diff --git a/README.md b/README.md
index 048742d6b..57520e56e 100644
--- a/README.md
+++ b/README.md
@@ -29,6 +29,119 @@ Read [instructions for Windows](https://github.com/MailCore/mailcore2/blob/maste
 
 Read [instructions for Linux](https://github.com/MailCore/mailcore2/blob/master/build-linux/README.md).
 
+## Updating mailcore2 on Windows (Spark prebuilt flow) ##
+
+Windows builds of spark-core do **not** compile mailcore2 C++ from source.
+`Build-SparkCore.ps1` clones this repo at a pinned tag and runs
+`build-windows-5.10\Build-SwiftMailcore.ps1`, which compiles only the Swift
+bindings (`src/swift`) and downloads the prebuilt C++ libraries from S3
+(`Get-Mailcore2.ps1`, `mailcore2-all-<N>.zip`). **Any C++ change reaches
+Windows only through a new prebuilt archive** — merging a PR or moving a tag
+is not enough.
+
+### Prerequisites ###
+
+Everything below is preinstalled in the CI image
+`ghcr.io/readdle/spark-js-addon-windows-builder` — building inside it is the
+easiest path. On a bare machine you need:
+
+- VS2022 Build Tools with an MSVC toolset whose STL accepts the Swift
+  toolchain's clang. Swift 5.10.1 ships clang 16, so the toolset must be
+  **14.39 (17.9) or older** — the 14.40+ STL requires clang 17 and fails with
+  `STL1000`. Install the side-by-side component
+  `Microsoft.VisualStudio.Component.VC.14.39.17.9.x86.x64` and make it the
+  default via `VC\Auxiliary\Build\Microsoft.VCToolsVersion.default.txt` if a
+  newer toolset is also installed.
+- Windows SDK **10.0.18362** (version 1903, from the Windows SDK archive) —
+  the RD build modules pin it when configuring the VS environment.
+- RD PowerShell modules (`RDBuildCMake`, `RDBuildMSVC`, `RDDependency`) in
+  `PSModulePath`.
+- Swift **5.10.1** toolchain (provides `clang-cl` and the Windows SDK with
+  dispatch/BlocksRuntime).
+- ICU 69.1 at `C:\Library\icu-69.1\usr`, libxml2 2.11.5 at
+  `C:\Library\libxml2-2.11.5\usr` (paths are hardcoded in the script).
+- `SPARK_PREBUILT_KEY` env var — download token for
+  `spark-prebuilt-binaries.s3.amazonaws.com` (zlib/sasl/openssl prebuilts).
+- ssh access to `git@github.com:readdle/{ctemplate,libetpan,tidy-html5}`.
+
+### Build ###
+
+```powershell
+$env:SPARK_PREBUILT_KEY = "<key>"
+powershell -ExecutionPolicy Bypass -File .\build-windows-5.10\Build-Mailcore2.ps1 -Install
+```
+
+The script clones and builds ctemplate/libetpan/tidy, downloads the binary
+deps, then builds mailcore2/CMailCore with CMake + Ninja using `clang-cl`
+from the Swift toolchain. `-Install` lays the result out in `.build\install`
+(`bin`, `include`, `lib`, `etc`).
+
+- After a **failed** run, delete `.build` before retrying — stale CMake
+  caches keep the old configuration (wrong install prefix, wrong build type)
+  and produce confusing errors.
+- Verify what was built: `type .build\install\etc\mailcore2-git-rev` must be
+  the commit you intend to ship.
+
+### Package ###
+
+The zip must contain a single top-level folder named exactly `mailcore2-all`
+(that is the path `Get-Mailcore2.ps1` extracts):
+
+```powershell
+cd .\.build
+Copy-Item -Recurse install mailcore2-all
+tar -a -cf mailcore2-all-<N+1>.zip mailcore2-all
+```
+
+Sanity check against the current archive: `tar -tf` both files and compare
+the top-level layout.
+
+### Upload ###
+
+The bucket is `spark-prebuilt-binaries` in **eu-central-1**.
+`SPARK_PREBUILT_KEY` is a download-only token — uploads need real AWS
+credentials:
+
+```bash
+aws s3 cp mailcore2-all-<N+1>.zip s3://spark-prebuilt-binaries/mailcore2-all-<N+1>.zip --region eu-central-1
+```
+
+Verify the build can fetch it the same way the script does:
+
+```powershell
+Invoke-RestMethod -Method Head -Uri "https://spark-prebuilt-binaries.s3.amazonaws.com/mailcore2-all-<N+1>.zip" -UserAgent $env:SPARK_PREBUILT_KEY
+```
+
+Never overwrite an existing `mailcore2-all-<N>.zip` — older tags keep
+downloading it by name.
+
+### Switch builds to the new prebuilt ###
+
+1. In this repo: bump `$PrebuiltMailcoreVersion` in
+   `build-windows-5.10/Get-Mailcore2.ps1`, PR into `spark2`.
+2. Tag the merge with the next `2.1.x` tag. The bump **must be inside the
+   tag** — spark-core runs `Get-Mailcore2.ps1` from its mailcore checkout at
+   that tag, so a tag without the bump silently downloads the previous
+   archive.
+3. In `spark-core-mono`, update every mailcore pin to the new tag — the
+   versions must match across platforms:
+   - `spark-core/build-scripts/Windows-5.10/Build-SparkCore.ps1`
+     (`GitBranch` of the MailCore dependency) — Windows;
+   - `spark-core/Package.swift` and `spark-core/SparkCoreNano/Package.swift`
+     (`.package(... branch:)`) — Mac/Android SPM;
+   - `spark-js-addon/scripts/mac/configure.rb` (`RemoteSwiftPackage`) — the
+     source the addon workspace is generated from;
+   - `spark-js-addon/SparkCoreAddon.xcworkspace/xcshareddata/swiftpm/Package.resolved`
+     — autogenerated from `configure.rb` but tracked in git; update the
+     `branch`/`revision` pair so CI resolves without a regeneration step.
+
+### Local testing without a prebuilt ###
+
+To test unreleased C++ changes, add `-BuildMailcore2` to the
+`Build-SwiftMailcore.ps1` invocation in `Build-SparkCore.ps1` — mailcore2 is
+then compiled from the pinned checkout instead of downloading the archive.
+Slower (~10–15 min extra), for test builds only; remove it before release.
+
 ## Basic IMAP Usage ##
 
 Using MailCore 2 is just a little more complex conceptually than the original MailCore.  All fetch requests in MailCore 2 are made asynchronously through a queue.  What does this mean?  Well, let's take a look at a simple example: