diff --git a/tools/spectral/ipa/__tests__/IPA125OneOfMustHaveDiscriminator.test.js b/tools/spectral/ipa/__tests__/IPA125OneOfMustHaveDiscriminator.test.js
new file mode 100644
index 0000000000..339b067997
--- /dev/null
+++ b/tools/spectral/ipa/__tests__/IPA125OneOfMustHaveDiscriminator.test.js
@@ -0,0 +1,185 @@
+import testRule from './__helpers__/testRule';
+import { DiagnosticSeverity } from '@stoplight/types';
+
+const componentSchemas = {
+  schemas: {
+    Dog: {
+      type: 'object',
+      properties: {
+        breed: { type: 'string' },
+        age: { type: 'integer' },
+      },
+    },
+    Cat: {
+      type: 'object',
+      properties: {
+        color: { type: 'string' },
+        livesLeft: { type: 'integer' },
+      },
+    },
+    Bird: {
+      type: 'object',
+      properties: {
+        species: { type: 'string' },
+        wingspan: { type: 'number' },
+      },
+    },
+    Fish: {
+      type: 'object',
+      properties: {
+        species: { type: 'string' },
+        waterType: { type: 'string' },
+      },
+    },
+  },
+};
+
+testRule('xgen-IPA-125-oneOf-must-have-discriminator', [
+  {
+    name: 'valid oneOf with discriminator and matching mapping',
+    document: {
+      components: componentSchemas,
+      schemas: {
+        Animal: {
+          oneOf: [{ $ref: '#/components/schemas/Dog' }, { $ref: '#/components/schemas/Cat' }],
+          discriminator: {
+            propertyName: 'type',
+            mapping: {
+              dog: '#/components/schemas/Dog',
+              cat: '#/components/schemas/Cat',
+            },
+          },
+        },
+      },
+    },
+    errors: [],
+  },
+  {
+    name: 'invalid oneOf with discriminator but mismatched mapping',
+    document: {
+      components: componentSchemas,
+      schemas: {
+        Animal: {
+          oneOf: [{ $ref: '#/components/schemas/Dog' }, { $ref: '#/components/schemas/Cat' }],
+          discriminator: {
+            propertyName: 'type',
+            mapping: {
+              dog: '#/components/schemas/Dog',
+              bird: '#/components/schemas/Bird',
+            },
+          },
+        },
+      },
+    },
+    errors: [
+      {
+        code: 'xgen-IPA-125-oneOf-must-have-discriminator',
+        message:
+          'The discriminator mapping must match the oneOf references. Unmatched Discriminator mappings with oneOf references: #/components/schemas/Bird',
+        path: ['schemas', 'Animal'],
+        severity: DiagnosticSeverity.Error,
+      },
+    ],
+  },
+  {
+    name: 'invalid oneOf without discriminator',
+    document: {
+      components: componentSchemas,
+      schemas: {
+        Animal: {
+          oneOf: [{ $ref: '#/components/schemas/Dog' }, { $ref: '#/components/schemas/Cat' }],
+        },
+      },
+    },
+    errors: [
+      {
+        code: 'xgen-IPA-125-oneOf-must-have-discriminator',
+        message: 'The schema has oneOf but no discriminator property.',
+        path: ['schemas', 'Animal'],
+        severity: DiagnosticSeverity.Error,
+      },
+    ],
+  },
+  {
+    name: 'invalid oneOf with non-object discriminator',
+    document: {
+      components: componentSchemas,
+      schemas: {
+        Animal: {
+          oneOf: [{ $ref: '#/components/schemas/Dog' }, { $ref: '#/components/schemas/Cat' }],
+          discriminator: "I'm a string, not an object!",
+        },
+      },
+    },
+    errors: [
+      {
+        code: 'xgen-IPA-125-oneOf-must-have-discriminator',
+        message: 'Discriminator property is not an object.',
+        path: ['schemas', 'Animal'],
+        severity: DiagnosticSeverity.Error,
+      },
+    ],
+  },
+  {
+    name: 'invalid oneOf with discriminator but no propertyName',
+    document: {
+      components: componentSchemas,
+      schemas: {
+        Animal: {
+          oneOf: [{ $ref: '#/components/schemas/Dog' }, { $ref: '#/components/schemas/Cat' }],
+          discriminator: {
+            mapping: {
+              dog: '#/components/schemas/Dog',
+              cat: '#/components/schemas/Cat',
+            },
+          },
+        },
+      },
+    },
+    errors: [
+      {
+        code: 'xgen-IPA-125-oneOf-must-have-discriminator',
+        message: 'Discriminator has no propertyName defined.',
+        path: ['schemas', 'Animal'],
+        severity: DiagnosticSeverity.Error,
+      },
+    ],
+  },
+  {
+    name: 'invalid oneOf with discriminator but no mapping',
+    document: {
+      components: componentSchemas,
+      schemas: {
+        Animal: {
+          oneOf: [{ $ref: '#/components/schemas/Dog' }, { $ref: '#/components/schemas/Cat' }],
+          discriminator: {
+            propertyName: 'type',
+          },
+        },
+      },
+    },
+    errors: [
+      {
+        code: 'xgen-IPA-125-oneOf-must-have-discriminator',
+        message: 'Discriminator must have a mapping object.',
+        path: ['schemas', 'Animal'],
+        severity: DiagnosticSeverity.Error,
+      },
+    ],
+  },
+  {
+    name: 'oneOf with discriminator exemption',
+    document: {
+      components: componentSchemas,
+      schemas: {
+        Animal: {
+          oneOf: [{ $ref: '#/components/schemas/Dog' }, { $ref: '#/components/schemas/Cat' }],
+          'x-xgen-IPA-exception': {
+            'xgen-IPA-125-oneOf-must-have-discriminator': 'reason for exemption',
+          },
+        },
+      },
+    },
+    errors: [],
+  },
+]);
diff --git a/tools/spectral/ipa/__tests__/__helpers__/testRule.js b/tools/spectral/ipa/__tests__/__helpers__/testRule.js
index 824cb2687c..d3cc2e8240 100644
--- a/tools/spectral/ipa/__tests__/__helpers__/testRule.js
+++ b/tools/spectral/ipa/__tests__/__helpers__/testRule.js
@@ -4,6 +4,7 @@ import { describe, expect, it } from '@jest/globals';
 import { Spectral, Document } from '@stoplight/spectral-core';
 import { httpAndFileResolver } from '@stoplight/spectral-ref-resolver';
 import { bundleAndLoadRuleset } from '@stoplight/spectral-ruleset-bundler/with-loader';
+import { fail } from 'node:assert';
 
 export default (ruleName, tests) => {
   describe(`Rule ${ruleName}`, () => {
@@ -13,6 +14,12 @@ export default (ruleName, tests) => {
         const doc = testCase.document instanceof Document ? testCase.document : JSON.stringify(testCase.document);
         const errors = await s.run(doc);
 
+        if (testCase.errors.length !== errors.length) {
+          fail(`Expected errors do not match actual errors
+            (${testCase.errors.length} !== ${errors.length})
+           Expected errors: ${JSON.stringify(testCase.errors, undefined, 2)}
+           Actual errors:  ${JSON.stringify(errors, undefined, 2)}`);
+        }
         expect(errors.length).toEqual(testCase.errors.length);
 
         errors.forEach((error, index) => {
diff --git a/tools/spectral/ipa/rulesets/IPA-125.yaml b/tools/spectral/ipa/rulesets/IPA-125.yaml
new file mode 100644
index 0000000000..5f0154f8fd
--- /dev/null
+++ b/tools/spectral/ipa/rulesets/IPA-125.yaml
@@ -0,0 +1,41 @@
+# IPA-125: Single Type in Request and Response
+# http://go/ipa/125
+
+functions:
+  - IPA125OneOfMustHaveDiscriminator
+
+rules:
+  xgen-IPA-125-oneOf-must-have-discriminator:
+    description: |
+      Each oneOf property must include a discriminator property to define the exact type.
+
+      ##### Implementation details
+      Rule checks for the following conditions:
+        - Applies only to schemas with `oneOf` containing references
+        - Ensures a `discriminator` property is present with a valid `propertyName`
+        - Validates that `discriminator.mapping` contains exactly the same number of entries as `oneOf` references
+        - Validates that each `discriminator.mapping` value matches a reference in the `oneOf` array
+        - Ignores `oneOf` definitions with inline schemas
+
+      ##### Matching Logic
+      - The `discriminator.mapping` must have the same number of entries as there are references in the `oneOf` array
+      - Each value in the `discriminator.mapping` must match one of the `$ref` values in the `oneOf` array
+      - Each `$ref` in the `oneOf` array must have a corresponding entry in the `discriminator.mapping`
+      - Example:
+        ```yaml
+        oneOf:
+          - $ref: '#/components/schemas/Dog'
+          - $ref: '#/components/schemas/Cat'
+        discriminator:
+          propertyName: type
+          mapping:
+            dog: '#/components/schemas/Dog'
+            cat: '#/components/schemas/Cat'
+        ```
+        This is valid because there are exactly 2 mappings for 2 oneOf references, and all values match.
+
+    message: '{{error}} https://mdb.link/mongodb-atlas-openapi-validation#xgen-IPA-125-oneOf-must-have-discriminator'
+    severity: error
+    given: '$..[?(@.oneOf)]'
+    then:
+      function: 'IPA125OneOfMustHaveDiscriminator'
diff --git a/tools/spectral/ipa/rulesets/functions/IPA125OneOfMustHaveDiscriminator.js b/tools/spectral/ipa/rulesets/functions/IPA125OneOfMustHaveDiscriminator.js
new file mode 100644
index 0000000000..e1a6bdf256
--- /dev/null
+++ b/tools/spectral/ipa/rulesets/functions/IPA125OneOfMustHaveDiscriminator.js
@@ -0,0 +1,61 @@
+import { collectAdoption, collectAndReturnViolation } from './utils/collectionUtils.js';
+import { resolveObject } from './utils/componentUtils.js';
+import { hasException } from './utils/exceptions.js';
+
+const RULE_NAME = 'xgen-IPA-125-oneOf-must-have-discriminator';
+const MISSING_DISCRIMINATOR_MESSAGE = 'The schema has oneOf but no discriminator property.';
+const INVALID_DISCRIMINATOR_MESSAGE = 'Discriminator property is not an object.';
+const MISSING_PROPERTY_NAME_MESSAGE = 'Discriminator has no propertyName defined.';
+const MISSING_MAPPING_MESSAGE = 'Discriminator must have a mapping object.';
+const MAPPING_ERROR_MESSAGE =
+  'The discriminator mapping must match the oneOf references. Unmatched Discriminator mappings with oneOf references:';
+
+export default (input, _, { path, documentInventory }) => {
+  const oas = documentInventory.unresolved; // Use unresolved document to access raw $ref
+  const schema = resolveObject(oas, path);
+
+  if (!schema || !schema.oneOf || !Array.isArray(schema.oneOf)) {
+    return;
+  }
+
+  // Check for exception first
+  if (hasException(schema, RULE_NAME)) {
+    return;
+  }
+
+  // Check if all oneOf items are objects with a $ref property
+  const allReferences = schema.oneOf.every((item) => typeof item === 'object' && item.$ref);
+  if (!allReferences) {
+    return;
+  }
+
+  // Validate the presence of a discriminator with more specific error messages
+  if (!schema.discriminator) {
+    return collectAndReturnViolation(path, RULE_NAME, [{ message: MISSING_DISCRIMINATOR_MESSAGE }]);
+  }
+
+  if (typeof schema.discriminator !== 'object') {
+    return collectAndReturnViolation(path, RULE_NAME, [{ message: INVALID_DISCRIMINATOR_MESSAGE }]);
+  }
+
+  if (!schema.discriminator.propertyName) {
+    return collectAndReturnViolation(path, RULE_NAME, [{ message: MISSING_PROPERTY_NAME_MESSAGE }]);
+  }
+
+  if (!schema.discriminator.mapping) {
+    return collectAndReturnViolation(path, RULE_NAME, [{ message: MISSING_MAPPING_MESSAGE }]);
+  }
+
+  const oneOfRefs = schema.oneOf.map((item) => item.$ref);
+  const mappingValues = Object.values(schema.discriminator.mapping);
+
+  // Check for discriminator mappings that don't match any oneOf reference
+  const unmatchedMappings = mappingValues.filter((mapping) => !oneOfRefs.includes(mapping));
+  if (unmatchedMappings.length > 0) {
+    return collectAndReturnViolation(path, RULE_NAME, [
+      { message: `${MAPPING_ERROR_MESSAGE} ${unmatchedMappings.join(', ')}` },
+    ]);
+  }
+
+  collectAdoption(path, RULE_NAME);
+};