diff --git a/docs/07-workspace.md b/docs/07-workspace.md
index 89e3d7016..44d7c1804 100644
--- a/docs/07-workspace.md
+++ b/docs/07-workspace.md
@@ -290,6 +290,14 @@ share it are planned and compiled together. Before 2026.10.5.2 a virtual root's
 profiles reached no member and were ignored without a diagnostic, and a rooted
 workspace's applied only when its own package was the first one selected.
 
+A selected member's `[target.<selector>.build] dialect_cxxflags` remains a
+graph-wide setting under that condition, including in a workspace plan. Matching
+rows reach the standard-library module, scanning and every C++ translation unit;
+nonmatching rows contribute nothing. Workspace grouping includes the predicates
+and flag lists, so members with different declarations use separate plans.
+Conditional sources and ordinary compile flags remain attributes of the member,
+and an ordinary dependency's dialect flags do not change its consumer's graph.
+
 **The root package is a member.** In a workspace whose root carries
 `[package]`, the root package receives `[workspace.package]` and
 `[workspace.build]` once, as every other member does (2026.10.5.2+), so its
diff --git a/docs/zh/07-workspace.md b/docs/zh/07-workspace.md
index a9e3e05fb..dfdca12d5 100644
--- a/docs/zh/07-workspace.md
+++ b/docs/zh/07-workspace.md
@@ -230,6 +230,12 @@ c++23；什么都不写的成员得到 c++26。这两种情况值相同而意图
 显式选择加入，因为依赖是解析图上的一条边：隐式继承一条边，会在成员自己的
 manifest 只字未提的情况下改变它解析到什么。
 
+成员被选中构建时，`[target.<selector>.build] dialect_cxxflags` 在工作空间计划中
+仍然是带条件的全图设置。匹配的条件会作用于标准库模块、扫描和所有 C++ 编译单元，
+不匹配的条件不贡献参数。工作空间分组包含条件及参数列表，因此声明不同的成员使用
+不同计划。条件源文件和普通编译参数仍属于成员自身；普通依赖的方言参数不会改变
+消费者的模块图。
+
 **追加的向量能覆盖什么。** 成员的词在命令行上排在工作空间的词之后。编译器按
 "后者胜出"处理的标志因此可以通过重写来覆盖：`-fno-exceptions` 之后的
 `-fexceptions`、`-Wx` 之后的 `-Wno-x`、`-O0` 之后的 `-O2`。头文件目录按顺序搜索，
diff --git a/src/project.cppm b/src/project.cppm
index 0906b2d12..583d15b21 100644
--- a/src/project.cppm
+++ b/src/project.cppm
@@ -765,6 +765,14 @@ export std::string root_position_key(const mcpp::manifest::Manifest& m) {
     field("standard", m.package.standard);
     auto const& b = m.buildConfig;
     list("dialect_cxxflags", b.dialectCxxflags);
+    // Conditional dialect flags are root-position values too. Grouping runs
+    // before the target is resolved, so retain the predicate and declaration
+    // order rather than evaluating a member's rows against the host.
+    for (auto const& cc : m.conditionalConfigs) {
+        if (cc.dialectCxxflags.empty()) continue;
+        field("dialect_predicate", cc.predicate);
+        list("conditional_dialect_cxxflags", cc.dialectCxxflags);
+    }
     field("cxx_runtime", b.cxxRuntime);
     field("cxx_runtime_tests", b.cxxRuntimeTests);
     field("cxx_runtime_shared", b.cxxRuntimeShared);
@@ -846,6 +854,16 @@ virtual_workspace_root(const mcpp::manifest::Manifest& workspace,
     b.cxxRuntimeShared = f.cxxRuntimeShared;
     b.target = f.target;
     b.dialectCxxflags = f.dialectCxxflags;
+    // The virtual root is created before conditional merging. Carry only the
+    // member's graph-wide dialect declarations: its sources, flags, runtime
+    // files and dependencies still belong to the member's own node (#786).
+    for (auto const& cc : first.conditionalConfigs) {
+        if (cc.dialectCxxflags.empty()) continue;
+        mcpp::manifest::ConditionalConfig row;
+        row.predicate = cc.predicate;
+        row.dialectCxxflags = cc.dialectCxxflags;
+        v.conditionalConfigs.push_back(std::move(row));
+    }
     b.abiThreads = f.abiThreads;
     b.abiThreadsDeclared = f.abiThreadsDeclared;
     b.abiExceptions = f.abiExceptions;
diff --git a/tests/e2e/890_workspace_conditional_dialect.sh b/tests/e2e/890_workspace_conditional_dialect.sh
new file mode 100644
index 000000000..c4fe2120e
--- /dev/null
+++ b/tests/e2e/890_workspace_conditional_dialect.sh
@@ -0,0 +1,100 @@
+#!/usr/bin/env bash
+# requires: unix-shell
+# #786: a selected member's architecture-specific dialect reaches the virtual
+# workspace root, std precompilation and every TU. Different declarations must
+# separate workspace plans; an ordinary dependency cannot change the dialect.
+set -e
+source "$(dirname "$0")/_host_path.sh"
+
+TMP=$(mktemp -d)
+trap 'rm -rf "$TMP"' EXIT
+fail() { echo "FAIL: $1"; shift; for f in "$@"; do echo "--- $f ---"; cat "$f" 2>/dev/null; done; exit 1; }
+MCPP="${MCPP:-mcpp}"
+REGISTRY_HOST=$(host_path "${MCPP_HOME:-$HOME/.mcpp}/registry")
+export MCPP_HOME="$TMP/mcpp-home"
+mkdir -p "$MCPP_HOME"
+cat > "$MCPP_HOME/config.toml" <<EOF
+[xlings]
+home = "$REGISTRY_HOST"
+EOF
+
+case "$(uname -m)" in
+    x86_64|amd64) ARCH=x86_64; OTHER=aarch64 ;;
+    aarch64|arm64) ARCH=aarch64; OTHER=x86_64 ;;
+    *) fail "unsupported fixture host architecture: $(uname -m)" ;;
+esac
+
+mkdir -p "$TMP/ws/app" "$TMP/ws/other" "$TMP/ws/dep"
+cd "$TMP/ws"
+cat > mcpp.toml <<'TOML'
+[workspace]
+members = ["app", "other"]
+[workspace.build]
+dialect_cxxflags = ["-DBASE786"]
+TOML
+cat > dep/mcpp.toml <<EOF
+[package]
+name = "dep786"
+version = "0.1.0"
+standard = "c++23"
+[build]
+sources = ["dep.cppm"]
+[targets.dep786]
+kind = "lib"
+[target.'cfg(arch = "$ARCH")'.build]
+dialect_cxxflags = ["-DDEP_ONLY786"]
+EOF
+printf 'export module dep786;\nexport int value786() { return 42; }\n' > dep/dep.cppm
+write_member() {
+    local name="$1" value="$2"
+    cat > "$name/mcpp.toml" <<EOF
+[package]
+name = "$name"
+version = "0.1.0"
+standard = "c++23"
+[build]
+sources = ["main.cpp"]
+[targets.$name]
+kind = "bin"
+main = "main.cpp"
+[dependencies]
+dep786 = { path = "../dep" }
+[target.'cfg(arch = "$ARCH")'.build]
+dialect_cxxflags = ["-DARCH786=$value"]
+[target.'cfg(arch = "$OTHER")'.build]
+dialect_cxxflags = ["-DNONMATCH786"]
+EOF
+    cat > "$name/main.cpp" <<EOF
+#ifndef BASE786
+#error workspace dialect missing
+#endif
+#if ARCH786 != $value
+#error member architecture dialect missing or taken from another member
+#endif
+#if defined(NONMATCH786) || defined(DEP_ONLY786)
+#error nonmatching or dependency dialect leaked
+#endif
+import std;
+import dep786;
+int main() { std::println("{}", value786()); return 0; }
+EOF
+}
+write_member app 1
+write_member other 2
+
+"$MCPP" build -p app > selected.log 2>&1 || fail "selected-member build lost its dialect" selected.log
+grep -q -- '-DARCH786=1' compile_commands.json || fail "CDB lost the conditional dialect" compile_commands.json
+grep -rq -- '-DARCH786=1' "$MCPP_HOME/build-cache/v1/std" || fail "std precompile lost the conditional dialect" selected.log
+if grep -q -- '-DNONMATCH786\|-DDEP_ONLY786' compile_commands.json; then
+    fail "nonmatching/dependency dialect reached compiler commands" compile_commands.json
+fi
+
+cd app
+"$MCPP" build > inside.log 2>&1 || fail "building inside a member lost its dialect" inside.log
+cd ..
+"$MCPP" build --workspace > workspace.log 2>&1 || fail "workspace grouping conflated distinct dialects" workspace.log
+graphs=$(find target -name build.ninja | wc -l | tr -d ' ')
+[ "$graphs" = 2 ] || fail "expected two configurations for different member dialects, got $graphs" workspace.log
+grep -rq -- '-DARCH786=2' "$MCPP_HOME/build-cache/v1/std" || fail "second member reused the first member's std configuration" workspace.log
+
+echo "PASS: 890 workspace conditional dialect"
diff --git a/tests/unit/test_workspace_plan.cpp b/tests/unit/test_workspace_plan.cpp
index 297e93698..0a08c1c96 100644
--- a/tests/unit/test_workspace_plan.cpp
+++ b/tests/unit/test_workspace_plan.cpp
@@ -91,6 +91,50 @@ TEST(WorkspacePlan, TheVirtualRootHoldsNoPackageContent) {
     EXPECT_EQ(mcpp::project::root_position_key(v), mcpp::project::root_position_key(first));
 }
 
+TEST(WorkspacePlan, ConditionalDialectFlagsSeparateConfigurations) {
+    auto a = member("a");
+    auto b = member("b");
+    mcpp::manifest::ConditionalConfig row;
+    row.predicate = "i686-windows-msvc";
+    row.dialectCxxflags = {"-DARCH_FLAG=1"};
+    a.conditionalConfigs.push_back(row);
+    EXPECT_NE(mcpp::project::root_position_key(a), mcpp::project::root_position_key(b));
+    b.conditionalConfigs.push_back(row);
+    EXPECT_EQ(mcpp::project::root_position_key(a), mcpp::project::root_position_key(b));
+    b.conditionalConfigs.front().inputs.cxxflags = {"-DMEMBER_ONLY"};
+    b.conditionalConfigs.front().inputs.sources = {"arch.cpp"};
+    EXPECT_EQ(mcpp::project::root_position_key(a), mcpp::project::root_position_key(b));
+    b.conditionalConfigs.front().dialectCxxflags = {"-DARCH_FLAG=2"};
+    EXPECT_NE(mcpp::project::root_position_key(a), mcpp::project::root_position_key(b));
+    b.conditionalConfigs.front().dialectCxxflags = row.dialectCxxflags;
+    b.conditionalConfigs.front().predicate = "x86_64-windows-msvc";
+    EXPECT_NE(mcpp::project::root_position_key(a), mcpp::project::root_position_key(b));
+}
+
+TEST(WorkspacePlan, VirtualRootPreservesOnlyConditionalDialectDeclarations) {
+    mcpp::manifest::Manifest ws;
+    auto first = member("a");
+    mcpp::manifest::ConditionalConfig row;
+    row.predicate = "i686-windows-msvc";
+    row.dialectCxxflags = {"-DARCH_FLAG"};
+    row.inputs.cxxflags = {"-DMEMBER_ONLY"};
+    row.inputs.sources = {"arch.cpp"};
+    row.libraries = {"member_runtime"};
+    first.conditionalConfigs.push_back(row);
+    mcpp::manifest::ConditionalConfig packageOnly;
+    packageOnly.predicate = "windows";
+    packageOnly.inputs.cxxflags = {"-DOTHER_MEMBER_FLAG"};
+    first.conditionalConfigs.push_back(packageOnly);
+    const auto v = mcpp::project::virtual_workspace_root(ws, first, "/ws");
+    ASSERT_EQ(v.conditionalConfigs.size(), 1u);
+    EXPECT_EQ(v.conditionalConfigs.front().predicate, row.predicate);
+    EXPECT_EQ(v.conditionalConfigs.front().dialectCxxflags, row.dialectCxxflags);
+    EXPECT_TRUE(v.conditionalConfigs.front().inputs.cxxflags.empty());
+    EXPECT_TRUE(v.conditionalConfigs.front().inputs.sources.empty());
+    EXPECT_TRUE(v.conditionalConfigs.front().libraries.empty());
+    EXPECT_EQ(mcpp::project::root_position_key(v), mcpp::project::root_position_key(first));
+}
+
 // A member's products are in `bin/<package name>/`, qualified when another
 // member of the workspace has the same name; the workspace's own package
 // keeps `bin/`.