY_VIEWER_HOST = "::1";
+    server = startViewerServer(0, null, null);
+    await waitForListening(server);
+    const addr = server.address() as AddressInfo;
+    expect(addr.address).toBe("::1");
+  });
+});