diff --git a/packages/api/src/mcp/zod.spec.ts b/packages/api/src/mcp/zod.spec.ts
index c1ab8c902f..7eb9c162a5 100644
--- a/packages/api/src/mcp/zod.spec.ts
+++ b/packages/api/src/mcp/zod.spec.ts
@@ -1485,4 +1485,334 @@ describe('convertJsonSchemaToZod', () => {
       expect(resolvedKeywords.additionalProperties).toBe(false);
     });
   });
+
+  describe('Bare object schema handling for dynamic properties', () => {
+    it('should handle object type without explicit properties but expecting dynamic field definitions', () => {
+      // This simulates the Kintone add_fields tool schema
+      const schema: JsonSchemaType = {
+        type: 'object',
+        properties: {
+          app_id: {
+            type: 'number',
+            description: 'アプリID',
+          },
+          properties: {
+            type: 'object',
+            description: 'フィールドの設定（各フィールドには code, type, label の指定が必須）',
+          },
+        },
+        required: ['app_id', 'properties'],
+      };
+
+      const zodSchema = convertWithResolvedRefs(schema);
+
+      // Test case 1: Basic field definition
+      const testData1 = {
+        app_id: 810,
+        properties: {
+          minutes_id: {
+            code: 'minutes_id',
+            type: 'SINGLE_LINE_TEXT',
+            label: 'minutes_id',
+          },
+        },
+      };
+
+      // WITH THE FIX: Bare object schemas now act as passthrough
+      const result = zodSchema?.parse(testData1);
+      expect(result).toEqual(testData1); // Properties pass through!
+    });
+
+    it('should work when properties field has additionalProperties true', () => {
+      const schema: JsonSchemaType = {
+        type: 'object',
+        properties: {
+          app_id: {
+            type: 'number',
+            description: 'アプリID',
+          },
+          properties: {
+            type: 'object',
+            description: 'フィールドの設定（各フィールドには code, type, label の指定が必須）',
+            additionalProperties: true,
+          },
+        },
+        required: ['app_id', 'properties'],
+      };
+
+      const zodSchema = convertWithResolvedRefs(schema);
+
+      const testData = {
+        app_id: 810,
+        properties: {
+          minutes_id: {
+            code: 'minutes_id',
+            type: 'SINGLE_LINE_TEXT',
+            label: 'minutes_id',
+          },
+        },
+      };
+
+      const result = zodSchema?.parse(testData);
+      expect(result).toEqual(testData);
+      expect(result?.properties?.minutes_id).toBeDefined();
+    });
+
+    it('should work with proper field type definitions in additionalProperties', () => {
+      const schema: JsonSchemaType = {
+        type: 'object',
+        properties: {
+          app_id: {
+            type: 'number',
+            description: 'アプリID',
+          },
+          properties: {
+            type: 'object',
+            description: 'フィールドの設定（各フィールドには code, type, label の指定が必須）',
+            additionalProperties: {
+              type: 'object',
+              properties: {
+                type: { type: 'string' },
+                code: { type: 'string' },
+                label: { type: 'string' },
+                required: { type: 'boolean' },
+                options: {
+                  type: 'object',
+                  additionalProperties: {
+                    type: 'object',
+                    properties: {
+                      label: { type: 'string' },
+                      index: { type: 'string' },
+                    },
+                  },
+                },
+              },
+              required: ['type', 'code', 'label'],
+            },
+          },
+        },
+        required: ['app_id', 'properties'],
+      };
+
+      const zodSchema = convertWithResolvedRefs(schema);
+
+      // Test case 1: Simple text field
+      const testData1 = {
+        app_id: 810,
+        properties: {
+          minutes_id: {
+            code: 'minutes_id',
+            type: 'SINGLE_LINE_TEXT',
+            label: 'minutes_id',
+            required: false,
+          },
+        },
+      };
+
+      const result1 = zodSchema?.parse(testData1);
+      expect(result1).toEqual(testData1);
+
+      // Test case 2: Dropdown field with options
+      const testData2 = {
+        app_id: 820,
+        properties: {
+          status: {
+            type: 'DROP_DOWN',
+            code: 'status',
+            label: 'Status',
+            options: {
+              'Not Started': {
+                label: 'Not Started',
+                index: '0',
+              },
+              'In Progress': {
+                label: 'In Progress',
+                index: '1',
+              },
+            },
+          },
+        },
+      };
+
+      const result2 = zodSchema?.parse(testData2);
+      expect(result2).toEqual(testData2);
+
+      // Test case 3: Multiple fields
+      const testData3 = {
+        app_id: 123,
+        properties: {
+          number_field: {
+            type: 'NUMBER',
+            code: 'number_field',
+            label: '数値フィールド',
+          },
+          text_field: {
+            type: 'SINGLE_LINE_TEXT',
+            code: 'text_field',
+            label: 'テキストフィールド',
+          },
+        },
+      };
+
+      const result3 = zodSchema?.parse(testData3);
+      expect(result3).toEqual(testData3);
+    });
+
+    it('should handle the actual reported failing case', () => {
+      // This is the exact schema that's failing for the user
+      const schema: JsonSchemaType = {
+        type: 'object',
+        properties: {
+          app_id: {
+            type: 'number',
+            description: 'アプリID',
+          },
+          properties: {
+            type: 'object',
+            description: 'フィールドの設定（各フィールドには code, type, label の指定が必須）',
+          },
+        },
+        required: ['app_id', 'properties'],
+      };
+
+      const zodSchema = convertWithResolvedRefs(schema);
+
+      // The exact data the user is trying to send
+      const userData = {
+        app_id: 810,
+        properties: {
+          minutes_id: {
+            code: 'minutes_id',
+            type: 'SINGLE_LINE_TEXT',
+            label: 'minutes_id',
+            required: false,
+          },
+        },
+      };
+
+      // WITH THE FIX: The properties now pass through correctly!
+      const result = zodSchema?.parse(userData);
+      expect(result).toEqual(userData);
+
+      // This fixes the error "properties requires at least one field definition"
+      // The MCP server now receives the full properties object
+    });
+
+    it('should demonstrate fix by treating bare object type as passthrough', () => {
+      // Test what happens if we modify the conversion to treat bare object types
+      // without properties as passthrough schemas
+      const schema: JsonSchemaType = {
+        type: 'object',
+        properties: {
+          app_id: {
+            type: 'number',
+            description: 'アプリID',
+          },
+          properties: {
+            type: 'object',
+            description: 'フィールドの設定（各フィールドには code, type, label の指定が必須）',
+          },
+        },
+        required: ['app_id', 'properties'],
+      };
+
+      // For now, we'll simulate the fix by adding additionalProperties
+      const fixedSchema: JsonSchemaType = {
+        ...schema,
+        properties: {
+          ...schema.properties,
+          properties: {
+            ...(schema.properties!.properties as JsonSchemaType),
+            additionalProperties: true,
+          },
+        },
+      };
+
+      const zodSchema = convertWithResolvedRefs(fixedSchema);
+
+      const userData = {
+        app_id: 810,
+        properties: {
+          minutes_id: {
+            code: 'minutes_id',
+            type: 'SINGLE_LINE_TEXT',
+            label: 'minutes_id',
+            required: false,
+          },
+        },
+      };
+
+      const result = zodSchema?.parse(userData);
+      expect(result).toEqual(userData);
+    });
+
+    it('should NOT treat object schemas with $ref or complex properties as bare objects', () => {
+      // This test ensures our fix doesn't affect schemas with $ref or other complex structures
+      const schemaWithRef = {
+        type: 'object' as const,
+        properties: {
+          data: {
+            type: 'object' as const,
+            // This has anyOf with $ref - should NOT be treated as a bare object
+            anyOf: [{ $ref: '#/$defs/dataSchema' }, { type: 'null' as const }],
+          },
+        },
+        $defs: {
+          dataSchema: {
+            type: 'object' as const,
+            additionalProperties: {
+              type: 'string' as const,
+            },
+          },
+        },
+      };
+
+      // Convert without resolving refs
+      const zodSchema = convertJsonSchemaToZod(schemaWithRef as any, {
+        transformOneOfAnyOf: true,
+      });
+
+      const testData = {
+        data: {
+          field1: 'value1',
+          field2: 'value2',
+        },
+      };
+
+      // Without ref resolution, the data field should be stripped/empty
+      const result = zodSchema?.parse(testData);
+      expect(result?.data).toEqual({});
+    });
+
+    it('should NOT treat object schemas with oneOf/anyOf as bare objects', () => {
+      // Ensure schemas with oneOf/anyOf are not treated as bare objects
+      const schemaWithOneOf = {
+        type: 'object' as const,
+        properties: {
+          config: {
+            type: 'object' as const,
+            // Empty properties but has oneOf - should NOT be passthrough
+            oneOf: [
+              { properties: { type: { const: 'A' } } },
+              { properties: { type: { const: 'B' } } },
+            ],
+          } as any,
+        },
+      };
+
+      const zodSchema = convertWithResolvedRefs(schemaWithOneOf as any, {
+        transformOneOfAnyOf: true,
+      });
+
+      const testData = {
+        config: {
+          randomField: 'should not pass through',
+        },
+      };
+
+      // The random field should be stripped because this isn't a bare object
+      const result = zodSchema?.parse(testData);
+      expect(result?.config).toEqual({});
+    });
+  });
 });
diff --git a/packages/api/src/mcp/zod.ts b/packages/api/src/mcp/zod.ts
index bd0a6e1f1b..cff63cd0ae 100644
--- a/packages/api/src/mcp/zod.ts
+++ b/packages/api/src/mcp/zod.ts
@@ -361,6 +361,18 @@ export function convertJsonSchemaToZod(
     const shape: Record<string, z.ZodType> = {};
     const properties = schema.properties ?? {};
 
+    /** Check if this is a bare object schema with no properties defined
+    and no explicit additionalProperties setting */
+    const isBareObjectSchema =
+      Object.keys(properties).length === 0 &&
+      schema.additionalProperties === undefined &&
+      !schema.patternProperties &&
+      !schema.propertyNames &&
+      !schema.$ref &&
+      !schema.allOf &&
+      !schema.anyOf &&
+      !schema.oneOf;
+
     for (const [key, value] of Object.entries(properties)) {
       // Handle nested oneOf/anyOf if transformOneOfAnyOf is enabled
       if (transformOneOfAnyOf) {
@@ -436,8 +448,9 @@ export function convertJsonSchemaToZod(
     }
 
     // Handle additionalProperties for open-ended objects
-    if (schema.additionalProperties === true) {
+    if (schema.additionalProperties === true || isBareObjectSchema) {
       // This allows any additional properties with any type
+      // Bare object schemas are treated as passthrough to allow dynamic properties
       zodSchema = objectSchema.passthrough();
     } else if (typeof schema.additionalProperties === 'object') {
       // For specific additional property types