diff --git a/tools/ui/eslint.config.js b/tools/ui/eslint.config.js
index 6b3b1b5c04..b90376b6c1 100644
--- a/tools/ui/eslint.config.js
+++ b/tools/ui/eslint.config.js
@@ -46,7 +46,14 @@ export default ts.config(
 	},
 	{
 		// Exclude generated build output and Storybook files from ESLint
-		ignores: ['dist/**', 'build/**', '.svelte-kit/**', 'test-results/**', '.storybook/**/*']
+		ignores: [
+			'dist/**',
+			'build/**',
+			'.svelte-kit/**',
+			'test-results/**',
+			'.storybook/**/*',
+			'src/lib/services/sandbox-worker.js'
+		]
 	},
 	storybook.configs['flat/recommended']
 );
diff --git a/tools/ui/src/lib/constants/index.ts b/tools/ui/src/lib/constants/index.ts
index c4334132c9..9ab864cd0e 100644
--- a/tools/ui/src/lib/constants/index.ts
+++ b/tools/ui/src/lib/constants/index.ts
@@ -37,6 +37,7 @@ export * from './model-id';
 export * from './precision';
 export * from './processing-info';
 export * from './routes';
+export * from './sandbox';
 export * from './settings-keys';
 export * from './settings-registry';
 export * from './supported-file-types';
diff --git a/tools/ui/src/lib/constants/sandbox.ts b/tools/ui/src/lib/constants/sandbox.ts
new file mode 100644
index 0000000000..30e769509e
--- /dev/null
+++ b/tools/ui/src/lib/constants/sandbox.ts
@@ -0,0 +1,39 @@
+import { JsonSchemaType, ToolCallType } from '$lib/enums';
+import type { OpenAIToolDefinition } from '$lib/types';
+
+export const SANDBOX_TOOL_NAME = 'run_javascript';
+
+export const SANDBOX_TIMEOUT_MS_DEFAULT = 10000;
+
+export const SANDBOX_TIMEOUT_MS_MAX = 30000;
+
+export const SANDBOX_OUTPUT_MAX_CHARS = 8192;
+
+export const SANDBOX_EMPTY_OUTPUT = '(no output)';
+
+export const SANDBOX_TRUNCATION_NOTICE = '[output truncated]';
+
+export const SANDBOX_TOOL_DEFINITION: OpenAIToolDefinition = {
+	type: ToolCallType.FUNCTION,
+	function: {
+		name: SANDBOX_TOOL_NAME,
+		description:
+			'Execute JavaScript in a sandboxed browser worker (no DOM, no page access). ' +
+			'Top level await is supported. Use console.log to print intermediate values; ' +
+			'a top level return statement is captured as the result.',
+		parameters: {
+			type: JsonSchemaType.OBJECT,
+			properties: {
+				code: {
+					type: JsonSchemaType.STRING,
+					description: 'JavaScript source to execute'
+				},
+				timeout_ms: {
+					type: JsonSchemaType.NUMBER,
+					description: `Execution timeout in milliseconds, default ${SANDBOX_TIMEOUT_MS_DEFAULT}, max ${SANDBOX_TIMEOUT_MS_MAX}`
+				}
+			},
+			required: ['code']
+		}
+	}
+};
diff --git a/tools/ui/src/lib/constants/settings-keys.ts b/tools/ui/src/lib/constants/settings-keys.ts
index 5fff9f94c2..3e19166dfd 100644
--- a/tools/ui/src/lib/constants/settings-keys.ts
+++ b/tools/ui/src/lib/constants/settings-keys.ts
@@ -69,6 +69,7 @@ export const SETTINGS_KEYS = {
 	ENABLE_THINKING: 'enableThinking',
 	SHOW_RAW_OUTPUT_SWITCH: 'showRawOutputSwitch',
 	// PY_INTERPRETER_ENABLED: 'pyInterpreterEnabled',
+	JS_SANDBOX_ENABLED: 'jsSandboxEnabled',
 	CUSTOM_JSON: 'customJson',
 	CUSTOM_CSS: 'customCss'
 } as const;
diff --git a/tools/ui/src/lib/constants/settings-registry.ts b/tools/ui/src/lib/constants/settings-registry.ts
index 9246b97033..910e5c2df0 100644
--- a/tools/ui/src/lib/constants/settings-registry.ts
+++ b/tools/ui/src/lib/constants/settings-registry.ts
@@ -690,6 +690,14 @@ const SETTINGS_REGISTRY: Record<string, SettingsSectionEntry> = {
 					paramType: SyncableParameterType.BOOLEAN
 				}
 			},
+			{
+				key: SETTINGS_KEYS.JS_SANDBOX_ENABLED,
+				label: 'JavaScript sandbox tool',
+				help: 'Expose a run_javascript tool to the model. Code runs in a Web Worker inside a sandboxed iframe with an opaque origin, isolated from the WebUI and its API, with a hard timeout.',
+				defaultValue: false,
+				type: SettingsFieldType.CHECKBOX,
+				section: SETTINGS_SECTION_SLUGS.DEVELOPER
+			},
 			{
 				key: SETTINGS_KEYS.CUSTOM_JSON,
 				label: 'Custom JSON',
diff --git a/tools/ui/src/lib/constants/tools.ts b/tools/ui/src/lib/constants/tools.ts
index efc3476cd7..4d9385f9f5 100644
--- a/tools/ui/src/lib/constants/tools.ts
+++ b/tools/ui/src/lib/constants/tools.ts
@@ -2,10 +2,12 @@ import { ToolSource } from '$lib/enums/tools.enums';
 
 export const TOOL_GROUP_LABELS = {
 	[ToolSource.BUILTIN]: 'Built-in',
-	[ToolSource.CUSTOM]: 'JSON Schema'
+	[ToolSource.CUSTOM]: 'JSON Schema',
+	[ToolSource.FRONTEND]: 'Browser'
 } as const;
 
 export const TOOL_SERVER_LABELS = {
 	[ToolSource.BUILTIN]: 'Built-in Tools',
-	[ToolSource.CUSTOM]: 'Custom Tools'
+	[ToolSource.CUSTOM]: 'Custom Tools',
+	[ToolSource.FRONTEND]: 'Browser Tools'
 } as const;
diff --git a/tools/ui/src/lib/enums/mcp.enums.ts b/tools/ui/src/lib/enums/mcp.enums.ts
index d2c27e1a0c..3d9a2070dc 100644
--- a/tools/ui/src/lib/enums/mcp.enums.ts
+++ b/tools/ui/src/lib/enums/mcp.enums.ts
@@ -54,7 +54,9 @@ export enum MCPContentType {
  * JSON Schema types used in MCP tool definitions
  */
 export enum JsonSchemaType {
-	OBJECT = 'object'
+	OBJECT = 'object',
+	STRING = 'string',
+	NUMBER = 'number'
 }
 
 /**
diff --git a/tools/ui/src/lib/enums/tools.enums.ts b/tools/ui/src/lib/enums/tools.enums.ts
index 4b2cdab320..120c1d471d 100644
--- a/tools/ui/src/lib/enums/tools.enums.ts
+++ b/tools/ui/src/lib/enums/tools.enums.ts
@@ -1,7 +1,8 @@
 export enum ToolSource {
 	BUILTIN = 'builtin',
 	MCP = 'mcp',
-	CUSTOM = 'custom'
+	CUSTOM = 'custom',
+	FRONTEND = 'frontend'
 }
 
 export enum ToolPermissionDecision {
diff --git a/tools/ui/src/lib/services/index.ts b/tools/ui/src/lib/services/index.ts
index 475e6419bb..386f740b8f 100644
--- a/tools/ui/src/lib/services/index.ts
+++ b/tools/ui/src/lib/services/index.ts
@@ -261,6 +261,26 @@ export { ParameterSyncService } from './parameter-sync.service';
  */
 export { MCPService } from './mcp.service';
 
+/**
+ * **SandboxService** - Frontend JavaScript execution in a browser sandbox
+ *
+ * Stateless executor for the run_javascript frontend tool. Model generated
+ * code runs in a Web Worker spawned inside a sandboxed iframe with an opaque
+ * origin: no access to the app origin, its storage or its API, and outgoing
+ * requests carry a null origin. The code never touches a main thread, so the
+ * parent enforces the timeout by removing the iframe, which terminates the
+ * worker at the browser level.
+ *
+ * **Architecture & Relationships:**
+ * - **SandboxService** (this class): Stateless sandbox execution
+ * - **toolsStore**: Exposes the tool definition when the sandbox is enabled
+ * - **agenticStore**: Dispatches ToolSource.FRONTEND calls here
+ *
+ * @see SANDBOX_TOOL_DEFINITION in constants/sandbox.ts - tool schema sent to the LLM
+ * @see agenticStore in stores/agentic.svelte.ts - tool dispatch
+ */
+export { SandboxService } from './sandbox.service';
+
 /**
  * **RouterService** — Dynamic route URL construction utility
  *
diff --git a/tools/ui/src/lib/services/sandbox-harness.ts b/tools/ui/src/lib/services/sandbox-harness.ts
new file mode 100644
index 0000000000..27b05e24b4
--- /dev/null
+++ b/tools/ui/src/lib/services/sandbox-harness.ts
@@ -0,0 +1,25 @@
+import WORKER_SHIM from './sandbox-worker.js?raw';
+
+/**
+ * Harness loaded as srcdoc into a sandboxed iframe (allow-scripts only).
+ * The opaque origin is the security boundary: no access to the app origin,
+ * its storage or its API. The harness spawns a worker so model code never
+ * runs on a main thread, which makes the parent timeout enforceable by
+ * removing the iframe.
+ */
+export const SANDBOX_HARNESS_HTML = `<!doctype html><script>
+const SHIM = ${JSON.stringify(WORKER_SHIM)};
+addEventListener('message', (event) => {
+	const respond = (payload) => parent.postMessage(payload, '*');
+	let worker;
+	try {
+		worker = new Worker(URL.createObjectURL(new Blob([SHIM], { type: 'text/javascript' })));
+	} catch (err) {
+		respond({ logs: [], result: null, error: 'Worker creation failed: ' + err });
+		return;
+	}
+	worker.onmessage = (msg) => respond(msg.data);
+	worker.onerror = (err) => respond({ logs: [], result: null, error: String(err.message || err) });
+	worker.postMessage({ code: event.data.code });
+});
+</script>`;
diff --git a/tools/ui/src/lib/services/sandbox-worker.js b/tools/ui/src/lib/services/sandbox-worker.js
new file mode 100644
index 0000000000..689b9211eb
--- /dev/null
+++ b/tools/ui/src/lib/services/sandbox-worker.js
@@ -0,0 +1,30 @@
+const logs = [];
+const fmt = (value) => {
+	if (typeof value === 'string') return value;
+	try {
+		return JSON.stringify(value);
+	} catch {
+		return String(value);
+	}
+};
+const capture =
+	(level, prefix) =>
+	(...args) => {
+		logs.push(prefix + args.map(fmt).join(' '));
+	};
+console.log = capture('log', '');
+console.info = capture('info', '');
+console.debug = capture('debug', '');
+console.warn = capture('warn', 'warn: ');
+console.error = capture('error', 'error: ');
+self.onmessage = async (event) => {
+	const reply = { logs, result: null, error: null };
+	try {
+		const AsyncFunction = Object.getPrototypeOf(async function () {}).constructor;
+		const value = await new AsyncFunction(event.data.code)();
+		if (value !== undefined) reply.result = fmt(value);
+	} catch (err) {
+		reply.error = err instanceof Error ? err.stack || err.message : String(err);
+	}
+	self.postMessage(reply);
+};
diff --git a/tools/ui/src/lib/services/sandbox.service.ts b/tools/ui/src/lib/services/sandbox.service.ts
new file mode 100644
index 0000000000..ef0e6cf4eb
--- /dev/null
+++ b/tools/ui/src/lib/services/sandbox.service.ts
@@ -0,0 +1,112 @@
+import {
+	NEWLINE_SEPARATOR,
+	SANDBOX_EMPTY_OUTPUT,
+	SANDBOX_OUTPUT_MAX_CHARS,
+	SANDBOX_TIMEOUT_MS_DEFAULT,
+	SANDBOX_TIMEOUT_MS_MAX,
+	SANDBOX_TOOL_NAME,
+	SANDBOX_TRUNCATION_NOTICE
+} from '$lib/constants';
+import { SANDBOX_HARNESS_HTML } from './sandbox-harness';
+import type { ToolExecutionResult } from '$lib/types';
+
+interface SandboxReply {
+	logs?: unknown;
+	result?: unknown;
+	error?: unknown;
+}
+
+function formatReply(reply: SandboxReply): ToolExecutionResult {
+	const lines: string[] = [];
+
+	if (Array.isArray(reply.logs)) {
+		for (const line of reply.logs) lines.push(String(line));
+	}
+
+	if (reply.error != null) {
+		lines.push(`Error: ${String(reply.error)}`);
+	} else if (reply.result != null) {
+		lines.push(`=> ${String(reply.result)}`);
+	}
+
+	let content = lines.join(NEWLINE_SEPARATOR);
+	if (!content) content = SANDBOX_EMPTY_OUTPUT;
+	if (content.length > SANDBOX_OUTPUT_MAX_CHARS) {
+		content = `${content.slice(0, SANDBOX_OUTPUT_MAX_CHARS)}${NEWLINE_SEPARATOR}${SANDBOX_TRUNCATION_NOTICE}`;
+	}
+
+	return { content, isError: reply.error != null };
+}
+
+export class SandboxService {
+	/**
+	 * Execute a frontend sandbox tool call and return its output.
+	 * One disposable iframe per execution, removed on completion,
+	 * timeout or abort. Removing the iframe terminates the worker
+	 * at the browser level, so runaway code cannot outlive it.
+	 */
+	static executeTool(
+		toolName: string,
+		params: Record<string, unknown>,
+		signal?: AbortSignal
+	): Promise<ToolExecutionResult> {
+		if (toolName !== SANDBOX_TOOL_NAME) {
+			return Promise.resolve({ content: `Unknown frontend tool: ${toolName}`, isError: true });
+		}
+
+		const code = typeof params.code === 'string' ? params.code : '';
+		if (!code) {
+			return Promise.resolve({ content: 'Missing required parameter: code', isError: true });
+		}
+
+		const requested = Number(params.timeout_ms);
+		const timeoutMs =
+			Number.isFinite(requested) && requested > 0
+				? Math.min(requested, SANDBOX_TIMEOUT_MS_MAX)
+				: SANDBOX_TIMEOUT_MS_DEFAULT;
+
+		return new Promise<ToolExecutionResult>((resolve, reject) => {
+			const iframe = document.createElement('iframe');
+			iframe.setAttribute('sandbox', 'allow-scripts');
+			iframe.style.display = 'none';
+			iframe.srcdoc = SANDBOX_HARNESS_HTML;
+
+			let settled = false;
+
+			const cleanup = () => {
+				settled = true;
+				clearTimeout(timer);
+				window.removeEventListener('message', onMessage);
+				signal?.removeEventListener('abort', onAbort);
+				iframe.remove();
+			};
+
+			const finish = (result: ToolExecutionResult) => {
+				if (settled) return;
+				cleanup();
+				resolve(result);
+			};
+
+			const onAbort = () => {
+				if (settled) return;
+				cleanup();
+				reject(new DOMException('Sandbox execution aborted', 'AbortError'));
+			};
+
+			const onMessage = (event: MessageEvent) => {
+				if (event.source !== iframe.contentWindow) return;
+				finish(formatReply((event.data ?? {}) as SandboxReply));
+			};
+
+			const timer = setTimeout(
+				() => finish({ content: `Execution timed out after ${timeoutMs} ms`, isError: true }),
+				timeoutMs
+			);
+
+			window.addEventListener('message', onMessage);
+			signal?.addEventListener('abort', onAbort);
+			iframe.onload = () => iframe.contentWindow?.postMessage({ code }, '*');
+			document.body.appendChild(iframe);
+		});
+	}
+}
diff --git a/tools/ui/src/lib/stores/agentic.svelte.ts b/tools/ui/src/lib/stores/agentic.svelte.ts
index 947737d7c1..5579cc1e5a 100644
--- a/tools/ui/src/lib/stores/agentic.svelte.ts
+++ b/tools/ui/src/lib/stores/agentic.svelte.ts
@@ -29,6 +29,7 @@ import { permissionsStore } from '$lib/stores/permissions.svelte';
 import { ToolSource, ToolPermissionDecision } from '$lib/enums';
 import { SvelteMap } from 'svelte/reactivity';
 import { ToolsService } from '$lib/services/tools.service';
+import { SandboxService } from '$lib/services/sandbox.service';
 import { isAbortError } from '$lib/utils';
 import { DEFAULT_AGENTIC_CONFIG, NEWLINE_SEPARATOR } from '$lib/constants';
 import {
@@ -784,6 +785,13 @@ class AgenticStore {
 
 							result = executionResult.content;
 
+							if (executionResult.isError) toolSuccess = false;
+						} else if (toolSource === ToolSource.FRONTEND) {
+							const args = this.parseToolArguments(toolCall.function.arguments);
+							const executionResult = await SandboxService.executeTool(toolName, args, signal);
+
+							result = executionResult.content;
+
 							if (executionResult.isError) toolSuccess = false;
 						} else {
 							const mcpCall: MCPToolCall = {
diff --git a/tools/ui/src/lib/stores/tools.svelte.ts b/tools/ui/src/lib/stores/tools.svelte.ts
index 82e41f0bf5..9f0101a82e 100644
--- a/tools/ui/src/lib/stores/tools.svelte.ts
+++ b/tools/ui/src/lib/stores/tools.svelte.ts
@@ -5,6 +5,7 @@ import { HealthCheckStatus, JsonSchemaType, ToolCallType, ToolSource } from '$li
 import { config } from '$lib/stores/settings.svelte';
 import {
 	DISABLED_TOOL_KEYS_LOCALSTORAGE_KEY,
+	SANDBOX_TOOL_DEFINITION,
 	TOOL_GROUP_LABELS,
 	TOOL_SERVER_LABELS
 } from '$lib/constants';
@@ -18,6 +19,8 @@ function toolKey(source: ToolSource, name: string, serverId?: string): string {
 			return serverId ? `mcp-${serverId}:${name}` : `mcp:${name}`;
 		case ToolSource.CUSTOM:
 			return `custom:${name}`;
+		case ToolSource.FRONTEND:
+			return `frontend:${name}`;
 		default:
 			return `builtin:${name}`;
 	}
@@ -82,6 +85,10 @@ class ToolsStore {
 		return mcpStore.getToolDefinitionsForLLM();
 	}
 
+	get frontendTools(): OpenAIToolDefinition[] {
+		return config().jsSandboxEnabled ? [SANDBOX_TOOL_DEFINITION] : [];
+	}
+
 	get customTools(): OpenAIToolDefinition[] {
 		const raw = config().customJson;
 		if (!raw || typeof raw !== 'string') return [];
@@ -156,6 +163,15 @@ class ToolsStore {
 			push({ source: ToolSource.BUILTIN, key: toolKey(ToolSource.BUILTIN, name), definition: def });
 		}
 
+		for (const def of this.frontendTools) {
+			const name = def.function.name;
+			push({
+				source: ToolSource.FRONTEND,
+				key: toolKey(ToolSource.FRONTEND, name),
+				definition: def
+			});
+		}
+
 		for (const { serverId, serverName, definition } of this.mcpEntries()) {
 			const name = definition.function.name;
 			push({
@@ -208,6 +224,8 @@ class ToolsStore {
 				return entry.serverName ?? '';
 			case ToolSource.CUSTOM:
 				return TOOL_GROUP_LABELS[ToolSource.CUSTOM];
+			case ToolSource.FRONTEND:
+				return TOOL_GROUP_LABELS[ToolSource.FRONTEND];
 			default:
 				return TOOL_GROUP_LABELS[ToolSource.BUILTIN];
 		}
@@ -237,6 +255,7 @@ class ToolsStore {
 		};
 
 		for (const def of this._builtinTools) take(def);
+		for (const def of this.frontendTools) take(def);
 		for (const def of mcpStore.getToolDefinitionsForLLM()) take(def);
 		for (const def of this.customTools) take(def);
 
@@ -346,6 +365,7 @@ class ToolsStore {
 		if (entry.serverName) return mcpStore.getServerDisplayName(entry.serverName);
 		if (entry.source === ToolSource.BUILTIN) return TOOL_SERVER_LABELS[ToolSource.BUILTIN];
 		if (entry.source === ToolSource.CUSTOM) return TOOL_SERVER_LABELS[ToolSource.CUSTOM];
+		if (entry.source === ToolSource.FRONTEND) return TOOL_SERVER_LABELS[ToolSource.FRONTEND];
 		return '';
 	}
 
diff --git a/tools/ui/tsconfig.json b/tools/ui/tsconfig.json
index 7c585f4db2..51f55971e9 100644
--- a/tools/ui/tsconfig.json
+++ b/tools/ui/tsconfig.json
@@ -24,7 +24,8 @@
 		"tests/**/*.svelte",
 		".storybook/**/*.ts",
 		".storybook/**/*.svelte"
-	]
+	],
+	"exclude": ["src/lib/services/sandbox-worker.js"]
 	// Path aliases are handled by https://svelte.dev/docs/kit/configuration#alias
 	// except $lib which is handled by https://svelte.dev/docs/kit/configuration#files
 	//