diff --git a/packages/synapse-core/src/sp/upload-streaming.ts b/packages/synapse-core/src/sp/upload-streaming.ts
index 859d3191..d681d5e0 100644
--- a/packages/synapse-core/src/sp/upload-streaming.ts
+++ b/packages/synapse-core/src/sp/upload-streaming.ts
@@ -89,7 +89,7 @@ export async function uploadPieceStreaming(
     ? new Blob([options.data as Uint8Array<ArrayBuffer>]).stream()
     : (options.data as ReadableStream) // ReadableStream types dont match between browsers and Node.js
 
-  let size = isUint8Array(options.data) ? options.data.length : options.size
+  const size = isUint8Array(options.data) ? options.data.length : options.size
 
   // Add size tracking and progress reporting
   let bytesUploaded = 0
@@ -140,10 +140,15 @@ export async function uploadPieceStreaming(
   // both execute regardless of path.
   let fetchBody: ReadableStream | Blob
   let fetchOptions: Record<string, string> = {}
+  // PUT /pdp/piece/uploads/{uuid}
+  const headers: Record<string, string> = {
+    'Content-Type': 'application/octet-stream',
+  }
 
   if (supportsStreamingFetchBody()) {
     fetchBody = bodyStream
     fetchOptions = { duplex: 'half' }
+    // Length on a ReadableStream body is conveyed via chunked encoding.
   } else {
     const chunks: Uint8Array[] = []
     let totalSize = 0
@@ -155,17 +160,7 @@ export async function uploadPieceStreaming(
       totalSize += value.length
     }
     fetchBody = new Blob(chunks as BlobPart[])
-    // Override Content-Length with the actual accumulated size since we now
-    // know it precisely, even for ReadableStream inputs without a pre-set size
-    if (size == null) {
-      size = totalSize
-    }
-  }
-
-  // PUT /pdp/piece/uploads/{uuid}
-  const headers: Record<string, string> = {
-    'Content-Type': 'application/octet-stream',
-    ...(size == null ? {} : { 'Content-Length': size.toString() }),
+    headers['Content-Length'] = (size ?? totalSize).toString()
   }
 
   const uploadResponse = await request.put(new URL(`pdp/piece/uploads/${uploadUuid}`, options.serviceURL), {
diff --git a/packages/synapse-core/src/utils/streams.ts b/packages/synapse-core/src/utils/streams.ts
index e7900903..b61336c8 100644
--- a/packages/synapse-core/src/utils/streams.ts
+++ b/packages/synapse-core/src/utils/streams.ts
@@ -142,7 +142,8 @@ export function supportsStreamingFetchBody(): boolean {
   if (_supportsStreamBody !== undefined) return _supportsStreamBody
   try {
     let duplexAccessed = false
-    const hasContentType = new Request('', {
+    // Absolute URL required: Node throws on `''` (no base URL).
+    const hasContentType = new Request('http://x', {
       body: new ReadableStream(),
       method: 'POST',
       get duplex() {
diff --git a/packages/synapse-core/test/sp.test.ts b/packages/synapse-core/test/sp.test.ts
index d30ed028..a080ea1d 100644
--- a/packages/synapse-core/test/sp.test.ts
+++ b/packages/synapse-core/test/sp.test.ts
@@ -1145,6 +1145,33 @@ InvalidSignature(address expected, address actual)
       assert.strictEqual(progressCalls[progressCalls.length - 1], testData.length)
     })
 
+    it('should not set Content-Length on a streaming ReadableStream body', async () => {
+      // Length on a streaming body is conveyed via chunked encoding.
+      const pieceCid = Piece.parse(mockPieceCidStr)
+      const testData = new Uint8Array(SIZE_CONSTANTS.MIN_UPLOAD_SIZE).fill(0x42)
+      let capturedHeaders: Headers | undefined
+
+      server.use(
+        postPieceUploadsHandler(mockUuid),
+        http.put(`https://pdp.example.com/pdp/piece/uploads/${mockUuid}`, async ({ request }) => {
+          capturedHeaders = request.headers
+          // Drain the body so the upstream pipeline runs to completion.
+          await request.arrayBuffer()
+          return HttpResponse.text('No Content', { status: 204 })
+        }),
+        finalizePieceUploadHandler(mockUuid, mockPieceCidStr)
+      )
+
+      await uploadPieceStreaming({
+        serviceURL: 'https://pdp.example.com',
+        data: testData,
+        pieceCid,
+      })
+
+      assert.isDefined(capturedHeaders)
+      assert.strictEqual(capturedHeaders?.get('content-length'), null)
+    })
+
     it('should fail when session creation returns error', async () => {
       const pieceCid = Piece.parse(mockPieceCidStr)
       const testData = new Uint8Array(SIZE_CONSTANTS.MIN_UPLOAD_SIZE).fill(0x42)