From 344dcd0ccaf2085f277153dc0eca97b3940e5531 Mon Sep 17 00:00:00 2001
From: Woohyun Park <woohyunpark@toktokhan.dev>
Date: Tue, 11 Aug 2026 10:53:53 +0900
Subject: [PATCH] Infer object type from additionalProperties

---
 src/schema-parser/schema-parser.ts            |  3 ++
 .../basic.test.ts                             | 35 +++++++++++++++++++
 .../schema.json                               | 32 +++++++++++++++++
 3 files changed, 70 insertions(+)
 create mode 100644 tests/spec/additional-properties-without-type/basic.test.ts
 create mode 100644 tests/spec/additional-properties-without-type/schema.json

diff --git a/src/schema-parser/schema-parser.ts b/src/schema-parser/schema-parser.ts
index b6306b8a..045049d9 100644
--- a/src/schema-parser/schema-parser.ts
+++ b/src/schema-parser/schema-parser.ts
@@ -215,6 +215,9 @@ export class SchemaParser {
       ) {
         this.schema.type = SCHEMA_TYPES.ARRAY;
       }
+      if (this.schema.additionalProperties && !this.schema.type) {
+        this.schema.type = SCHEMA_TYPES.OBJECT;
+      }
       // schema is enum with one null value
       if (
         Array.isArray(this.schema.enum) &&
diff --git a/tests/spec/additional-properties-without-type/basic.test.ts b/tests/spec/additional-properties-without-type/basic.test.ts
new file mode 100644
index 00000000..1739d97c
--- /dev/null
+++ b/tests/spec/additional-properties-without-type/basic.test.ts
@@ -0,0 +1,35 @@
+import * as fs from "node:fs/promises";
+import * as os from "node:os";
+import * as path from "node:path";
+import { afterAll, beforeAll, describe, expect, test } from "vitest";
+import { generateApi } from "../../../src/index.js";
+
+describe("additionalProperties without type", async () => {
+  let tmpdir = "";
+
+  beforeAll(async () => {
+    tmpdir = await fs.mkdtemp(path.join(os.tmpdir(), "swagger-typescript-api"));
+  });
+
+  afterAll(async () => {
+    await fs.rm(tmpdir, { recursive: true });
+  });
+
+  test("infers dictionary types", async () => {
+    await generateApi({
+      fileName: "schema",
+      input: path.resolve(import.meta.dirname, "schema.json"),
+      output: tmpdir,
+      silent: true,
+      generateClient: false,
+    });
+
+    const content = await fs.readFile(path.join(tmpdir, "schema.ts"), {
+      encoding: "utf8",
+    });
+
+    expect(content).toContain("values: Record<string, string[]>;");
+    expect(content).toContain("open?: Record<string, any>;");
+    expect(content).toContain("closed?: any;");
+  });
+});
diff --git a/tests/spec/additional-properties-without-type/schema.json b/tests/spec/additional-properties-without-type/schema.json
new file mode 100644
index 00000000..42e54457
--- /dev/null
+++ b/tests/spec/additional-properties-without-type/schema.json
@@ -0,0 +1,32 @@
+{
+  "openapi": "3.0.0",
+  "info": {
+    "title": "Additional properties without an explicit type",
+    "version": "1.0.0"
+  },
+  "paths": {},
+  "components": {
+    "schemas": {
+      "Widget": {
+        "type": "object",
+        "required": ["values"],
+        "properties": {
+          "values": {
+            "additionalProperties": {
+              "type": "array",
+              "items": {
+                "type": "string"
+              }
+            }
+          },
+          "open": {
+            "additionalProperties": true
+          },
+          "closed": {
+            "additionalProperties": false
+          }
+        }
+      }
+    }
+  }
+}