[BUG] Non-streaming retry receives event-stream response (HTTP/2 stream mixup, Intel Mac)

### Preflight Checklist

- [x] I have searched [existing issues](https://github.com/anthropics/claude-code/issues?q=is%3Aissue%20state%3Aopen%20label%3Abug) and this hasn't been reported yet
- [x] This is a single bug report (please file separate reports for different bugs)
- [x] I am using the latest version of Claude Code

### What's Wrong?

**Environment**
- Claude Code: 2.1.236 and 2.1.237
- Node.js: 22.17.0 (nvm, x86_64)
- macOS: 15.7.4 (Intel)
- Cloudflare POP: ICN

**Symptom**
Intermittent, session-sticky. Once triggered, persists until process restart.
Same version on Apple Silicon Mac on same network: no issue.

**Error**
API returned an empty or malformed response (HTTP 200)
- 1st attempt (streaming): 0 stream events, no request-id
- 2nd attempt (non-streaming retry): HTTP 200, content-type event-stream,
64685 bytes — streaming response returned for non-streaming request

**Ruled out**
Proxy, VPN, SSL inspection, firewall, macOS network filter,
MDM, DNS, /etc/hosts, ~/.claude state, auth re-login, reboot,
different network (hotspot), Node.js reinstall, different directory

### What Should Happen?

Streaming requests should receive SSE event streams with a valid
Anthropic request-id. When a streaming request fails and Claude Code
retries as a non-streaming request (stream: false), the retry response
should be a JSON object (content-type: application/json), not an
event-stream. Each HTTP/2 stream should be fully independent — a failed
streaming request's response must never leak into a subsequent
non-streaming retry on the same connection.

### Error Messages/Logs

```shell
API Error: API returned an empty or malformed response (HTTP 200) — check for a proxy or gateway intercepting the request. Response: content-type event-stream, body is an event stream (the non-streaming request was answered with a stream), 2795 bytes, request-id req_011CeDCC5p1UT6L6ifTfnBAU, server cloudflare, intermediary headers anthropic-* cf-cache-status cf-ray. This was the non-streaming retry of streaming request (no Anthropic request-id), which failed with: other; 0 stream events received.
```

### Steps to Reproduce

1. Environment: macOS 15.7.4, Intel x86_64, Node.js 22.17.0 (nvm),
Claude Code 2.1.237
2. Launch `claude` in any directory (no specific project config required)
3. Send any prompt
4. Intermittently, the streaming request fails with 0 SSE events and
no Anthropic request-id
5. Claude Code automatically retries as a non-streaming request
6. The retry receives HTTP 200 with content-type: event-stream
(64685 bytes) instead of expected application/json
7. Error displayed:
"API returned an empty or malformed response (HTTP 200) —
check for a proxy or gateway intercepting the request"
8. Once triggered, every subsequent request in the same session fails
with the same error
9. Restarting the `claude` process temporarily resolves until
the next occurrence

### Claude Model

Opus

### Is this a regression?

Yes, this worked in a previous version

### Last Working Version

_No response_

### Claude Code Version

2.1.237

### Platform

Anthropic API

### Operating System

macOS

### Terminal/Shell

Terminal.app (macOS)

### Additional Information

_No response_

No source files or tests are named. Start by reproducing the failure with Claude Code 2.1.237 on Intel macOS using the listed streaming-then-retry sequence, then trace the HTTP/2 request and retry handling. Done means failed streams do not affect later requests and non-streaming retries consistently return JSON rather than event-stream responses.

Escrito por el modelo de indexación a partir del texto del issue.