@@ -457,6 +457,15 @@ jobs:
       - name: "Compile database e2e that needs llvm (783, 784, 785, 786)"
         run: |
           set -o pipefail
+          # The scripts read the database with python3, which this container
+          # lacks. It is not a toolchain: the job's property (no host compiler,
+          # no host C runtime) was asserted before the checkout and still holds.
+          if ! command -v python3 >/dev/null 2>&1; then
+            apt-get install -y -qq python3 >/dev/null
+            if command -v gcc >/dev/null 2>&1; then
+              echo "::error::installing python3 brought a host compiler"; exit 1
+            fi
+          fi
           export PATH="$HOME/.xlings/subos/current/bin:$PATH"
           export MCPP
           run_and_assert() {   # $1 = script; the remaining arguments are lines it must print
 
@@ -66,19 +66,21 @@ grep -q 'tests[\/][\/]*test_smoke.cpp' compile_commands.json || {
     exit 1
 }
 if command -v python3 >/dev/null 2>&1; then
-    # The devkit path is passed in directly rather than derived from the CDB's
-    # `directory`: since C3 (design 2026-09-26 §3.3), `directory` is the
-    # OUTPUT directory the compiler runs in, not the project root, so a
-    # sibling fixture's path can no longer be recovered from it.
-    python3 - compile_commands.json "$TMP/devkit/include" <<'PY'
+    # The devkit path is derived from the entry's own `file`, in the spelling
+    # mcpp wrote: the shell's `$TMP` is not it on Windows (an MSYS root) or on
+    # macOS (`/var` against the resolved `/private/var`). `directory` no longer
+    # serves, because since C3 (design 2026-09-26 §3.3) it is the output
+    # directory the compiler runs in, not the project root.
+    python3 - compile_commands.json <<'PY'
 import json, sys
 entries = json.load(open(sys.argv[1], encoding="utf-8"))
-devkit_include = sys.argv[2]
 normal = lambda p: p.replace("\\", "/").rstrip("/")
 test = next(e for e in entries if normal(e["file"]).endswith("/tests/test_smoke.cpp"))
 main = next(e for e in entries if normal(e["file"]).endswith("/src/main.cpp"))
 args = test["arguments"]
-expected_include = normal(devkit_include).casefold()
+project = normal(test["file"])[: -len("/tests/test_smoke.cpp")]
+fixture = project.rsplit("/", 1)[0]
+expected_include = f"{fixture}/devkit/include".casefold()
 include_args = {
     normal(a[2:]).casefold()
     for a in args
 
@@ -15,12 +15,8 @@ trap 'rm -rf "$TMP"' EXIT
 cd "$TMP"
 "$MCPP" new app > /dev/null
 cd app
-# Pinned rather than left to the machine's default: the property under test
-# does not depend on which toolchain is used, and pinning keeps the test's
-# reading independent of shared state outside this tree.
-TCFLAG=(--toolchain gcc@16.1.0)
 
-"$MCPP" build "${TCFLAG[@]}" > build1.log 2>&1 || { cat build1.log; echo "FAIL: the first build failed"; exit 1; }
+"$MCPP" build > build1.log 2>&1 || { cat build1.log; echo "FAIL: the first build failed"; exit 1; }
 [[ -s compile_commands.json ]] || { echo "FAIL: no compile_commands.json after the first build"; exit 1; }
 
 config_cdb=$(find target -name compile_commands.json | head -1)
@@ -29,7 +25,7 @@ before=$(cat "$config_cdb")
 
 rm compile_commands.json
 
-"$MCPP" build "${TCFLAG[@]}" > build2.log 2>&1 || {
+"$MCPP" build > build2.log 2>&1 || {
     cat build2.log
     echo "FAIL: the build after deleting the root database failed"
     exit 1
@@ -45,7 +41,9 @@ rm compile_commands.json
 # real prepare pass has nothing to compile -- the restore must not trigger
 # one. A plan prints a "Compiling <pkg>" line; the fast path prints only
 # "Finished ... in <time>".
-if grep -q "Compiling" build2.log; then
+# The fast path replays ELF builds only (#400); on macOS and Windows the
+# second build plans again, and the full path's writer restores the file.
+if [[ "$(uname -s)" == Linux ]] && grep -q "Compiling" build2.log; then
     echo "FAIL: restoring the root database ran a full plan"
     cat build2.log
     exit 1
@@ -61,7 +59,7 @@ after=$(cat compile_commands.json)
 # The restore survives a SECOND round-trip too (not a one-shot fluke of the
 # cache entry the first build happened to leave behind).
 rm compile_commands.json
-"$MCPP" build "${TCFLAG[@]}" > build3.log 2>&1 || { cat build3.log; echo "FAIL: the third build failed"; exit 1; }
+"$MCPP" build > build3.log 2>&1 || { cat build3.log; echo "FAIL: the third build failed"; exit 1; }
 [[ -f compile_commands.json ]] || { echo "FAIL: the second delete was not restored"; exit 1; }
 
 echo "OK"
@@ -14,11 +14,7 @@ trap 'rm -rf "$TMP"' EXIT
 cd "$TMP"
 "$MCPP" new app > /dev/null
 cd app
-# Pinned rather than left to the machine's default: the property under test
-# does not depend on which toolchain is used, and pinning keeps the test's
-# reading independent of shared state outside this tree.
-TCFLAG=(--toolchain gcc@16.1.0)
-"$MCPP" build "${TCFLAG[@]}" > /dev/null 2>&1
+"$MCPP" build > /dev/null 2>&1
 
 # Another writer's database: two entries with no `output` field at all -- the
 # shape a hand-written or xmake-style database uses, and never mcpp's own
@@ -34,7 +30,7 @@ EOF
 
 # Force a real prepare pass (not the fast path), so the replace actually runs.
 touch src/main.cpp
-out=$("$MCPP" build "${TCFLAG[@]}" 2>&1) || { echo "$out"; echo "FAIL: build failed"; exit 1; }
+out=$("$MCPP" build 2>&1) || { echo "$out"; echo "FAIL: build failed"; exit 1; }
 
 warn_count=$(printf '%s\n' "$out" | grep -c 'held 2 entries mcpp did not write')
 [[ "$warn_count" -eq 1 ]] || {
@@ -66,7 +62,7 @@ grep -q "main.cpp" compile_commands.json || {
 
 # A second build over the now-mcpp-owned file warns no further: nothing
 # foreign is left to replace.
-out2=$("$MCPP" build "${TCFLAG[@]}" --no-cache 2>&1) || { echo "$out2"; echo "FAIL: second build failed"; exit 1; }
+out2=$("$MCPP" build --no-cache 2>&1) || { echo "$out2"; echo "FAIL: second build failed"; exit 1; }
 if printf '%s\n' "$out2" | grep -q "did not write"; then
     echo "FAIL: warned again on a file that already held only mcpp's configuration"
     echo "$out2"
 
@@ -869,12 +869,22 @@ TEST(BuildDirectives, PrepareActionWithNoOutputDirIsRefused) {
 // replayed `prepare` payload decodes to the same role and output_dir a fresh
 // run would -- the cache hit must replay both exactly.
 TEST(BuildDirectives, PrepareActionSurvivesTheCacheRoundTrip) {
+    // The paths are JSON strings: a Windows path's backslashes are escaped,
+    // as the bundled module's serializer escapes them.
+    auto json_string = [](std::string_view v) {
+        std::string out;
+        for (char c : v) {
+            if (c == '\\' || c == '"') out.push_back('\\');
+            out.push_back(c);
+        }
+        return out;
+    };
     const std::string payload = std::format(
         "{{\"id\":\"prep\",\"role\":\"prepare\","
         "\"description\":\"\",\"blocking\":false,\"output_dir\":\"{}\","
         "\"inputs\":[],\"outputs\":[\"{}\"],"
         "\"command\":[\"install.sh\"],\"provides\":[],\"imports\":[],\"targets\":[]}}",
-        under_root("prefix"), under_root("prep.stamp"));
+        json_string(under_root("prefix")), json_string(under_root("prep.stamp")));
     dirs::Directives replayed;
     ASSERT_TRUE(dirs::accept_cache_record(replayed, "action", payload));
     auto a = dirs::decode_action(replayed.at(dirs::Slot::Actions).front());
 
@@ -635,8 +635,10 @@ TEST(CompileCommandsEmit, AProjectUnitsDirectoryIsTheOutputDirectory) {
 
     auto invs = unit_invocations(plan, flags);
     ASSERT_EQ(invs.size(), 1u);
-    EXPECT_EQ(invs[0].directory, plan.outputDir.string());
-    EXPECT_NE(invs[0].directory, plan.projectRoot.string());
+    // Compared as paths: the field is written in the native spelling, which
+    // on Windows turns this fixture's `/` into `\`.
+    EXPECT_EQ(std::filesystem::path(invs[0].directory), plan.outputDir);
+    EXPECT_NE(std::filesystem::path(invs[0].directory), plan.projectRoot);
 }
 
 // ── C4: the module interface language flag ──────────────────────────────────