diff --git a/web/backend/api/models.go b/web/backend/api/models.go
index ce7719906..38a55948b 100644
--- a/web/backend/api/models.go
+++ b/web/backend/api/models.go
@@ -204,8 +204,13 @@ func (h *Handler) handleUpdateModel(w http.ResponseWriter, r *http.Request) {
 	} else {
 		mc.ModelConfig.SetAPIKey(mc.APIKey)
 	}
+	// Preserve existing ExtraBody when omitted (nil), but clear it when
+	// the frontend sends an empty object {} to indicate the field should
+	// be removed.
 	if mc.ExtraBody == nil {
 		mc.ExtraBody = cfg.ModelList[idx].ExtraBody
+	} else if len(mc.ExtraBody) == 0 {
+		mc.ExtraBody = nil
 	}
 
 	cfg.ModelList[idx] = &mc.ModelConfig
diff --git a/web/frontend/src/components/models/add-model-sheet.tsx b/web/frontend/src/components/models/add-model-sheet.tsx
index c760bc672..c0c48994f 100644
--- a/web/frontend/src/components/models/add-model-sheet.tsx
+++ b/web/frontend/src/components/models/add-model-sheet.tsx
@@ -12,6 +12,7 @@ import {
 } from "@/components/shared-form"
 import { Button } from "@/components/ui/button"
 import { Input } from "@/components/ui/input"
+import { Textarea } from "@/components/ui/textarea"
 import {
   Sheet,
   SheetContent,
@@ -34,6 +35,7 @@ interface AddForm {
   maxTokensField: string
   requestTimeout: string
   thinkingLevel: string
+  extraBody: string
 }
 
 const EMPTY_ADD_FORM: AddForm = {
@@ -49,6 +51,7 @@ const EMPTY_ADD_FORM: AddForm = {
   maxTokensField: "",
   requestTimeout: "",
   thinkingLevel: "",
+  extraBody: "",
 }
 
 interface AddModelSheetProps {
@@ -100,7 +103,7 @@ export function AddModelSheet({
   }
 
   const setField =
-    (key: keyof AddForm) => (e: React.ChangeEvent<HTMLInputElement>) => {
+    (key: keyof AddForm) => (e: React.ChangeEvent<HTMLInputElement | HTMLTextAreaElement>) => {
       setForm((f) => ({ ...f, [key]: e.target.value }))
       if (fieldErrors[key]) {
         setFieldErrors((prev) => ({ ...prev, [key]: undefined }))
@@ -129,6 +132,9 @@ export function AddModelSheet({
           ? Number(form.requestTimeout)
           : undefined,
         thinking_level: form.thinkingLevel.trim() || undefined,
+        extra_body: form.extraBody.trim()
+          ? JSON.parse(form.extraBody.trim())
+          : undefined,
       })
       if (setAsDefault) {
         await setDefaultModel(modelName)
@@ -305,6 +311,18 @@ export function AddModelSheet({
                   placeholder="max_completion_tokens"
                 />
               </Field>
+
+              <Field
+                label={t("models.field.extraBody")}
+                hint={t("models.field.extraBodyHint")}
+              >
+                <Textarea
+                  value={form.extraBody}
+                  onChange={setField("extraBody")}
+                  placeholder='{"key": "value"}'
+                  rows={3}
+                />
+              </Field>
             </AdvancedSection>
 
             {serverError && (
diff --git a/web/frontend/src/components/models/edit-model-sheet.tsx b/web/frontend/src/components/models/edit-model-sheet.tsx
index 237991a9f..13678f03d 100644
--- a/web/frontend/src/components/models/edit-model-sheet.tsx
+++ b/web/frontend/src/components/models/edit-model-sheet.tsx
@@ -12,6 +12,7 @@ import {
 } from "@/components/shared-form"
 import { Button } from "@/components/ui/button"
 import { Input } from "@/components/ui/input"
+import { Textarea } from "@/components/ui/textarea"
 import {
   Sheet,
   SheetContent,
@@ -32,6 +33,7 @@ interface EditForm {
   maxTokensField: string
   requestTimeout: string
   thinkingLevel: string
+  extraBody: string
 }
 
 interface EditModelSheetProps {
@@ -59,6 +61,7 @@ export function EditModelSheet({
     maxTokensField: "",
     requestTimeout: "",
     thinkingLevel: "",
+    extraBody: "",
   })
   const [saving, setSaving] = useState(false)
   const [setAsDefault, setSetAsDefault] = useState(false)
@@ -79,6 +82,9 @@ export function EditModelSheet({
           ? String(model.request_timeout)
           : "",
         thinkingLevel: model.thinking_level ?? "",
+        extraBody: model.extra_body
+          ? JSON.stringify(model.extra_body, null, 2)
+          : "",
       })
       setSetAsDefault(model.is_default)
       setError("")
@@ -86,7 +92,7 @@ export function EditModelSheet({
   }, [model])
 
   const setField =
-    (key: keyof EditForm) => (e: React.ChangeEvent<HTMLInputElement>) =>
+    (key: keyof EditForm) => (e: React.ChangeEvent<HTMLInputElement | HTMLTextAreaElement>) =>
       setForm((f) => ({ ...f, [key]: e.target.value }))
 
   const handleSave = async () => {
@@ -109,6 +115,9 @@ export function EditModelSheet({
           ? Number(form.requestTimeout)
           : undefined,
         thinking_level: form.thinkingLevel || undefined,
+        extra_body: form.extraBody.trim()
+          ? JSON.parse(form.extraBody.trim())
+          : {},
       })
       if (setAsDefault && !model.is_default) {
         await setDefaultModel(model.model_name)
@@ -273,6 +282,18 @@ export function EditModelSheet({
                   placeholder="max_completion_tokens"
                 />
               </Field>
+
+              <Field
+                label={t("models.field.extraBody")}
+                hint={t("models.field.extraBodyHint")}
+              >
+                <Textarea
+                  value={form.extraBody}
+                  onChange={setField("extraBody")}
+                  placeholder='{"key": "value"}'
+                  rows={3}
+                />
+              </Field>
             </AdvancedSection>
 
             {error && (
diff --git a/web/frontend/src/i18n/locales/en.json b/web/frontend/src/i18n/locales/en.json
index eebf6e9fc..d15cde693 100644
--- a/web/frontend/src/i18n/locales/en.json
+++ b/web/frontend/src/i18n/locales/en.json
@@ -209,7 +209,9 @@
       "thinkingLevel": "Thinking Level",
       "thinkingLevelHint": "Extended thinking budget: off, low, medium, high, xhigh, adaptive.",
       "maxTokensField": "Max Tokens Field",
-      "maxTokensFieldHint": "Override the request field name for max tokens, e.g. max_completion_tokens."
+      "maxTokensFieldHint": "Override the request field name for max tokens, e.g. max_completion_tokens.",
+      "extraBody": "Extra Body",
+      "extraBodyHint": "Additional JSON fields to inject into the request body, e.g. {\"reasoning_split\": true}."
     },
     "edit": {
       "title": "Configure {{name}}",
diff --git a/web/frontend/src/i18n/locales/zh.json b/web/frontend/src/i18n/locales/zh.json
index 848bea15f..7eb14e983 100644
--- a/web/frontend/src/i18n/locales/zh.json
+++ b/web/frontend/src/i18n/locales/zh.json
@@ -209,7 +209,9 @@
       "thinkingLevel": "思考级别",
       "thinkingLevelHint": "扩展思考预算：off、low、medium、high、xhigh、adaptive。",
       "maxTokensField": "Max Tokens 字段名",
-      "maxTokensFieldHint": "覆盖请求中 max_tokens 的字段名，例如 max_completion_tokens。"
+      "maxTokensFieldHint": "覆盖请求中 max_tokens 的字段名，例如 max_completion_tokens。",
+      "extraBody": "Extra Body",
+      "extraBodyHint": "要注入到请求体中的额外 JSON 字段，例如 {\"reasoning_split\": true}。"
     },
     "edit": {
       "title": "配置 {{name}}",