diff --git a/CHANGELOG.md b/CHANGELOG.md
index ad23ec93..626d5fcb 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,6 +3,30 @@
 > 本文件追踪 `mcpp-community/mcpp` 公开仓的版本演进。
 > 格式参考 [Keep a Changelog](https://keepachangelog.com/zh-CN/1.1.0/)。
 
+## [未发布]
+
+### 修复
+
+- **暂存跨主版本副本时,与源码相邻的私有头没有跟着走。**
+
+  当同一个包的两个主版本同时出现在一张图里,解析器会把其中一份改名暂存到
+  `target/.mangled/` 下。暂存只搬源码 —— 这对经 `[build].include_dirs`
+  找到的头是对的,它们靠绝对化后的路径仍指回原处;但对**写在源码旁边**的私有头
+  不对:`#include "detail.h"` 是相对**包含它的那个文件所在目录**解析的,搬走源码
+  就搬走了搜索起点。包从来没有声明过这条路径,因为它从来不需要。
+
+  ```
+  target/.mangled/openkal-opensbi/__self__/src/time.cpp:44:10:
+      fatal error: 'sbi.h' file not found
+  ```
+
+  ⚠️ 这条诊断指向的东西全是错的:路径是作者没写过的暂存目录,头文件就躺在源码
+  期待的位置,而触发它的构建没有要求任何不寻常的事情 —— 一张图里有两个主版本是
+  受支持的安排,这是它最普通的后果。
+
+  现在,凡是包含了被暂存源码的目录,其中未被暂存的文件一并原样带过去。没有被
+  暂存源码的目录不会被访问,所以代价与暂存量成正比。
+
 ## [2026.8.24.5] — 2026-08-25
 
 ### 改进
diff --git a/src/build/prepare.cppm b/src/build/prepare.cppm
index cc48dcbc..bac66cd9 100644
--- a/src/build/prepare.cppm
+++ b/src/build/prepare.cppm
@@ -3912,9 +3912,30 @@ prepare_build(bool print_fingerprint,
     // version mangling fallback (Level 1) so two cross-major copies of
     // the same package can coexist with distinct module names.
     //
-    // Headers (referenced via `[build].include_dirs`) are NOT staged —
-    // those keep pointing at the original install dir via absolutized
-    // include paths.
+    // Headers reached through `[build].include_dirs` are NOT staged — those
+    // keep pointing at the original install dir via absolutized include paths.
+    //
+    // ⭐ HEADERS BESIDE A SOURCE ARE A DIFFERENT CASE, AND THEY ARE STAGED.
+    //
+    // `#include "detail.h"` is resolved relative to the directory of the file
+    // holding the directive, so moving the source moves the search. No
+    // `include_dirs` entry is involved and absolutizing one cannot help: the
+    // package never declared a path because it never needed one. Measured
+    // before this, on a package whose `src/time.cpp` includes `src/sbi.h`:
+    //
+    //     target/.mangled/openkal-opensbi/__self__/src/time.cpp:44:10:
+    //         fatal error: 'sbi.h' file not found
+    //
+    // ⚠️ THE DIAGNOSIS THIS PRODUCES POINTS AT THE WRONG THING. The path in it
+    // is a staging directory the author never wrote, for a header sitting
+    // exactly where the source expects it, and the build that triggered it
+    // asked for nothing unusual — two majors of one dependency is a supported
+    // arrangement, and this is its most ordinary consequence.
+    //
+    // What is copied is every file in a directory that contains a staged
+    // source and is not itself staged, verbatim: rewriting applies to module
+    // declarations, and a header has none. Directories with no staged source
+    // are not visited, so this stays proportional to what is being staged.
     auto stage_with_rewrite = [&](const std::filesystem::path& srcRoot,
                                   const std::filesystem::path& dstRoot,
                                   const mcpp::manifest::Manifest& depManifest,
@@ -3948,6 +3969,29 @@ prepare_build(bool print_fingerprint,
                 "stage: cannot write '{}'", dst.string()));
             os << out;
         }
+
+        // The files beside those sources, carried across unchanged so a quoted
+        // include still finds what it named.
+        std::set<std::filesystem::path> sourceDirs;
+        for (auto const& f : *sources) sourceDirs.insert(f.parent_path());
+        for (auto const& dir : sourceDirs) {
+            for (auto const& entry : std::filesystem::directory_iterator(dir, ec)) {
+                if (ec) break;
+                if (!entry.is_regular_file()) continue;
+                if (sources->contains(entry.path())) continue;
+                auto rel = std::filesystem::relative(entry.path(), srcRoot, ec);
+                if (ec) continue;
+                auto dst = dstRoot / rel;
+                std::filesystem::create_directories(dst.parent_path(), ec);
+                std::filesystem::copy_file(
+                    entry.path(), dst,
+                    std::filesystem::copy_options::overwrite_existing, ec);
+                if (ec) return std::unexpected(std::format(
+                    "stage: cannot copy '{}': {}",
+                    entry.path().string(), ec.message()));
+            }
+            ec.clear();
+        }
         return {};
     };
 
diff --git a/tests/e2e/33_multi_version_mangling.sh b/tests/e2e/33_multi_version_mangling.sh
index 56537a16..f84495cb 100755
--- a/tests/e2e/33_multi_version_mangling.sh
+++ b/tests/e2e/33_multi_version_mangling.sh
@@ -62,10 +62,39 @@ mkdir -p "$TMP/libB" && cd "$TMP/libB"
 cd libB
 rm -f src/main.cpp
 cat > src/libB.cppm <<'EOF'
+module;
+// Declared in the global module fragment, so it has the same linkage as its
+// definition in libB_impl.cpp — a non-module translation unit. Declaring it
+// after `export module` would attach it to libB and leave the definition
+// unattached, which is a mistake in a fixture rather than in what it tests.
+int libB_impl_v();
 export module libB;
 import mcpplibs.cmdline;   // resolver rewrites this to the mangled secondary
 import std;
-export int libB_v() { return 2; }
+export int libB_v() { return libB_impl_v(); }
+EOF
+# ⭐ A PRIVATE HEADER BESIDE A SOURCE, WHICH IS WHAT STAGING USED TO DROP.
+#
+# libB is the copy the resolver stages under `target/.mangled/`, and a quoted
+# include is resolved relative to the directory of the file holding it — so
+# moving the source moves the search. No `include_dirs` entry is involved and
+# absolutizing one cannot help: a package with its header beside its source
+# never declared a path because it never needed one.
+#
+# Measured before the fix, on this fixture and on a real package alike:
+#
+#     .mangled/libB/__self__/src/libB_impl.cpp:1:10:
+#         fatal error: libB_detail.h: No such file or directory
+#
+# The failure names a staging directory the author never wrote, for a header
+# sitting exactly where the source expects it.
+cat > src/libB_detail.h <<'EOF'
+#pragma once
+inline int libB_detail_v() { return 2; }
+EOF
+cat > src/libB_impl.cpp <<'EOF'
+#include "libB_detail.h"
+int libB_impl_v() { return libB_detail_v(); }
 EOF
 cat > mcpp.toml <<'EOF'
 [package]
@@ -122,6 +151,16 @@ find target -name 'mcpplibs.cmdline.gcm' | grep -q . || {
     find target -name '*.gcm' | head -10
     exit 1; }
 
+# The staged copy carries the header beside the staged source. Asserted on
+# disk as well as through the build, because a build that happened to find the
+# header through some other path would pass while leaving this broken.
+staged="$(find target/.mangled -name 'libB_detail.h' | head -1)"
+[ -n "$staged" ] || {
+    echo "the header beside the staged source was not staged with it"
+    find target/.mangled -type f | head -20
+    exit 1; }
+echo "  ok  the header beside the staged source came with it ($staged)"
+
 out="$("$MCPP" run 2>&1 | tail -1)"
 [[ "$out" == "a=1 b=2" ]] || { echo "unexpected output: $out"; exit 1; }