diff --git a/CHANGELOG.md b/CHANGELOG.md
index 294c64aaf..9cb6155ff 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -36,6 +36,7 @@
 - Config: add `channels.<provider>.configWrites` gating for channel-initiated config writes; migrate Slack channel IDs.
 
 ### Fixes
+
 #### Agents
 - Agents: make user time zone and 24-hour time explicit in the system prompt. (#859) — thanks @CashWilliams.
 - Agents: strip downgraded tool call text without eating adjacent replies and filter thinking-tag leaks. (#905) — thanks @erikpr1994.
@@ -72,6 +73,7 @@
 - WhatsApp: normalize user JIDs with device suffix for allowlist checks in groups. (#838) — thanks @peschee.
 
 #### Gateway / Daemon / Sessions
+- Gateway/UI: ship session defaults in the hello snapshot so the Control UI canonicalizes main session keys (no bare `main` alias).
 - Gateway: forward termination signals to respawned CLI child processes to avoid orphaned systemd runs. (#933) — thanks @roshanasingh4.
 - Sessions: return deep clones (`structuredClone`) so cached session entries can't be mutated. (#934) — thanks @ronak-guliani.
 - Heartbeat: keep `updatedAt` monotonic when restoring heartbeat sessions. (#934) — thanks @ronak-guliani.
diff --git a/src/gateway/protocol/schema/snapshot.ts b/src/gateway/protocol/schema/snapshot.ts
index 3d624e125..efd899eca 100644
--- a/src/gateway/protocol/schema/snapshot.ts
+++ b/src/gateway/protocol/schema/snapshot.ts
@@ -22,6 +22,16 @@ export const PresenceEntrySchema = Type.Object(
 
 export const HealthSnapshotSchema = Type.Any();
 
+export const SessionDefaultsSchema = Type.Object(
+  {
+    defaultAgentId: NonEmptyString,
+    mainKey: NonEmptyString,
+    mainSessionKey: NonEmptyString,
+    scope: Type.Optional(NonEmptyString),
+  },
+  { additionalProperties: false },
+);
+
 export const StateVersionSchema = Type.Object(
   {
     presence: Type.Integer({ minimum: 0 }),
@@ -38,6 +48,7 @@ export const SnapshotSchema = Type.Object(
     uptimeMs: Type.Integer({ minimum: 0 }),
     configPath: Type.Optional(NonEmptyString),
     stateDir: Type.Optional(NonEmptyString),
+    sessionDefaults: Type.Optional(SessionDefaultsSchema),
   },
   { additionalProperties: false },
 );
diff --git a/src/gateway/server/health-state.ts b/src/gateway/server/health-state.ts
index a6d0c8774..62ff24616 100644
--- a/src/gateway/server/health-state.ts
+++ b/src/gateway/server/health-state.ts
@@ -1,5 +1,8 @@
+import { resolveDefaultAgentId } from "../../agents/agent-scope.js";
 import { getHealthSnapshot, type HealthSummary } from "../../commands/health.js";
-import { CONFIG_PATH_CLAWDBOT, STATE_DIR_CLAWDBOT } from "../../config/config.js";
+import { CONFIG_PATH_CLAWDBOT, STATE_DIR_CLAWDBOT, loadConfig } from "../../config/config.js";
+import { resolveMainSessionKey } from "../../config/sessions.js";
+import { normalizeMainKey } from "../../routing/session-key.js";
 import { listSystemPresence } from "../../infra/system-presence.js";
 import type { Snapshot } from "../protocol/index.js";
 
@@ -10,6 +13,11 @@ let healthRefresh: Promise<HealthSummary> | null = null;
 let broadcastHealthUpdate: ((snap: HealthSummary) => void) | null = null;
 
 export function buildGatewaySnapshot(): Snapshot {
+  const cfg = loadConfig();
+  const defaultAgentId = resolveDefaultAgentId(cfg);
+  const mainKey = normalizeMainKey(cfg.session?.mainKey);
+  const mainSessionKey = resolveMainSessionKey(cfg);
+  const scope = cfg.session?.scope ?? "per-sender";
   const presence = listSystemPresence();
   const uptimeMs = Math.round(process.uptime() * 1000);
   // Health is async; caller should await getHealthSnapshot and replace later if needed.
@@ -22,6 +30,12 @@ export function buildGatewaySnapshot(): Snapshot {
     // Surface resolved paths so UIs can display the true config location.
     configPath: CONFIG_PATH_CLAWDBOT,
     stateDir: STATE_DIR_CLAWDBOT,
+    sessionDefaults: {
+      defaultAgentId,
+      mainKey,
+      mainSessionKey,
+      scope,
+    },
   };
 }
 
diff --git a/ui/src/ui/app-gateway.ts b/ui/src/ui/app-gateway.ts
index 076275793..61c9af086 100644
--- a/ui/src/ui/app-gateway.ts
+++ b/ui/src/ui/app-gateway.ts
@@ -5,14 +5,20 @@ import { GatewayBrowserClient } from "./gateway";
 import type { EventLogEntry } from "./app-events";
 import type { PresenceEntry, HealthSnapshot, StatusSummary } from "./types";
 import type { Tab } from "./navigation";
+import type { UiSettings } from "./storage";
 import { handleAgentEvent, resetToolStream, type AgentEventPayload } from "./app-tool-stream";
 import { flushChatQueueForEvent } from "./app-chat";
-import { loadCron, refreshActiveTab, setLastActiveSessionKey } from "./app-settings";
+import {
+  applySettings,
+  loadCron,
+  refreshActiveTab,
+  setLastActiveSessionKey,
+} from "./app-settings";
 import { handleChatEvent, type ChatEventPayload } from "./controllers/chat";
 import type { ClawdbotApp } from "./app";
 
 type GatewayHost = {
-  settings: { gatewayUrl: string; token: string };
+  settings: UiSettings;
   password: string;
   client: GatewayBrowserClient | null;
   connected: boolean;
@@ -29,6 +35,60 @@ type GatewayHost = {
   chatRunId: string | null;
 };
 
+type SessionDefaultsSnapshot = {
+  defaultAgentId?: string;
+  mainKey?: string;
+  mainSessionKey?: string;
+  scope?: string;
+};
+
+function normalizeSessionKeyForDefaults(
+  value: string | undefined,
+  defaults: SessionDefaultsSnapshot,
+): string {
+  const raw = (value ?? "").trim();
+  const mainSessionKey = defaults.mainSessionKey?.trim();
+  if (!mainSessionKey) return raw;
+  if (!raw) return mainSessionKey;
+  const mainKey = defaults.mainKey?.trim() || "main";
+  const defaultAgentId = defaults.defaultAgentId?.trim();
+  const isAlias =
+    raw === "main" ||
+    raw === mainKey ||
+    (defaultAgentId &&
+      (raw === `agent:${defaultAgentId}:main` ||
+        raw === `agent:${defaultAgentId}:${mainKey}`));
+  return isAlias ? mainSessionKey : raw;
+}
+
+function applySessionDefaults(host: GatewayHost, defaults?: SessionDefaultsSnapshot) {
+  if (!defaults?.mainSessionKey) return;
+  const resolvedSessionKey = normalizeSessionKeyForDefaults(host.sessionKey, defaults);
+  const resolvedSettingsSessionKey = normalizeSessionKeyForDefaults(
+    host.settings.sessionKey,
+    defaults,
+  );
+  const resolvedLastActiveSessionKey = normalizeSessionKeyForDefaults(
+    host.settings.lastActiveSessionKey,
+    defaults,
+  );
+  const nextSessionKey = resolvedSessionKey || resolvedSettingsSessionKey || host.sessionKey;
+  const nextSettings = {
+    ...host.settings,
+    sessionKey: resolvedSettingsSessionKey || nextSessionKey,
+    lastActiveSessionKey: resolvedLastActiveSessionKey || nextSessionKey,
+  };
+  const shouldUpdateSettings =
+    nextSettings.sessionKey !== host.settings.sessionKey ||
+    nextSettings.lastActiveSessionKey !== host.settings.lastActiveSessionKey;
+  if (nextSessionKey !== host.sessionKey) {
+    host.sessionKey = nextSessionKey;
+  }
+  if (shouldUpdateSettings) {
+    applySettings(host as unknown as Parameters<typeof applySettings>[0], nextSettings);
+  }
+}
+
 export function connectGateway(host: GatewayHost) {
   host.lastError = null;
   host.hello = null;
@@ -113,7 +173,11 @@ export function handleGatewayEvent(host: GatewayHost, evt: GatewayEventFrame) {
 
 export function applySnapshot(host: GatewayHost, hello: GatewayHelloOk) {
   const snapshot = hello.snapshot as
-    | { presence?: PresenceEntry[]; health?: HealthSnapshot }
+    | {
+        presence?: PresenceEntry[];
+        health?: HealthSnapshot;
+        sessionDefaults?: SessionDefaultsSnapshot;
+      }
     | undefined;
   if (snapshot?.presence && Array.isArray(snapshot.presence)) {
     host.presenceEntries = snapshot.presence;
@@ -121,4 +185,7 @@ export function applySnapshot(host: GatewayHost, hello: GatewayHelloOk) {
   if (snapshot?.health) {
     host.debugHealth = snapshot.health;
   }
+  if (snapshot?.sessionDefaults) {
+    applySessionDefaults(host, snapshot.sessionDefaults);
+  }
 }