Summary: Headroom doesn't work with DeepSeek's Anthropic-compatible endpoint

[kakuola]

I tested Headroom v1.4.0 (Docker) as a proxy between Claude Code and DeepSeek's Anthropic-compatible API (https://api.deepseek.com/anthropic). Here's what happened:
1. Compression proxy mode (default): Claude Code hangs / becomes unresponsive. The compressed requests produced by Headroom appear incompatible with DeepSeek's Anthropic-compatible layer — responses never come back properly.
2. Passthrough mode (--no-optimize): Works fine (streaming and all), but obviously no compression happens — it's just a transparent proxy with no benefit.
3. MCP tool mode (headroom mcp serve): Works correctly, but the ContentRouter returns "router:noop" for content under a certain token threshold, meaning short conversations or small files never get compressed. Only large inputs (thousands of tokens) trigger actual compression. Not useful as an automatic background optimizer for daily chat.
Root cause: DeepSeek's Anthropic-compatible endpoint is not 100% API-compatible with native Anthropic. Any middleware that modifies/recompresses requests breaks the communication.
Verdict: If you're using DeepSeek as your backend, Headroom's proxy compression mode is a no-go. MCP tool mode technically works but delivers very little practical value for typical usage.