diff --git a/.gts-spec b/.gts-spec
index 56ca20d..4c929a2 160000
--- a/.gts-spec
+++ b/.gts-spec
@@ -1 +1 @@
-Subproject commit 56ca20d89df2c2d8e70773da33482e4c748c446d
+Subproject commit 4c929a2f3a2fecdb2d226096ac52a50459bdea7c
diff --git a/README.md b/README.md
index acf3490..27c46c9 100644
--- a/README.md
+++ b/README.md
@@ -24,7 +24,7 @@ Featureset:
 - [x] **OP#9 - Version Casting**: Transform instances between compatible MINOR versions
 - [x] **OP#10 - Query Execution**: Filter identifier collections using the GTS query language
 - [x] **OP#11 - Attribute Access**: Retrieve property values and metadata using the attribute selector (`@`)
-- [ ] **OP#12 - Schema Validation**: Validate schema against its precedent schema
+- [x] **OP#12 - Schema Validation**: Validate schema against its precedent schema
 
 Other GTS spec [Reference Implementation](https://github.com/globaltypesystem/gts-spec/blob/main/README.md#9-reference-implementation-recommendations) recommended features support:
 
@@ -32,8 +32,8 @@ Other GTS spec [Reference Implementation](https://github.com/globaltypesystem/gt
 - [x] **CLI** - command-line interface for all GTS operations
 - [x] **Web server** - a non-production web-server with REST API for the operations processing and testing
 - [x] **x-gts-ref** - to support special GTS entity reference annotation in schemas
-- [ ] **YAML support** - to support YAML files (*.yml, *.yaml) as input files
-- [ ] **TypeSpec support** - add [typespec.io](https://typespec.io/) files (*.tsp) support
+- [ ] **YAML support** - to support YAML files (`*.yml`, `*.yaml`) as input files
+- [ ] **TypeSpec support** - add [typespec.io](https://typespec.io/) files (`*.tsp`) support
 - [ ] **UUID for instances** - to support UUID as ID in JSON instances
 
 ## Usage
@@ -60,7 +60,7 @@ import { extractID } from '@globaltypesystem/gts-ts';
 
 const content = {
   gtsId: 'gts.vendor.pkg.ns.type.v1.0',
-  name: 'My Entity'
+  name: 'My Entity',
 };
 
 const extracted = extractID(content);
@@ -76,10 +76,7 @@ if (parsed.ok) {
 }
 
 // OP#4 - Pattern Matching
-const matchResult = matchIDPattern(
-  'gts.vendor.pkg.ns.type.v1.0',
-  'gts.vendor.pkg.*'
-);
+const matchResult = matchIDPattern('gts.vendor.pkg.ns.type.v1.0', 'gts.vendor.pkg.*');
 if (matchResult.match) {
   console.log('Pattern matched!');
 }
@@ -100,7 +97,7 @@ const gts = new GTS();
 // Register an entity
 const entity = {
   gtsId: 'gts.vendor.pkg.ns.type.v1.0',
-  name: 'My Entity'
+  name: 'My Entity',
 };
 
 gts.register(entity);
@@ -117,20 +114,13 @@ console.log(`Relationships: ${relationships.relationships}`);
 console.log(`Broken references: ${relationships.brokenReferences}`);
 
 // OP#8 - Check compatibility
-const compatResult = gts.checkCompatibility(
-  'gts.vendor.pkg.ns.type.v1~',
-  'gts.vendor.pkg.ns.type.v2~',
-  'backward'
-);
+const compatResult = gts.checkCompatibility('gts.vendor.pkg.ns.type.v1~', 'gts.vendor.pkg.ns.type.v2~', 'backward');
 if (compatResult.compatible) {
   console.log('Schemas are compatible');
 }
 
 // OP#9 - Cast instance to different version
-const castResult = gts.castInstance(
-  'gts.vendor.pkg.ns.type.v1.0',
-  'gts.vendor.pkg.ns.type.v2~'
-);
+const castResult = gts.castInstance('gts.vendor.pkg.ns.type.v1.0', 'gts.vendor.pkg.ns.type.v2~');
 if (castResult.ok) {
   console.log('Instance casted successfully');
 }
@@ -220,6 +210,7 @@ npx gts-server --host 127.0.0.1 --port 8000 --verbose 2
 ### API Endpoints
 
 #### Entity Management
+
 - `GET /entities` - List all entities
 - `GET /entities/:id` - Get specific entity
 - `POST /entities` - Add new entity
@@ -227,6 +218,7 @@ npx gts-server --host 127.0.0.1 --port 8000 --verbose 2
 - `POST /schemas` - Add new schema
 
 #### GTS Operations
+
 - `GET /validate-id?id=<gts_id>` - Validate GTS ID (OP#1)
 - `POST /extract-id` - Extract GTS ID from JSON (OP#2)
 - `GET /parse-id?id=<gts_id>` - Parse GTS ID (OP#3)
@@ -238,8 +230,11 @@ npx gts-server --host 127.0.0.1 --port 8000 --verbose 2
 - `POST /cast` - Cast instance (OP#9)
 - `GET /query?expr=<expression>&limit=<limit>` - Query entities (OP#10)
 - `GET /attr?path=<path>` - Get attribute value (OP#11)
+- `POST /validate-schema` - Validate schema against parent schema (OP#12)
+- `POST /validate-entity` - Validate entity (schema or instance) (OP#12)
 
 #### Other
+
 - `GET /health` - Health check
 - `GET /openapi` - OpenAPI specification
 
diff --git a/package-lock.json b/package-lock.json
index 4fcfad5..dad1511 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -1,19 +1,18 @@
 {
   "name": "@globaltypesystem/gts-ts",
-  "version": "0.1.1",
+  "version": "0.2.0",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "@globaltypesystem/gts-ts",
-      "version": "0.1.1",
+      "version": "0.2.0",
       "license": "Apache-2.0",
       "dependencies": {
         "ajv": "^8.12.0",
         "ajv-formats": "^2.1.1",
         "commander": "^12.0.0",
-        "fastify": "^4.26.0",
-        "fastify-cors": "^6.1.0",
+        "fastify": "^5.7.3",
         "uuid": "^9.0.1"
       },
       "bin": {
@@ -686,44 +685,131 @@
       }
     },
     "node_modules/@fastify/ajv-compiler": {
-      "version": "3.6.0",
-      "resolved": "https://registry.npmjs.org/@fastify/ajv-compiler/-/ajv-compiler-3.6.0.tgz",
-      "integrity": "sha512-LwdXQJjmMD+GwLOkP7TVC68qa+pSSogeWWmznRJ/coyTcfe9qA05AHFSe1eZFwK6q+xVRpChnvFUkf1iYaSZsQ==",
+      "version": "4.0.5",
+      "resolved": "https://registry.npmjs.org/@fastify/ajv-compiler/-/ajv-compiler-4.0.5.tgz",
+      "integrity": "sha512-KoWKW+MhvfTRWL4qrhUwAAZoaChluo0m0vbiJlGMt2GXvL4LVPQEjt8kSpHI3IBq5Rez8fg+XeH3cneztq+C7A==",
+      "funding": [
+        {
+          "type": "github",
+          "url": "https://github.com/sponsors/fastify"
+        },
+        {
+          "type": "opencollective",
+          "url": "https://opencollective.com/fastify"
+        }
+      ],
       "license": "MIT",
       "dependencies": {
-        "ajv": "^8.11.0",
-        "ajv-formats": "^2.1.1",
-        "fast-uri": "^2.0.0"
+        "ajv": "^8.12.0",
+        "ajv-formats": "^3.0.1",
+        "fast-uri": "^3.0.0"
       }
     },
-    "node_modules/@fastify/ajv-compiler/node_modules/fast-uri": {
-      "version": "2.4.0",
-      "resolved": "https://registry.npmjs.org/fast-uri/-/fast-uri-2.4.0.tgz",
-      "integrity": "sha512-ypuAmmMKInk5q7XcepxlnUWDLWv4GFtaJqAzWKqn62IpQ3pejtr5dTVbt3vwqVaMKmkNR55sTT+CqUKIaT21BA==",
-      "license": "MIT"
+    "node_modules/@fastify/ajv-compiler/node_modules/ajv-formats": {
+      "version": "3.0.1",
+      "resolved": "https://registry.npmjs.org/ajv-formats/-/ajv-formats-3.0.1.tgz",
+      "integrity": "sha512-8iUql50EUR+uUcdRQ3HDqa6EVyo3docL8g5WJ3FNcWmu62IbkGUue/pEyLBW8VGKKucTPgqeks4fIU1DA4yowQ==",
+      "license": "MIT",
+      "dependencies": {
+        "ajv": "^8.0.0"
+      },
+      "peerDependencies": {
+        "ajv": "^8.0.0"
+      },
+      "peerDependenciesMeta": {
+        "ajv": {
+          "optional": true
+        }
+      }
     },
     "node_modules/@fastify/error": {
-      "version": "3.4.1",
-      "resolved": "https://registry.npmjs.org/@fastify/error/-/error-3.4.1.tgz",
-      "integrity": "sha512-wWSvph+29GR783IhmvdwWnN4bUxTD01Vm5Xad4i7i1VuAOItLvbPAb69sb0IQ2N57yprvhNIwAP5B6xfKTmjmQ==",
+      "version": "4.2.0",
+      "resolved": "https://registry.npmjs.org/@fastify/error/-/error-4.2.0.tgz",
+      "integrity": "sha512-RSo3sVDXfHskiBZKBPRgnQTtIqpi/7zhJOEmAxCiBcM7d0uwdGdxLlsCaLzGs8v8NnxIRlfG0N51p5yFaOentQ==",
+      "funding": [
+        {
+          "type": "github",
+          "url": "https://github.com/sponsors/fastify"
+        },
+        {
+          "type": "opencollective",
+          "url": "https://opencollective.com/fastify"
+        }
+      ],
       "license": "MIT"
     },
     "node_modules/@fastify/fast-json-stringify-compiler": {
-      "version": "4.3.0",
-      "resolved": "https://registry.npmjs.org/@fastify/fast-json-stringify-compiler/-/fast-json-stringify-compiler-4.3.0.tgz",
-      "integrity": "sha512-aZAXGYo6m22Fk1zZzEUKBvut/CIIQe/BapEORnxiD5Qr0kPHqqI69NtEMCme74h+at72sPhbkb4ZrLd1W3KRLA==",
+      "version": "5.0.3",
+      "resolved": "https://registry.npmjs.org/@fastify/fast-json-stringify-compiler/-/fast-json-stringify-compiler-5.0.3.tgz",
+      "integrity": "sha512-uik7yYHkLr6fxd8hJSZ8c+xF4WafPK+XzneQDPU+D10r5X19GW8lJcom2YijX2+qtFF1ENJlHXKFM9ouXNJYgQ==",
+      "funding": [
+        {
+          "type": "github",
+          "url": "https://github.com/sponsors/fastify"
+        },
+        {
+          "type": "opencollective",
+          "url": "https://opencollective.com/fastify"
+        }
+      ],
       "license": "MIT",
       "dependencies": {
-        "fast-json-stringify": "^5.7.0"
+        "fast-json-stringify": "^6.0.0"
       }
     },
+    "node_modules/@fastify/forwarded": {
+      "version": "3.0.1",
+      "resolved": "https://registry.npmjs.org/@fastify/forwarded/-/forwarded-3.0.1.tgz",
+      "integrity": "sha512-JqDochHFqXs3C3Ml3gOY58zM7OqO9ENqPo0UqAjAjH8L01fRZqwX9iLeX34//kiJubF7r2ZQHtBRU36vONbLlw==",
+      "funding": [
+        {
+          "type": "github",
+          "url": "https://github.com/sponsors/fastify"
+        },
+        {
+          "type": "opencollective",
+          "url": "https://opencollective.com/fastify"
+        }
+      ],
+      "license": "MIT"
+    },
     "node_modules/@fastify/merge-json-schemas": {
-      "version": "0.1.1",
-      "resolved": "https://registry.npmjs.org/@fastify/merge-json-schemas/-/merge-json-schemas-0.1.1.tgz",
-      "integrity": "sha512-fERDVz7topgNjtXsJTTW1JKLy0rhuLRcquYqNR9rF7OcVpCa2OVW49ZPDIhaRRCaUuvVxI+N416xUoF76HNSXA==",
+      "version": "0.2.1",
+      "resolved": "https://registry.npmjs.org/@fastify/merge-json-schemas/-/merge-json-schemas-0.2.1.tgz",
+      "integrity": "sha512-OA3KGBCy6KtIvLf8DINC5880o5iBlDX4SxzLQS8HorJAbqluzLRn80UXU0bxZn7UOFhFgpRJDasfwn9nG4FG4A==",
+      "funding": [
+        {
+          "type": "github",
+          "url": "https://github.com/sponsors/fastify"
+        },
+        {
+          "type": "opencollective",
+          "url": "https://opencollective.com/fastify"
+        }
+      ],
+      "license": "MIT",
+      "dependencies": {
+        "dequal": "^2.0.3"
+      }
+    },
+    "node_modules/@fastify/proxy-addr": {
+      "version": "5.1.0",
+      "resolved": "https://registry.npmjs.org/@fastify/proxy-addr/-/proxy-addr-5.1.0.tgz",
+      "integrity": "sha512-INS+6gh91cLUjB+PVHfu1UqcB76Sqtpyp7bnL+FYojhjygvOPA9ctiD/JDKsyD9Xgu4hUhCSJBPig/w7duNajw==",
+      "funding": [
+        {
+          "type": "github",
+          "url": "https://github.com/sponsors/fastify"
+        },
+        {
+          "type": "opencollective",
+          "url": "https://opencollective.com/fastify"
+        }
+      ],
       "license": "MIT",
       "dependencies": {
-        "fast-deep-equal": "^3.1.3"
+        "@fastify/forwarded": "^3.0.0",
+        "ipaddr.js": "^2.1.0"
       }
     },
     "node_modules/@humanwhocodes/config-array": {
@@ -1445,7 +1531,6 @@
       "integrity": "sha512-WJtwWJu7UdlvzEAUm484QNg5eAoq5QR08KDNx7g45Usrs2NtOPiX8ugDqmKdXkyL03rBqU5dYNYVQetEpBHq2g==",
       "dev": true,
       "license": "MIT",
-      "peer": true,
       "dependencies": {
         "undici-types": "~6.21.0"
       }
@@ -1521,7 +1606,6 @@
       "integrity": "sha512-4Z+L8I2OqhZV8qA132M4wNL30ypZGYOQVBfMgxDH/K5UX0PNqTu1c6za9ST5r9+tavvHiTWmBnKzpCJ/GlVFtg==",
       "dev": true,
       "license": "BSD-2-Clause",
-      "peer": true,
       "dependencies": {
         "@typescript-eslint/scope-manager": "7.18.0",
         "@typescript-eslint/types": "7.18.0",
@@ -1694,7 +1778,6 @@
       "integrity": "sha512-NZyJarBfL7nWwIq+FDL6Zp/yHEhePMNnnJ0y3qfieCrmNvYct8uvtiV41UvlSe6apAfk0fY1FbWx+NwfmpvtTg==",
       "dev": true,
       "license": "MIT",
-      "peer": true,
       "bin": {
         "acorn": "bin/acorn"
       },
@@ -1861,12 +1944,22 @@
       }
     },
     "node_modules/avvio": {
-      "version": "8.4.0",
-      "resolved": "https://registry.npmjs.org/avvio/-/avvio-8.4.0.tgz",
-      "integrity": "sha512-CDSwaxINFy59iNwhYnkvALBwZiTydGkOecZyPkqBpABYR1KqGEsET0VOOYDwtleZSUIdeY36DC2bSZ24CO1igA==",
+      "version": "9.2.0",
+      "resolved": "https://registry.npmjs.org/avvio/-/avvio-9.2.0.tgz",
+      "integrity": "sha512-2t/sy01ArdHHE0vRH5Hsay+RtCZt3dLPji7W7/MMOCEgze5b7SNDC4j5H6FnVgPkI1MTNFGzHdHrVXDDl7QSSQ==",
+      "funding": [
+        {
+          "type": "github",
+          "url": "https://github.com/sponsors/fastify"
+        },
+        {
+          "type": "opencollective",
+          "url": "https://opencollective.com/fastify"
+        }
+      ],
       "license": "MIT",
       "dependencies": {
-        "@fastify/error": "^3.3.0",
+        "@fastify/error": "^4.0.0",
         "fastq": "^1.17.1"
       }
     },
@@ -2056,7 +2149,6 @@
         }
       ],
       "license": "MIT",
-      "peer": true,
       "dependencies": {
         "baseline-browser-mapping": "^2.9.0",
         "caniuse-lite": "^1.0.30001759",
@@ -2269,12 +2361,16 @@
       "license": "MIT"
     },
     "node_modules/cookie": {
-      "version": "0.7.2",
-      "resolved": "https://registry.npmjs.org/cookie/-/cookie-0.7.2.tgz",
-      "integrity": "sha512-yki5XnKuf750l50uGTllt6kKILY4nQ1eNIQatoXEByZ5dWgnKqbnqmTrBE5B4N7lrMJKQ2ytWMiTO2o0v6Ew/w==",
+      "version": "1.1.1",
+      "resolved": "https://registry.npmjs.org/cookie/-/cookie-1.1.1.tgz",
+      "integrity": "sha512-ei8Aos7ja0weRpFzJnEA9UHJ/7XQmqglbRwnf2ATjcB9Wq874VKH9kfjjirM6UhU2/E5fFYadylyhFldcqSidQ==",
       "license": "MIT",
       "engines": {
-        "node": ">= 0.6"
+        "node": ">=18"
+      },
+      "funding": {
+        "type": "opencollective",
+        "url": "https://opencollective.com/express"
       }
     },
     "node_modules/create-jest": {
@@ -2371,6 +2467,15 @@
         "node": ">=0.10.0"
       }
     },
+    "node_modules/dequal": {
+      "version": "2.0.3",
+      "resolved": "https://registry.npmjs.org/dequal/-/dequal-2.0.3.tgz",
+      "integrity": "sha512-0je+qPKHEMohvfRTCEo3CrPG6cAzAYgmzKyxRiYSSDkS6eGJdyVJm7WaYA5ECaAD9wLB2T4EEeymA5aFVcYXCA==",
+      "license": "MIT",
+      "engines": {
+        "node": ">=6"
+      }
+    },
     "node_modules/detect-newline": {
       "version": "3.1.0",
       "resolved": "https://registry.npmjs.org/detect-newline/-/detect-newline-3.1.0.tgz",
@@ -2494,7 +2599,6 @@
       "deprecated": "This version is no longer supported. Please see https://eslint.org/version-support for other options.",
       "dev": true,
       "license": "MIT",
-      "peer": true,
       "dependencies": {
         "@eslint-community/eslint-utils": "^4.2.0",
         "@eslint-community/regexpp": "^4.6.1",
@@ -2751,12 +2855,6 @@
         "node": "^14.15.0 || ^16.10.0 || >=18.0.0"
       }
     },
-    "node_modules/fast-content-type-parse": {
-      "version": "1.1.0",
-      "resolved": "https://registry.npmjs.org/fast-content-type-parse/-/fast-content-type-parse-1.1.0.tgz",
-      "integrity": "sha512-fBHHqSTFLVnR61C+gltJuE5GkVQMV0S2nqUO8TJ+5Z3qAKG8vAx4FKai1s5jq/inV1+sREynIWSuQ6HgoSXpDQ==",
-      "license": "MIT"
-    },
     "node_modules/fast-decode-uri-component": {
       "version": "1.0.1",
       "resolved": "https://registry.npmjs.org/fast-decode-uri-component/-/fast-decode-uri-component-1.0.1.tgz",
@@ -2807,17 +2905,26 @@
       "license": "MIT"
     },
     "node_modules/fast-json-stringify": {
-      "version": "5.16.1",
-      "resolved": "https://registry.npmjs.org/fast-json-stringify/-/fast-json-stringify-5.16.1.tgz",
-      "integrity": "sha512-KAdnLvy1yu/XrRtP+LJnxbBGrhN+xXu+gt3EUvZhYGKCr3lFHq/7UFJHHFgmJKoqlh6B40bZLEv7w46B0mqn1g==",
+      "version": "6.3.0",
+      "resolved": "https://registry.npmjs.org/fast-json-stringify/-/fast-json-stringify-6.3.0.tgz",
+      "integrity": "sha512-oRCntNDY/329HJPlmdNLIdogNtt6Vyjb1WuT01Soss3slIdyUp8kAcDU3saQTOquEK8KFVfwIIF7FebxUAu+yA==",
+      "funding": [
+        {
+          "type": "github",
+          "url": "https://github.com/sponsors/fastify"
+        },
+        {
+          "type": "opencollective",
+          "url": "https://opencollective.com/fastify"
+        }
+      ],
       "license": "MIT",
       "dependencies": {
-        "@fastify/merge-json-schemas": "^0.1.0",
-        "ajv": "^8.10.0",
+        "@fastify/merge-json-schemas": "^0.2.0",
+        "ajv": "^8.12.0",
         "ajv-formats": "^3.0.1",
-        "fast-deep-equal": "^3.1.3",
-        "fast-uri": "^2.1.0",
-        "json-schema-ref-resolver": "^1.0.1",
+        "fast-uri": "^3.0.0",
+        "json-schema-ref-resolver": "^3.0.0",
         "rfdc": "^1.2.0"
       }
     },
@@ -2838,12 +2945,6 @@
         }
       }
     },
-    "node_modules/fast-json-stringify/node_modules/fast-uri": {
-      "version": "2.4.0",
-      "resolved": "https://registry.npmjs.org/fast-uri/-/fast-uri-2.4.0.tgz",
-      "integrity": "sha512-ypuAmmMKInk5q7XcepxlnUWDLWv4GFtaJqAzWKqn62IpQ3pejtr5dTVbt3vwqVaMKmkNR55sTT+CqUKIaT21BA==",
-      "license": "MIT"
-    },
     "node_modules/fast-levenshtein": {
       "version": "2.0.6",
       "resolved": "https://registry.npmjs.org/fast-levenshtein/-/fast-levenshtein-2.0.6.tgz",
@@ -2877,9 +2978,9 @@
       "license": "BSD-3-Clause"
     },
     "node_modules/fastify": {
-      "version": "4.29.1",
-      "resolved": "https://registry.npmjs.org/fastify/-/fastify-4.29.1.tgz",
-      "integrity": "sha512-m2kMNHIG92tSNWv+Z3UeTR9AWLLuo7KctC7mlFPtMEVrfjIhmQhkQnT9v15qA/BfVq3vvj134Y0jl9SBje3jXQ==",
+      "version": "5.7.4",
+      "resolved": "https://registry.npmjs.org/fastify/-/fastify-5.7.4.tgz",
+      "integrity": "sha512-e6l5NsRdaEP8rdD8VR0ErJASeyaRbzXYpmkrpr2SuvuMq6Si3lvsaVy5C+7gLanEkvjpMDzBXWE5HPeb/hgTxA==",
       "funding": [
         {
           "type": "github",
@@ -2892,58 +2993,23 @@
       ],
       "license": "MIT",
       "dependencies": {
-        "@fastify/ajv-compiler": "^3.5.0",
-        "@fastify/error": "^3.4.0",
-        "@fastify/fast-json-stringify-compiler": "^4.3.0",
+        "@fastify/ajv-compiler": "^4.0.5",
+        "@fastify/error": "^4.0.0",
+        "@fastify/fast-json-stringify-compiler": "^5.0.0",
+        "@fastify/proxy-addr": "^5.0.0",
         "abstract-logging": "^2.0.1",
-        "avvio": "^8.3.0",
-        "fast-content-type-parse": "^1.1.0",
-        "fast-json-stringify": "^5.8.0",
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diff --git a/package.json b/package.json
index 03751cb..ca675bb 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "@globaltypesystem/gts-ts",
-  "version": "0.1.1",
+  "version": "0.2.0",
   "description": "TypeScript library for working with GTS (Global Type System) identifiers and JSON/JSON Schema artifacts",
   "main": "dist/index.js",
   "types": "dist/index.d.ts",
@@ -47,8 +47,7 @@
     "ajv": "^8.12.0",
     "ajv-formats": "^2.1.1",
     "commander": "^12.0.0",
-    "fastify": "^4.26.0",
-    "fastify-cors": "^6.1.0",
+    "fastify": "^5.7.3",
     "uuid": "^9.0.1"
   }
 }
diff --git a/src/server/server.ts b/src/server/server.ts
index 491946b..822ba8b 100644
--- a/src/server/server.ts
+++ b/src/server/server.ts
@@ -13,6 +13,8 @@ import {
   CompatibilityParams,
   CastBody,
   QueryParams,
+  ValidateSchemaBody,
+  ValidateEntityBody,
 } from './types';
 import * as gts from '../index';
 
@@ -99,6 +101,12 @@ export class GtsServer {
     // OP#11 - Attribute Access
     this.fastify.get('/attr', this.handleAttribute.bind(this));
 
+    // OP#12 - Validate Schema
+    this.fastify.post('/validate-schema', this.handleValidateSchema.bind(this));
+
+    // OP#12 - Validate Entity (unified)
+    this.fastify.post('/validate-entity', this.handleValidateEntity.bind(this));
+
     // OpenAPI spec
     this.fastify.get('/openapi', this.handleOpenAPI.bind(this));
   }
@@ -584,6 +592,42 @@ export class GtsServer {
     return this.store['store'].getAttribute(gtsId, path);
   }
 
+  // OP#12 - Validate Schema
+  private async handleValidateSchema(
+    request: FastifyRequest<{ Body: ValidateSchemaBody }>,
+    _reply: FastifyReply
+  ): Promise<any> {
+    const { schema_id } = request.body;
+    if (!schema_id) {
+      return { ok: false, error: 'Missing required field: schema_id' };
+    }
+    return this.store['store'].validateSchemaAgainstParent(schema_id);
+  }
+
+  // OP#12 - Validate Entity (unified)
+  private async handleValidateEntity(
+    request: FastifyRequest<{ Body: ValidateEntityBody }>,
+    _reply: FastifyReply
+  ): Promise<any> {
+    const id = request.body.entity_id || request.body.gts_id;
+    if (!id) {
+      return { ok: false, error: 'Missing required field: entity_id or gts_id' };
+    }
+
+    const entity = this.store['store'].get(id);
+    if (!entity) {
+      return { ok: false, error: `Entity not found: ${id}` };
+    }
+
+    if (entity.isSchema) {
+      const result = this.store['store'].validateSchemaAgainstParent(id);
+      return { ...result, entity_type: 'schema' };
+    } else {
+      const result = this.store.validateInstance(id);
+      return { ...result, entity_type: 'instance' };
+    }
+  }
+
   // OpenAPI Specification
   private async handleOpenAPI(): Promise<any> {
     return {
diff --git a/src/server/types.ts b/src/server/types.ts
index 12d54d8..671a2f3 100644
--- a/src/server/types.ts
+++ b/src/server/types.ts
@@ -72,3 +72,12 @@ export interface AttributeParams {
   gts_id: string;
   path: string;
 }
+
+export interface ValidateSchemaBody {
+  schema_id: string;
+}
+
+export interface ValidateEntityBody {
+  entity_id?: string;
+  gts_id?: string;
+}
diff --git a/src/store.ts b/src/store.ts
index 3d205fa..1b6bb0e 100644
--- a/src/store.ts
+++ b/src/store.ts
@@ -4,6 +4,13 @@ import { Gts } from './gts';
 import { GtsExtractor } from './extract';
 import { XGtsRefValidator } from './x-gts-ref';
 
+interface ResolvedSchema {
+  properties: Record<string, any>;
+  required: string[];
+  additionalProperties?: boolean;
+  type?: string;
+}
+
 export class GtsStore {
   private byId: Map<string, JsonEntity> = new Map();
   private config: GtsConfig;
@@ -194,6 +201,9 @@ export class GtsStore {
     const normalized: any = {};
 
     for (const [key, value] of Object.entries(obj)) {
+      // Strip x-gts-ref so Ajv never sees the unknown keyword
+      if (key === 'x-gts-ref') continue;
+
       let newKey = key;
       let newValue = value;
 
@@ -221,6 +231,25 @@ export class GtsStore {
       normalized[newKey] = newValue;
     }
 
+    // Clean up combinator arrays: remove subschemas that were x-gts-ref-only (now empty after stripping)
+    for (const combinator of ['oneOf', 'anyOf', 'allOf']) {
+      if (Array.isArray(normalized[combinator])) {
+        normalized[combinator] = normalized[combinator].filter((_sub: any, idx: number) => {
+          const original = (obj as any)[combinator]?.[idx];
+          const isXGtsRefOnly =
+            original &&
+            typeof original === 'object' &&
+            !Array.isArray(original) &&
+            Object.keys(original).length === 1 &&
+            original['x-gts-ref'] !== undefined;
+          return !isXGtsRefOnly;
+        });
+        if (normalized[combinator].length === 0) {
+          delete normalized[combinator];
+        }
+      }
+    }
+
     // Normalize $id values
     if (normalized['$id'] && typeof normalized['$id'] === 'string') {
       if (normalized['$id'].startsWith(GTS_URI_PREFIX)) {
@@ -1053,6 +1082,425 @@ export class GtsStore {
     return unique.sort();
   }
 
+  validateSchemaAgainstParent(schemaId: string): ValidationResult {
+    const entity = this.get(schemaId);
+    if (!entity) {
+      return { id: schemaId, ok: false, error: `Entity not found: ${schemaId}` };
+    }
+    if (!entity.isSchema) {
+      return { id: schemaId, ok: false, error: `Entity is not a schema: ${schemaId}` };
+    }
+
+    const content = entity.content;
+
+    // Find parent reference in allOf
+    const parentRef = this.findParentRef(content);
+    if (!parentRef) {
+      // Base schema with no parent → always valid
+      return { id: schemaId, ok: true, error: '' };
+    }
+
+    // Resolve parent entity
+    const parentId = parentRef.startsWith(GTS_URI_PREFIX) ? parentRef.substring(GTS_URI_PREFIX.length) : parentRef;
+    const parentEntity = this.get(parentId);
+    if (!parentEntity) {
+      return { id: schemaId, ok: false, error: `Parent schema not found: ${parentId}` };
+    }
+    if (!parentEntity.isSchema || !parentEntity.content) {
+      return { id: schemaId, ok: false, error: `Parent entity is not a schema: ${parentId}` };
+    }
+
+    // Resolve parent's effective (fully flattened) schema
+    const resolvedParent = this.resolveSchemaFully(parentEntity.content);
+
+    // Extract overlay from derived schema (non-$ref subschemas in allOf + top-level)
+    const overlay = this.extractOverlay(content);
+
+    // Compare overlay against resolved parent
+    const errors = this.compareOverlayToBase(overlay, resolvedParent, '');
+    if (errors.length > 0) {
+      return { id: schemaId, ok: false, error: errors.join('; ') };
+    }
+
+    return { id: schemaId, ok: true, error: '' };
+  }
+
+  private findParentRef(schema: any): string | null {
+    if (!schema || !schema.allOf || !Array.isArray(schema.allOf)) {
+      return null;
+    }
+    for (const sub of schema.allOf) {
+      if (sub && typeof sub === 'object') {
+        const ref = sub['$$ref'] || sub['$ref'];
+        if (typeof ref === 'string') {
+          return ref;
+        }
+      }
+    }
+    return null;
+  }
+
+  private resolveSchemaFully(schema: any, visited: Set<string> = new Set()): ResolvedSchema {
+    const result: ResolvedSchema = {
+      properties: {},
+      required: [],
+      additionalProperties: undefined,
+      type: schema.type,
+    };
+
+    // If this schema has allOf, resolve each part
+    if (schema.allOf && Array.isArray(schema.allOf)) {
+      for (const sub of schema.allOf) {
+        const ref = sub['$$ref'] || sub['$ref'];
+        if (typeof ref === 'string') {
+          // Resolve referenced schema
+          const refId = ref.startsWith(GTS_URI_PREFIX) ? ref.substring(GTS_URI_PREFIX.length) : ref;
+          if (visited.has(refId)) {
+            continue;
+          }
+          visited.add(refId);
+          const refEntity = this.get(refId);
+          if (refEntity && refEntity.content) {
+            const resolved = this.resolveSchemaFully(refEntity.content, visited);
+            Object.assign(result.properties, resolved.properties);
+            if (resolved.required) {
+              result.required.push(...resolved.required);
+            }
+            if (resolved.additionalProperties !== undefined) {
+              result.additionalProperties = resolved.additionalProperties;
+            }
+            if (resolved.type && !result.type) {
+              result.type = resolved.type;
+            }
+          }
+        } else {
+          // Non-ref subschema - merge it
+          const resolved = this.resolveSchemaFully(sub, visited);
+          // For overlay properties, merge them (they override)
+          for (const [propName, propSchema] of Object.entries(resolved.properties || {})) {
+            if (result.properties[propName]) {
+              // Merge property constraints - overlay tightens base
+              result.properties[propName] = this.mergePropertySchemas(result.properties[propName], propSchema);
+            } else {
+              result.properties[propName] = propSchema;
+            }
+          }
+          if (resolved.required) {
+            result.required.push(...resolved.required);
+          }
+          if (resolved.additionalProperties !== undefined) {
+            result.additionalProperties = resolved.additionalProperties;
+          }
+        }
+      }
+    }
+
+    // Add direct properties
+    if (schema.properties) {
+      for (const [propName, propSchema] of Object.entries(schema.properties)) {
+        if (result.properties[propName]) {
+          result.properties[propName] = this.mergePropertySchemas(result.properties[propName], propSchema);
+        } else {
+          result.properties[propName] = propSchema;
+        }
+      }
+    }
+
+    // Add direct required
+    if (schema.required && Array.isArray(schema.required)) {
+      result.required.push(...schema.required);
+    }
+
+    // Direct additionalProperties
+    if (schema.additionalProperties !== undefined) {
+      result.additionalProperties = schema.additionalProperties;
+    }
+
+    // Deduplicate required
+    result.required = Array.from(new Set(result.required));
+
+    return result;
+  }
+
+  private mergePropertySchemas(base: any, overlay: any): any {
+    if (base === false || overlay === false) {
+      return false;
+    }
+    if (typeof base !== 'object' || typeof overlay !== 'object') {
+      return overlay;
+    }
+    const merged: any = { ...base };
+    for (const [key, val] of Object.entries(overlay)) {
+      if (key === 'properties' && merged.properties) {
+        merged.properties = { ...merged.properties, ...(val as any) };
+      } else if (key === 'required' && merged.required) {
+        const mergedReq = new Set([...(merged.required as string[]), ...(val as string[])]);
+        merged.required = Array.from(mergedReq);
+      } else {
+        merged[key] = val;
+      }
+    }
+    return merged;
+  }
+
+  private extractOverlay(schema: any): ResolvedSchema {
+    const overlay: ResolvedSchema = {
+      properties: {},
+      required: [],
+      additionalProperties: undefined,
+    };
+
+    if (schema.allOf && Array.isArray(schema.allOf)) {
+      for (const sub of schema.allOf) {
+        const ref = sub['$$ref'] || sub['$ref'];
+        if (typeof ref === 'string') {
+          continue; // Skip ref subschemas
+        }
+        // This is a non-ref overlay subschema
+        if (sub.properties) {
+          for (const [propName, propSchema] of Object.entries(sub.properties)) {
+            overlay.properties[propName] = overlay.properties[propName]
+              ? this.mergePropertySchemas(overlay.properties[propName], propSchema)
+              : propSchema;
+          }
+        }
+        if (sub.required && Array.isArray(sub.required)) {
+          overlay.required.push(...sub.required);
+        }
+        if (sub.additionalProperties !== undefined) {
+          overlay.additionalProperties = sub.additionalProperties;
+        }
+      }
+    }
+
+    // Add top-level properties (outside allOf)
+    if (schema.properties) {
+      for (const [propName, propSchema] of Object.entries(schema.properties)) {
+        overlay.properties[propName] = overlay.properties[propName]
+          ? this.mergePropertySchemas(overlay.properties[propName], propSchema)
+          : propSchema;
+      }
+    }
+    if (schema.required && Array.isArray(schema.required)) {
+      overlay.required.push(...schema.required);
+    }
+    if (schema.additionalProperties !== undefined && overlay.additionalProperties === undefined) {
+      overlay.additionalProperties = schema.additionalProperties;
+    }
+
+    return overlay;
+  }
+
+  private compareOverlayToBase(overlay: ResolvedSchema, baseResolved: ResolvedSchema, path: string): string[] {
+    const errors: string[] = [];
+    const overlayProps = overlay.properties || {};
+    const baseProps = baseResolved.properties || {};
+
+    for (const [propName, propSchema] of Object.entries(overlayProps)) {
+      const propPath = path ? `${path}.${propName}` : propName;
+
+      // Property schema set to false
+      if (propSchema === false) {
+        if (baseProps[propName] !== undefined) {
+          errors.push(`Property '${propPath}' is set to false but exists in base`);
+        }
+        continue;
+      }
+
+      const baseProp = baseProps[propName];
+
+      if (baseProp === undefined || baseProp === null) {
+        // New property not in base
+        if (baseResolved.additionalProperties === false) {
+          errors.push(`Property '${propPath}' not in base and base has additionalProperties: false`);
+        }
+        continue;
+      }
+
+      if (baseProp === false) {
+        // Base already set property to false, overlay can't use it
+        errors.push(`Property '${propPath}' is forbidden in base`);
+        continue;
+      }
+
+      // Both base and overlay have this property — compare constraints
+      if (typeof propSchema === 'object' && propSchema !== null) {
+        errors.push(...this.comparePropertyConstraints(propSchema, baseProp, propPath));
+      }
+    }
+
+    // Check additionalProperties
+    if (baseResolved.additionalProperties === false) {
+      if (overlay.additionalProperties === true) {
+        errors.push('Cannot loosen additionalProperties from false to true');
+      } else if (overlay.additionalProperties === undefined) {
+        errors.push('Base has additionalProperties: false but derived does not restate it');
+      }
+    }
+
+    return errors;
+  }
+
+  private comparePropertyConstraints(derived: any, base: any, propPath: string): string[] {
+    const errors: string[] = [];
+
+    if (typeof base !== 'object' || base === null) {
+      return errors;
+    }
+
+    // Type check
+    const baseType = base.type;
+    const derivedType = derived.type;
+    if (baseType !== undefined && derivedType !== undefined) {
+      if (Array.isArray(derivedType)) {
+        // Derived has array type — widening (fail)
+        if (!Array.isArray(baseType)) {
+          errors.push(`Property '${propPath}' widens type from '${baseType}' to array`);
+          return errors;
+        }
+      }
+      if (Array.isArray(baseType)) {
+        if (!Array.isArray(derivedType)) {
+          // Could be narrowing from array type
+          if (!baseType.includes(derivedType)) {
+            errors.push(`Property '${propPath}' type '${derivedType}' not in base types [${baseType}]`);
+            return errors;
+          }
+        }
+      } else if (!Array.isArray(derivedType)) {
+        // Both scalar types
+        if (baseType !== derivedType) {
+          errors.push(`Property '${propPath}' type changed from '${baseType}' to '${derivedType}'`);
+          return errors;
+        }
+      }
+    }
+
+    // Determine if the overlay adds any NEW constraint keywords not in the base.
+    // Under allOf semantics, base constraints are preserved. Drops are only flagged
+    // when the overlay doesn't introduce any new tightening constraints.
+    const CONSTRAINT_KEYWORDS = [
+      'maxLength',
+      'minLength',
+      'maximum',
+      'minimum',
+      'maxItems',
+      'minItems',
+      'enum',
+      'const',
+      'pattern',
+      'items',
+    ];
+    const baseConstraintKeys = new Set(CONSTRAINT_KEYWORDS.filter((kw) => base[kw] !== undefined));
+    const derivedConstraintKeys = new Set(CONSTRAINT_KEYWORDS.filter((kw) => derived[kw] !== undefined));
+    const hasNewConstraints = [...derivedConstraintKeys].some((kw) => !baseConstraintKeys.has(kw));
+
+    // Max constraints (tightening = lower value OK; loosening = higher value FAIL)
+    for (const kw of ['maxLength', 'maximum', 'maxItems']) {
+      if (base[kw] !== undefined) {
+        if (derived[kw] === undefined) {
+          if (!hasNewConstraints) {
+            errors.push(`Property '${propPath}' drops constraint '${kw}'`);
+          }
+        } else if (derived[kw] > base[kw]) {
+          errors.push(`Property '${propPath}' loosens '${kw}' from ${base[kw]} to ${derived[kw]}`);
+        }
+      }
+    }
+
+    // Min constraints (tightening = higher value OK; loosening = lower value FAIL)
+    for (const kw of ['minLength', 'minimum', 'minItems']) {
+      if (base[kw] !== undefined) {
+        if (derived[kw] === undefined) {
+          if (!hasNewConstraints) {
+            errors.push(`Property '${propPath}' drops constraint '${kw}'`);
+          }
+        } else if (derived[kw] < base[kw]) {
+          errors.push(`Property '${propPath}' loosens '${kw}' from ${base[kw]} to ${derived[kw]}`);
+        }
+      }
+    }
+
+    // Enum check
+    if (base.enum !== undefined) {
+      if (derived.enum === undefined) {
+        if (!hasNewConstraints) {
+          errors.push(`Property '${propPath}' drops constraint 'enum'`);
+        }
+      } else {
+        const baseSet = new Set(base.enum.map((v: any) => JSON.stringify(v)));
+        for (const val of derived.enum) {
+          if (!baseSet.has(JSON.stringify(val))) {
+            errors.push(`Property '${propPath}' enum value '${val}' not in base enum`);
+          }
+        }
+      }
+    }
+
+    // Const check
+    if (base.const !== undefined) {
+      if (derived.const === undefined) {
+        if (!hasNewConstraints) {
+          errors.push(`Property '${propPath}' drops constraint 'const'`);
+        }
+      } else if (JSON.stringify(base.const) !== JSON.stringify(derived.const)) {
+        errors.push(
+          `Property '${propPath}' const conflict: ${JSON.stringify(derived.const)} vs base ${JSON.stringify(base.const)}`
+        );
+      }
+    }
+    // Check const in derived against base numeric constraints
+    if (derived.const !== undefined && typeof derived.const === 'number') {
+      if (base.minimum !== undefined && derived.const < base.minimum) {
+        errors.push(`Property '${propPath}' const ${derived.const} violates base minimum ${base.minimum}`);
+      }
+      if (base.maximum !== undefined && derived.const > base.maximum) {
+        errors.push(`Property '${propPath}' const ${derived.const} violates base maximum ${base.maximum}`);
+      }
+    }
+
+    // Pattern check
+    if (base.pattern !== undefined) {
+      if (derived.pattern === undefined) {
+        if (!hasNewConstraints) {
+          errors.push(`Property '${propPath}' drops constraint 'pattern'`);
+        }
+      } else if (base.pattern !== derived.pattern) {
+        errors.push(`Property '${propPath}' pattern changed from '${base.pattern}' to '${derived.pattern}'`);
+      }
+    }
+
+    // Items check (array items)
+    if (base.items !== undefined) {
+      if (derived.items === undefined) {
+        if (!hasNewConstraints) {
+          errors.push(`Property '${propPath}' drops constraint 'items'`);
+        }
+      } else if (typeof base.items === 'object' && typeof derived.items === 'object') {
+        errors.push(...this.comparePropertyConstraints(derived.items, base.items, `${propPath}.items`));
+      }
+    }
+
+    // Nested object: recursively compare
+    if (base.type === 'object' && derived.type === 'object') {
+      if (base.properties || derived.properties) {
+        const nestedOverlay = {
+          properties: derived.properties || {},
+          required: derived.required || [],
+          additionalProperties: derived.additionalProperties,
+        };
+        const nestedBase = {
+          properties: base.properties || {},
+          required: base.required || [],
+          additionalProperties: base.additionalProperties,
+        };
+        errors.push(...this.compareOverlayToBase(nestedOverlay, nestedBase, propPath));
+      }
+    }
+
+    return errors;
+  }
+
   getAttribute(gtsId: string, path: string): any {
     const entity = this.get(gtsId);
     if (!entity) {
diff --git a/src/x-gts-ref.ts b/src/x-gts-ref.ts
index ba4875f..697b249 100644
--- a/src/x-gts-ref.ts
+++ b/src/x-gts-ref.ts
@@ -81,6 +81,56 @@ export class XGtsRefValidator {
         });
       }
     }
+
+    // Recurse into combinator subschemas
+    if (Array.isArray(schema.allOf)) {
+      for (const subSchema of schema.allOf) {
+        this.visitInstance(instance, subSchema, path, rootSchema, errors);
+      }
+    }
+
+    if (Array.isArray(schema.anyOf)) {
+      // Only enforce when all branches have x-gts-ref; mixed branches may be valid via non-x-gts-ref path (Ajv handles that)
+      const refBranches = schema.anyOf.filter((s: any) => this.containsXGtsRef(s));
+      if (refBranches.length > 0 && refBranches.length === schema.anyOf.length) {
+        const branchResults = refBranches.map((subSchema: any) => {
+          const branchErrors: XGtsRefValidationError[] = [];
+          this.visitInstance(instance, subSchema, path, rootSchema, branchErrors);
+          return branchErrors;
+        });
+        const anyPassed = branchResults.some((errs: XGtsRefValidationError[]) => errs.length === 0);
+        if (!anyPassed) {
+          for (const branchErrors of branchResults) {
+            errors.push(...branchErrors);
+          }
+        }
+      }
+    }
+
+    if (Array.isArray(schema.oneOf)) {
+      // Only enforce when all branches have x-gts-ref; mixed branches can't be coordinated with Ajv's branch selection
+      const refBranches = schema.oneOf.filter((s: any) => this.containsXGtsRef(s));
+      if (refBranches.length > 0 && refBranches.length === schema.oneOf.length) {
+        const branchResults = refBranches.map((subSchema: any) => {
+          const branchErrors: XGtsRefValidationError[] = [];
+          this.visitInstance(instance, subSchema, path, rootSchema, branchErrors);
+          return branchErrors;
+        });
+        const passingCount = branchResults.filter((errs: XGtsRefValidationError[]) => errs.length === 0).length;
+        if (passingCount === 0) {
+          for (const branchErrors of branchResults) {
+            errors.push(...branchErrors);
+          }
+        } else if (passingCount > 1) {
+          errors.push({
+            fieldPath: path || '/',
+            value: instance,
+            refPattern: '',
+            reason: `Value matches ${passingCount} oneOf branches but must match exactly one`,
+          });
+        }
+      }
+    }
   }
 
   private visitSchema(schema: any, path: string, rootSchema: any, errors: XGtsRefValidationError[]): void {
@@ -300,6 +350,19 @@ export class XGtsRefValidator {
     return null;
   }
 
+  private containsXGtsRef(schema: any): boolean {
+    if (!schema || typeof schema !== 'object') return false;
+    if (schema['x-gts-ref'] !== undefined) return true;
+    for (const value of Object.values(schema)) {
+      if (Array.isArray(value)) {
+        if (value.some((item) => this.containsXGtsRef(item))) return true;
+      } else if (value && typeof value === 'object') {
+        if (this.containsXGtsRef(value)) return true;
+      }
+    }
+    return false;
+  }
+
   /**
    * Strip the "gts://" prefix from a value if present
    */
diff --git a/tests/gts.test.ts b/tests/gts.test.ts
index 7c60b3c..82d1110 100644
--- a/tests/gts.test.ts
+++ b/tests/gts.test.ts
@@ -414,6 +414,8 @@ describe('GTS Store Operations', () => {
     });
   });
 
+  // x-gts-ref combinator tests (oneOf/anyOf/allOf) are in the canonical gts-spec test suite
+
   describe('OP#12 - Wildcard Validation (v0.7)', () => {
     test('validates wildcard patterns', () => {
       const result = validateGtsID('gts.vendor.pkg.*');