middleapi · dinwwwh · Jun 16, 2025 · Jun 16, 2025 · Jun 16, 2025
diff --git a/apps/content/docs/openapi/openapi-specification.md b/apps/content/docs/openapi/openapi-specification.md
@@ -136,12 +136,19 @@ const spec = await generator.generate(router, {
       strategy: 'output',
       schema: PetSchema,
     },
+    UndefinedError: {
+      error: 'UndefinedError'
+    }
   },
 })
 ```
 
 :::info
-The `strategy` option determines which schema definition to use when input and output types differ (defaults to `input`). This is needed because we cannot use the same `$ref` for both input and output in this case.
+
+- The `strategy` option determines which schema definition to use when input and output types differ (defaults to `input`). This is needed because we cannot use the same `$ref` for both input and output in this case.
+
+- `UndefinedError` is used for undefined errors, which is very useful when using [Type-Safe Error Handling](/docs/error-handling#type‐safe-error-handling).
+
 :::
 
 ## Excluding Procedures

diff --git a/packages/openapi/src/openapi-generator.test.ts b/packages/openapi/src/openapi-generator.test.ts
@@ -1082,6 +1082,9 @@ describe('openAPIGenerator', () => {
         OutputDetailedStructure: {
           schema: OutputDetailedStructure,
         },
+        UndefinedError2: {
+          error: 'UndefinedError',
+        },
       },
     })
 
@@ -1155,11 +1158,22 @@ describe('openAPIGenerator', () => {
               },
             ],
           },
+          UndefinedError2: {
+            type: 'object',
+            properties: {
+              defined: { const: false },
+              code: { type: 'string' },
+              status: { type: 'number' },
+              message: { type: 'string' },
+              data: {},
+            },
+            required: ['defined', 'code', 'status', 'message'],
+          },
         },
       })
     })
 
-    it('works with schema that input & output is same', async () => {
+    it('works with schema that input & output is same + error', async () => {
       expect(spec.paths!['/user']).toEqual({
         post: {
           requestBody: {
@@ -1197,15 +1211,7 @@ describe('openAPIGenerator', () => {
                         required: ['defined', 'code', 'status', 'message', 'data'],
                       },
                       {
-                        type: 'object',
-                        properties: {
-                          defined: { const: false },
-                          code: { type: 'string' },
-                          status: { type: 'number' },
-                          message: { type: 'string' },
-                          data: {},
-                        },
-                        required: ['defined', 'code', 'status', 'message'],
+                        $ref: '#/components/schemas/UndefinedError2',
                       },
                     ],
                   },
@@ -1218,7 +1224,7 @@ describe('openAPIGenerator', () => {
       })
     })
 
-    it('works with schema that input & output is different', async () => {
+    it('works with schema that input & output is different + error', async () => {
       expect(spec.paths!['/pet']).toEqual({
         post: {
           operationId: 'pet',
@@ -1263,15 +1269,7 @@ describe('openAPIGenerator', () => {
                         required: ['defined', 'code', 'status', 'message', 'data'],
                       },
                       {
-                        type: 'object',
-                        properties: {
-                          defined: { const: false },
-                          code: { type: 'string' },
-                          status: { type: 'number' },
-                          message: { type: 'string' },
-                          data: {},
-                        },
-                        required: ['defined', 'code', 'status', 'message'],
+                        $ref: '#/components/schemas/UndefinedError2',
                       },
                     ],
                   },

diff --git a/packages/openapi/src/openapi-generator.ts b/packages/openapi/src/openapi-generator.ts
@@ -14,7 +14,7 @@ import { checkParamsSchema, resolveOpenAPIJsonSchemaRef, toOpenAPIContent, toOpe
 import { CompositeSchemaConverter } from './schema-converter'
 import { applySchemaOptionality, expandUnionSchema, isAnySchema, isObjectSchema, separateObjectSchema } from './schema-utils'
 
-class OpenAPIGeneratorError extends Error {}
+class OpenAPIGeneratorError extends Error { }
 
 export interface OpenAPIGeneratorOptions extends StandardOpenAPIJsonSerializerOptions {
   schemaConverters?: ConditionalSchemaConverter[]
@@ -55,6 +55,9 @@ export interface OpenAPIGeneratorGenerateOptions extends Partial<Omit<OpenAPI.Do
      */
     strategy?: SchemaConvertOptions['strategy']
     schema: AnySchema
+  } | {
+    error: 'UndefinedError'
+    schema?: never
   }>
 }
 
@@ -88,7 +91,7 @@ export class OpenAPIGenerator {
       commonSchemas: undefined,
     } as OpenAPI.Document
 
-    const baseSchemaConvertOptions = await this.#resolveCommonSchemas(doc, options.commonSchemas)
+    const { baseSchemaConvertOptions, undefinedErrorJsonSchema } = await this.#resolveCommonSchemas(doc, options.commonSchemas)
 
     const contracts: { contract: AnyContractProcedure, path: readonly string[] }[] = []
 
@@ -125,7 +128,7 @@ export class OpenAPIGenerator {
 
           await this.#request(doc, operationObjectRef, def, baseSchemaConvertOptions)
           await this.#successResponse(doc, operationObjectRef, def, baseSchemaConvertOptions)
-          await this.#errorResponse(operationObjectRef, def, baseSchemaConvertOptions)
+          await this.#errorResponse(operationObjectRef, def, baseSchemaConvertOptions, undefinedErrorJsonSchema)
         }
 
         doc.paths ??= {}
@@ -152,14 +155,34 @@ export class OpenAPIGenerator {
     return this.serializer.serialize(doc)[0] as OpenAPI.Document
   }
 
-  async #resolveCommonSchemas(doc: OpenAPI.Document, commonSchemas: OpenAPIGeneratorGenerateOptions['commonSchemas']): Promise<Pick<SchemaConvertOptions, 'components'>> {
-    const baseOptions: { components?: SchemaConverterComponent[] } = {}
+  async #resolveCommonSchemas(doc: OpenAPI.Document, commonSchemas: OpenAPIGeneratorGenerateOptions['commonSchemas']): Promise<{
+    baseSchemaConvertOptions: Pick<SchemaConvertOptions, 'components'>
+    undefinedErrorJsonSchema: JSONSchema
+  }> {
+    let undefinedErrorJsonSchema: JSONSchema = {
+      type: 'object',
+      properties: {
+        defined: { const: false },
+        code: { type: 'string' },
+        status: { type: 'number' },
+        message: { type: 'string' },
+        data: {},
+      },
+      required: ['defined', 'code', 'status', 'message'],
+    }
+    const baseSchemaConvertOptions: { components?: SchemaConverterComponent[] } = {}
 
     if (commonSchemas) {
-      baseOptions.components = []
+      baseSchemaConvertOptions.components = []
 
       for (const key in commonSchemas) {
-        const { schema, strategy = 'input' } = commonSchemas[key]!
+        const options = commonSchemas[key]!
+
+        if (options.schema === undefined) {
+          continue
+        }
+
+        const { schema, strategy = 'input' } = options
 
         const [required, json] = await this.converter.convert(schema, { strategy })
 
@@ -180,7 +203,7 @@ export class OpenAPIGenerator {
           }
         }
 
-        baseOptions.components.push({
+        baseSchemaConvertOptions.components.push({
           schema,
           required,
           ref: `#/components/schemas/${key}`,
@@ -192,11 +215,23 @@ export class OpenAPIGenerator {
       doc.components.schemas ??= {}
 
       for (const key in commonSchemas) {
-        const { schema, strategy = 'input' } = commonSchemas[key]!
+        const options = commonSchemas[key]!
+
+        if (options.schema === undefined) {
+          if (options.error === 'UndefinedError') {
+            doc.components.schemas[key] = toOpenAPISchema(undefinedErrorJsonSchema)
+            undefinedErrorJsonSchema = { $ref: `#/components/schemas/${key}` }
+          }
+
+          continue
+        }
+
+        const { schema, strategy = 'input' } = options
+
         const [, json] = await this.converter.convert(
           schema,
           {
-            ...baseOptions,
+            ...baseSchemaConvertOptions,
             strategy,
             minStructureDepthForRef: 1, // not allow use $ref for root schemas
           },
@@ -205,7 +240,7 @@ export class OpenAPIGenerator {
       }
     }
 
-    return baseOptions
+    return { baseSchemaConvertOptions, undefinedErrorJsonSchema }
   }
 
   async #request(
@@ -472,6 +507,7 @@ export class OpenAPIGenerator {
     ref: OpenAPI.OperationObject,
     def: AnyContractProcedure['~orpc'],
     baseSchemaConvertOptions: Pick<SchemaConvertOptions, 'components'>,
+    undefinedErrorSchema: JSONSchema,
   ): Promise<void> {
     const errorMap = def.errorMap as ErrorMap
 
@@ -513,17 +549,7 @@ export class OpenAPIGenerator {
         content: toOpenAPIContent({
           oneOf: [
             ...schemas,
-            {
-              type: 'object',
-              properties: {
-                defined: { const: false },
-                code: { type: 'string' },
-                status: { type: 'number' },
-                message: { type: 'string' },
-                data: {},
-              },
-              required: ['defined', 'code', 'status', 'message'],
-            },
+            undefinedErrorSchema,
           ],
         }),
       }

diff --git a/playgrounds/astro/src/pages/api/[...rest].ts b/playgrounds/astro/src/pages/api/[...rest].ts
@@ -34,6 +34,7 @@ const handler = new OpenAPIHandler(router, {
           NewPlanet: { schema: NewPlanetSchema },
           UpdatePlanet: { schema: UpdatePlanetSchema },
           Planet: { schema: PlanetSchema },
+          UndefinedError: { error: 'UndefinedError' },
         },
         security: [{ bearerAuth: [] }],
         components: {

diff --git a/playgrounds/contract-first/src/main.ts b/playgrounds/contract-first/src/main.ts
@@ -35,6 +35,7 @@ const openAPIHandler = new OpenAPIHandler(router, {
           NewPlanet: { schema: NewPlanetSchema },
           UpdatePlanet: { schema: UpdatePlanetSchema },
           Planet: { schema: PlanetSchema },
+          UndefinedError: { error: 'UndefinedError' },
         },
         security: [{ bearerAuth: [] }],
         components: {

diff --git a/playgrounds/nest/src/reference/reference.service.ts b/playgrounds/nest/src/reference/reference.service.ts
@@ -35,6 +35,7 @@ export class ReferenceService {
         NewPlanet: { schema: NewPlanetSchema },
         UpdatePlanet: { schema: UpdatePlanetSchema },
         Planet: { schema: PlanetSchema },
+        UndefinedError: { error: 'UndefinedError' },
       },
       servers: [
         { url: 'http://localhost:3000' },

diff --git a/playgrounds/next/src/app/api/[[...rest]]/route.ts b/playgrounds/next/src/app/api/[[...rest]]/route.ts
@@ -33,6 +33,7 @@ const openAPIHandler = new OpenAPIHandler(router, {
           NewPlanet: { schema: NewPlanetSchema },
           UpdatePlanet: { schema: UpdatePlanetSchema },
           Planet: { schema: PlanetSchema },
+          UndefinedError: { error: 'UndefinedError' },
         },
         security: [{ bearerAuth: [] }],
         components: {

diff --git a/playgrounds/nuxt/server/routes/api/[...].ts b/playgrounds/nuxt/server/routes/api/[...].ts
@@ -32,6 +32,7 @@ const openAPIHandler = new OpenAPIHandler(router, {
           NewPlanet: { schema: NewPlanetSchema },
           UpdatePlanet: { schema: UpdatePlanetSchema },
           Planet: { schema: PlanetSchema },
+          UndefinedError: { error: 'UndefinedError' },
         },
         security: [{ bearerAuth: [] }],
         components: {

diff --git a/playgrounds/solid-start/src/routes/api/[...rest].ts b/playgrounds/solid-start/src/routes/api/[...rest].ts
@@ -34,6 +34,7 @@ const handler = new OpenAPIHandler(router, {
           NewPlanet: { schema: NewPlanetSchema },
           UpdatePlanet: { schema: UpdatePlanetSchema },
           Planet: { schema: PlanetSchema },
+          UndefinedError: { error: 'UndefinedError' },
         },
         security: [{ bearerAuth: [] }],
         components: {

diff --git a/playgrounds/svelte-kit/src/routes/api/[...rest]/+server.ts b/playgrounds/svelte-kit/src/routes/api/[...rest]/+server.ts
@@ -34,6 +34,7 @@ const handler = new OpenAPIHandler(router, {
           NewPlanet: { schema: NewPlanetSchema },
           UpdatePlanet: { schema: UpdatePlanetSchema },
           Planet: { schema: PlanetSchema },
+          UndefinedError: { error: 'UndefinedError' },
         },
         security: [{ bearerAuth: [] }],
         components: {

diff --git a/playgrounds/tanstack-start/src/routes/api/$.ts b/playgrounds/tanstack-start/src/routes/api/$.ts
@@ -35,6 +35,7 @@ const handler = new OpenAPIHandler(router, {
           NewPlanet: { schema: NewPlanetSchema },
           UpdatePlanet: { schema: UpdatePlanetSchema },
           Planet: { schema: PlanetSchema },
+          UndefinedError: { error: 'UndefinedError' },
         },
         security: [{ bearerAuth: [] }],
         components: {