Issue Review: cloudflare/workers-sdk#9522

Summary

Miniflare doesn't support zstd compressed subrequests - zstd responses are not decompressed and the Content-Encoding header is improperly modified to MF-Content-Encoding.

Findings

• Created: 2025-06-07
• Updated: 2025-06-08
• Version: 4.19.1 → 4.20260120.0 (current)
• Component: Miniflare
• Labels: bug
• Comments: 1 (confirming the issue)

Key Evidence

• Bug is clearly documented: The issue explains that unlike gzip/brotli, zstd-compressed responses are not automatically decompressed by undici/miniflare
• Code reference valid: The problematic code at packages/miniflare/src/index.ts (lines 2445-2449) still exists and behaves as described
• Confirmed by another user: A second user (johnpyp) reproduced the issue in a basic proxy worker scenario
• No fix found: No merged PRs reference this issue, no changelog entries mention zstd
• Related issue still open: Issue #8004 (miniflare aggressively buffers responses) is also still open and related
• Two separate problems:
  1. zstd-compressed responses aren't decompressed (undici doesn't support zstd decompression)
  2. Content-Encoding: zstd header is changed to MF-Content-Encoding: zstd, breaking proxy pass-through scenarios

Recommendation

Status: KEEP OPEN

Reasoning: This is a valid, confirmed bug with no evidence of a fix. The issue is clearly documented, has been reproduced by another user, and the problematic code still exists in the codebase. The bug affects users trying to proxy zstd-compressed content or read zstd responses within workers during local development.

Action: Keep open for engineering attention. Consider adding a miniflare label for better categorization.