Malformed_HTTP_Response

[AmeerTheInteger]

Unable to route claude code's traffic via burpsuite proxy tab. Recevied "Malformed_HTTP_Response"

Steps to reproduce
CWD contains .mcp.json

• export HTTP_PROXY=http://127.0.0.1:8080
• export HTTPS_PROXY=http://127.0.0.1:8080
• export NODE_EXTRA_CA_CERTS=/path/to/burp_ca.pem
• claude

[hoquanganh09]

Fix: Add NO_PROXY=127.0.0.1,localhost

Root cause: When HTTP_PROXY is set, Node.js 22 (global fetch) routes all HTTP requests through Burp proxy — including the SSE connection to 127.0.0.1:9876 (Burp's MCP extension). Burp receives this local request but can't forward it correctly → Malformed_HTTP_Response.

Fix — add NO_PROXY to exclude localhost:

export HTTP_PROXY=http://127.0.0.1:8080
export HTTPS_PROXY=http://127.0.0.1:8080
export NO_PROXY=127.0.0.1,localhost
export NODE_EXTRA_CA_CERTS=/path/to/burp_ca.pem
claude

Verified on Node.js v22.11.0 / Windows 11:

• 127.0.0.1:9876 (MCP SSE) → connects directly ✓
• External HTTPS (e.g. Anthropic API) → goes through Burp proxy ✓

The Burp CA cert step is still required for HTTPS interception to work.