MCP proxy intermittently blocked by Cloudflare WAF (capture_thought on custom MCP server)

Description

The mcp-proxy.anthropic.com endpoint intermittently returns Cloudflare WAF block pages (HTTP 403) when calling MCP tools on a custom MCP server (Open Brain, hosted on Supabase).

Behavior

• capture_thought and search_thoughts calls through the MCP proxy sometimes succeed and sometimes get blocked by Cloudflare
• Short payloads occasionally get through; longer payloads with technical content (IP addresses, shell commands, URLs) seem more likely to trigger the block
• Once a block occurs, subsequent requests (even short ones) get blocked for ~15-30 minutes, suggesting IP-level rate limiting after the initial WAF trigger
• The block page says "You are unable to access anthropic.com"

Environment

• Claude Code 2.1.90, Linux (Ubuntu 26.04), native install
• Residential IPv6 connection (SLAAC address)
• MCP server: Open Brain v1.0.0 (custom, hosted on Supabase)
• Proxy endpoint: mcp-proxy.anthropic.com

Cloudflare Ray IDs

• 9e6a3dee8bea6806 (first block — longer payload)
• 9e6a42588b846806 (second attempt — longer payload)
• 9e6a470f4c0e6806 (third attempt — short payload, still blocked)

Notes

• Direct curl to mcp-proxy.anthropic.com returns normal responses (404 on GET, as expected)
• Both IPv4 and IPv6 connectivity to anthropic.com and api.anthropic.com work fine
• The block is specifically on MCP POST requests through the proxy
• Low-reputation IPv6 addresses may be more susceptible to Cloudflare WAF false positives