diff --git a/agentex-ui/components/primary-content/prompt-input.tsx b/agentex-ui/components/primary-content/prompt-input.tsx
index 5d872c4..aab272d 100644
--- a/agentex-ui/components/primary-content/prompt-input.tsx
+++ b/agentex-ui/components/primary-content/prompt-input.tsx
@@ -11,6 +11,7 @@ import { DataContent, TextContent } from 'agentex/resources';
 import { ArrowUp } from 'lucide-react';
 
 import { useAgentexClient } from '@/components/providers';
+import { Button } from '@/components/ui/button';
 import { IconButton } from '@/components/ui/icon-button';
 import { Switch } from '@/components/ui/switch';
 import { toast } from '@/components/ui/toast';
@@ -21,6 +22,7 @@ import {
 } from '@/hooks/use-safe-search-params';
 import { useSendMessage } from '@/hooks/use-task-messages';
 import { useTask } from '@/hooks/use-tasks';
+import { parseOptionalJsonObject } from '@/lib/json-utils';
 import { TaskStatusEnum } from '@/lib/types';
 
 type PromptInputProps = {
@@ -49,6 +51,8 @@ export function PromptInput({ prompt, setPrompt }: PromptInputProps) {
   const { taskID, agentName, updateParams } = useSafeSearchParams();
   const [isClient, setIsClient] = useState(false);
   const [isSendingJSON, setIsSendingJSON] = useState(false);
+  const [isTaskParamsEnabled, setIsTaskParamsEnabled] = useState(false);
+  const [taskParamsPrompt, setTaskParamsPrompt] = useState('');
 
   const { agentexClient } = useAgentexClient();
 
@@ -58,6 +62,7 @@ export function PromptInput({ prompt, setPrompt }: PromptInputProps) {
 
   const textInputRef = useRef<HTMLInputElement>(null);
   const codeMirrorViewRef = useRef<EditorView | null>(null);
+  const taskParamsCodeMirrorViewRef = useRef<EditorView | null>(null);
 
   const isTaskTerminal = useMemo(() => {
     if (!taskID || !task) return false;
@@ -116,18 +121,34 @@ export function PromptInput({ prompt, setPrompt }: PromptInputProps) {
       }
     }
 
+    let taskParams: Record<string, unknown> | undefined;
+    if (!currentTaskId && isTaskParamsEnabled) {
+      try {
+        taskParams = parseOptionalJsonObject(taskParamsPrompt);
+      } catch (error) {
+        toast.error({
+          title: 'Invalid task creation params',
+          message:
+            error instanceof Error ? error.message : 'Invalid JSON object',
+        });
+        return;
+      }
+    }
+
     setPrompt('');
 
     if (!currentTaskId) {
       const task = await createTaskMutation.mutateAsync({
         agentName: agentName,
-        params: {
+        params: taskParams ?? {
           description: prompt,
           content: currentPrompt,
         },
       });
       currentTaskId = task.id;
       updateParams({ [SearchParamKey.TASK_ID]: currentTaskId });
+      setIsTaskParamsEnabled(false);
+      setTaskParamsPrompt('');
     }
 
     const content: TextContent | DataContent = isSendingJSON
@@ -159,10 +180,22 @@ export function PromptInput({ prompt, setPrompt }: PromptInputProps) {
     sendMessageMutation,
     setPrompt,
     isSendingJSON,
+    isTaskParamsEnabled,
+    taskParamsPrompt,
   ]);
 
   return (
     <div className="flex w-full max-w-3xl flex-col gap-2">
+      {!taskID && (
+        <TaskCreationParamsEditor
+          value={taskParamsPrompt}
+          setValue={setTaskParamsPrompt}
+          isEnabled={isTaskParamsEnabled}
+          setIsEnabled={setIsTaskParamsEnabled}
+          isDisabled={isDisabled}
+          codeMirrorViewRef={taskParamsCodeMirrorViewRef}
+        />
+      )}
       <div
         className={`border-input dark:bg-input ${isDisabled ? 'bg-muted scale-90 cursor-not-allowed' : 'scale-100'} flex w-full items-center justify-between rounded-4xl border py-2 pr-2 pl-6 shadow-sm transition-transform duration-300 disabled:cursor-not-allowed`}
       >
@@ -211,6 +244,100 @@ export function PromptInput({ prompt, setPrompt }: PromptInputProps) {
   );
 }
 
+const TaskCreationParamsEditor = ({
+  value,
+  setValue,
+  isEnabled,
+  setIsEnabled,
+  isDisabled,
+  codeMirrorViewRef,
+}: {
+  value: string;
+  setValue: (value: string) => void;
+  isEnabled: boolean;
+  setIsEnabled: (isEnabled: boolean) => void;
+  isDisabled: boolean;
+  codeMirrorViewRef: React.MutableRefObject<EditorView | null>;
+}) => {
+  const handleAdd = useCallback(() => {
+    setIsEnabled(true);
+  }, [setIsEnabled]);
+
+  const handleRemove = useCallback(() => {
+    setIsEnabled(false);
+    setValue('');
+  }, [setIsEnabled, setValue]);
+
+  useEffect(() => {
+    if (!isEnabled) return;
+
+    requestAnimationFrame(() => {
+      codeMirrorViewRef.current?.focus();
+    });
+  }, [codeMirrorViewRef, isEnabled]);
+
+  return (
+    <div
+      className="text-muted-foreground flex flex-col gap-2 px-4 text-sm"
+      style={{
+        opacity: isDisabled ? 0 : 1,
+        transition: 'opacity 0.2s',
+      }}
+    >
+      {isEnabled ? (
+        <div className="border-input bg-background dark:bg-input/30 rounded-xl border p-2 shadow-sm">
+          <div className="mb-2 flex items-center justify-between px-1">
+            <span className="text-xs font-medium">Task creation params</span>
+            <Button
+              type="button"
+              variant="ghost"
+              size="sm"
+              className="text-muted-foreground hover:text-foreground h-auto px-0 py-0 text-xs"
+              onClick={handleRemove}
+              disabled={isDisabled}
+            >
+              Remove
+            </Button>
+          </div>
+          <CodeMirror
+            className="text-sm"
+            value={value}
+            onChange={(nextValue: string) => setValue(nextValue)}
+            onCreateEditor={view => {
+              codeMirrorViewRef.current = view;
+            }}
+            extensions={[json(), noOutlineTheme, closeBrackets()]}
+            placeholder='{ "container_id": "..." }'
+            basicSetup={{
+              lineNumbers: false,
+              foldGutter: false,
+              highlightActiveLineGutter: false,
+              highlightActiveLine: false,
+            }}
+            editable={!isDisabled}
+            theme="none"
+            maxHeight="180px"
+          />
+          <p className="text-muted-foreground mt-2 px-1 text-xs">
+            Optional JSON object sent only when this GUI starts a new task.
+          </p>
+        </div>
+      ) : (
+        <Button
+          type="button"
+          variant="ghost"
+          size="sm"
+          className="text-muted-foreground hover:text-foreground h-auto w-fit px-0 py-0 text-xs"
+          onClick={handleAdd}
+          disabled={isDisabled}
+        >
+          + Add task creation params
+        </Button>
+      )}
+    </div>
+  );
+};
+
 const TextInput = ({
   prompt,
   setPrompt,
diff --git a/agentex-ui/lib/json-utils.test.ts b/agentex-ui/lib/json-utils.test.ts
index 0256692..0676b12 100644
--- a/agentex-ui/lib/json-utils.test.ts
+++ b/agentex-ui/lib/json-utils.test.ts
@@ -1,6 +1,6 @@
 import { describe, it, expect } from 'vitest';
 
-import { serializeValue } from '@/lib/json-utils';
+import { parseOptionalJsonObject, serializeValue } from '@/lib/json-utils';
 import type { JsonValue } from '@/lib/types';
 
 describe('serializeValue', () => {
@@ -77,3 +77,34 @@ describe('serializeValue', () => {
     expect(result).toBe('0');
   });
 });
+
+describe('parseOptionalJsonObject', () => {
+  it('returns undefined for empty input', () => {
+    expect(parseOptionalJsonObject('')).toBeUndefined();
+    expect(parseOptionalJsonObject('   ')).toBeUndefined();
+  });
+
+  it('parses a valid JSON object', () => {
+    expect(parseOptionalJsonObject('{ "container_id": "abc123" }')).toEqual({
+      container_id: 'abc123',
+    });
+  });
+
+  it('rejects invalid JSON', () => {
+    expect(() => parseOptionalJsonObject('{ bad json }')).toThrow(
+      'Invalid JSON'
+    );
+  });
+
+  it('rejects arrays', () => {
+    expect(() => parseOptionalJsonObject('[]')).toThrow(
+      'Expected a JSON object'
+    );
+  });
+
+  it('rejects null', () => {
+    expect(() => parseOptionalJsonObject('null')).toThrow(
+      'Expected a JSON object'
+    );
+  });
+});
diff --git a/agentex-ui/lib/json-utils.ts b/agentex-ui/lib/json-utils.ts
index ee17338..d4b08e3 100644
--- a/agentex-ui/lib/json-utils.ts
+++ b/agentex-ui/lib/json-utils.ts
@@ -1,5 +1,7 @@
 import type { JsonValue } from '@/lib/types';
 
+export type JsonObject = { [key: string]: JsonValue };
+
 export function serializeValue(data: JsonValue): string {
   if (typeof data === 'object' && data !== null) {
     return JSON.stringify(data, null, 2);
@@ -9,3 +11,30 @@ export function serializeValue(data: JsonValue): string {
   }
   return String(data);
 }
+
+export function parseJsonObject(input: string): JsonObject {
+  let parsedValue: unknown;
+  try {
+    parsedValue = JSON.parse(input) as unknown;
+  } catch (error) {
+    throw new Error('Invalid JSON', { cause: error });
+  }
+
+  if (
+    parsedValue === null ||
+    typeof parsedValue !== 'object' ||
+    Array.isArray(parsedValue)
+  ) {
+    throw new Error('Expected a JSON object');
+  }
+
+  return parsedValue as JsonObject;
+}
+
+export function parseOptionalJsonObject(input: string): JsonObject | undefined {
+  if (!input.trim()) {
+    return undefined;
+  }
+
+  return parseJsonObject(input);
+}