diff --git a/README.md b/README.md
index 375ef88..2e25d96 100644
--- a/README.md
+++ b/README.md
@@ -60,7 +60,8 @@ the sweep marks pending rows `manual` so nothing hangs forever.
 Use **Server keys** to add, replace, or remove a Plutonium key for an exact
 server ID such as `t5_zombie_theater_2`. IDs use a supported map slug and a
 lobby number from 1 to 20; the server does not need to be deployed first.
-During deployment, choose the map and lobby. A saved key is reused automatically,
+During deployment, choose the map. The lowest available lobby is assigned
+automatically, with the exact server ID shown before deployment. Its saved key is reused automatically,
 and any newly entered key is saved for that ID, including if deployment fails.
 Destroying a server preserves its saved key. Updating or removing a saved key
 does not change a running server; changes apply to future deployments.
diff --git a/src/lib/server/fleet/create.ts b/src/lib/server/fleet/create.ts
index ce10bed..729cabe 100644
--- a/src/lib/server/fleet/create.ts
+++ b/src/lib/server/fleet/create.ts
@@ -48,7 +48,7 @@ export interface CreateServerInput {
     gameMode: GameMode;   // 'zombie' — only value supported today
     mapSlug: string;      // 'cosmodrome'
     plutoKey?: string;    // saved for this server ID when supplied
-    lobby?: number;      // explicit stable identity; otherwise first available
+    expectedName?: string; // optional preview guard; never selects a lobby
     createdBy?: string;   // 'ui' by default
 }
 
@@ -160,15 +160,14 @@ async function createServerUnlocked(input: CreateServerInput): Promise<CreateSer
     }
 
     const baseName = `t5_${input.gameMode}_${mapEntry.slug}`;
-    if (input.lobby !== undefined && (!Number.isInteger(input.lobby) || input.lobby < 1 || input.lobby > 20)) {
-        return { ok: false, kind: 'bad-request', message: 'Lobby must be an integer from 1 to 20' };
-    }
-    const lobby = input.lobby ?? nextLobbyNumber(baseName);
+    const lobby = nextLobbyNumber(baseName);
     if (lobby === null) {
         return { ok: false, kind: 'no-slot', message: `all lobby slots (1..20) for ${baseName} are taken` };
     }
     const name = `${baseName}_${lobby}`;
-    if (getServer(name)) return { ok: false, kind: 'no-slot', message: `${name} is already deployed` };
+    if (input.expectedName !== undefined && input.expectedName !== name) {
+        return { ok: false, kind: 'no-slot', message: 'Available lobby changed. Review the updated server ID and key, then deploy again.' };
+    }
     const plutoKey = input.plutoKey?.trim() || getServerKey(name);
     if (!plutoKey) return { ok: false, kind: 'bad-request', message: `No saved key for ${name}. Enter a Plutonium server key.` };
 
diff --git a/src/lib/ui/components/DeployServerDialog.svelte b/src/lib/ui/components/DeployServerDialog.svelte
index 4957c3d..453827f 100644
--- a/src/lib/ui/components/DeployServerDialog.svelte
+++ b/src/lib/ui/components/DeployServerDialog.svelte
@@ -22,7 +22,6 @@
     let gameMode = $state<'zombie'>('zombie');
     let mapSlug = $state<string>('');
     let plutoKey = $state('');
-    let lobby = $state('1');
     let savedIds = $state<string[]>([]);
     let deployedIds = $state<string[]>([]);
     let keysLoaded = $state(false);
@@ -77,21 +76,12 @@
         if (e.target === dialog) close();
     }
 
+    let lobby = $derived(Array.from({ length: 20 }, (_, i) => i + 1)
+        .find(n => !deployedIds.includes(`t5_${gameMode}_${mapSlug}_${n}`)) ?? null);
     let selectedMap = $derived(maps.find((m) => m.slug === mapSlug));
-    let previewName = $derived(selectedMap ? `t5_${gameMode}_${selectedMap.slug}_${lobby}` : '');
+    let previewName = $derived(selectedMap && keysLoaded && lobby !== null ? `t5_${gameMode}_${selectedMap.slug}_${lobby}` : '');
 
     let hasSavedKey = $derived(savedIds.includes(previewName));
-    let alreadyDeployed = $derived(deployedIds.includes(previewName));
-    let lobbyOptions = $derived(Array.from({ length: 20 }, (_, i) => {
-        const value = String(i + 1);
-        const id = `t5_${gameMode}_${mapSlug}_${value}`;
-        return { value, label: `${value}${deployedIds.includes(id) ? ' (deployed)' : savedIds.includes(id) ? ' (saved key)' : ''}` };
-    }));
-    $effect(() => {
-        const prefix = `t5_${gameMode}_${mapSlug}_`;
-        lobby = String(Array.from({ length: 20 }, (_, i) => i + 1)
-            .find(n => !deployedIds.includes(`${prefix}${n}`)) ?? 1);
-    });
     $effect(() => { mapSlug; lobby; plutoKey = ''; });
 
     let modeOptions = [{ value: 'zombie', label: 'Zombies' }];
@@ -99,7 +89,7 @@
 
     async function submit(e: SubmitEvent): Promise<void> {
         e.preventDefault();
-        if (deploying) return;
+        if (deploying || !keysLoaded || lobby === null) return;
         if (!selectedMap) { flash = 'Pick a map'; return; }
         if (!hasSavedKey && plutoKey.trim().length === 0) { flash = 'Pluto server key is required'; return; }
         deploying = true;
@@ -108,11 +98,12 @@
             const res = await fetch('/api/servers', {
                 method: 'POST',
                 headers: { 'Content-Type': 'application/json' },
-                body: JSON.stringify({ gameMode, mapSlug: selectedMap.slug, lobby: Number(lobby), plutoKey: plutoKey.trim() || undefined })
+                body: JSON.stringify({ gameMode, mapSlug: selectedMap.slug, expectedName: previewName, plutoKey: plutoKey.trim() || undefined })
             });
             const body = (await res.json()) as { ok: boolean; name?: string; port?: number; error?: string; kind?: string };
             if (!body.ok || !body.name) {
                 flash = body.error ?? 'Deploy failed';
+                if (body.kind === 'no-slot') await loadKeys();
                 deploying = false;
                 return;
             }
@@ -150,10 +141,6 @@
                     <Select bind:value={mapSlug} options={mapOptions} disabled={deploying} />
                 </Field>
 
-                <Field label="Lobby number" hint="Choose the same lobby to reuse its server ID and saved key.">
-                    <Select bind:value={lobby} options={lobbyOptions} disabled={deploying} />
-                </Field>
-
                 <Field label="Pluto server key" hint={hasSavedKey ? 'Saved key ready. Leave blank to reuse it, or enter a replacement.' : 'Saved for this server ID and reused on future deployments.'}>
                     <Input
                         type="password"
@@ -168,7 +155,7 @@
             <div class="preview">
                 <span class="preview-k">Will create</span>
                 <span class="preview-v">{previewName || '—'}</span>
-                <span class="preview-sub">{alreadyDeployed ? "Already deployed; choose another lobby" : "Port auto-assigned"}</span>
+                <span class="preview-sub">{!keysLoaded ? "Loading available lobby..." : lobby === null ? "All lobby slots for this map are in use" : "Lowest available lobby; port auto-assigned"}</span>
             </div>
 
             <a href="/server-keys" onclick={close}>Manage saved keys</a>
@@ -177,7 +164,7 @@
                 <Button
                     type="submit"
                     variant="primary"
-                    disabled={deploying || !selectedMap || !keysLoaded || alreadyDeployed}
+                    disabled={deploying || !selectedMap || !keysLoaded || lobby === null}
                     loading={deploying}
                 >
                     Deploy server
@@ -251,7 +238,7 @@
         gap: var(--space-4);
     }
     .fields :global(.field):nth-child(1),
-    .fields :global(.field):nth-child(5) {
+    .fields :global(.field):nth-child(4) {
         grid-column: 1 / -1;
     }
 
diff --git a/src/routes/api/servers/+server.ts b/src/routes/api/servers/+server.ts
index b3bd6b9..4f199f5 100644
--- a/src/routes/api/servers/+server.ts
+++ b/src/routes/api/servers/+server.ts
@@ -43,11 +43,11 @@ export async function POST({ request }: { request: Request }) {
         gameMode?: string;
         mapSlug?: string;
         plutoKey?: string;
-        lobby?: number;
+        expectedName?: string;
     };
-    if (!body || typeof body.gameMode !== 'string' || typeof body.mapSlug !== 'string' || (body.plutoKey !== undefined && typeof body.plutoKey !== 'string') || (body.lobby !== undefined && typeof body.lobby !== 'number')) {
+    if (!body || typeof body.gameMode !== 'string' || typeof body.mapSlug !== 'string' || (body.plutoKey !== undefined && typeof body.plutoKey !== 'string') || (body.expectedName !== undefined && typeof body.expectedName !== 'string')) {
         return json(
-            { ok: false, error: 'gameMode and mapSlug are required strings; plutoKey and lobby are optional', kind: 'bad-request' },
+            { ok: false, error: 'gameMode and mapSlug are required strings; plutoKey and expectedName are optional strings', kind: 'bad-request' },
             { status: 400 }
         );
     }
@@ -55,7 +55,7 @@ export async function POST({ request }: { request: Request }) {
         gameMode: body.gameMode as GameMode,
         mapSlug: body.mapSlug,
         plutoKey: body.plutoKey,
-        lobby: body.lobby,
+        expectedName: body.expectedName,
         createdBy: 'ui'
     });
     if (!result.ok) {
diff --git a/tests/unit/create-server.test.ts b/tests/unit/create-server.test.ts
index 80cf42d..ec9adf2 100644
--- a/tests/unit/create-server.test.ts
+++ b/tests/unit/create-server.test.ts
@@ -127,15 +127,16 @@ describe('persistent server keys', () => {
         const { komodoCreateDeployment, komodoInspectHealthy } = await import('../../src/lib/server/integrations/komodo.js');
         vi.mocked(komodoInspectHealthy).mockResolvedValue({ ok: false, kind: 'not-found', message: 'gone' } as never);
         const name = 't5_zombie_cosmodrome_2';
-        expect(await createServer({ ...input, lobby: 2 })).toMatchObject({ ok: true, name });
+        expect(await createServer(input)).toMatchObject({ ok: true, name: serverName });
+        expect(await createServer(input)).toMatchObject({ ok: true, name });
         expect(getServerKey(name)).toBe('test-key');
         expect(await destroyServer(name)).toMatchObject({ ok: true });
-        expect(listServerKeys()).toEqual([expect.objectContaining({ server_id: name, deployed: 0 })]);
+        expect(listServerKeys()).toContainEqual(expect.objectContaining({ server_id: name, deployed: 0 }));
         expect(JSON.stringify(listServerKeys())).not.toContain('test-key');
-        expect(await createServer({ gameMode: 'zombie', mapSlug: 'cosmodrome', lobby: 2 })).toMatchObject({ ok: true, name });
+        expect(await createServer({ gameMode: 'zombie', mapSlug: 'cosmodrome', expectedName: name })).toMatchObject({ ok: true, name });
         const spec = vi.mocked(komodoCreateDeployment).mock.calls.at(-1)![0];
         expect(spec.environment).toContainEqual({ variable: 'PLUTO_SERVER_KEY', value: 'test-key' });
-        expect(await createServer({ gameMode: 'zombie', mapSlug: 'cosmodrome', lobby: 1 })).toMatchObject({ ok: false, kind: 'bad-request' });
+        expect(await createServer({ gameMode: 'zombie', mapSlug: 'cosmodrome' })).toMatchObject({ ok: false, kind: 'bad-request' });
     });
 
     it('updates and removes keys independently of deployments and survives reopening the database', async () => {
@@ -153,15 +154,19 @@ describe('persistent server keys', () => {
         expect(() => saveServerKey(serverName, 'two words')).toThrow();
     });
 
-    it('rejects invalid or occupied lobbies before external operations', async () => {
-        const { createServer } = await import('../../src/lib/server/fleet/create.js');
-        const { komodoCreateDeployment } = await import('../../src/lib/server/integrations/komodo.js');
-        for (const lobby of [0, 21, 1.5]) {
-            expect(await createServer({ ...input, lobby })).toMatchObject({ ok: false, kind: 'bad-request' });
-        }
-        expect(komodoCreateDeployment).not.toHaveBeenCalled();
-        expect(await createServer({ ...input, lobby: 2 })).toMatchObject({ ok: true });
-        expect(await createServer({ ...input, lobby: 2 })).toMatchObject({ ok: false, kind: 'no-slot' });
-        expect(komodoCreateDeployment).toHaveBeenCalledTimes(1);
+    it('fills the lowest gap and rejects stale previews before saving a key or deploying', async () => {
+        const { createServer, destroyServer } = await import('../../src/lib/server/fleet/create.js');
+        const { getServerKey } = await import('../../src/lib/server/fleet/server-keys.js');
+        const { komodoCreateDeployment, komodoInspectHealthy } = await import('../../src/lib/server/integrations/komodo.js');
+        vi.mocked(komodoInspectHealthy).mockResolvedValue({ ok: false } as never);
+        expect(await createServer(input)).toMatchObject({ ok: true, name: serverName });
+        expect(await createServer(input)).toMatchObject({ ok: true, name: 't5_zombie_cosmodrome_2' });
+        await destroyServer(serverName);
+        expect(await createServer({ ...input, plutoKey: 'wrong-key', expectedName: 't5_zombie_cosmodrome_3' }))
+            .toMatchObject({ ok: false, kind: 'no-slot' });
+        expect(getServerKey(serverName)).toBe('test-key');
+        expect(komodoCreateDeployment).toHaveBeenCalledTimes(2);
+        expect(await createServer({ gameMode: 'zombie', mapSlug: 'cosmodrome', expectedName: serverName }))
+            .toMatchObject({ ok: true, name: serverName });
     });
 });