diff --git a/install-scripts/README.md b/install-scripts/README.md
index 5cf17dc..b451c44 100644
--- a/install-scripts/README.md
+++ b/install-scripts/README.md
@@ -18,6 +18,11 @@ files beneath it and install command wrappers or symlinks into `$DEV_TOOLS_DIR/b
 directory is prepended to implementation agents' `PATH`. Environment changes made by a script do
 not persist into implementation turns.
 
+The supplied `dotnet` wrapper keeps the SDK itself in `DEV_TOOLS_DIR`, but places the writable
+.NET CLI home and NuGet package cache under `/tmp`. Codex implementation sandboxes do not expose a
+normal user home and receive read-only access to installed tools, so runtime caches cannot live
+beside the SDK.
+
 The configured scripts run in list order after the repository is cloned (or an existing agent PR
 branch is synchronized) and before the first implementation turn for `implement`, `iterate`, and
 `fix`. They do not rerun between implementation and review-revision turns within one job. Scripts
diff --git a/install-scripts/dotnet b/install-scripts/dotnet
index 9fe78d7..2db526a 100644
--- a/install-scripts/dotnet
+++ b/install-scripts/dotnet
@@ -25,6 +25,13 @@ PYTHON
 printf '%s\n' \
     '#!/bin/sh' \
     "export DOTNET_ROOT='$install_dir'" \
+    'export DOTNET_CLI_HOME="${DOTNET_CLI_HOME:-/tmp/agentci-dotnet}"' \
+    'export NUGET_PACKAGES="${NUGET_PACKAGES:-/tmp/agentci-nuget/packages}"' \
+    'export DOTNET_CLI_TELEMETRY_OPTOUT=1' \
+    'export DOTNET_SKIP_FIRST_TIME_EXPERIENCE=1' \
+    'export DOTNET_NOLOGO=1' \
+    'export DOTNET_CLI_WORKLOAD_UPDATE_NOTIFY_DISABLE=1' \
+    'mkdir -p "$DOTNET_CLI_HOME" "$NUGET_PACKAGES"' \
     "exec '$install_dir/dotnet' \"\$@\"" \
     > "$bin_dir/dotnet"
 chmod 0755 "$bin_dir/dotnet"
diff --git a/tests/test_install_scripts.py b/tests/test_install_scripts.py
new file mode 100644
index 0000000..b1de10d
--- /dev/null
+++ b/tests/test_install_scripts.py
@@ -0,0 +1,10 @@
+from pathlib import Path
+
+
+def test_dotnet_wrapper_uses_sandbox_writable_runtime_directories() -> None:
+    root = Path(__file__).parents[1]
+    script = (root / "install-scripts" / "dotnet").read_text()
+
+    assert 'DOTNET_CLI_HOME="${DOTNET_CLI_HOME:-/tmp/agentci-dotnet}"' in script
+    assert 'NUGET_PACKAGES="${NUGET_PACKAGES:-/tmp/agentci-nuget/packages}"' in script
+    assert 'mkdir -p "$DOTNET_CLI_HOME" "$NUGET_PACKAGES"' in script