From 51387eade9b01008cdd0d66640b15ba99a4f6068 Mon Sep 17 00:00:00 2001
From: LeSingh1 <sshaurya914@gmail.com>
Date: Fri, 10 Jul 2026 07:34:44 -0700
Subject: [PATCH 1/2] fix: expose InvalidJSONFieldError and NoFormData in
 multipartErrors

---
 index.js                    |  4 ++-
 test/multipart-json.test.js | 70 +++++++++++++++++++++++++++++++++++--
 types/index.d.ts            |  3 ++
 3 files changed, 74 insertions(+), 3 deletions(-)

diff --git a/index.js b/index.js
index e22ddc03..1e33f7b3 100644
--- a/index.js
+++ b/index.js
@@ -181,7 +181,9 @@ function fastifyMultipart (fastify, options, done) {
     InvalidMultipartContentTypeError,
     RequestFileTooLargeError,
     FileBufferNotFoundError,
-    PrematureCloseError
+    PrematureCloseError,
+    InvalidJSONFieldError,
+    NoFormData
   })
 
   fastify.addContentTypeParser('multipart/form-data', setMultipart)
diff --git a/test/multipart-json.test.js b/test/multipart-json.test.js
index 9e9da6fb..3c3c09f7 100644
--- a/test/multipart-json.test.js
+++ b/test/multipart-json.test.js
@@ -141,7 +141,7 @@ test('should not parse JSON fields forms if non-json content-type is set', funct
 })
 
 test('should throw error when parsing JSON fields failed', function (t, done) {
-  t.plan(2)
+  t.plan(3)
 
   const fastify = Fastify()
   t.after(() => fastify.close())
@@ -188,7 +188,7 @@ test('should throw error when parsing JSON fields failed', function (t, done) {
 })
 
 test('should always reject JSON parsing if the value was truncated', function (t, done) {
-  t.plan(2)
+  t.plan(3)
 
   const fastify = Fastify()
   t.after(() => fastify.close())
@@ -409,3 +409,69 @@ test('should return 400 when the field validation fails', function (t, done) {
     form.pipe(req)
   })
 })
+
+test('multipartErrors should expose InvalidJSONFieldError', async function (t) {
+  t.plan(2)
+
+  const fastify = Fastify()
+  t.after(() => fastify.close())
+
+  fastify.register(multipart)
+  await fastify.ready()
+
+  t.assert.ok(fastify.multipartErrors.InvalidJSONFieldError, 'InvalidJSONFieldError is exposed on multipartErrors')
+  t.assert.strictEqual(typeof fastify.multipartErrors.InvalidJSONFieldError, 'function')
+})
+
+test('thrown InvalidJSONFieldError should be an instance of multipartErrors.InvalidJSONFieldError', function (t, done) {
+  t.plan(3)
+
+  const fastify = Fastify()
+  t.after(() => fastify.close())
+
+  fastify.register(multipart)
+
+  fastify.post('/', async function (req, reply) {
+    try {
+      for await (const part of req.parts()) {
+        t.assert.strictEqual(typeof part.value, 'string')
+      }
+      reply.code(200).send()
+    } catch (error) {
+      t.assert.ok(error instanceof fastify.multipartErrors.InvalidJSONFieldError)
+      t.assert.strictEqual(error.code, 'FST_INVALID_JSON_FIELD_ERROR')
+      reply.code(error.statusCode).send()
+    }
+  })
+
+  fastify.listen({ port: 0 }, function () {
+    const boundary = 'testboundary'
+    const body = [
+      '--' + boundary,
+      'Content-Disposition: form-data; name="jsonfield"',
+      'Content-Type: application/json',
+      '',
+      'not valid json',
+      '--' + boundary + '--',
+      ''
+    ].join('\r\n')
+
+    const opts = {
+      hostname: '127.0.0.1',
+      port: fastify.server.address().port,
+      path: '/',
+      method: 'POST',
+      headers: {
+        'content-type': 'multipart/form-data; boundary=' + boundary,
+        'content-length': Buffer.byteLength(body)
+      }
+    }
+
+    const req = http.request(opts, res => {
+      t.assert.strictEqual(res.statusCode, 406)
+      res.resume()
+      res.on('end', done)
+    })
+    req.end(body)
+  })
+})
diff --git a/types/index.d.ts b/types/index.d.ts
index 6a8a5426..26e9545c 100644
--- a/types/index.d.ts
+++ b/types/index.d.ts
@@ -62,7 +62,10 @@ interface MultipartErrors {
   PrototypeViolationError: FastifyErrorConstructor;
   InvalidMultipartContentTypeError: FastifyErrorConstructor;
   RequestFileTooLargeError: FastifyErrorConstructor;
+  FileBufferNotFoundError: FastifyErrorConstructor;
   PrematureCloseError: FastifyErrorConstructor;
+  InvalidJSONFieldError: FastifyErrorConstructor;
+  NoFormData: FastifyErrorConstructor;
 }
 
 declare namespace fastifyMultipart {

From 0a03ca67fc059b37041d864ae880acaa5dfcd7fa Mon Sep 17 00:00:00 2001
From: LeSingh1 <sshaurya914@gmail.com>
Date: Mon, 13 Jul 2026 12:35:03 -0700
Subject: [PATCH 2/2] test: assert InvalidJSONFieldError and NoFormData in
 multipartErrors type tests

---
 types/index.tst.ts | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/types/index.tst.ts b/types/index.tst.ts
index 949de7bf..11aac220 100644
--- a/types/index.tst.ts
+++ b/types/index.tst.ts
@@ -165,7 +165,9 @@ const runServer = async () => {
 
     expect(app.multipartErrors.FieldsLimitError).type.toBe<FastifyErrorConstructor>()
     expect(app.multipartErrors.FilesLimitError).type.toBe<FastifyErrorConstructor>()
+    expect(app.multipartErrors.InvalidJSONFieldError).type.toBe<FastifyErrorConstructor>()
     expect(app.multipartErrors.InvalidMultipartContentTypeError).type.toBe<FastifyErrorConstructor>()
+    expect(app.multipartErrors.NoFormData).type.toBe<FastifyErrorConstructor>()
     expect(app.multipartErrors.PartsLimitError).type.toBe<FastifyErrorConstructor>()
     expect(app.multipartErrors.PrototypeViolationError).type.toBe<FastifyErrorConstructor>()
     expect(app.multipartErrors.RequestFileTooLargeError).type.toBe<FastifyErrorConstructor>()