feat: sync field

relay/common/override.go

@@ -34,7 +34,7 @@ type ConditionOperation struct {
 
 type ParamOperation struct {
 	Path       string               `json:"path"`
-	Mode       string               `json:"mode"` // delete, set, move, copy, prepend, append, trim_prefix, trim_suffix, ensure_prefix, ensure_suffix, trim_space, to_lower, to_upper, replace, regex_replace, return_error, prune_objects, set_header, delete_header, copy_header, move_header
+	Mode       string               `json:"mode"` // delete, set, move, copy, prepend, append, trim_prefix, trim_suffix, ensure_prefix, ensure_suffix, trim_space, to_lower, to_upper, replace, regex_replace, return_error, prune_objects, set_header, delete_header, copy_header, move_header, sync_fields
 	Value      interface{}          `json:"value"`
 	KeepOrigin bool                 `json:"keep_origin"`
 	From       string               `json:"from,omitempty"`
@@ -494,6 +494,11 @@ func applyOperations(jsonStr string, operations []ParamOperation, conditionConte
 			if err == nil {
 				contextJSON, err = marshalContextJSON(context)
 			}
+		case "sync_fields":
+			result, err = syncFieldsBetweenTargets(result, context, op.From, op.To)
+			if err == nil {
+				contextJSON, err = marshalContextJSON(context)
+			}
 		default:
 			return "", fmt.Errorf("unknown operation: %s", op.Mode)
 		}
@@ -673,6 +678,119 @@ func deleteHeaderOverrideInContext(context map[string]interface{}, headerName st
 	return nil
 }
 
+type syncTarget struct {
+	kind string
+	key  string
+}
+
+func parseSyncTarget(spec string) (syncTarget, error) {
+	raw := strings.TrimSpace(spec)
+	if raw == "" {
+		return syncTarget{}, fmt.Errorf("sync_fields target is required")
+	}
+
+	idx := strings.Index(raw, ":")
+	if idx < 0 {
+		// Backward compatibility: treat bare value as JSON path.
+		return syncTarget{
+			kind: "json",
+			key:  raw,
+		}, nil
+	}
+
+	kind := strings.ToLower(strings.TrimSpace(raw[:idx]))
+	key := strings.TrimSpace(raw[idx+1:])
+	if key == "" {
+		return syncTarget{}, fmt.Errorf("sync_fields target key is required: %s", raw)
+	}
+
+	switch kind {
+	case "json", "body":
+		return syncTarget{
+			kind: "json",
+			key:  key,
+		}, nil
+	case "header":
+		return syncTarget{
+			kind: "header",
+			key:  key,
+		}, nil
+	default:
+		return syncTarget{}, fmt.Errorf("sync_fields target prefix is invalid: %s", raw)
+	}
+}
+
+func readSyncTargetValue(jsonStr string, context map[string]interface{}, target syncTarget) (interface{}, bool, error) {
+	switch target.kind {
+	case "json":
+		path := processNegativeIndex(jsonStr, target.key)
+		value := gjson.Get(jsonStr, path)
+		if !value.Exists() || value.Type == gjson.Null {
+			return nil, false, nil
+		}
+		if value.Type == gjson.String && strings.TrimSpace(value.String()) == "" {
+			return nil, false, nil
+		}
+		return value.Value(), true, nil
+	case "header":
+		value, ok := getHeaderValueFromContext(context, target.key)
+		if !ok || strings.TrimSpace(value) == "" {
+			return nil, false, nil
+		}
+		return value, true, nil
+	default:
+		return nil, false, fmt.Errorf("unsupported sync_fields target kind: %s", target.kind)
+	}
+}
+
+func writeSyncTargetValue(jsonStr string, context map[string]interface{}, target syncTarget, value interface{}) (string, error) {
+	switch target.kind {
+	case "json":
+		path := processNegativeIndex(jsonStr, target.key)
+		nextJSON, err := sjson.Set(jsonStr, path, value)
+		if err != nil {
+			return "", err
+		}
+		return nextJSON, nil
+	case "header":
+		if err := setHeaderOverrideInContext(context, target.key, value, false); err != nil {
+			return "", err
+		}
+		return jsonStr, nil
+	default:
+		return "", fmt.Errorf("unsupported sync_fields target kind: %s", target.kind)
+	}
+}
+
+func syncFieldsBetweenTargets(jsonStr string, context map[string]interface{}, fromSpec string, toSpec string) (string, error) {
+	fromTarget, err := parseSyncTarget(fromSpec)
+	if err != nil {
+		return "", err
+	}
+	toTarget, err := parseSyncTarget(toSpec)
+	if err != nil {
+		return "", err
+	}
+
+	fromValue, fromExists, err := readSyncTargetValue(jsonStr, context, fromTarget)
+	if err != nil {
+		return "", err
+	}
+	toValue, toExists, err := readSyncTargetValue(jsonStr, context, toTarget)
+	if err != nil {
+		return "", err
+	}
+
+	// If one side exists and the other side is missing, sync the missing side.
+	if fromExists && !toExists {
+		return writeSyncTargetValue(jsonStr, context, toTarget, fromValue)
+	}
+	if toExists && !fromExists {
+		return writeSyncTargetValue(jsonStr, context, fromTarget, toValue)
+	}
+	return jsonStr, nil
+}
+
 func ensureMapKeyInContext(context map[string]interface{}, key string) map[string]interface{} {
 	if context == nil {
 		return map[string]interface{}{}

relay/common/override_test.go

@@ -1057,6 +1057,113 @@ func TestApplyParamOverrideCopyHeaderFromRequestHeaders(t *testing.T) {
 	assertJSONEqual(t, `{"temperature":0.1}`, string(out))
 }
 
+func TestApplyParamOverrideSyncFieldsHeaderToJSON(t *testing.T) {
+	input := []byte(`{"model":"gpt-4"}`)
+	override := map[string]interface{}{
+		"operations": []interface{}{
+			map[string]interface{}{
+				"mode": "sync_fields",
+				"from": "header:session_id",
+				"to":   "json:prompt_cache_key",
+			},
+		},
+	}
+	ctx := map[string]interface{}{
+		"request_headers_raw": map[string]interface{}{
+			"session_id": "sess-123",
+		},
+		"request_headers": map[string]interface{}{
+			"session_id": "sess-123",
+		},
+	}
+
+	out, err := ApplyParamOverride(input, override, ctx)
+	if err != nil {
+		t.Fatalf("ApplyParamOverride returned error: %v", err)
+	}
+	assertJSONEqual(t, `{"model":"gpt-4","prompt_cache_key":"sess-123"}`, string(out))
+}
+
+func TestApplyParamOverrideSyncFieldsJSONToHeader(t *testing.T) {
+	input := []byte(`{"model":"gpt-4","prompt_cache_key":"cache-abc"}`)
+	override := map[string]interface{}{
+		"operations": []interface{}{
+			map[string]interface{}{
+				"mode": "sync_fields",
+				"from": "header:session_id",
+				"to":   "json:prompt_cache_key",
+			},
+		},
+	}
+	ctx := map[string]interface{}{}
+
+	out, err := ApplyParamOverride(input, override, ctx)
+	if err != nil {
+		t.Fatalf("ApplyParamOverride returned error: %v", err)
+	}
+	assertJSONEqual(t, `{"model":"gpt-4","prompt_cache_key":"cache-abc"}`, string(out))
+
+	headers, ok := ctx["header_override"].(map[string]interface{})
+	if !ok {
+		t.Fatalf("expected header_override context map")
+	}
+	if headers["session_id"] != "cache-abc" {
+		t.Fatalf("expected session_id to be synced from prompt_cache_key, got: %v", headers["session_id"])
+	}
+}
+
+func TestApplyParamOverrideSyncFieldsNoChangeWhenBothExist(t *testing.T) {
+	input := []byte(`{"model":"gpt-4","prompt_cache_key":"cache-body"}`)
+	override := map[string]interface{}{
+		"operations": []interface{}{
+			map[string]interface{}{
+				"mode": "sync_fields",
+				"from": "header:session_id",
+				"to":   "json:prompt_cache_key",
+			},
+		},
+	}
+	ctx := map[string]interface{}{
+		"request_headers_raw": map[string]interface{}{
+			"session_id": "cache-header",
+		},
+		"request_headers": map[string]interface{}{
+			"session_id": "cache-header",
+		},
+	}
+
+	out, err := ApplyParamOverride(input, override, ctx)
+	if err != nil {
+		t.Fatalf("ApplyParamOverride returned error: %v", err)
+	}
+	assertJSONEqual(t, `{"model":"gpt-4","prompt_cache_key":"cache-body"}`, string(out))
+
+	headers, _ := ctx["header_override"].(map[string]interface{})
+	if headers != nil {
+		if _, exists := headers["session_id"]; exists {
+			t.Fatalf("expected no override when both sides already have value")
+		}
+	}
+}
+
+func TestApplyParamOverrideSyncFieldsInvalidTarget(t *testing.T) {
+	input := []byte(`{"model":"gpt-4"}`)
+	override := map[string]interface{}{
+		"operations": []interface{}{
+			map[string]interface{}{
+				"mode": "sync_fields",
+				"from": "foo:session_id",
+				"to":   "json:prompt_cache_key",
+			},
+		},
+	}
+
+	_, err := ApplyParamOverride(input, override, nil)
+	if err == nil {
+		t.Fatalf("expected error, got nil")
+	}
+}
+
 func TestApplyParamOverrideSetHeaderKeepOrigin(t *testing.T) {
 	input := []byte(`{"temperature":0.7}`)
 	override := map[string]interface{}{

web/src/components/table/channels/modals/ParamOverrideEditorModal.jsx

@@ -58,6 +58,7 @@ const OPERATION_MODE_OPTIONS = [
   { label: 'JSON · to_upper', value: 'to_upper' },
   { label: 'Control · return_error', value: 'return_error' },
   { label: 'Control · prune_objects', value: 'prune_objects' },
+  { label: 'Control · sync_fields', value: 'sync_fields' },
   { label: 'Header · set_header', value: 'set_header' },
   { label: 'Header · delete_header', value: 'delete_header' },
   { label: 'Header · copy_header', value: 'copy_header' },
@@ -101,6 +102,7 @@ const MODE_META = {
   to_upper: { path: true },
   return_error: { value: true },
   prune_objects: { pathOptional: true, value: true },
+  sync_fields: { from: true, to: true },
   set_header: { path: true, value: true, keepOrigin: true },
   delete_header: { path: true },
   copy_header: { from: true, to: true, keepOrigin: true, pathAlias: true },
@@ -124,9 +126,16 @@ const FROM_REQUIRED_MODES = new Set([
   'regex_replace',
   'copy_header',
   'move_header',
+  'sync_fields',
 ]);
 
-const TO_REQUIRED_MODES = new Set(['copy', 'move', 'copy_header', 'move_header']);
+const TO_REQUIRED_MODES = new Set([
+  'copy',
+  'move',
+  'copy_header',
+  'move_header',
+  'sync_fields',
+]);
 
 const MODE_DESCRIPTIONS = {
   set: 'Set JSON value at path',
@@ -146,12 +155,18 @@ const MODE_DESCRIPTIONS = {
   to_upper: 'Convert string to upper case',
   return_error: 'Stop processing and return custom error',
   prune_objects: 'Remove objects matching conditions',
+  sync_fields: 'Sync two fields when one exists and the other is missing',
   set_header: 'Set runtime override header',
   delete_header: 'Delete runtime override header',
   copy_header: 'Copy header from from -> to',
   move_header: 'Move header from from -> to',
 };
 
+const SYNC_TARGET_TYPE_OPTIONS = [
+  { label: 'JSON', value: 'json' },
+  { label: 'Header', value: 'header' },
+];
+
 const OPERATION_PATH_SUGGESTIONS = [
   'model',
   'temperature',
@@ -355,6 +370,13 @@ const PARAM_OVERRIDE_JSON_SCHEMA = {
           },
           {
             if: {
+              properties: { mode: { const: 'sync_fields' } },
+              required: ['mode'],
+            },
+            then: { required: ['from', 'to'] },
+          },
+          {
+            if: {
               properties: { mode: { const: 'set_header' } },
               required: ['mode'],
             },
@@ -415,6 +437,26 @@ const parseLooseValue = (valueText) => {
   }
 };
 
+const parseSyncTargetSpec = (spec) => {
+  const raw = String(spec ?? '').trim();
+  if (!raw) return { type: 'json', key: '' };
+  const idx = raw.indexOf(':');
+  if (idx < 0) return { type: 'json', key: raw };
+  const prefix = raw.slice(0, idx).trim().toLowerCase();
+  const key = raw.slice(idx + 1).trim();
+  if (prefix === 'header') {
+    return { type: 'header', key };
+  }
+  return { type: 'json', key };
+};
+
+const buildSyncTargetSpec = (type, key) => {
+  const normalizedType = type === 'header' ? 'header' : 'json';
+  const normalizedKey = String(key ?? '').trim();
+  if (!normalizedKey) return '';
+  return `${normalizedType}:${normalizedKey}`;
+};
+
 const normalizeCondition = (condition = {}) => ({
   id: nextLocalId(),
   path: typeof condition.path === 'string' ? condition.path : '',
@@ -1028,6 +1070,14 @@ const ParamOverrideEditorModal = ({ visible, value, onSave, onCancel }) => {
                     const mode = operation.mode || 'set';
                     const meta = MODE_META[mode] || MODE_META.set;
                     const conditions = operation.conditions || [];
+                    const syncFromTarget =
+                      mode === 'sync_fields'
+                        ? parseSyncTargetSpec(operation.from)
+                        : null;
+                    const syncToTarget =
+                      mode === 'sync_fields'
+                        ? parseSyncTargetSpec(operation.to)
+                        : null;
                     return (
                       <Card key={operation.id} className='!rounded-xl border'>
                         <div className='flex items-center justify-between mb-2'>
@@ -1146,7 +1196,107 @@ const ParamOverrideEditorModal = ({ visible, value, onSave, onCancel }) => {
                           </div>
                         ) : null}
 
-                        {meta.from || meta.to === false || meta.to ? (
+                        {mode === 'sync_fields' ? (
+                          <div className='mt-2'>
+                            <Text type='tertiary' size='small'>
+                              sync endpoints
+                            </Text>
+                            <Row gutter={12} style={{ marginTop: 6 }}>
+                              <Col xs={24} md={12}>
+                                <Text type='tertiary' size='small'>
+                                  from endpoint
+                                </Text>
+                                <div className='flex gap-2'>
+                                  <Select
+                                    value={syncFromTarget?.type || 'json'}
+                                    optionList={SYNC_TARGET_TYPE_OPTIONS}
+                                    style={{ width: 120 }}
+                                    onChange={(nextType) =>
+                                      updateOperation(operation.id, {
+                                        from: buildSyncTargetSpec(
+                                          nextType,
+                                          syncFromTarget?.key || '',
+                                        ),
+                                      })
+                                    }
+                                  />
+                                  <Input
+                                    value={syncFromTarget?.key || ''}
+                                    placeholder='session_id'
+                                    onChange={(nextKey) =>
+                                      updateOperation(operation.id, {
+                                        from: buildSyncTargetSpec(
+                                          syncFromTarget?.type || 'json',
+                                          nextKey,
+                                        ),
+                                      })
+                                    }
+                                  />
+                                </div>
+                              </Col>
+                              <Col xs={24} md={12}>
+                                <Text type='tertiary' size='small'>
+                                  to endpoint
+                                </Text>
+                                <div className='flex gap-2'>
+                                  <Select
+                                    value={syncToTarget?.type || 'json'}
+                                    optionList={SYNC_TARGET_TYPE_OPTIONS}
+                                    style={{ width: 120 }}
+                                    onChange={(nextType) =>
+                                      updateOperation(operation.id, {
+                                        to: buildSyncTargetSpec(
+                                          nextType,
+                                          syncToTarget?.key || '',
+                                        ),
+                                      })
+                                    }
+                                  />
+                                  <Input
+                                    value={syncToTarget?.key || ''}
+                                    placeholder='prompt_cache_key'
+                                    onChange={(nextKey) =>
+                                      updateOperation(operation.id, {
+                                        to: buildSyncTargetSpec(
+                                          syncToTarget?.type || 'json',
+                                          nextKey,
+                                        ),
+                                      })
+                                    }
+                                  />
+                                </div>
+                              </Col>
+                            </Row>
+                            <Space wrap style={{ marginTop: 8 }}>
+                              <Tag
+                                size='small'
+                                color='cyan'
+                                className='cursor-pointer'
+                                onClick={() =>
+                                  updateOperation(operation.id, {
+                                    from: 'header:session_id',
+                                    to: 'json:prompt_cache_key',
+                                  })
+                                }
+                              >
+                                {'header:session_id -> json:prompt_cache_key'}
+                              </Tag>
+                              <Tag
+                                size='small'
+                                color='cyan'
+                                className='cursor-pointer'
+                                onClick={() =>
+                                  updateOperation(operation.id, {
+                                    from: 'json:prompt_cache_key',
+                                    to: 'header:session_id',
+                                  })
+                                }
+                              >
+                                {'json:prompt_cache_key -> header:session_id'}
+                              </Tag>
+                            </Space>
+                          </div>
+                        ) : meta.from || meta.to === false || meta.to ? (
                           <Row gutter={12} style={{ marginTop: 8 }}>
                             {meta.from || meta.to === false ? (
                               <Col xs={24} md={12}>