diff --git a/build/winter/runtime.native.d.ts b/build/winter/runtime.native.d.ts
index 85c4227e4e37003dd3769a590e73a69874de5fac..e389ca8edd2310df9ce29684991465c275e804c5 100644
--- a/build/winter/runtime.native.d.ts
+++ b/build/winter/runtime.native.d.ts
@@ -1,3 +1,2 @@
 import 'react-native/Libraries/Core/InitializeCore';
-import '../../types';
 //# sourceMappingURL=runtime.native.d.ts.map
\ No newline at end of file
diff --git a/src/winter/fetch/RequestUtils.ts b/src/winter/fetch/RequestUtils.ts
index a93473fd2beaf2ab0f3880951d292e58e7288d10..c27911bd5cb56ba6051fb8ebf4194cced43bc0fd 100644
--- a/src/winter/fetch/RequestUtils.ts
+++ b/src/winter/fetch/RequestUtils.ts
@@ -50,7 +50,11 @@ function isBlob(obj: any): obj is Blob {
  */
 export async function normalizeBodyInitAsync(
   body: BodyInit | null | undefined
-): Promise<{ body: Uint8Array | null; overriddenHeaders?: NativeHeadersType }> {
+): Promise<{
+  body: Uint8Array | null;
+  overriddenHeaders?: NativeHeadersType;
+  fallbackHeaders?: NativeHeadersType;
+}> {
   if (body == null) {
     return { body: null };
   }
@@ -71,7 +75,12 @@ export async function normalizeBodyInitAsync(
   if (body instanceof Blob || isBlob(body)) {
     return {
       body: new Uint8Array(await blobToArrayBufferAsync(body)),
-      overriddenHeaders: [['Content-Type', body.type]],
+      /*
+       * Per the fetch spec, a blob's type is only a default for Content-Type:
+       * it must not replace a header the caller set explicitly, and an empty
+       * type contributes no header at all.
+       */
+      fallbackHeaders: body.type ? [['Content-Type', body.type]] : undefined,
     };
   }
 
@@ -137,6 +146,25 @@ export function overrideHeaders(
   return result;
 }
 
+/**
+ * Create a new header array by adding new headers only for keys not already
+ * present (by case-insensitive header key). Used for body-derived defaults
+ * that must not replace caller-provided headers.
+ */
+export function fillMissingHeaders(
+  headers: NativeHeadersType,
+  newHeaders: NativeHeadersType
+): NativeHeadersType {
+  const existingKeySet = new Set(headers.map(([key]) => key.toLocaleLowerCase()));
+  const result: NativeHeadersType = [...headers];
+  for (const [key, value] of newHeaders) {
+    if (!existingKeySet.has(key.toLocaleLowerCase())) {
+      result.push([key, value]);
+    }
+  }
+  return result;
+}
+
 /** Normalizes known HTTP methods to uppercase */
 export function normalizeMethod(method: string): string {
   const normalized = method.toUpperCase();
diff --git a/src/winter/fetch/fetch.ts b/src/winter/fetch/fetch.ts
index ac789c024c96379e239bf44903c47e6a8e540d63..4f953b845c145de02976e8162ba2f63f8bd4dfd5 100644
--- a/src/winter/fetch/fetch.ts
+++ b/src/winter/fetch/fetch.ts
@@ -3,6 +3,7 @@ import { FetchError } from './FetchErrors';
 import { FetchResponse, type AbortSubscriptionCleanupFunction } from './FetchResponse';
 import type { NativeRequest, NativeRequestInit } from './NativeRequest';
 import {
+  fillMissingHeaders,
   normalizeBodyInitAsync,
   normalizeHeadersInit,
   overrideHeaders,
@@ -62,10 +63,17 @@ export async function fetch(
 
   const request = new ExpoFetchModule.NativeRequest(response) as NativeRequest;
 
-  const { body: requestBody, overriddenHeaders } = await normalizeBodyInitAsync(body);
+  const {
+    body: requestBody,
+    overriddenHeaders,
+    fallbackHeaders,
+  } = await normalizeBodyInitAsync(body);
   if (overriddenHeaders) {
     headers = overrideHeaders(headers, overriddenHeaders);
   }
+  if (fallbackHeaders) {
+    headers = fillMissingHeaders(headers, fallbackHeaders);
+  }
 
   const nativeRequestInit: NativeRequestInit = {
     credentials: credentials ?? 'include',