fix: do not let a string type swallow array input in multi-type schemas

index.js

@@ -919,6 +919,10 @@ function buildMultiTypeSerializer (context, location, input) {
           `
         break
       case 'string': {
+        // An array has its own `toString`, so it would otherwise be captured by
+        // the duck-typing check below and serialized as a comma-joined string
+        // (dropping the JSON structure). Exclude arrays here so a sibling
+        // `array` type in the same `type` list can match.
         code += `
           ${statement}(
             typeof ${input} === "string" ||
@@ -927,6 +931,7 @@ function buildMultiTypeSerializer (context, location, input) {
             ${input} instanceof RegExp ||
             (
               typeof ${input} === "object" &&
+              !Array.isArray(${input}) &&
               typeof ${input}.toString === "function" &&
               ${input}.toString !== Object.prototype.toString
             )

test/typesArray.test.js

@@ -607,3 +607,36 @@ test('multi-type [object, array] round-trips an array of objects (JSON:API data)
   }
   t.assert.deepEqual(JSON.parse(stringify(objectInput)), objectInput)
 })
+
+test('multi-type [string, array] round-trips an array', (t) => {
+  t.plan(2)
+
+  const schema = {
+    type: 'object',
+    properties: {
+      data: {
+        type: ['string', 'array'],
+        items: {
+          type: 'object',
+          properties: {
+            id: { type: 'string' },
+            type: { type: 'string' }
+          }
+        }
+      }
+    }
+  }
+
+  const stringify = build(schema, { ajv: { allowUnionTypes: true } })
+
+  const arrayInput = {
+    data: [
+      { id: '1', type: 'article' },
+      { id: '2', type: 'article' }
+    ]
+  }
+  t.assert.deepEqual(JSON.parse(stringify(arrayInput)), arrayInput)
+
+  const stringInput = { data: 'plain' }
+  t.assert.deepEqual(JSON.parse(stringify(stringInput)), stringInput)
+})