@@ -81,6 +81,9 @@
   `[workspace.xlings.overrides]`.
 - Report a `build.mcpp` beside a workspace root without `[package]`, and the
   schema warnings of a workspace root when its members are built.
+- Spell every `${mcpp.*}` path token with forward slashes on every platform, so
+  the `${mcpp.out_dir}/gen/x.cppm` a build program writes is one separator on
+  Windows too instead of a native prefix beside the author's `/` (#393).
 
 ### Deprecated
 
 
@@ -1086,6 +1086,14 @@ none to rely on), and the only interpolations are a closed set:
 | `${mcpp.stage_dir}` *(2026.9.11.1+)* | the tree `mcpp pack` staged, absolute. `artifact` role only, and only under `mcpp pack --format <name>` |
 | `${mcpp.self}` *(2026.9.13.1+)* | the engine's own executable, absolute. An action's command is an argv with no shell, so a build program has no portable way to copy a file; the engine is present wherever a build runs, and `${mcpp.self} stage --verify content --output <dst> <src>` copies one file, creates the destination's parent, and writes only when the bytes differ. That argument shape is a contract from 2026.9.13.1 on; a build program that names it declares that release as its floor |
 
+Every path a `${mcpp.*}` token expands to is spelled with **forward slashes**.
+A token is spliced into the text the build program wrote, and that text spells
+its own separators as `/`; a native spelling on Windows would put a `C:\…`
+prefix beside that `/` inside one argv token. This is the spelling `mcpp` gives
+the compiler, the archiver and every ninja node name, so
+`${mcpp.out_dir}/gen/x.cppm` reads as one separator on Windows as well as on
+POSIX.
+
 The raw stdout protocol above remains the low-level substrate; `import mcpp;`
 is the typed layer over it.
 
 
@@ -922,6 +922,12 @@ mcpp 会写出 `<暂存树>.stage-manifest` —— 一个兄弟文件，永不
 | `${mcpp.stage_dir}` *(2026.9.11.1+)* | `mcpp pack` 暂存出的那棵树，绝对路径。仅 `artifact` role 可用，且仅在 `mcpp pack --format <name>` 下可用 |
 | `${mcpp.self}` *(2026.9.13.1+)* | 引擎自己的可执行文件，绝对路径。action 的命令是没有 shell 的 argv，构建程序因此没有可移植的拷贝手段；引擎在构建运行的每台机器上都在，`${mcpp.self} stage --verify content --output <dst> <src>` 拷贝一个文件、创建目标的父目录、只在字节不同时写入。这个参数形状自 2026.9.13.1 起是契约；写下它的构建程序即以该版本为下限 |
 
+`${mcpp.*}` 展开出的每个路径都用**正斜杠**拼写。token 会被拼进构建程序写下的
+文本，而那段文本用自己的分隔符 `/`；在 Windows 上按原生拼写就会把 `C:\…` 前缀
+和那个 `/` 放进同一个 argv token 里。这与 mcpp 给编译器、归档器和每个 ninja
+节点名的一贯拼写相同，因此 `${mcpp.out_dir}/gen/x.cppm` 在 Windows 上和在 POSIX
+上读起来是同一种分隔符。
+
 上面的裸 stdout 协议仍是底层基底；`import mcpp;` 是其上的类型化层。
 
 ### 读取解析后的依赖图：`graph_file`（2026.9.16.1+）
 
@@ -981,9 +981,19 @@ static std::expected<void, std::string> step13_build_graph_actions(PrepareState&
                 for (std::size_t p; (p = s.find(what)) != std::string::npos; )
                     s.replace(p, what.size(), with);
             };
-            rep("${mcpp.out_dir}",    ctx.plan.outputDir.string());
-            rep("${mcpp.bin_dir}",    binDir.string());
-            rep("${mcpp.compile_db}", ctx.plan.compileDbPath.string());
+            // EVERY `${mcpp.*}` PATH TOKEN IS SPELLED GENERICALLY -- `\`-free on
+            // Windows too (mcpp#393). A token is spliced into text the build
+            // program wrote, and that text writes its own separators as `/`
+            // (`${mcpp.out_dir}/gen/x.cppm`); a native `C:\…` prefix beside
+            // that `/` is a mixed spelling, and the command line carries it to
+            // the tool as one string. Generic is also the spelling the engines
+            // around us already use: `escape_ninja_path` spells every ninja node
+            // name and every tool path this way (the compiler, `ar`, `mcpp`
+            // itself), and `${mcpp.target_file:}` below always has. POSIX output
+            // is byte-identical.
+            rep("${mcpp.out_dir}",    ctx.plan.outputDir.generic_string());
+            rep("${mcpp.bin_dir}",    binDir.generic_string());
+            rep("${mcpp.compile_db}", ctx.plan.compileDbPath.generic_string());
             // The engine's own executable, absolute (2026.9.13.1+). An action
             // whose command is an argv with no shell has no portable way to
             // copy, touch or compare a file, and the engine is the one
@@ -995,7 +1005,7 @@ static std::expected<void, std::string> step13_build_graph_actions(PrepareState&
             // wrapper's: a version change regenerates build.ninja, and a
             // binary moved under an unchanged version leaves a stale path,
             // exactly as it would for the compiler.
-            rep("${mcpp.self}",       mcpp::platform::fs::self_exe_path().string());
+            rep("${mcpp.self}",       mcpp::platform::fs::self_exe_path().generic_string());
             // ABSOLUTE, unlike `${mcpp.target_file:}` and for the same reason
             // stated the other way round: the staged tree lives outside the
             // build directory and no ninja edge produces it, so there is no
@@ -1011,7 +1021,7 @@ static std::expected<void, std::string> step13_build_graph_actions(PrepareState&
                 } else {
                     thisActionUsesStageDir = true;
                 }
-                rep("${mcpp.stage_dir}", stage ? stage->dir.string() : std::string{});
+                rep("${mcpp.stage_dir}", stage ? stage->dir.generic_string() : std::string{});
             }
             constexpr std::string_view kTf = "${mcpp.target_file:";
             for (std::size_t p; (p = s.find(kTf)) != std::string::npos; ) {
 
@@ -71,6 +71,37 @@ if grep -q 'mcpp\.self' "$NINJA"; then
     echo "FAIL: \${mcpp.self} reached build.ninja unsubstituted"; exit 1
 fi
 
+# ── every `${mcpp.*}` path token expands to the GENERIC spelling ───────────
+#
+# A token is spliced into text the build program wrote, and that text writes
+# its own separators as `/` (`${mcpp.out_dir}/copied/deeper/app-copy`); a
+# native `C:\…` prefix beside that `/` is a mixed spelling, and the command
+# line reaches the tool as one string (mcpp#393). Every token this action names
+# -- `${mcpp.self}`, `${mcpp.out_dir}` and `${mcpp.target_file:app}` -- is an
+# engine path, so on Windows a backslash left in the argv IS the regression.
+# On POSIX the two spellings are byte-identical, so this leg only bites on the
+# shard the test exists for; the `# requires:`-free note at the top is why it
+# is here and not in a unit test.
+#
+# The ARGV is read out of the rule's `command =` line, past the wrapper the
+# engine adds when a check/stamp or an env/cwd is in play (`... -- <argv>`); a
+# bare action has no wrapper, and then the whole line is the argv. The EDGE
+# line cannot show the bug: `escape_ninja_path` generic-izes every node name
+# whatever the token spelled.
+CMD_LINES=$(
+    awk '/^rule mcpp_action_/{seen=1; next}
+         seen && /^  command = /{sub(/^  command = /, ""); print; seen=0}' "$NINJA"
+)
+[ -n "$CMD_LINES" ] || { echo "FAIL: no mcpp_action_ rule in build.ninja"; exit 1; }
+while IFS= read -r line; do
+    argv="${line#* -- }"
+    case "$argv" in
+        *\\*) printf 'FAIL: a ${mcpp.*} path token expanded to a native spelling:\n  %s\n' "$argv"
+              exit 1;;
+    esac
+done <<< "$CMD_LINES"
+echo "ok: \${mcpp.*} path tokens are spelled with forward slashes"
+
 # The copy exists, in a directory the action never created itself, and is the
 # program byte for byte: the only way it got there is the engine's `stage`.
 COPY=$(find target -path '*/copied/deeper/app-copy' -print -quit)