diff --git a/src/backend/tests/utils/ssh-algorithms.test.ts b/src/backend/tests/utils/ssh-algorithms.test.ts
new file mode 100644
index 000000000..a6c1561bd
--- /dev/null
+++ b/src/backend/tests/utils/ssh-algorithms.test.ts
@@ -0,0 +1,137 @@
+import { generateKeyPairSync } from "node:crypto";
+import net from "node:net";
+import { Client, Server } from "ssh2";
+import { describe, expect, it } from "vitest";
+import { buildSSHAlgorithms } from "../../utils/ssh-algorithms.js";
+
+// An SSH peer advertising only DH group exchange. Stop after the client's
+// plaintext GEX request, before expensive prime validation or authentication.
+async function requestedGroupBits(): Promise<number> {
+  const name = (value: string) => {
+    const data = Buffer.from(value);
+    const size = Buffer.alloc(4);
+    size.writeUInt32BE(data.length);
+    return Buffer.concat([size, data]);
+  };
+  const payload = Buffer.concat([
+    Buffer.from([20]),
+    Buffer.alloc(16),
+    ...[
+      "diffie-hellman-group-exchange-sha256",
+      "rsa-sha2-256",
+      "aes256-gcm@openssh.com,aes128-gcm@openssh.com",
+      "aes256-gcm@openssh.com,aes128-gcm@openssh.com",
+      "hmac-sha2-256",
+      "hmac-sha2-256",
+      "none",
+      "none",
+      "",
+      "",
+    ].map(name),
+    Buffer.alloc(5),
+  ]);
+  const padding = 8 - ((payload.length + 5) % 8) + 8;
+  const header = Buffer.alloc(5);
+  header.writeUInt32BE(payload.length + padding + 1);
+  header[4] = padding;
+  const client = new Client();
+  let peer: net.Socket | undefined;
+  let timer: ReturnType<typeof setTimeout> | undefined;
+  const server = net.createServer();
+  try {
+    const result = new Promise<number>((resolve, reject) => {
+      timer = setTimeout(() => reject(new Error("No SSH group request")), 3000);
+      client.on("error", reject);
+      server.on("connection", (socket) => {
+        peer = socket;
+        let buffer = Buffer.alloc(0);
+        let banner = false;
+        socket.write(
+          Buffer.concat([
+            Buffer.from("SSH-2.0-OpenSSH_7.4\r\n"),
+            header,
+            payload,
+            Buffer.alloc(padding),
+          ]),
+        );
+        socket.on("data", (chunk) => {
+          buffer = Buffer.concat([buffer, chunk]);
+          if (!banner) {
+            const end = buffer.indexOf("\n");
+            if (end < 0) return;
+            buffer = buffer.subarray(end + 1);
+            banner = true;
+          }
+          while (buffer.length >= 5) {
+            const length = buffer.readUInt32BE(0) + 4;
+            if (buffer.length < length) return;
+            if (buffer[5] === 34) resolve(buffer.readUInt32BE(10));
+            buffer = buffer.subarray(length);
+          }
+        });
+      });
+    });
+    await new Promise<void>((resolve) =>
+      server.listen(0, "127.0.0.1", resolve),
+    );
+    client.connect({
+      host: "127.0.0.1",
+      port: (server.address() as net.AddressInfo).port,
+      username: "test",
+      algorithms: buildSSHAlgorithms(false),
+    });
+    return await result;
+  } finally {
+    clearTimeout(timer);
+    client.destroy();
+    peer?.destroy();
+    await new Promise<void>((resolve) => server.close(() => resolve()));
+  }
+}
+
+describe("SSH cipher preference", () => {
+  it("requests a 3072-bit group from a GEX-only peer offering both GCM ciphers", async () => {
+    expect(await requestedGroupBits()).toBe(3072);
+  });
+
+  it("still connects when a server only supports AES-256-GCM", async () => {
+    const { privateKey } = generateKeyPairSync("rsa", { modulusLength: 2048 });
+    const server = new Server(
+      {
+        hostKeys: [privateKey.export({ type: "pkcs1", format: "pem" })],
+        algorithms: { cipher: ["aes256-gcm@openssh.com"] },
+      },
+      (peer) => {
+        peer.on("authentication", (auth) => auth.accept());
+      },
+    );
+    const client = new Client();
+    try {
+      await new Promise<void>((resolve) =>
+        server.listen(0, "127.0.0.1", resolve),
+      );
+      await new Promise<void>((resolve, reject) => {
+        client.once("ready", resolve).once("error", reject);
+        client.connect({
+          host: "127.0.0.1",
+          port: (server.address() as net.AddressInfo).port,
+          username: "test",
+          readyTimeout: 2000,
+          algorithms: buildSSHAlgorithms(false),
+        });
+      });
+    } finally {
+      client.destroy();
+      await new Promise<void>((resolve) => server.close(() => resolve()));
+    }
+  });
+
+  it("keeps legacy algorithms opt-in", () => {
+    expect(buildSSHAlgorithms(false).kex).not.toContain(
+      "diffie-hellman-group-exchange-sha1",
+    );
+    expect(buildSSHAlgorithms(true).kex).toContain(
+      "diffie-hellman-group-exchange-sha1",
+    );
+  });
+});
diff --git a/src/backend/utils/ssh-algorithms.ts b/src/backend/utils/ssh-algorithms.ts
index 2e5d6a8f2..a88b0ba64 100644
--- a/src/backend/utils/ssh-algorithms.ts
+++ b/src/backend/utils/ssh-algorithms.ts
@@ -97,8 +97,10 @@ export function buildSSHAlgorithms(
   ];
   const cipher = filterCiphers([
     "chacha20-poly1305@openssh.com",
-    "aes256-gcm@openssh.com",
+    // ssh2 sizes DH group exchange from the cipher key length. Prefer
+    // AES-128-GCM to avoid 8192-bit prime validation on GEX-only hosts.
     "aes128-gcm@openssh.com",
+    "aes256-gcm@openssh.com",
     "aes256-ctr",
     "aes192-ctr",
     "aes128-ctr",