diff --git a/.changeset/pin-boto3-for-non-aws-s3.md b/.changeset/pin-boto3-for-non-aws-s3.md
new file mode 100644
index 000000000..c079b58f3
--- /dev/null
+++ b/.changeset/pin-boto3-for-non-aws-s3.md
@@ -0,0 +1,7 @@
+---
+"worker-comfyui": patch
+---
+
+fix: pin `boto3<1.40` so S3 uploads to non-AWS providers (Cloudflare R2, Google Cloud Storage) keep working
+
+boto3 1.40 ships a botocore release that enforces stricter AWS-only auth flows; uploads to S3-compatible endpoints like GCS and R2 began failing with `SignatureDoesNotMatch`. The fix is a version pin until upstream `runpod` and `aioboto3` stabilize on the new botocore. Reporter on #156 isolated this exactly: `boto3==1.35.40` works, `1.40.1` breaks.
diff --git a/Dockerfile b/Dockerfile
index e4fcf7d23..999b18d3a 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -71,8 +71,11 @@ ADD src/extra_model_paths.yaml ./
 # Go back to the root
 WORKDIR /
 
-# Install Python runtime dependencies for the handler
-RUN uv pip install runpod requests websocket-client
+# Install Python runtime dependencies for the handler.
+# Pin boto3 < 1.40 to keep S3 uploads working against non-AWS S3 endpoints
+# (Cloudflare R2, Google Cloud Storage). boto3 1.40+ ships a botocore that
+# enforces stricter AWS-only auth flows; see issue #156.
+RUN uv pip install runpod requests websocket-client 'boto3<1.40'
 
 # Add application code and scripts
 ADD src/start.sh src/network_volume.py handler.py test_input.json ./