diff --git a/CHANGELOG.md b/CHANGELOG.md
index 6a4e646..a8daf10 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,15 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html)
 (pre-1.0: minor bumps may include breaking changes to the sim's behavior).
 
+## [0.20.4] - 2026-07-07
+
+### Docs
+- README: added a Platforms section — Linux (x86-64/ARM incl. Raspberry Pi OS,
+  Debian, Ubuntu) and macOS (Apple Silicon/Intel), Python 3.11+; both verified
+  with the full test suite (macOS re-verified at v0.20.3: fresh clone from the
+  public repo, 908 tests green). Sandbox mode is Linux/macOS-only (root);
+  Windows untested.
+
 ## [0.20.3] - 2026-07-07
 
 ### Fixed
diff --git a/README.md b/README.md
index 4d0e3f3..619397f 100644
--- a/README.md
+++ b/README.md
@@ -39,6 +39,12 @@ leaves/border-leaves (green), controllers (amber), and the inter-site ISN
 cloud (slate) — solid orange ISN uplinks, plus a physical-links table with
 the /31 point-to-point ISN addressing. See §5's `aci-sim graph` for how to generate your own.*
 
+**Platforms** — Linux (x86-64 & ARM: Raspberry Pi OS, Debian, Ubuntu — a
+Raspberry Pi 4 is enough) and macOS (Apple Silicon & Intel). Python 3.11+.
+The full test suite runs green on both. The default port mode runs anywhere
+Python does; sandbox mode (real management IPs on `:443`) needs root and is
+Linux/macOS only. Windows is untested.
+
 ### 60-second quickstart
 
 ```bash
diff --git a/pyproject.toml b/pyproject.toml
index 22a4c7a..be165e3 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -1,6 +1,6 @@
 [project]
 name = "aci-sim"
-version = "0.20.3"
+version = "0.20.4"
 description = "Faithful REST simulator of a 2-site Cisco ACI fabric (per-site APIC + ND/NDO)"
 readme = "README.md"
 license = "PolyForm-Noncommercial-1.0.0"