diff --git a/frontend/src/settings/ToolConfig.test.tsx b/frontend/src/settings/ToolConfig.test.tsx
index 4786e102..4a5467b8 100644
--- a/frontend/src/settings/ToolConfig.test.tsx
+++ b/frontend/src/settings/ToolConfig.test.tsx
@@ -567,6 +567,30 @@ describe('ToolConfig', () => {
       expect(credential && disabled(credential)).toBe(true);
     });
 
+    it('locks fixed values for anyone but the owner', async () => {
+      // Even an editor who may change credentials: the server refuses a fixed
+      // value from anyone but the owner.
+      await render({
+        ...userTool,
+        access: 'editor',
+        allowed_actions: ['edit', 'edit_credentials', 'use', 'use_in_own'],
+      } as UserToolType);
+      await act(async () => {
+        (
+          container.querySelector('[class*="cursor-pointer"]') as HTMLElement
+        ).click();
+      });
+      const filled = container.querySelector(
+        '[aria-label="settings.tools.filledByLLM"]',
+      );
+      expect(filled?.hasAttribute('disabled')).toBe(true);
+      const tableInputs = Array.from(
+        container.querySelectorAll<HTMLInputElement>('table input[data-slot]'),
+      );
+      // The description stays editable; the value is the fixed one.
+      expect(tableInputs.at(-1)?.disabled).toBe(true);
+    });
+
     it('says why the credentials are locked for an editor without edit_credentials', async () => {
       await render({ ...configTool, ...editorNoCreds } as UserToolType);
       const note = container.querySelector('[data-slot="alert"]');
diff --git a/frontend/src/settings/ToolConfig.tsx b/frontend/src/settings/ToolConfig.tsx
index c2e51a79..76cf5a33 100644
--- a/frontend/src/settings/ToolConfig.tsx
+++ b/frontend/src/settings/ToolConfig.tsx
@@ -58,11 +58,14 @@ const BODY_TYPE_HINT_KEYS: Record<string, string> = {
 /**
  * What the caller may change on the open tool (`utils/accessUtils` `can`):
  * `canEdit` covers the name and the actions, `canEditCredentials` the
- * secrets, URLs and header / query values.
+ * secrets, URLs and header / query values, and `canFixValues` whether a
+ * parameter is filled by the AI or fixed, and its fixed value (the owner's
+ * alone: the server refuses anyone else).
  */
 const ToolAccessContext = React.createContext({
   canEdit: true,
   canEditCredentials: true,
+  canFixValues: true,
 });
 
 /** Maps a body content type to its hint's locale key suffix (JSON by default). */
@@ -133,9 +136,10 @@ export default function ToolConfig({
     can(tool, 'edit_credentials') && !sharedOAuth && !ownerOnlyConnection;
   // Neither: the tool opens as a read-only view with no Save.
   const readOnly = !canEdit && !canEditCredentials;
+  const canFixValues = isOwner(tool);
   const access = React.useMemo(
-    () => ({ canEdit, canEditCredentials }),
-    [canEdit, canEditCredentials],
+    () => ({ canEdit, canEditCredentials, canFixValues }),
+    [canEdit, canEditCredentials, canFixValues],
   );
 
   const toggleUserActionExpand = (index: number) => {
@@ -681,6 +685,7 @@ export default function ToolConfig({
                                               <Checkbox
                                                 size="sm"
                                                 checked={param[1].filled_by_llm}
+                                                disabled={!canFixValues}
                                                 id={uniqueKey}
                                                 aria-label={t(
                                                   'settings.tools.filledByLLM',
@@ -738,7 +743,10 @@ export default function ToolConfig({
                                           <Input
                                             value={param[1].value}
                                             key={uniqueKey}
-                                            disabled={param[1].filled_by_llm}
+                                            disabled={
+                                              param[1].filled_by_llm ||
+                                              !canFixValues
+                                            }
                                             size="sm"
                                             onChange={(e) => {
                                               setTool({
@@ -1293,7 +1301,8 @@ function APIActionTable({
   ) => void;
 }) {
   const { t } = useTranslation();
-  const { canEditCredentials } = React.useContext(ToolAccessContext);
+  const { canEditCredentials, canFixValues } =
+    React.useContext(ToolAccessContext);
   const idPrefix = React.useId();
 
   const [action, setAction] = React.useState<APIActionType>(apiAction);
@@ -1557,6 +1566,7 @@ function APIActionTable({
                       id={`${idPrefix}-${section}-${index}-filled-by-llm`}
                       aria-label={t('settings.tools.filledByLLM')}
                       checked={param.filled_by_llm}
+                      disabled={!canFixValues}
                       onCheckedChange={(checked) =>
                         handlePropertyChange(
                           section,
@@ -1588,6 +1598,7 @@ function APIActionTable({
                   value={param.value}
                   disabled={
                     param.filled_by_llm ||
+                    !canFixValues ||
                     (section === 'query_params' && !canEditCredentials)
                   }
                   onChange={(e) =>