openapi3: emit minItems/maxItems on tuple types

.chronus/changes/tuple-openapi3-fixed-length-2026-8-9-13-1-9.md

@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/openapi3"
+---
+
+Emit `minItems` and `maxItems` for tuple types so generated schemas enforce the tuple's exact length.

packages/openapi3/src/schema-emitter-3-1.ts

@@ -389,9 +389,12 @@ export class OpenAPI31SchemaEmitter extends OpenAPI3SchemaEmitterBase<OpenAPISch
   }
 
   tupleLiteral(tuple: Tuple): EmitterOutput<Record<string, any>> {
+    const size = tuple.values.length;
     return new ObjectBuilder({
       type: "array",
       prefixItems: this.emitter.emitTupleLiteralValues(tuple),
+      minItems: size,
+      maxItems: size,
     });
   }
 

packages/openapi3/src/schema-emitter.ts

@@ -845,7 +845,8 @@ export class OpenAPI3SchemaEmitterBase<
   }
 
   tupleLiteral(tuple: Tuple): EmitterOutput<Record<string, any>> {
-    return { type: "array", items: {} };
+    const size = tuple.values.length;
+    return { type: "array", items: {}, minItems: size, maxItems: size };
   }
   #getSchemaForScalar(scalar: Scalar): Schema {
     let result: Schema = {} as Schema;

packages/openapi3/test/array.test.ts

@@ -268,21 +268,43 @@ worksFor(["3.0.0"], ({ oapiForModel }) => {
     deepStrictEqual(res.schemas.Pet.properties.names, {
       type: "array",
       items: {},
+      minItems: 2,
+      maxItems: 2,
       default: ["bismarck", 12],
     });
 
     deepStrictEqual(res.schemas.Pet.properties.decimals, {
       type: "array",
       items: {},
+      minItems: 2,
+      maxItems: 2,
       default: ["hi", 456.7],
     });
 
     deepStrictEqual(res.schemas.Pet.properties.decimal128s, {
       type: "array",
       items: {},
+      minItems: 2,
+      maxItems: 2,
       default: ["hi", 456.7],
     });
   });
+
+  it("emits minItems and maxItems of 0 for empty tuple", async () => {
+    const res = await oapiForModel(
+      "Pet",
+      `
+      model Pet { names: [] };
+      `,
+    );
+
+    deepStrictEqual(res.schemas.Pet.properties.names, {
+      type: "array",
+      items: {},
+      minItems: 0,
+      maxItems: 0,
+    });
+  });
 });
 
 worksFor(["3.1.0"], ({ oapiForModel }) => {
@@ -369,18 +391,24 @@ worksFor(["3.1.0"], ({ oapiForModel }) => {
     deepStrictEqual(res.schemas.Pet.properties.names, {
       type: "array",
       prefixItems: [{ type: "string" }, { type: "integer", format: "int32" }],
+      minItems: 2,
+      maxItems: 2,
       default: ["bismarck", 12],
     });
 
     deepStrictEqual(res.schemas.Pet.properties.decimals, {
       type: "array",
       prefixItems: [{ type: "string" }, { type: "number", format: "decimal" }],
+      minItems: 2,
+      maxItems: 2,
       default: ["hi", 456.7],
     });
 
     deepStrictEqual(res.schemas.Pet.properties.decimal128s, {
       type: "array",
       prefixItems: [{ type: "string" }, { type: "number", format: "decimal128" }],
+      minItems: 2,
+      maxItems: 2,
       default: ["hi", 456.7],
     });
   });
@@ -401,6 +429,8 @@ worksFor(["3.1.0"], ({ oapiForModel }) => {
         { type: "string", enum: ["bismark"] },
         { type: "number", enum: [32] },
       ],
+      minItems: 2,
+      maxItems: 2,
     });
   });
 });
@@ -418,6 +448,8 @@ worksFor(["3.1.0", "3.2.0"], ({ oapiForModel }) => {
     deepStrictEqual(res.schemas.Pet.properties.names, {
       type: "array",
       prefixItems: [{ $ref: "#/components/schemas/Bar" }],
+      minItems: 1,
+      maxItems: 1,
     });
   });
 
@@ -432,6 +464,24 @@ worksFor(["3.1.0", "3.2.0"], ({ oapiForModel }) => {
     deepStrictEqual(res.schemas.Pet.properties.names, {
       type: "array",
       prefixItems: [{ $ref: "#/components/schemas/Pet" }],
+      minItems: 1,
+      maxItems: 1,
+    });
+  });
+
+  it("emits minItems and maxItems of 0 for empty tuple", async () => {
+    const res = await oapiForModel(
+      "Pet",
+      `
+      model Pet { names: [] };
+      `,
+    );
+
+    deepStrictEqual(res.schemas.Pet.properties.names, {
+      type: "array",
+      prefixItems: [],
+      minItems: 0,
+      maxItems: 0,
     });
   });
 });

packages/samples/test/output/authentication/@typespec/openapi3/openapi.TypeSpec.ServiceAuth.yaml

@@ -47,6 +47,8 @@ components:
         scopes:
           type: array
           items: {}
+          minItems: 2
+          maxItems: 2
   securitySchemes:
     BearerAuth:
       type: http