diff --git a/tools/ui/src/lib/services/migration.service.ts b/tools/ui/src/lib/services/migration.service.ts
index 152b78cd3..d7709bc6b 100644
--- a/tools/ui/src/lib/services/migration.service.ts
+++ b/tools/ui/src/lib/services/migration.service.ts
@@ -551,13 +551,49 @@ const mcpDefaultEnabledMigration: Migration = {
 	}
 };
 
+const CONFIG_TYPES_MIGRATION_ID = 'config-type-normalization-v1';
+
+const configTypesMigration: Migration = {
+	id: CONFIG_TYPES_MIGRATION_ID,
+	description: 'Coerce legacy string-encoded booleans in persisted config to real booleans',
+
+	async run(): Promise<void> {
+		const configRaw = localStorage.getItem(CONFIG_LOCALSTORAGE_KEY);
+		if (configRaw === null) return;
+
+		const config = JSON.parse(configRaw);
+		let changed = false;
+
+		// Pre-schema configs persisted booleans as the strings "true"/"false", which the
+		// strict server schema now rejects. Coerce those back to real booleans. No config
+		// string field holds exactly "true"/"false", so the match is unambiguous.
+		for (const key of Object.keys(config)) {
+			if (config[key] === 'true') {
+				config[key] = true;
+				changed = true;
+			} else if (config[key] === 'false') {
+				config[key] = false;
+				changed = true;
+			}
+		}
+
+		if (changed) {
+			localStorage.setItem(CONFIG_LOCALSTORAGE_KEY, JSON.stringify(config));
+		}
+
+		if (import.meta.env.DEV && import.meta.env.VITE_DEBUG)
+			console.log(`[Migration] Config types: coerced string booleans (changed=${changed})`);
+	}
+};
+
 const migrations: Migration[] = [
 	localStorageMigration,
 	idxdbMigration,
 	legacyMessageMigration,
 	themeMigration,
 	customJsonKeyMigration,
-	mcpDefaultEnabledMigration
+	mcpDefaultEnabledMigration,
+	configTypesMigration
 ];
 
 export const MigrationService = {
diff --git a/tools/ui/src/lib/stores/conversations.svelte.ts b/tools/ui/src/lib/stores/conversations.svelte.ts
index ef8d61309..486202207 100644
--- a/tools/ui/src/lib/stores/conversations.svelte.ts
+++ b/tools/ui/src/lib/stores/conversations.svelte.ts
@@ -114,14 +114,13 @@ class ConversationsStore {
 
 	/** Load thinking-enabled default from localStorage */
 	private static loadThinkingDefaults(): boolean {
-		if (typeof globalThis.localStorage === 'undefined') return false;
+		if (typeof globalThis.localStorage === 'undefined') return true;
 		try {
 			const raw = localStorage.getItem(THINKING_ENABLED_DEFAULT_LOCALSTORAGE_KEY);
-			if (!raw) return false;
-			const parsed = raw === 'true';
-			return typeof parsed === 'boolean' ? parsed : false;
+			if (!raw) return true;
+			return raw === 'true';
 		} catch {
-			return false;
+			return true;
 		}
 	}
 
@@ -333,7 +332,7 @@ class ConversationsStore {
 			}
 
 			this.pendingMcpServerOverrides = [];
-			this.pendingThinkingEnabled = false;
+			this.pendingThinkingEnabled = ConversationsStore.loadThinkingDefaults();
 			this.activeConversation = conversation;
 
 			if (conversation.currNode) {