From 45e67006c356e130761bc623bc6c96741a5a2fc3 Mon Sep 17 00:00:00 2001
From: Mark Probst <mark.probst@gmail.com>
Date: Mon, 20 Jul 2026 16:38:29 -0400
Subject: [PATCH] fix(json-schema): recognize $defs when deriving type names
 (#2778)

The definitionName getter in JSONSchemaInput.ts only recognized the
legacy "definitions" keyword, so types defined under the standard
"$defs" keyword (draft 2019-09+) never had their schema-given name
treated as a given name. combineNames then merged the type's other
candidate names by common prefix, producing truncated/wrong names
(e.g. "LightParams" became "Light").

Fix: also recognize "$defs" in definitionName.

Adds a schema fixture (light.schema/light.1.json) exercising the
round trip, and a focused unit test asserting the generated
TypeScript interface keeps its given name, since fixture tests don't
observe generated symbol names.

Co-Authored-By: gpt-5.6-sol via pi <noreply@openai.com>
---
 .../src/input/JSONSchemaInput.ts              |  2 +-
 test/inputs/schema/light.1.json               |  7 +++
 test/inputs/schema/light.schema               | 21 ++++++++
 test/unit/json-schema-definitions.test.ts     | 51 +++++++++++++++++++
 4 files changed, 80 insertions(+), 1 deletion(-)
 create mode 100644 test/inputs/schema/light.1.json
 create mode 100644 test/inputs/schema/light.schema
 create mode 100644 test/unit/json-schema-definitions.test.ts

diff --git a/packages/quicktype-core/src/input/JSONSchemaInput.ts b/packages/quicktype-core/src/input/JSONSchemaInput.ts
index d3a0e4feef..30678943dc 100644
--- a/packages/quicktype-core/src/input/JSONSchemaInput.ts
+++ b/packages/quicktype-core/src/input/JSONSchemaInput.ts
@@ -306,7 +306,7 @@ export class Ref {
     public get definitionName(): string | undefined {
         const pe = arrayGetFromEnd(this.path, 2);
         if (pe === undefined) return undefined;
-        if (keyOrIndex(pe) === "definitions")
+        if (keyOrIndex(pe) === "definitions" || keyOrIndex(pe) === "$defs")
             return keyOrIndex(defined(arrayLast(this.path)));
         return undefined;
     }
diff --git a/test/inputs/schema/light.1.json b/test/inputs/schema/light.1.json
new file mode 100644
index 0000000000..478f25fe74
--- /dev/null
+++ b/test/inputs/schema/light.1.json
@@ -0,0 +1,7 @@
+{
+  "LightParams": {
+    "outlet_id": "outlet-1",
+    "app_id": "app-1",
+    "rgba": "255,255,255,1"
+  }
+}
diff --git a/test/inputs/schema/light.schema b/test/inputs/schema/light.schema
new file mode 100644
index 0000000000..42c4b43b64
--- /dev/null
+++ b/test/inputs/schema/light.schema
@@ -0,0 +1,21 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "$defs": {
+    "LightParams": {
+      "type": "object",
+      "properties": {
+        "outlet_id": { "type": "string" },
+        "app_id": { "type": "string" },
+        "rgba": { "type": "string" }
+      },
+      "additionalProperties": false,
+      "required": ["outlet_id", "app_id", "rgba"]
+    }
+  },
+  "type": "object",
+  "properties": {
+    "LightParams": { "$ref": "#/$defs/LightParams" }
+  },
+  "additionalProperties": false,
+  "required": ["LightParams"]
+}
diff --git a/test/unit/json-schema-definitions.test.ts b/test/unit/json-schema-definitions.test.ts
new file mode 100644
index 0000000000..f63702d495
--- /dev/null
+++ b/test/unit/json-schema-definitions.test.ts
@@ -0,0 +1,51 @@
+import {
+    FetchingJSONSchemaStore,
+    InputData,
+    JSONSchemaInput,
+    quicktype,
+} from "quicktype-core";
+import { expect, test } from "vitest";
+
+const lightSchema = {
+    $schema: "https://json-schema.org/draft/2020-12/schema",
+    $defs: {
+        LightParams: {
+            type: "object",
+            properties: {
+                outlet_id: { type: "string" },
+                app_id: { type: "string" },
+                rgba: { type: "string" },
+            },
+            additionalProperties: false,
+            required: ["outlet_id", "app_id", "rgba"],
+        },
+    },
+    type: "object",
+    properties: {
+        LightParams: { $ref: "#/$defs/LightParams" },
+    },
+    additionalProperties: false,
+    required: ["LightParams"],
+};
+
+test("a type under $defs keeps its definition name (issue #2778)", async () => {
+    const schemaInput = new JSONSchemaInput(new FetchingJSONSchemaStore());
+    await schemaInput.addSource({
+        name: "LightSchema",
+        schema: JSON.stringify(lightSchema),
+    });
+    const inputData = new InputData();
+    inputData.addInput(schemaInput);
+
+    const result = await quicktype({
+        inputData,
+        lang: "typescript",
+        rendererOptions: { "just-types": "true" },
+    });
+    const output = result.lines.join("\n");
+
+    // The fixture exercises this schema end-to-end, but generated symbol names
+    // are not observable at runtime, so assert the regression here.
+    expect(output).toContain("LightParams: LightParams;");
+    expect(output).toContain("export interface LightParams {");
+});