diff --git a/crates/fbuild-build-mcu/src/ch32v/ch32v_compiler.rs b/crates/fbuild-build-mcu/src/ch32v/ch32v_compiler.rs
index b18cd8f2..494f6067 100644
--- a/crates/fbuild-build-mcu/src/ch32v/ch32v_compiler.rs
+++ b/crates/fbuild-build-mcu/src/ch32v/ch32v_compiler.rs
@@ -270,7 +270,7 @@ impl Compiler for Ch32vCompiler {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use crate::ch32v::mcu_config::{apply_board_isa, get_ch32v_config_for_mcu};
+    use crate::ch32v::mcu_config::{get_ch32v_config_for_mcu, with_board_isa};
 
     fn test_compiler() -> Ch32vCompiler {
         let mut defines = HashMap::new();
@@ -302,8 +302,11 @@ mod tests {
 
     #[test]
     fn test_common_flags_use_board_isa() {
-        let mut config = get_ch32v_config_for_mcu("ch32v203").unwrap();
-        apply_board_isa(&mut config, Some("rv32imacxw"), Some("ilp32"));
+        let config = with_board_isa(
+            get_ch32v_config_for_mcu("ch32v203").unwrap(),
+            Some("rv32imacxw"),
+            Some("ilp32"),
+        );
         let mut compiler = test_compiler();
         compiler.mcu_config = config;
         let flags = compiler.c_flags();
diff --git a/crates/fbuild-build-mcu/src/ch32v/mcu_config.rs b/crates/fbuild-build-mcu/src/ch32v/mcu_config.rs
index b2e6f300..1818a203 100644
--- a/crates/fbuild-build-mcu/src/ch32v/mcu_config.rs
+++ b/crates/fbuild-build-mcu/src/ch32v/mcu_config.rs
@@ -96,42 +96,67 @@ pub fn normalize_march(march: &str) -> String {
     normalized
 }
 
-/// Apply board ISA and ABI values to compiler and linker flags.
-pub fn apply_board_isa(config: &mut Ch32vMcuConfig, march: Option<&str>, mabi: Option<&str>) {
-    apply_board_isa_for_toolchain(config, march, mabi, "riscv-none-elf");
+/// The recipe with the board's ISA and ABI applied to compiler and linker
+/// flags, for the xPack `riscv-none-elf` compiler.
+pub fn with_board_isa(
+    config: Ch32vMcuConfig,
+    march: Option<&str>,
+    mabi: Option<&str>,
+) -> Ch32vMcuConfig {
+    with_board_isa_for_toolchain(config, march, mabi, "riscv-none-elf")
 }
 
-/// Preserve WCH's vendor ISA extensions for its own compiler; only the xPack
-/// compiler requires the stripped/modernized `zicsr` spelling.
-pub fn apply_board_isa_for_toolchain(
-    config: &mut Ch32vMcuConfig,
+/// The recipe with the board's ISA and ABI applied to compiler and linker
+/// flags, spelled for the compiler with `executable_prefix`.
+pub fn with_board_isa_for_toolchain(
+    config: Ch32vMcuConfig,
     march: Option<&str>,
     mabi: Option<&str>,
     executable_prefix: &str,
-) {
-    fn replace(flags: &mut [String], prefix: &str, value: &str) {
-        for flag in flags {
-            if flag.starts_with(prefix) {
-                *flag = format!("{prefix}{value}");
-            }
-        }
+) -> Ch32vMcuConfig {
+    let config = match march {
+        Some(march) => with_flag_value(
+            config,
+            "-march=",
+            &march_for_toolchain(march, executable_prefix),
+        ),
+        None => config,
+    };
+    match mabi {
+        Some(mabi) => with_flag_value(config, "-mabi=", mabi),
+        None => config,
     }
+}
 
-    if let Some(march) = march {
-        let normalized = if executable_prefix == "riscv-none-elf" {
-            normalize_march(march)
-        } else {
-            march.to_ascii_lowercase()
-        };
-        replace(&mut config.compiler_flags.common, "-march=", &normalized);
-        replace(&mut config.linker_flags, "-march=", &normalized);
-    }
-    if let Some(mabi) = mabi {
-        replace(&mut config.compiler_flags.common, "-mabi=", mabi);
-        replace(&mut config.linker_flags, "-mabi=", mabi);
+/// Preserve WCH's vendor ISA extensions for its own compiler; only the xPack
+/// compiler requires the stripped/modernized `zicsr` spelling.
+fn march_for_toolchain(march: &str, executable_prefix: &str) -> String {
+    if executable_prefix == "riscv-none-elf" {
+        normalize_march(march)
+    } else {
+        march.to_ascii_lowercase()
     }
 }
 
+/// Set the value of every `prefix`-led compiler and linker flag.
+fn with_flag_value(mut config: Ch32vMcuConfig, prefix: &str, value: &str) -> Ch32vMcuConfig {
+    let set = |flags: Vec<String>| -> Vec<String> {
+        flags
+            .into_iter()
+            .map(|flag| {
+                if flag.starts_with(prefix) {
+                    format!("{prefix}{value}")
+                } else {
+                    flag
+                }
+            })
+            .collect()
+    };
+    config.compiler_flags.common = set(config.compiler_flags.common);
+    config.linker_flags = set(config.linker_flags);
+    config
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -146,9 +171,8 @@ mod tests {
 
     #[test]
     fn platformio_gcc8_preserves_wch_vendor_isa() {
-        let mut config = get_ch32v_config_for_mcu("ch32v003").unwrap();
-        apply_board_isa_for_toolchain(
-            &mut config,
+        let config = with_board_isa_for_toolchain(
+            get_ch32v_config_for_mcu("ch32v003").unwrap(),
             Some("rv32ecxw"),
             Some("ilp32e"),
             "riscv-none-embed",
@@ -169,9 +193,12 @@ mod tests {
     }
 
     #[test]
-    fn test_apply_board_isa_updates_compiler_and_linker() {
-        let mut config = get_ch32v_config_for_mcu("ch32v003").unwrap();
-        apply_board_isa(&mut config, Some("rv32imacxw"), Some("ilp32"));
+    fn test_with_board_isa_updates_compiler_and_linker() {
+        let config = with_board_isa(
+            get_ch32v_config_for_mcu("ch32v003").unwrap(),
+            Some("rv32imacxw"),
+            Some("ilp32"),
+        );
         assert!(
             config
                 .compiler_flags
@@ -193,10 +220,9 @@ mod tests {
     }
 
     #[test]
-    fn test_apply_board_isa_none_is_noop() {
-        let mut config = get_ch32v_config_for_mcu("ch32v003").unwrap();
-        let before = config.clone();
-        apply_board_isa(&mut config, None, None);
+    fn test_with_board_isa_none_is_noop() {
+        let before = get_ch32v_config_for_mcu("ch32v003").unwrap();
+        let config = with_board_isa(before.clone(), None, None);
         assert_eq!(config.compiler_flags.common, before.compiler_flags.common);
         assert_eq!(config.linker_flags, before.linker_flags);
     }
diff --git a/crates/fbuild-build-mcu/src/ch32v/orchestrator.rs b/crates/fbuild-build-mcu/src/ch32v/orchestrator.rs
index b0c75bf0..59d5194e 100644
--- a/crates/fbuild-build-mcu/src/ch32v/orchestrator.rs
+++ b/crates/fbuild-build-mcu/src/ch32v/orchestrator.rs
@@ -289,10 +289,9 @@ impl BuildOrchestrator for Ch32vOrchestrator {
         );
 
         // 7. Build include dirs + compiler
-        let mut mcu_config = super::mcu_config::get_ch32v_config_for_mcu(&series)?;
         let toolchain_prefix = toolchain.executable_prefix();
-        super::mcu_config::apply_board_isa_for_toolchain(
-            &mut mcu_config,
+        let mcu_config = super::mcu_config::with_board_isa_for_toolchain(
+            super::mcu_config::get_ch32v_config_for_mcu(&series)?,
             selected_march,
             selected_mabi,
             &toolchain_prefix,
diff --git a/crates/fbuild-build-mcu/src/ch32v/orchestrator_tests.rs b/crates/fbuild-build-mcu/src/ch32v/orchestrator_tests.rs
index c8186cca..f5833fb2 100644
--- a/crates/fbuild-build-mcu/src/ch32v/orchestrator_tests.rs
+++ b/crates/fbuild-build-mcu/src/ch32v/orchestrator_tests.rs
@@ -38,9 +38,8 @@ fn explicit_board_isa_and_abi_override_selected_platform_defaults() {
         preferred_board_value(None, "board_build.march", Some("rv32ecxw"), Some("rv32ec")),
         Some("rv32ecxw")
     );
-    let mut config = crate::ch32v::mcu_config::get_ch32v_config_for_mcu("ch32v003").unwrap();
-    crate::ch32v::mcu_config::apply_board_isa_for_toolchain(
-        &mut config,
+    let config = crate::ch32v::mcu_config::with_board_isa_for_toolchain(
+        crate::ch32v::mcu_config::get_ch32v_config_for_mcu("ch32v003").unwrap(),
         preferred_board_value(
             Some(&env),
             "board_build.march",