diff --git a/.editorconfig b/.editorconfig
new file mode 100644
index 0000000..6edc529
--- /dev/null
+++ b/.editorconfig
@@ -0,0 +1,7 @@
+root = true
+
+[*.cs]
+# CA1416: Validate platform compatibility. This app targets Windows only (System.Drawing,
+# registry, etc. usage throughout Fuchs/OCORE); the platform-compat analyzer's warnings here
+# are expected noise, not signal. See CLAUDE.md "Build & Test" for the pre-existing-warnings note.
+dotnet_diagnostic.CA1416.severity = none
diff --git a/.github/copilot-instructions.md b/.github/copilot-instructions.md
index c0e02f7..4cfdafe 100644
--- a/.github/copilot-instructions.md
+++ b/.github/copilot-instructions.md
@@ -15,6 +15,7 @@
 - Build app: `dotnet build Fuchs/Fuchs.csproj -c Debug`. Build all: `dotnet build Fuchs_Intranet.slnx -c Debug`.
 - Frontend assets are source-built: run the gulp tasks in `Fuchs/` (`npx gulp min`, or `npx gulp all` when copied/static assets also need refreshing) whenever JS or SCSS/CSS sources change. The generated files under `Fuchs/wwwroot/web/` are what the app serves.
 - Test: `dotnet test Fuchs.Tests/Fuchs.Tests.csproj -c Debug`.
+- Always build **and** run the test suite before committing. CA1416 (platform-compatibility) is suppressed solution-wide via the root `.editorconfig` — the app is Windows-only. The build may still emit other pre-existing analyzer warnings; only treat `: error` lines as failures.
 - Submodules include the OCORE projects and `eRechnungLib` (ZUGFeRD/Factur-X + XRechnung generation) — invoices are moving to eRechnung output.
 - Project structure (relative to `Fuchs/`):
   - `Controllers/` — `IntranetController` partials (no area)
diff --git a/CLAUDE.md b/CLAUDE.md
index 0ebd9a5..5dbdfc6 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -17,7 +17,7 @@
 - Build app: `dotnet build Fuchs/Fuchs.csproj -c Debug`. Build all: `dotnet build Fuchs_Intranet.slnx -c Debug`.
 - Frontend assets are source-built: run the gulp tasks in `Fuchs/` (`npx gulp min`, or `npx gulp all` when copied/static assets also need refreshing) whenever JS or SCSS/CSS sources change. The generated files under `Fuchs/wwwroot/web/` are what the app serves.
 - Test: `dotnet test Fuchs.Tests/Fuchs.Tests.csproj -c Debug`.
-- Always build **and** run the test suite before committing. The build emits many pre-existing analyzer/platform warnings (CA1416 etc.) — those are expected; only treat `: error` lines as failures.
+- Always build **and** run the test suite before committing. CA1416 (platform-compatibility) is suppressed solution-wide via the root `.editorconfig` — the app is Windows-only. The build may still emit other pre-existing analyzer warnings; only treat `: error` lines as failures.
 - Commit only when asked. Co-author trailer: `Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>`.
 - The working tree may contain an untracked `Fuchs_Database/` SQL project — it is **not** part of app changes; never `git add -A` it into an unrelated commit. Stage explicit paths.
 
diff --git a/CODEX.md b/CODEX.md
index 3ee73ce..d69afc5 100644
--- a/CODEX.md
+++ b/CODEX.md
@@ -17,7 +17,7 @@
 - Build app: `dotnet build Fuchs/Fuchs.csproj -c Debug`. Build all: `dotnet build Fuchs_Intranet.slnx -c Debug`.
 - Frontend assets are source-built: run the gulp tasks in `Fuchs/` (`npx gulp min`, or `npx gulp all` when copied/static assets also need refreshing) whenever JS or SCSS/CSS sources change. The generated files under `Fuchs/wwwroot/web/` are what the app serves.
 - Test: `dotnet test Fuchs.Tests/Fuchs.Tests.csproj -c Debug`.
-- Always build **and** run the test suite before committing. The build emits many pre-existing analyzer/platform warnings (CA1416 etc.) — those are expected; only treat `: error` lines as failures.
+- Always build **and** run the test suite before committing. CA1416 (platform-compatibility) is suppressed solution-wide via the root `.editorconfig` — the app is Windows-only. The build may still emit other pre-existing analyzer warnings; only treat `: error` lines as failures.
 - Commit only when asked. Co-author trailer, when requested: `Co-Authored-By: Codex <noreply@openai.com>`.
 - The working tree may contain an untracked `Fuchs_Database/` SQL project — it is **not** part of app changes; never `git add -A` it into an unrelated commit. Stage explicit paths.