diff --git a/.cursor/rules/csharp-style.md b/.cursor/rules/csharp-style.md
index 10f179f..5e11015 100644
--- a/.cursor/rules/csharp-style.md
+++ b/.cursor/rules/csharp-style.md
@@ -262,5 +262,9 @@ public async Task PostExample_ShouldReturnOk_WhenBodyValid()
 
 ## Tooling
 
-- Prefer fixes that satisfy built-in **.NET analyzers** / `dotnet format` (if adopted) rather than fighting IDE warnings without reason.
-- Remove **unnecessary `using` directives** (IDE0005 / CS8019) — with **implicit usings** and **global usings**, many `Microsoft.Extensions.*`, `System.Threading`, and `System.Linq` imports are redundant. Run `dotnet format <project>.csproj --diagnostics IDE0005 --severity info` before merge on touched projects.
+- **`Directory.Build.props`**: **`TreatWarningsAsErrors`** — any compiler or analyzer **warning** fails **`dotnet build`**.
+- **`.editorconfig`**: **`IDE0005`** / **`CS8019`** (unnecessary usings) at **warning** severity — caught by **`dotnet format`**, not duplicated in per-file usings when covered by **`GlobalUsings.cs`** or implicit usings.
+- Before commit on touched **`server/**/*.cs`**: pre-commit runs **`scripts/verify-dotnet-format.sh`**; CI runs the same after restore.
+- Fix redundant usings locally: `dotnet format server/NeonSprawl.Server/NeonSprawl.Server.csproj --severity warn` (and Tests project when applicable).
+- Prefer fixes that satisfy built-in **.NET analyzers** / `dotnet format` rather than fighting IDE warnings without reason.
+- When adding **`server/NeonSprawl.Server.Tests/**/*.cs`**, read **`GlobalUsings.cs`** first — do not copy sibling **`using`** blocks blindly; only import namespaces used in that file.
diff --git a/.editorconfig b/.editorconfig
new file mode 100644
index 0000000..d789d69
--- /dev/null
+++ b/.editorconfig
@@ -0,0 +1,13 @@
+# Neon Sprawl — shared editor and analyzer settings (C# server).
+# Pair with Directory.Build.props (TreatWarningsAsErrors + EnforceCodeStyleInBuild).
+root = true
+
+[*]
+charset = utf-8
+insert_final_newline = true
+trim_trailing_whitespace = true
+
+[*.cs]
+# Redundant usings duplicate GlobalUsings.cs / implicit usings (CS8019 / IDE0005).
+dotnet_diagnostic.IDE0005.severity = warning
+dotnet_diagnostic.CS8019.severity = warning
diff --git a/.github/workflows/dotnet.yml b/.github/workflows/dotnet.yml
index c5af815..8fb02e3 100644
--- a/.github/workflows/dotnet.yml
+++ b/.github/workflows/dotnet.yml
@@ -11,6 +11,8 @@ on:
     branches: [main]
     paths:
       - "NeonSprawl.sln"
+      - "Directory.Build.props"
+      - ".editorconfig"
       - "server/**"
       - "bruno/neon-sprawl-server/**"
       - ".github/workflows/dotnet.yml"
@@ -52,6 +54,8 @@ jobs:
           filters: |
             server:
               - "NeonSprawl.sln"
+              - "Directory.Build.props"
+              - ".editorconfig"
               - "server/**"
               - "bruno/neon-sprawl-server/**"
               - ".github/workflows/dotnet.yml"
@@ -70,6 +74,10 @@ jobs:
         if: github.event_name == 'push' || steps.paths.outputs.server == 'true'
         run: dotnet restore NeonSprawl.sln
 
+      - name: Verify dotnet format (IDE0005 / style)
+        if: github.event_name == 'push' || steps.paths.outputs.server == 'true'
+        run: ./scripts/verify-dotnet-format.sh
+
       - name: Build
         if: github.event_name == 'push' || steps.paths.outputs.server == 'true'
         run: dotnet build NeonSprawl.sln --no-restore --configuration Release
diff --git a/Directory.Build.props b/Directory.Build.props
new file mode 100644
index 0000000..2a6c53b
--- /dev/null
+++ b/Directory.Build.props
@@ -0,0 +1,7 @@
+<Project>
+  <PropertyGroup>
+    <!-- Compiler and analyzer warnings fail the build. Pair with .editorconfig severities. -->
+    <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
+    <!-- IDE0005 (unnecessary usings) is enforced via dotnet format in CI/pre-commit — see scripts/verify-dotnet-format.sh -->
+  </PropertyGroup>
+</Project>
diff --git a/client/README.md b/client/README.md
index b18052b..959d9c7 100644
--- a/client/README.md
+++ b/client/README.md
@@ -501,7 +501,7 @@ python3 -m venv .venv-gd
 
 This enables:
 
-- **pre-commit**: blocks commits when `client/scripts/*.gd` changes lack matching `client/test/*_test.gd` updates, or when server route/contract files change without corresponding `server/NeonSprawl.Server.Tests/` and `bruno/neon-sprawl-server/**/*.bru` updates.
+- **pre-commit**: blocks commits when `client/scripts/*.gd` changes lack matching `client/test/*_test.gd` updates, when server route/contract files change without corresponding `server/NeonSprawl.Server.Tests/` and `bruno/neon-sprawl-server/**/*.bru` updates, or when staged `server/**/*.cs` fails **`scripts/verify-dotnet-format.sh`** (unnecessary usings / warn-severity style — pair with **`TreatWarningsAsErrors`** in **`Directory.Build.props`**).
 - **pre-push**: when **`client/scripts/*.gd`** or **`client/test/*.gd`** changed in the commits being pushed, runs **`gdlint client/scripts client/test`** and **`gdformat --check client/scripts client/test`** (same scope as CI). Otherwise skips lint/format. Still requires a **clean working tree** on every push.
 
 The pre-push hook prefers **`.venv-gd/bin/`** (Unix) or **`.venv-gd/Scripts/`** (Windows venv) when present; without that venv or a global install, the hook **fails** when GDScript changed — same as CI. Pushes with no client `.gd` changes do not require gdtoolkit installed.
diff --git a/scripts/git-hooks/pre-commit b/scripts/git-hooks/pre-commit
index 426303c..c67dc16 100755
--- a/scripts/git-hooks/pre-commit
+++ b/scripts/git-hooks/pre-commit
@@ -46,3 +46,12 @@ if has_staged_match '^client/scripts/.*\.gd$'; then
 		exit 1
 	fi
 fi
+
+if has_staged_match '^server/.*\.cs$'; then
+	if ! "$repo_root/scripts/verify-dotnet-format.sh"; then
+		echo "pre-commit: dotnet format check failed (unnecessary usings / style)." >&2
+		echo "Fix with: dotnet format server/NeonSprawl.Server/NeonSprawl.Server.csproj --severity warn" >&2
+		echo "          dotnet format server/NeonSprawl.Server.Tests/NeonSprawl.Server.Tests.csproj --severity warn" >&2
+		exit 1
+	fi
+fi
diff --git a/scripts/verify-dotnet-format.sh b/scripts/verify-dotnet-format.sh
new file mode 100755
index 0000000..0d22fe1
--- /dev/null
+++ b/scripts/verify-dotnet-format.sh
@@ -0,0 +1,31 @@
+#!/usr/bin/env bash
+# Verify dotnet format (IDE0005 unnecessary usings + other warn-severity style) with no auto-fixes.
+set -euo pipefail
+
+repo_root="$(git rev-parse --show-toplevel 2>/dev/null || pwd)"
+cd "$repo_root"
+
+projects=(
+	server/NeonSprawl.Server/NeonSprawl.Server.csproj
+	server/NeonSprawl.Server.Tests/NeonSprawl.Server.Tests.csproj
+)
+
+try_format() {
+	local proj="$1"
+	dotnet format "$proj" --verify-no-changes --severity warn --no-restore
+}
+
+restore_needed=false
+for proj in "${projects[@]}"; do
+	if ! try_format "$proj" >/dev/null 2>&1; then
+		restore_needed=true
+		break
+	fi
+done
+
+if [[ "$restore_needed" == true ]]; then
+	dotnet restore NeonSprawl.sln --verbosity quiet
+	for proj in "${projects[@]}"; do
+		try_format "$proj"
+	done
+fi