diff --git a/CHANGELOG.md b/CHANGELOG.md
index eec6b8a..1f23b39 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+
+- Fix `csharp-ci.yaml`'s coverage shard crashing in `dotnet-coverage merge` (exit 134, `FileNotFoundException` for `Microsoft.CodeCoverage.Instrumentation`) when another job on the same self-hosted host installs a different `dotnet-coverage` version at the same moment. The `Install dotnet-coverage` step installed the tool globally (`dotnet tool install -g`), and every runner slot on a shared host (e.g. the four Hetzner slots) runs as the same Unix user, so they all shared one `~/.dotnet/tools`. Consumers pin different versions through `Microsoft.Testing.Extensions.CodeCoverage`, and the 2.3.x fix (#31) made each job uninstall and reinstall the global tool whenever its pin differed from what was there, pulling the binary out from under any other slot's `dotnet-coverage merge` that was running. `scripts/install-dotnet-coverage.sh` now installs the pinned version with `--tool-path` into `$RUNNER_TEMP/dotnet-coverage-<version>`, which is private to the job and wiped between jobs, and prepends that directory to `GITHUB_PATH`. It never touches the global tool, so no job can affect another job's copy, and whatever version an earlier job left globally no longer matters. The step fails loud if `RUNNER_TEMP` or `GITHUB_PATH` is unset. GitHub-hosted consumers see no change beyond the tool's install location.
+
 ## [2.4.1] - 2026-08-30
 
 ### Fixed
diff --git a/scripts/install-dotnet-coverage.sh b/scripts/install-dotnet-coverage.sh
index 4f8cdf4..dd31141 100755
--- a/scripts/install-dotnet-coverage.sh
+++ b/scripts/install-dotnet-coverage.sh
@@ -5,22 +5,9 @@ set -euo pipefail
 
 version="${1:?usage: install-dotnet-coverage.sh <version>}"
 dotnet_bin="${DOTNET_BIN:-dotnet}"
+job_tool_path="${RUNNER_TEMP:?RUNNER_TEMP must be set}/dotnet-coverage-${version}"
+github_path_file="${GITHUB_PATH:?GITHUB_PATH must be set}"
 
-installed="$("$dotnet_bin" tool list -g | awk '$1 == "dotnet-coverage" {print $2}')"
-
-if [ "$installed" = "$version" ]; then
-  echo "dotnet-coverage $version already installed"
-  exit 0
-fi
-
-if [ -n "$installed" ]; then
-  # `dotnet tool update` refuses to move to an older version, so a runner
-  # that already carries a newer dotnet-coverage than the caller's pin
-  # (shared self-hosted runners accumulate whatever the last caller
-  # installed) fails outright instead of matching the pin. Uninstalling
-  # first makes this idempotent in both directions.
-  echo "dotnet-coverage $installed installed, want $version — reinstalling"
-  "$dotnet_bin" tool uninstall -g dotnet-coverage
-fi
-
-"$dotnet_bin" tool install -g dotnet-coverage --version "$version"
+"$dotnet_bin" tool install dotnet-coverage --version "$version" --tool-path "$job_tool_path"
+echo "$job_tool_path" >> "$github_path_file"
+echo "dotnet-coverage $version installed to $job_tool_path"
diff --git a/test/install-dotnet-coverage_test.sh b/test/install-dotnet-coverage_test.sh
index 2a991ed..9966d9d 100755
--- a/test/install-dotnet-coverage_test.sh
+++ b/test/install-dotnet-coverage_test.sh
@@ -17,84 +17,59 @@ check() {
   fi
 }
 
-stub="$here/_stub_dotnet.sh"
+work="$(mktemp -d)"
+trap 'rm -rf "$work"' EXIT
+
+stub="$work/stub_dotnet.sh"
 cat > "$stub" <<'STUB'
 #!/usr/bin/env bash
-if [ "$1" = "tool" ] && [ "$2" = "list" ]; then
-  echo "Package Id      Version      Commands"
-  echo "-----------------------------------------"
-  if [ -n "${STUB_INSTALLED_VERSION:-}" ]; then
-    echo "dotnet-coverage      $STUB_INSTALLED_VERSION      dotnet-coverage"
-  fi
-  exit 0
-fi
-if [ "$1" = "tool" ] && [ "$2" = "uninstall" ]; then
-  echo "UNINSTALL" >> "$CALL_LOG"
-  exit 0
-fi
-if [ "$1" = "tool" ] && [ "$2" = "install" ]; then
-  shift 2
-  ver=""
-  while [ $# -gt 0 ]; do
-    if [ "$1" = "--version" ]; then ver="$2"; fi
-    shift
-  done
-  echo "INSTALL $ver" >> "$CALL_LOG"
-  exit 0
-fi
-echo "unexpected dotnet invocation: $*" >&2
-exit 1
+echo "$*" >> "$CALL_LOG"
+exit "${STUB_EXIT_CODE:-0}"
 STUB
 chmod +x "$stub"
 
-work="$(mktemp -d)"
 export CALL_LOG="$work/calls.log"
+runner_temp="$work/runner-temp"
+github_path="$work/github-path"
+mkdir -p "$runner_temp"
 
-# Not installed at all — the common fresh-runner case.
-: > "$CALL_LOG"
-set +e
-out="$(STUB_INSTALLED_VERSION= DOTNET_BIN="$stub" bash "$script" 18.8.0)"
-rc=$?
-set -e
-check "not-installed exit 0" 0 "$rc"
-check "not-installed calls" "INSTALL 18.8.0" "$(cat "$CALL_LOG")"
+run_script() {
+  : > "$CALL_LOG"
+  : > "$github_path"
+  set +e
+  RUNNER_TEMP="$runner_temp" GITHUB_PATH="$github_path" DOTNET_BIN="$stub" bash "$script" "$@" >/dev/null 2>&1
+  rc=$?
+  set -e
+}
 
-# Installed at an older version — the ordinary update case.
-: > "$CALL_LOG"
-set +e
-STUB_INSTALLED_VERSION=18.7.0 DOTNET_BIN="$stub" bash "$script" 18.8.0 >/dev/null
-rc=$?
-set -e
-check "older-installed exit 0" 0 "$rc"
-check "older-installed calls" "$(printf 'UNINSTALL\nINSTALL 18.8.0')" "$(cat "$CALL_LOG")"
+run_script 18.11.2
+check "install exit 0" 0 "$rc"
+check "installs the pinned version into a job-private tool path, never globally" \
+  "tool install dotnet-coverage --version 18.11.2 --tool-path $runner_temp/dotnet-coverage-18.11.2" \
+  "$(cat "$CALL_LOG")"
+check "puts the job-private tool path on GITHUB_PATH" \
+  "$runner_temp/dotnet-coverage-18.11.2" \
+  "$(cat "$github_path")"
 
-# Installed at a NEWER version than the pin — the actual regression this
-# script exists to fix (`dotnet tool update` refuses to downgrade and fails
-# outright here; a shared self-hosted runner can easily end up in this state
-# because a different caller's pin installed a newer version first).
-: > "$CALL_LOG"
-set +e
-STUB_INSTALLED_VERSION=18.9.0 DOTNET_BIN="$stub" bash "$script" 18.8.0 >/dev/null
-rc=$?
-set -e
-check "newer-installed exit 0" 0 "$rc"
-check "newer-installed calls" "$(printf 'UNINSTALL\nINSTALL 18.8.0')" "$(cat "$CALL_LOG")"
+run_script 18.11.0
+check "a different pin gets its own tool path" \
+  "tool install dotnet-coverage --version 18.11.0 --tool-path $runner_temp/dotnet-coverage-18.11.0" \
+  "$(cat "$CALL_LOG")"
 
-# Already at the exact pinned version — should be a pure no-op, no reinstall.
-: > "$CALL_LOG"
-set +e
-out="$(STUB_INSTALLED_VERSION=18.8.0 DOTNET_BIN="$stub" bash "$script" 18.8.0)"
-rc=$?
-set -e
-check "already-current exit 0" 0 "$rc"
-check "already-current calls nothing" "" "$(cat "$CALL_LOG")"
-check "already-current message" "dotnet-coverage 18.8.0 already installed" "$out"
+STUB_EXIT_CODE=1 run_script 18.11.2
+check "install failure exits non-zero" 1 "$rc"
+check "install failure leaves GITHUB_PATH untouched" "" "$(cat "$github_path")"
+
+run_script
+check "no-args exit 1" 1 "$rc"
+check "no-args calls nothing" "" "$(cat "$CALL_LOG")"
 
-# No version argument — usage error.
+: > "$CALL_LOG"
 set +e
-DOTNET_BIN="$stub" bash "$script" >/dev/null 2>&1
+env -u RUNNER_TEMP GITHUB_PATH="$github_path" DOTNET_BIN="$stub" bash "$script" 18.11.2 >/dev/null 2>&1
 rc=$?
 set -e
-check "no-args exit 1" 1 "$rc"
+check "missing RUNNER_TEMP exit 1" 1 "$rc"
+check "missing RUNNER_TEMP calls nothing" "" "$(cat "$CALL_LOG")"
 
 exit $fail