diff --git a/src/csv-loader/loader.ts b/src/csv-loader/loader.ts
index a0bb8f9..0d80348 100644
--- a/src/csv-loader/loader.ts
+++ b/src/csv-loader/loader.ts
@@ -82,7 +82,11 @@ function resolveReferenceId(
 }
 
 function parseReferenceIds(schema: ReferenceSchema, valueString: string): unknown {
-  const valueParser = new ReferenceValueParser(valueString.trim());
+  const trimmed = valueString.trim();
+  if (schema.isOptional && trimmed === '') {
+    return null;
+  }
+  const valueParser = new ReferenceValueParser(trimmed);
   const ids = valueParser.parseIds(schema.isArray);
   if (schema.isArray) {
     return ids;
@@ -136,6 +140,7 @@ function parseValueWithReferenceIds(
 function extractNestedReferenceIds(value: unknown, schema: Schema): unknown {
   switch (schema.type) {
     case 'reference':
+      if (value === null || value === undefined) return value;
       if (schema.isArray) {
         const ids = Array.isArray(value) ? value : [value];
         return ids.map(id => String(id));
@@ -258,6 +263,7 @@ function resolveNestedReferences(
 ): unknown {
   switch (schema.type) {
     case 'reference': {
+      if (value === null || value === undefined) return value;
       const { lookup } = loadReferenceTable(schema, refBaseDir, defaultPrimaryKey, currentFilePath);
       if (schema.isArray) {
         const ids = Array.isArray(value) ? value : [value];
@@ -379,9 +385,14 @@ function parseReferenceValue(
   defaultPrimaryKey: string,
   currentFilePath: string | undefined
 ): unknown {
+  const trimmed = valueString.trim();
+  if (schema.isOptional && trimmed === '') {
+    return null;
+  }
+
   const { lookup } = loadReferenceTable(schema, refBaseDir, defaultPrimaryKey, currentFilePath);
   
-  const valueParser = new ReferenceValueParser(valueString.trim());
+  const valueParser = new ReferenceValueParser(trimmed);
   const ids = valueParser.parseIds(schema.isArray);
   
   if (schema.isArray) {
@@ -497,10 +508,10 @@ function schemaToTypeString(schema: Schema, resourceNames?: Map<string, string>)
     case 'union':
       return schema.members.map(m => schemaToTypeString(m, resourceNames)).join(' | ');
     case 'reference': {
-      // Use the resource name mapping if provided, otherwise capitalize table name
       const typeName = resourceNames?.get(schema.tableName) ||
         schema.tableName.charAt(0).toUpperCase() + schema.tableName.slice(1);
-      return schema.isArray ? `readonly ${typeName}[]` : typeName;
+      const baseType = schema.isArray ? `readonly ${typeName}[]` : typeName;
+      return schema.isOptional ? `${baseType} | null` : baseType;
     }
     case 'array':
       if (schema.element.type === 'tuple') {
@@ -764,6 +775,12 @@ function generateSchemaResolutionCode(
   switch (schema.type) {
     case 'reference': {
       const lookup = lookupVar(schema.tableName);
+      if (schema.isOptional) {
+        if (schema.isArray) {
+          return `(${valueExpr} === null || ${valueExpr} === undefined ? ${valueExpr} : (Array.isArray(${valueExpr}) ? ${valueExpr}.map(id => ${lookup}.get(String(id))) : ${lookup}.get(String(${valueExpr}))))`;
+        }
+        return `(${valueExpr} === null || ${valueExpr} === undefined ? ${valueExpr} : ${lookup}.get(String(${valueExpr})))`;
+      }
       if (schema.isArray) {
         return `(Array.isArray(${valueExpr}) ? ${valueExpr}.map(id => ${lookup}.get(String(id))) : ${valueExpr})`;
       }
diff --git a/src/index.test.ts b/src/index.test.ts
index a16c651..1ea5c00 100644
--- a/src/index.test.ts
+++ b/src/index.test.ts
@@ -302,22 +302,37 @@ describe('parseSchema', () => {
 describe('Reference schemas (parseSchema)', () => {
   it('should parse single reference schema @tablename', () => {
     const schema = parseSchema('@users');
-    expect(schema).toEqual({ type: 'reference', tableName: 'users', isArray: false });
+    expect(schema).toEqual({ type: 'reference', tableName: 'users', isArray: false, isOptional: false });
   });
 
   it('should parse array reference schema @tablename[]', () => {
     const schema = parseSchema('@users[]');
-    expect(schema).toEqual({ type: 'reference', tableName: 'users', isArray: true });
+    expect(schema).toEqual({ type: 'reference', tableName: 'users', isArray: true, isOptional: false });
   });
 
   it('should parse reference with hyphens in table name', () => {
     const schema = parseSchema('@my-table');
-    expect(schema).toEqual({ type: 'reference', tableName: 'my-table', isArray: false });
+    expect(schema).toEqual({ type: 'reference', tableName: 'my-table', isArray: false, isOptional: false });
   });
 
   it('should parse reference with underscores in table name', () => {
     const schema = parseSchema('@my_table');
-    expect(schema).toEqual({ type: 'reference', tableName: 'my_table', isArray: false });
+    expect(schema).toEqual({ type: 'reference', tableName: 'my_table', isArray: false, isOptional: false });
+  });
+
+  it('should parse array reference schema @tablename[]', () => {
+    const schema = parseSchema('@users[]');
+    expect(schema).toEqual({ type: 'reference', tableName: 'users', isArray: true, isOptional: false });
+  });
+
+  it('should parse reference with hyphens in table name', () => {
+    const schema = parseSchema('@my-table');
+    expect(schema).toEqual({ type: 'reference', tableName: 'my-table', isArray: false, isOptional: false });
+  });
+
+  it('should parse reference with underscores in table name', () => {
+    const schema = parseSchema('@my_table');
+    expect(schema).toEqual({ type: 'reference', tableName: 'my_table', isArray: false, isOptional: false });
   });
 
   it('should throw ParseError for @ without table name', () => {
@@ -334,7 +349,7 @@ describe('Reference schemas (parseSchema)', () => {
     if (schema.type === 'tuple') {
       expect(schema.elements).toHaveLength(2);
       expect(schema.elements[0].schema).toEqual({ type: 'string' });
-      expect(schema.elements[1].schema).toEqual({ type: 'reference', tableName: 'users', isArray: false });
+      expect(schema.elements[1].schema).toEqual({ type: 'reference', tableName: 'users', isArray: false, isOptional: false });
     }
   });
 
@@ -344,7 +359,7 @@ describe('Reference schemas (parseSchema)', () => {
     if (schema.type === 'tuple') {
       expect(schema.elements).toHaveLength(2);
       expect(schema.elements[0].schema).toEqual({ type: 'string' });
-      expect(schema.elements[1].schema).toEqual({ type: 'reference', tableName: 'users', isArray: true });
+      expect(schema.elements[1].schema).toEqual({ type: 'reference', tableName: 'users', isArray: true, isOptional: false });
     }
   });
 
@@ -354,7 +369,7 @@ describe('Reference schemas (parseSchema)', () => {
     if (schema.type === 'tuple') {
       expect(schema.elements[1]).toEqual({
         name: 'author',
-        schema: { type: 'reference', tableName: 'users', isArray: false },
+        schema: { type: 'reference', tableName: 'users', isArray: false, isOptional: false },
       });
     }
   });
@@ -365,7 +380,7 @@ describe('Reference schemas (parseSchema)', () => {
     if (schema.type === 'array') {
       expect(schema.element.type).toBe('tuple');
       if (schema.element.type === 'tuple') {
-        expect(schema.element.elements[0].schema).toEqual({ type: 'reference', tableName: 'users', isArray: false });
+        expect(schema.element.elements[0].schema).toEqual({ type: 'reference', tableName: 'users', isArray: false, isOptional: false });
       }
     }
   });
@@ -375,7 +390,7 @@ describe('Reference schemas (parseSchema)', () => {
     expect(schema.type).toBe('union');
     if (schema.type === 'union') {
       expect(schema.members).toHaveLength(2);
-      expect(schema.members[0]).toEqual({ type: 'reference', tableName: 'users', isArray: false });
+      expect(schema.members[0]).toEqual({ type: 'reference', tableName: 'users', isArray: false, isOptional: false });
       expect(schema.members[1]).toEqual({ type: 'string' });
     }
   });
@@ -384,7 +399,7 @@ describe('Reference schemas (parseSchema)', () => {
     const schema = parseSchema('@users[] | string');
     expect(schema.type).toBe('union');
     if (schema.type === 'union') {
-      expect(schema.members[0]).toEqual({ type: 'reference', tableName: 'users', isArray: true });
+      expect(schema.members[0]).toEqual({ type: 'reference', tableName: 'users', isArray: true, isOptional: false });
     }
   });
 
@@ -393,8 +408,8 @@ describe('Reference schemas (parseSchema)', () => {
     expect(schema.type).toBe('union');
     if (schema.type === 'union') {
       expect(schema.members).toHaveLength(2);
-      expect(schema.members[0]).toEqual({ type: 'reference', tableName: 'users', isArray: false });
-      expect(schema.members[1]).toEqual({ type: 'reference', tableName: 'parts', isArray: false });
+      expect(schema.members[0]).toEqual({ type: 'reference', tableName: 'users', isArray: false, isOptional: false });
+      expect(schema.members[1]).toEqual({ type: 'reference', tableName: 'parts', isArray: false, isOptional: false });
     }
   });
 
@@ -404,11 +419,39 @@ describe('Reference schemas (parseSchema)', () => {
     if (schema.type === 'array') {
       expect(schema.element.type).toBe('union');
       if (schema.element.type === 'union') {
-        expect(schema.element.members[0]).toEqual({ type: 'reference', tableName: 'users', isArray: false });
-        expect(schema.element.members[1]).toEqual({ type: 'reference', tableName: 'parts', isArray: false });
+        expect(schema.element.members[0]).toEqual({ type: 'reference', tableName: 'users', isArray: false, isOptional: false });
+        expect(schema.element.members[1]).toEqual({ type: 'reference', tableName: 'parts', isArray: false, isOptional: false });
       }
     }
   });
+
+  it('should parse optional reference schema @tablename?', () => {
+    const schema = parseSchema('@users?');
+    expect(schema).toEqual({ type: 'reference', tableName: 'users', isArray: false, isOptional: true });
+  });
+
+  it('should parse optional array reference schema @tablename[]?', () => {
+    const schema = parseSchema('@users[]?');
+    expect(schema).toEqual({ type: 'reference', tableName: 'users', isArray: true, isOptional: true });
+  });
+
+  it('should parse optional reference schema @tablename?', () => {
+    const schema = parseSchema('@users?');
+    expect(schema).toEqual({ type: 'reference', tableName: 'users', isArray: false, isOptional: true });
+  });
+
+  it('should parse optional array reference schema @tablename[]?', () => {
+    const schema = parseSchema('@users[]?');
+    expect(schema).toEqual({ type: 'reference', tableName: 'users', isArray: true, isOptional: true });
+  });
+
+  it('should parse optional reference in union @users? | string', () => {
+    const schema = parseSchema('@users? | string');
+    expect(schema.type).toBe('union');
+    if (schema.type === 'union') {
+      expect(schema.members[0]).toEqual({ type: 'reference', tableName: 'users', isArray: false, isOptional: true });
+    }
+  });
 });
 
 describe('Reference value parsing (parseValue)', () => {
diff --git a/src/parser.ts b/src/parser.ts
index 6519a2e..e14d4f9 100644
--- a/src/parser.ts
+++ b/src/parser.ts
@@ -323,18 +323,25 @@ class Parser {
 
     // Check for array syntax
     if (this.consumeStr('[]')) {
+      this.skipWhitespace();
+      const isOptional = this.consumeStr('?');
       return {
         type: 'reference',
         tableName,
         isArray: true,
+        isOptional,
       };
     }
 
+    // Check for optional suffix
+    const isOptional = this.consumeStr('?');
+
     // Single reference (non-array)
     return {
       type: 'reference',
       tableName,
       isArray: false,
+      isOptional,
     };
   }
 }
diff --git a/src/types.ts b/src/types.ts
index 7bcb530..e2f8742 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -25,6 +25,8 @@ export interface ReferenceSchema {
   tableName: string;
   /** Whether it's an array reference */
   isArray: boolean;
+  /** Whether it's optional (allows empty input resolving to null) */
+  isOptional?: boolean;
 }
 
 export interface StringLiteralSchema {
diff --git a/src/validator.ts b/src/validator.ts
index 2c67c7c..1380f60 100644
--- a/src/validator.ts
+++ b/src/validator.ts
@@ -312,7 +312,14 @@ class ValueParser {
     return result;
   }
 
-  private parseReferenceValue(schema: ReferenceSchema): string | string[] {
+  private parseReferenceValue(schema: ReferenceSchema): string | string[] | null {
+    if (schema.isOptional) {
+      this.skipWhitespace();
+      if (this.pos >= this.input.length) {
+        return null;
+      }
+    }
+
     if (schema.isArray) {
       // Parse array of IDs: [id1; id2; id3]
       let hasOpenBracket = false;
@@ -413,7 +420,7 @@ export function createValidator(schema: Schema): (value: unknown) => boolean {
         if (!Array.isArray(value)) return false;
         return value.every((item) => createValidator(schema.element)(item));
       case 'reference':
-        // Reference can be a string (single ID) or array of strings (IDs)
+        if (schema.isOptional && value === null) return true;
         if (schema.isArray) {
           return Array.isArray(value) && value.every((id) => typeof id === 'string');
         }