diff --git a/src/crates/contracts/config-contracts/src/conversion.rs b/src/crates/contracts/config-contracts/src/conversion.rs
index 02e2c0b859..cd21f90caf 100644
--- a/src/crates/contracts/config-contracts/src/conversion.rs
+++ b/src/crates/contracts/config-contracts/src/conversion.rs
@@ -81,7 +81,11 @@ impl TryFrom<AIModelConfig> for AIConfig {
             ));
         }
 
-        let custom_request_body = if let Some(body_str) = &other.custom_request_body {
+        let custom_request_body = if let Some(body_str) = other
+            .custom_request_body
+            .as_deref()
+            .filter(|body| !body.trim().is_empty())
+        {
             match serde_json::from_str::<serde_json::Value>(body_str) {
                 Ok(value) => Some(value),
                 Err(e) => {
@@ -268,6 +272,55 @@ mod tests {
         assert_eq!(config.model, "test-model");
     }
 
+    #[test]
+    fn accepts_legacy_missing_null_and_blank_custom_request_bodies() {
+        for body in [
+            None,
+            Some(serde_json::Value::Null),
+            Some("".into()),
+            Some(" \n\t ".into()),
+        ] {
+            let mut payload = serde_json::to_value(base_model_config()).unwrap();
+            payload
+                .as_object_mut()
+                .unwrap()
+                .remove("custom_request_body");
+            if let Some(body) = body {
+                payload["custom_request_body"] = body;
+            }
+            let model: AIModelConfig = serde_json::from_value(payload).unwrap();
+            let round_trip: AIModelConfig =
+                serde_json::from_value(serde_json::to_value(model).unwrap()).unwrap();
+            let config =
+                AIConfig::try_from(round_trip).expect("legacy config should remain usable");
+            assert_eq!(config.custom_request_body, None);
+        }
+    }
+
+    #[test]
+    fn preserves_non_empty_custom_request_body_and_mode() {
+        let mut model = base_model_config();
+        model.custom_request_body = Some(" \n{\"temperature\":0.2}\n ".to_string());
+        model.custom_request_body_mode = Some("trim".to_string());
+
+        let config = AIConfig::try_from(model).expect("conversion should succeed");
+        assert_eq!(
+            config.custom_request_body,
+            Some(serde_json::json!({"temperature": 0.2}))
+        );
+        assert_eq!(config.custom_request_body_mode.as_deref(), Some("trim"));
+    }
+
+    #[test]
+    fn ignores_malformed_non_empty_custom_request_body() {
+        let mut model = base_model_config();
+        model.custom_request_body = Some("{".to_string());
+
+        let config =
+            AIConfig::try_from(model).expect("existing parse fallback should remain usable");
+        assert_eq!(config.custom_request_body, None);
+    }
+
     #[test]
     fn derives_the_largest_output_tier_within_one_quarter_of_context() {
         for (context_window, expected_max_tokens) in [
diff --git a/src/web-ui/src/app/scenes/settings/pages/ai/ModelSettingsPage.tsx b/src/web-ui/src/app/scenes/settings/pages/ai/ModelSettingsPage.tsx
index c997f1412b..25721200bd 100644
--- a/src/web-ui/src/app/scenes/settings/pages/ai/ModelSettingsPage.tsx
+++ b/src/web-ui/src/app/scenes/settings/pages/ai/ModelSettingsPage.tsx
@@ -28,6 +28,7 @@ import {
 import { ModelDiscoveryCoordinator } from '@/infrastructure/config/components/modelDiscoveryCoordinator';
 import {
   getModelEditorRequestSettings,
+  prepareModelEditorRequestSettingsForSave,
   updateModelEditorRequestSettings,
   type ModelEditorRequestSettings,
 } from '@/infrastructure/config/components/modelEditorRequestSettings';
@@ -1995,7 +1996,7 @@ const ModelSettingsPage: React.FC = () => {
             [PROVIDER_INSTANCE_METADATA_KEY]: providerInstanceId,
           },
           reasoning: draft.reasoning,
-          ...draft.requestSettings,
+          ...prepareModelEditorRequestSettingsForSave(draft.requestSettings),
           auth: providerConfig.auth || { type: 'api_key' },
         };
       });
diff --git a/src/web-ui/src/infrastructure/config/components/modelEditorRequestSettings.test.ts b/src/web-ui/src/infrastructure/config/components/modelEditorRequestSettings.test.ts
index d760aacdfc..a47f759f1d 100644
--- a/src/web-ui/src/infrastructure/config/components/modelEditorRequestSettings.test.ts
+++ b/src/web-ui/src/infrastructure/config/components/modelEditorRequestSettings.test.ts
@@ -1,7 +1,28 @@
 import { describe, expect, it } from 'vitest';
-import { getModelEditorRequestSettings, updateModelEditorRequestSettings } from './modelEditorRequestSettings';
+import { getModelEditorRequestSettings, prepareModelEditorRequestSettingsForSave, updateModelEditorRequestSettings } from './modelEditorRequestSettings';
 
 describe('model editor request drafts', () => {
+  it.each(['', ' \n\t '])('clears a previously saved custom body when the editor contains %j', body => {
+    const persisted = { custom_request_body: '{"temperature":0.2}', custom_request_body_mode: 'trim' as const };
+    const drafts = [{ key: 'model', requestSettings: getModelEditorRequestSettings(persisted) }];
+    const [edited] = updateModelEditorRequestSettings(drafts, { custom_request_body: body }, 'model');
+    const saved = { ...persisted, ...prepareModelEditorRequestSettingsForSave(edited.requestSettings) };
+    const reloaded = JSON.parse(JSON.stringify(saved));
+
+    expect(reloaded).not.toHaveProperty('custom_request_body');
+    expect(reloaded.custom_request_body_mode).toBe('trim');
+    expect(getModelEditorRequestSettings(reloaded).custom_request_body).toBeUndefined();
+    expect(edited.requestSettings.custom_request_body).toBe(body);
+    expect(persisted.custom_request_body).toBe('{"temperature":0.2}');
+  });
+
+  it('preserves non-empty custom JSON verbatim on save', () => {
+    const body = ' \n{"temperature":0.2}\n ';
+    expect(prepareModelEditorRequestSettingsForSave(getModelEditorRequestSettings({
+      custom_request_body: body,
+    })).custom_request_body).toBe(body);
+  });
+
   it('accepts older configurations with no optional request settings and isolates header edits', () => {
     expect(getModelEditorRequestSettings()).toMatchObject({
       inline_think_in_text: true,
diff --git a/src/web-ui/src/infrastructure/config/components/modelEditorRequestSettings.ts b/src/web-ui/src/infrastructure/config/components/modelEditorRequestSettings.ts
index c2e06656b6..004ca9b894 100644
--- a/src/web-ui/src/infrastructure/config/components/modelEditorRequestSettings.ts
+++ b/src/web-ui/src/infrastructure/config/components/modelEditorRequestSettings.ts
@@ -20,6 +20,13 @@ export function getModelEditorRequestSettings(config: Partial<AIModelConfig> = {
   };
 }
 
+export function prepareModelEditorRequestSettingsForSave(settings: ModelEditorRequestSettings): ModelEditorRequestSettings {
+  return {
+    ...settings,
+    custom_request_body: settings.custom_request_body?.trim() ? settings.custom_request_body : undefined,
+  };
+}
+
 export function updateModelEditorRequestSettings<T extends { key: string; requestSettings: ModelEditorRequestSettings }>(
   drafts: T[],
   updates: Partial<ModelEditorRequestSettings>,