ECONNRESET reliably triggered by image upload (even <1MB, single/first images)

Description

Repeated Unable to connect to API (ECONNRESET) errors, reproducibly triggered by sending images, even when the images are small (233KB–587KB PNGs) and even in a fresh session with no prior history.

Environment

• Claude Code version: 2.1.220 (macOS, darwin 25.5.0)
• Node: v22.20.0
• No proxy, no VPN active, no MDM/security software intercepting traffic
• ISP path includes a CGNAT hop (100.64.0.0/10 range) before reaching the public internet

Reproduction

1. Start a Claude Code session (fresh, no -c/continue).
2. Send a message containing 1+ images (PNG, 233KB–587KB each, well under any reasonable size limit).
3. Request fails with Unable to connect to API (ECONNRESET), retries up to 10 times, frequently exhausts all retries and fails outright.

Confirmed:

• Text-only turns in the same environment/network succeed reliably (curl to api.anthropic.com succeeds consistently, ping/traceroute show 0% packet loss, clean path).
• Failure reproduces across multiple unrelated project directories/sessions on the same machine.
• Failure reproduces on both a resumed (12MB+ history) session and a brand-new session with empty history — so it isn't just about accumulated conversation size.
• The one consistent variable across every failure is the presence of image content in the request.

Packet capture evidence

tcpdump during one occurrence showed the client mid-upload of a large request body, then the server (160.79.104.10, Anthropic's API edge) sending a short ~474-byte response packet followed immediately by a TCP RST, with the server resetting all subsequent packets from the client. This indicates the reset originates from the server/edge side after receiving (and apparently rejecting) the request — not a client-side network drop.

Impact

This makes any image-containing request unreliable, often requiring many retries or failing completely after exhausting the retry budget, interrupting active work.

Ask

Would appreciate:

• Confirmation of whether there's a request size/format constraint on image uploads that's being hit here (1.3MB total across 4 images seems far too small to be hitting a legitimate limit).
• Any server-side logs/insight into why the edge is resetting these specific requests.