diff --git a/flake.nix b/flake.nix
index adbe509..faddfb5 100644
--- a/flake.nix
+++ b/flake.nix
@@ -96,11 +96,8 @@
         # Exercise the installed host exactly as the loop launches it: the
         # caller has no Cargo on PATH, so only the packaged runtime toolchain
         # can run the native tests and the wasm32-wasip2 build.
-        checks.runtime-toolchain = pkgs.runCommand "sausage-runtime-toolchain" {
-          nativeBuildInputs = [ pkgs.expect ];
-        } ''
-          set -eux
-          echo "runtime-check: prepare fixture"
+        checks.runtime-toolchain = pkgs.runCommand "sausage-runtime-toolchain" { } ''
+          set -eu
           root="$TMPDIR/runtime-toolchain"
           world="$root/world"
           emptyPath="$root/empty-bin"
@@ -123,37 +120,39 @@
               assert_eq!(2 + 2, 4);
           }
           RUST
-          export SAUSAGE_BIN=${sausage}/bin/sausage
-          export WORLD="$world"
-          export EMPTY_PATH="$emptyPath"
-          echo "runtime-check: start expect with empty PATH"
-          expect <<'EOF'
-          log_user 1
-          set timeout 300
-          set env(PATH) $::env(EMPTY_PATH)
-          set env(HOME) $::env(WORLD)
-          set env(TMPDIR) [file dirname $::env(WORLD)]
-          set env(CARGO_HOME) [file join [file dirname $::env(WORLD)] cargo-home]
-          puts stderr "runtime-check: spawn packaged host"
-          spawn $::env(SAUSAGE_BIN) run $::env(WORLD)
-          send -- "@cargo {\"type\":\"build\",\"source\":\"/actor/console/public/probe\"}\r"
-          expect {
-              -re {"type":"build_failed"} {
-                  puts stderr "packaged cargo actor build failed with an empty caller PATH"
-                  exit 1
-              }
-              -re {"type":"built"} {
-                  expect {
-                      -re {"passed":1} { send -- "/quit\r" }
-                      timeout { puts stderr "native cargo test did not pass"; exit 1 }
-                  }
-              }
-              timeout { puts stderr "packaged host did not build the probe"; exit 1 }
+          echo "runtime-check: run packaged host with an empty PATH"
+          coproc host {
+            ${pkgs.coreutils}/bin/env -i PATH="$emptyPath" HOME="$root/home" TMPDIR="$root" \
+              CARGO_HOME="$root/cargo-home" ${sausage}/bin/sausage run "$world"
           }
-          expect eof
-          set result [wait]
-          if {[lindex $result 3] != 0} { exit [lindex $result 3] }
-          EOF
+          hostInput=''${host[1]}
+          hostOutput=''${host[0]}
+          hostPid=$host_PID
+          trap 'kill "$hostPid" 2>/dev/null || true' EXIT
+          printf '%s\n' '@cargo {"type":"build","source":"/actor/console/public/probe"}' >&"$hostInput"
+          built=0
+          while IFS= read -r -t 300 line <&"$hostOutput"; do
+            echo "$line"
+            case "$line" in
+              *'"type":"build_failed"'*)
+                echo "packaged cargo actor build failed with an empty caller PATH" >&2
+                exit 1
+                ;;
+              *'"type":"built"'*)
+                case "$line" in
+                  *'"passed":1'*) built=1 ;;
+                  *) echo "native test did not pass" >&2; exit 1 ;;
+                esac
+                break
+                ;;
+            esac
+          done
+          if [ "$built" -ne 1 ]; then
+            echo "packaged host did not build the probe" >&2
+            exit 1
+          fi
+          printf '%s\n' '/quit' >&"$hostInput"
+          wait "$hostPid"
           mkdir -p "$out"
           touch "$out/passed"
         '';