diff --git a/src/__tests__/utils.parseFilter.test.js b/src/__tests__/utils.parseFilter.test.js
index df7e562..fa2a5ba 100644
--- a/src/__tests__/utils.parseFilter.test.js
+++ b/src/__tests__/utils.parseFilter.test.js
@@ -112,6 +112,50 @@ describe('parseFilter number fields', () => {
   });
 });
 
+describe('parseFilter empty values', () => {
+  it("treats '' as null or missing", async () => {
+    await expect(parseFilter('_reference', "''", jobModel)).resolves.toEqual({
+      _reference: null,
+    });
+  });
+
+  it("treats ='' as equal to null or missing", async () => {
+    await expect(parseFilter('_reference', "=''", jobModel)).resolves.toEqual({
+      _reference: null,
+    });
+  });
+
+  it("treats <>'' as present and not null", async () => {
+    await expect(parseFilter('_reference', "<>''", jobModel)).resolves.toEqual({
+      _reference: { $ne: null },
+    });
+  });
+
+  it("treats '' as empty on number fields", async () => {
+    await expect(parseFilter('quantity', "''", jobModel)).resolves.toEqual({
+      quantity: null,
+    });
+  });
+
+  it("treats <>'' as not empty on date fields", async () => {
+    await expect(parseFilter('createdAt', "<>''", jobModel)).resolves.toEqual({
+      createdAt: { $ne: null },
+    });
+  });
+
+  it("treats '' as empty on object references", async () => {
+    await expect(parseFilter('gcodeFile', "''", jobModel)).resolves.toEqual({
+      gcodeFile: null,
+    });
+  });
+
+  it("treats <>'' as not empty on object references", async () => {
+    await expect(parseFilter('gcodeFile', "<>''", jobModel)).resolves.toEqual({
+      gcodeFile: { $ne: null },
+    });
+  });
+});
+
 describe('parseFilter date fields', () => {
   it('matches TODAY as the current calendar day', async () => {
     const now = new Date();
diff --git a/src/utils.js b/src/utils.js
index 43dffe0..f55b4e1 100644
--- a/src/utils.js
+++ b/src/utils.js
@@ -41,6 +41,7 @@ logger.level = config.server.logLevel;
 //   *        any number of characters  Co* / *Co / *Co*
 //   ?        a single character        Hans?n
 //   @        ignore case               @location (text matching is case-insensitive)
+//   ''       empty (null or missing)   '' / ='' / <>''
 //
 // Date-typed fields (schema Date) interpret bare values as calendar
 // days/datetimes, e.g. "22" => the whole of day 22 of the current month/year,
@@ -87,6 +88,17 @@ function buildWildcardRegexPattern(input) {
 }
 
 const NO_MATCH_CONDITION = { op: { $in: [] } };
+const EMPTY_OPERAND = "''";
+
+function isEmptyOperand(value) {
+  return String(value).trim() === EMPTY_OPERAND;
+}
+
+// MongoDB null equality matches both null and missing (undefined) fields.
+function emptyCondition(operator = 'eq') {
+  if (operator === 'ne') return { op: { $ne: null } };
+  return { value: null };
+}
 
 function isBooleanField(property, model = null) {
   const schema = model?.schema;
@@ -620,6 +632,7 @@ function isValidDateOperand(value) {
 function isValidDateFilterLeaf(rawToken) {
   const token = rawToken.trim();
   if (token === '') return true;
+  if (isEmptyOperand(token)) return true;
 
   const rangeIdx = token.indexOf('..');
   if (rangeIdx !== -1) {
@@ -634,6 +647,7 @@ function isValidDateFilterLeaf(rawToken) {
   for (const symbol of operators) {
     if (token.startsWith(symbol)) {
       const operand = stripIgnoreCase(token.slice(symbol.length).trim());
+      if ((symbol === '=' || symbol === '<>') && isEmptyOperand(operand)) return true;
       return operand !== '' && isValidDateOperand(operand);
     }
   }
@@ -733,6 +747,7 @@ function buildEquality(
   isNumberField = false,
   property = ''
 ) {
+  if (isEmptyOperand(value)) return emptyCondition('eq');
   if (isBooleanField) return buildBooleanEquality(value);
   if (isObjectIdField) {
     const { suffix } = parsePrefixedValue(value);
@@ -770,6 +785,10 @@ function buildComparison(
   isNumberField = false,
   property = ''
 ) {
+  if (isEmptyOperand(value)) {
+    if (name === 'eq' || name === 'ne') return emptyCondition(name);
+    return NO_MATCH_CONDITION;
+  }
   if (isBooleanField) return buildBooleanComparison(name, value);
   if (isObjectIdField) {
     const { suffix } = parsePrefixedValue(value);
@@ -856,6 +875,7 @@ function parseLeafCondition(
     if (isBooleanField || isObjectIdField || isNumberField) return NO_MATCH_CONDITION;
     return { op: buildRegexOp('^$') };
   }
+  if (isEmptyOperand(token)) return emptyCondition('eq');
 
   // Interval: a..b, ..b, a..
   const rangeIdx = token.indexOf('..');
@@ -1052,6 +1072,10 @@ function combineAndRefIds(children) {
 
 async function resolveRefOperand(operand, refModelEntry, operator = 'eq') {
   const lookupOperand = String(operand).trim();
+  if (isEmptyOperand(lookupOperand)) {
+    if (operator === 'eq' || operator === 'ne') return emptyCondition(operator);
+    return NO_MATCH_CONDITION;
+  }
   const { prefix, suffix, hadPrefix } = parsePrefixedValue(lookupOperand);
   if (hadPrefix) {
     const prefixEntry = getRefModelEntryFromPrefix(prefix);
@@ -1084,6 +1108,7 @@ async function resolveRefOperand(operand, refModelEntry, operator = 'eq') {
 async function resolveRefLeaf(rawToken, fallbackRefName) {
   const token = rawToken.trim();
   if (token === '') return NO_MATCH_CONDITION;
+  if (isEmptyOperand(token)) return emptyCondition('eq');
 
   const refModelEntry = getRefModelEntryForToken(token, fallbackRefName);
   const lookupToken = stripPrefixFromOperand(token);