diff --git a/.changeset/fix-zod-union-deduplication.md b/.changeset/fix-zod-union-deduplication.md
new file mode 100644
index 0000000000..a0c4ee6e1c
--- /dev/null
+++ b/.changeset/fix-zod-union-deduplication.md
@@ -0,0 +1,5 @@
+---
+"@hey-api/openapi-ts": patch
+---
+
+fix(parser): expand schema deduplication by including validation constraints in type ID
diff --git a/packages/openapi-ts-tests/specs/3.1.x/validators-string-constraints-union.json b/packages/openapi-ts-tests/specs/3.1.x/validators-string-constraints-union.json
new file mode 100644
index 0000000000..1f09f37833
--- /dev/null
+++ b/packages/openapi-ts-tests/specs/3.1.x/validators-string-constraints-union.json
@@ -0,0 +1,27 @@
+{
+  "openapi": "3.1.0",
+  "info": {
+    "title": "String Constraints Union Test",
+    "version": "1.0.0"
+  },
+  "components": {
+    "schemas": {
+      "LocaleOrLanguage": {
+        "anyOf": [
+          {
+            "type": "string",
+            "minLength": 5,
+            "maxLength": 5,
+            "description": "Combination of ISO 639-1 and ISO 3166-1 alpha-2 separated by a hyphen."
+          },
+          {
+            "type": "string",
+            "minLength": 2,
+            "maxLength": 2,
+            "description": "ISO 639-1 language code."
+          }
+        ]
+      }
+    }
+  }
+}
diff --git a/packages/openapi-ts-tests/zod/v3/__snapshots__/3.1.x/mini/validators-string-constraints-union/zod.gen.ts b/packages/openapi-ts-tests/zod/v3/__snapshots__/3.1.x/mini/validators-string-constraints-union/zod.gen.ts
new file mode 100644
index 0000000000..1e8242e45d
--- /dev/null
+++ b/packages/openapi-ts-tests/zod/v3/__snapshots__/3.1.x/mini/validators-string-constraints-union/zod.gen.ts
@@ -0,0 +1,8 @@
+// This file is auto-generated by @hey-api/openapi-ts
+
+import * as z from 'zod/v4-mini';
+
+export const zLocaleOrLanguage = z.union([
+    z.string().check(z.length(5)),
+    z.string().check(z.length(2))
+]);
diff --git a/packages/openapi-ts-tests/zod/v3/__snapshots__/3.1.x/v3/validators-string-constraints-union/zod.gen.ts b/packages/openapi-ts-tests/zod/v3/__snapshots__/3.1.x/v3/validators-string-constraints-union/zod.gen.ts
new file mode 100644
index 0000000000..5027df72f8
--- /dev/null
+++ b/packages/openapi-ts-tests/zod/v3/__snapshots__/3.1.x/v3/validators-string-constraints-union/zod.gen.ts
@@ -0,0 +1,8 @@
+// This file is auto-generated by @hey-api/openapi-ts
+
+import { z } from 'zod';
+
+export const zLocaleOrLanguage = z.union([
+    z.string().length(5),
+    z.string().length(2)
+]);
diff --git a/packages/openapi-ts-tests/zod/v3/__snapshots__/3.1.x/v4/validators-string-constraints-union/zod.gen.ts b/packages/openapi-ts-tests/zod/v3/__snapshots__/3.1.x/v4/validators-string-constraints-union/zod.gen.ts
new file mode 100644
index 0000000000..531f15719b
--- /dev/null
+++ b/packages/openapi-ts-tests/zod/v3/__snapshots__/3.1.x/v4/validators-string-constraints-union/zod.gen.ts
@@ -0,0 +1,8 @@
+// This file is auto-generated by @hey-api/openapi-ts
+
+import { z } from 'zod/v4';
+
+export const zLocaleOrLanguage = z.union([
+    z.string().length(5),
+    z.string().length(2)
+]);
diff --git a/packages/openapi-ts-tests/zod/v3/test/3.1.x.test.ts b/packages/openapi-ts-tests/zod/v3/test/3.1.x.test.ts
index 8ec15f08d0..4fedd7c557 100644
--- a/packages/openapi-ts-tests/zod/v3/test/3.1.x.test.ts
+++ b/packages/openapi-ts-tests/zod/v3/test/3.1.x.test.ts
@@ -138,6 +138,13 @@ for (const zodVersion of zodVersions) {
         description:
           "validator schemas with merged unions (can't use .merge())",
       },
+      {
+        config: createConfig({
+          input: 'validators-string-constraints-union.json',
+          output: 'validators-string-constraints-union',
+        }),
+        description: 'validator schemas with string constraints union',
+      },
     ];
 
     it.each(scenarios)('$description', async ({ config }) => {
diff --git a/packages/openapi-ts-tests/zod/v4/__snapshots__/3.1.x/mini/validators-string-constraints-union/zod.gen.ts b/packages/openapi-ts-tests/zod/v4/__snapshots__/3.1.x/mini/validators-string-constraints-union/zod.gen.ts
new file mode 100644
index 0000000000..99addfb0b9
--- /dev/null
+++ b/packages/openapi-ts-tests/zod/v4/__snapshots__/3.1.x/mini/validators-string-constraints-union/zod.gen.ts
@@ -0,0 +1,8 @@
+// This file is auto-generated by @hey-api/openapi-ts
+
+import * as z from 'zod/mini';
+
+export const zLocaleOrLanguage = z.union([
+    z.string().check(z.length(5)),
+    z.string().check(z.length(2))
+]);
diff --git a/packages/openapi-ts-tests/zod/v4/__snapshots__/3.1.x/v3/validators-string-constraints-union/zod.gen.ts b/packages/openapi-ts-tests/zod/v4/__snapshots__/3.1.x/v3/validators-string-constraints-union/zod.gen.ts
new file mode 100644
index 0000000000..6a9afba9d6
--- /dev/null
+++ b/packages/openapi-ts-tests/zod/v4/__snapshots__/3.1.x/v3/validators-string-constraints-union/zod.gen.ts
@@ -0,0 +1,8 @@
+// This file is auto-generated by @hey-api/openapi-ts
+
+import { z } from 'zod/v3';
+
+export const zLocaleOrLanguage = z.union([
+    z.string().length(5),
+    z.string().length(2)
+]);
diff --git a/packages/openapi-ts-tests/zod/v4/__snapshots__/3.1.x/v4/validators-string-constraints-union/zod.gen.ts b/packages/openapi-ts-tests/zod/v4/__snapshots__/3.1.x/v4/validators-string-constraints-union/zod.gen.ts
new file mode 100644
index 0000000000..5027df72f8
--- /dev/null
+++ b/packages/openapi-ts-tests/zod/v4/__snapshots__/3.1.x/v4/validators-string-constraints-union/zod.gen.ts
@@ -0,0 +1,8 @@
+// This file is auto-generated by @hey-api/openapi-ts
+
+import { z } from 'zod';
+
+export const zLocaleOrLanguage = z.union([
+    z.string().length(5),
+    z.string().length(2)
+]);
diff --git a/packages/openapi-ts-tests/zod/v4/test/3.1.x.test.ts b/packages/openapi-ts-tests/zod/v4/test/3.1.x.test.ts
index 8ec15f08d0..4fedd7c557 100644
--- a/packages/openapi-ts-tests/zod/v4/test/3.1.x.test.ts
+++ b/packages/openapi-ts-tests/zod/v4/test/3.1.x.test.ts
@@ -138,6 +138,13 @@ for (const zodVersion of zodVersions) {
         description:
           "validator schemas with merged unions (can't use .merge())",
       },
+      {
+        config: createConfig({
+          input: 'validators-string-constraints-union.json',
+          output: 'validators-string-constraints-union',
+        }),
+        description: 'validator schemas with string constraints union',
+      },
     ];
 
     it.each(scenarios)('$description', async ({ config }) => {
diff --git a/packages/openapi-ts/src/ir/schema.ts b/packages/openapi-ts/src/ir/schema.ts
index b9e42c669e..b257a3a450 100644
--- a/packages/openapi-ts/src/ir/schema.ts
+++ b/packages/openapi-ts/src/ir/schema.ts
@@ -42,7 +42,25 @@ export const deduplicateSchema = <T extends IR.SchemaObject>({
         item.format !== undefined && detectFormat
           ? `format-${item.format}`
           : '';
-      const typeId = `${item.$ref ?? ''}${item.type ?? ''}${constant}${format}`;
+
+      // Include validation constraints in the type ID to avoid incorrect deduplication
+      const constraints = [
+        item.minLength !== undefined ? `minLength-${item.minLength}` : '',
+        item.maxLength !== undefined ? `maxLength-${item.maxLength}` : '',
+        item.minimum !== undefined ? `minimum-${item.minimum}` : '',
+        item.maximum !== undefined ? `maximum-${item.maximum}` : '',
+        item.exclusiveMinimum !== undefined
+          ? `exclusiveMinimum-${item.exclusiveMinimum}`
+          : '',
+        item.exclusiveMaximum !== undefined
+          ? `exclusiveMaximum-${item.exclusiveMaximum}`
+          : '',
+        item.minItems !== undefined ? `minItems-${item.minItems}` : '',
+        item.maxItems !== undefined ? `maxItems-${item.maxItems}` : '',
+        item.pattern !== undefined ? `pattern-${item.pattern}` : '',
+      ].join('');
+
+      const typeId = `${item.$ref ?? ''}${item.type ?? ''}${constant}${format}${constraints}`;
       if (!typeIds.includes(typeId)) {
         typeIds.push(typeId);
         uniqueItems.push(item);