Lineage

Undertow is the third implementation in a family that pins one wire protocol across three runtimes:

Server	Runtime	Relationship
Levee	Elixir / BEAM	Origin of the Phoenix-protocol surface; its integration suites are Undertow’s drop-in parity bar
Floodgate	Gleam / BEAM	The reference Undertow was ported from; source of the golden wire fixtures
Undertow	.NET	This server — wire-compatible with both

The port

Undertow was ported from Floodgate at commit 22cf469, with golden fixtures captured at 2687b5f. The plan and design record — docs/plans/2026-08-06-undertow-plan.md in the repository — is unusually candid engineering history: it was re-pinned twice because the reference moved underneath it, and in four cases the advice inverted (ping timeouts, 42["close"], op-history caps and idle eviction, and per-document sequencing all became parity requirements after first being things to skip).

The port also surfaced a live bug in the reference: the Gleam server’s key-sorted IClient echo tripped the Fluid container loader’s assert 0x4b2 in real browser flows. The finding, and the fix that landed in both servers, are recorded in Divergence notes.

Extraction

This repository was extracted from tylerbutler/levee at commit ec92d7d; the filtered history preserves the original Undertow commits.

Compatibility contract

Environment variables mirror Floodgate’s one for one, spelled UNDERTOW_* with a transitional FLOODGATE_* fallback, so one compose file can drive either binary during shadow-diff runs. The tools/Undertow.WireDiff recorder and differ exist precisely to keep that claim honest.