diff --git a/deploy/Dockerfile b/deploy/Dockerfile
index 6edd0d4..82fd0f5 100644
--- a/deploy/Dockerfile
+++ b/deploy/Dockerfile
@@ -14,6 +14,11 @@ COPY pyproject.toml README.md ./
 COPY src ./src
 
 RUN pip install --upgrade pip && pip install .
+# httpx 0.28 ships a regression that raises
+#   "Attempted to send an sync request with an AsyncClient instance"
+# on multipart POSTs (used by bookfusion's /uploads/init). Pin to 0.27.x
+# until the upstream fix lands.
+RUN pip install --no-cache-dir "httpx>=0.27,<0.28"
 
 # Non-root user for read-only library access.
 RUN useradd -r -u 1000 -m bookfusion
diff --git a/src/bookfusion_mcp/bookfusion_api.py b/src/bookfusion_mcp/bookfusion_api.py
index 894a086..02b728c 100644
--- a/src/bookfusion_mcp/bookfusion_api.py
+++ b/src/bookfusion_mcp/bookfusion_api.py
@@ -139,14 +139,34 @@ class BookFusionClient:
         expect_json: bool = True,
     ) -> Any:
         url = url_override or self._url(path)
+        # httpx builds an IteratorByteStream (sync) whenever `data` is a list
+        # of tuples — even when files are present — and AsyncClient rejects
+        # those at send time. We must always pass a dict; duplicate keys are
+        # collapsed to a list value, which httpx encodes as repeated fields
+        # (form-urlencoded or multipart, depending on `files`).
+        req_data: Any = data
+        if data is not None:
+            req_data = {}
+            for k, v in data:
+                if k in req_data:
+                    existing = req_data[k]
+                    req_data[k] = existing + [v] if isinstance(existing, list) else [existing, v]
+                else:
+                    req_data[k] = v
         try:
-            resp = await self._client.request(
+            req = self._client.build_request(
                 method,
                 url,
                 params=params,
-                data=data,
+                data=req_data,
                 files=files,
             )
+            # Presigned upload URLs (e.g. S3) embed their own signature in
+            # the query string and reject any extra Authorization header.
+            # Our BookFusion-wide Basic auth must not leak there.
+            if url_override:
+                req.headers.pop("Authorization", None)
+            resp = await self._client.send(req)
         except httpx.HTTPError as exc:
             raise BookFusionError(f"Network error calling {method} {url}: {exc}") from exc