diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index dbe4ba5dd2..c761fab9c1 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -84,6 +84,7 @@ We review as capacity allows — focused PRs that follow this guide move fastest
 | Node.js | 24+ | Required for desktop app commands and `just ci` |
 | pnpm | 10+ | Required for desktop app commands and `just ci` |
 | Flutter | 3.41+ | Required for mobile app — install via [flutter.dev](https://docs.flutter.dev/get-started/install) |
+| Xcode | 27.0 recommended; 26 supported | Required for iOS builds; see [iOS toolchain setup](mobile/README.md#ios-toolchain) |
 | Docker | 24+ | For Postgres, Redis, MinIO |
 | `just` | latest | Task runner — `cargo install just` |
 | `lefthook` | 2.1.3 (Hermit-pinned) | Auto-installed by `just hooks` — no manual install needed |
diff --git a/mobile/README.md b/mobile/README.md
index b447a7fbe6..c2e7e6f4aa 100644
--- a/mobile/README.md
+++ b/mobile/README.md
@@ -21,6 +21,34 @@ external Flutter SDK has touched `mobile/.dart_tool`, rerun `mobile-install`
 with the pinned SDK before building so `flutter_test`, `sky_engine`, and the
 engine all come from the same Flutter version.
 
+### iOS toolchain
+
+Use Xcode 27.0 for iOS development and build validation on an Apple silicon
+Mac running macOS Tahoe 26.6 or later, as required by the
+[Xcode 27 release notes](https://developer.apple.com/documentation/xcode-release-notes/xcode-27-release-notes).
+On Intel Macs or hosts that cannot meet those requirements, use a compatible
+Xcode 26 version, which remains supported. Activate the repository's pinned
+Flutter SDK with Hermit as described above.
+
+When multiple Xcode versions are installed, select the intended version for
+the command with `DEVELOPER_DIR`, for example:
+
+```bash
+DEVELOPER_DIR=/Applications/Xcode-27.0.0.app/Contents/Developer xcodebuild -version
+```
+
+Adjust the path to your installation. Keep the compiler and SDK from the same
+Xcode installation. The pinned Flutter native-assets tooling can discover the
+globally selected SDK even when `DEVELOPER_DIR` selects another compiler, so
+use a build machine with a matching default Xcode when validating either
+Xcode 26 or Xcode 27.
+
+The native cryptography dependency is pinned upstream C source in
+`ios/BuzzPushKit/Sources/CSecp256k1`; it requires no Swift build-plugin approval.
+GitHub's mobile CI currently uses the Xcode default of its `macos-latest`
+runner. The internal release toolchain is configured separately in
+[buzz-releases](https://github.com/squareup/buzz-releases).
+
 ## Run
 
 ```bash