diff --git a/crates/fbuild-build/README.md b/crates/fbuild-build/README.md
index da737db6..7acc0e8d 100644
--- a/crates/fbuild-build/README.md
+++ b/crates/fbuild-build/README.md
@@ -13,6 +13,29 @@ Build orchestration, compilation, linking for all platforms (AVR, ESP32, RP2040,
 - **parallel** — Parallel compilation with job control
 - **source_scanner** — Source file discovery (sketch, core, variant)
 
+## Native `extra_scripts` Boundary
+
+Native mode intentionally supports only a narrow subset of PlatformIO `extra_scripts`.
+The goal is to preserve ordinary flag and path mutations without trying to emulate all of SCons.
+
+Supported in native mode:
+
+- `pre:` and `post:` script entries
+- `Import("env")` in PRE/POST scripts
+- `Import("projenv")` in POST scripts only
+- `Append`, `AppendUnique`, `Prepend`, and `Replace` over `CPPDEFINES`, `CPPPATH`, `CCFLAGS`, `CFLAGS`, `CXXFLAGS`, `ASFLAGS`, `LINKFLAGS`, `LIBPATH`, and `LIBS`
+- helper shims such as `Dump`, `BoardConfig`, `PioPlatform`, `Flatten`, `VerboseAction`, and `Execute`
+
+Rejected or out of scope:
+
+- unsupported script prefixes
+- PRE scripts that request `projenv`
+- mutations on unsupported SCons scopes
+- unsupported `Import(...)` targets
+- builders, middleware, upload/package hooks, and other arbitrary Python-driven build behavior
+
+Unsupported `extra_scripts` behavior fails the native build early with a recommendation to use `--platformio`.
+
 ## Compile Database (compile_commands.json)
 
 After every build, fbuild generates a [JSON Compilation Database](https://clang.llvm.org/docs/JSONCompilationDatabase.html) so that clangd and VS Code IntelliSense can resolve includes to actual source files.
diff --git a/crates/fbuild-build/src/README.md b/crates/fbuild-build/src/README.md
index c9c500e2..09683fa6 100644
--- a/crates/fbuild-build/src/README.md
+++ b/crates/fbuild-build/src/README.md
@@ -14,6 +14,17 @@ Build orchestration for all supported embedded platforms.
 - **`build_output.rs`** -- Uniform build log formatting across all platforms
 - **`zccache.rs`** -- Optional zccache compiler cache wrapper integration
 
+## Native `extra_scripts` Boundary
+
+The native replay path intentionally supports a narrow subset of PlatformIO script behavior:
+
+- Supported script prefixes: `pre:` and `post:`
+- Supported imports: `Import("env")`; `Import("projenv")` only in post scripts
+- Supported mutation scopes: `CPPDEFINES`, `CPPPATH`, `CCFLAGS`, `CFLAGS`, `CXXFLAGS`, `ASFLAGS`, `LINKFLAGS`, `LIBPATH`, `LIBS`
+- Supported no-op helpers: `AddPreAction`, `AddPostAction`, `AlwaysBuild`, `Alias`, `Depends`
+
+Anything outside that matrix fails fast with a `use --platformio` recommendation so unsupported scripts do not silently produce a partial build.
+
 ## Platform Subdirectories
 
 - **`avr/`** -- AVR ATmega (Arduino Uno, Mega, Nano)
diff --git a/crates/fbuild-build/src/script_runtime.rs b/crates/fbuild-build/src/script_runtime.rs
index 37187806..f609384a 100644
--- a/crates/fbuild-build/src/script_runtime.rs
+++ b/crates/fbuild-build/src/script_runtime.rs
@@ -1,3 +1,10 @@
+//! Native compatibility layer for a constrained subset of PlatformIO `extra_scripts`.
+//!
+//! Supported shapes are intentionally narrow: `pre:`/`post:` entries, `Import("env")`
+//! in PRE/POST scripts, `Import("projenv")` in POST scripts only, and flag/path
+//! mutations over the known compiler/linker scopes. Unsupported behavior fails fast
+//! with a recommendation to use `--platformio`.
+
 use std::collections::HashMap;
 use std::path::Path;
 use std::process::Command;
@@ -340,6 +347,40 @@ mod tests {
     use crate::flag_overlay::ScriptScopeState;
     use std::fs;
 
+    fn write_runtime_project(
+        extra_scripts: &str,
+        script_name: &str,
+        script_body: &str,
+    ) -> tempfile::TempDir {
+        let temp = tempfile::tempdir().unwrap();
+        let project_dir = temp.path();
+        fs::write(
+            project_dir.join("platformio.ini"),
+            format!(
+                "\
+[env:demo]
+platform = atmelavr
+board = uno
+framework = arduino
+extra_scripts = {}
+",
+                extra_scripts
+            ),
+        )
+        .unwrap();
+        fs::write(project_dir.join(script_name), script_body).unwrap();
+        temp
+    }
+
+    fn resolve_runtime_error(project_dir: &Path) -> String {
+        let config =
+            fbuild_config::PlatformIOConfig::from_path(&project_dir.join("platformio.ini"))
+                .unwrap();
+        resolve_extra_script_overlay(project_dir, "demo", &config)
+            .unwrap_err()
+            .to_string()
+    }
+
     #[test]
     fn test_cppdefines_to_flags_string_and_kv() {
         let flags = cppdefines_to_flags(&[
@@ -604,4 +645,86 @@ env.Append(CPPDEFINES=[\"PIO_PLATFORM_SHIM_OK\"])
             .common
             .contains(&"-DPIO_PLATFORM_SHIM_OK".to_string()));
     }
+
+    #[test]
+    fn test_resolve_extra_script_overlay_rejects_unsupported_import_name() {
+        if find_python().is_none() {
+            return;
+        }
+
+        let temp = write_runtime_project(
+            "post:bad_import_test.py",
+            "bad_import_test.py",
+            "\
+Import(\"board\")
+",
+        );
+        let err = resolve_runtime_error(temp.path());
+        assert!(err.contains("Import('board') is not supported"), "{err}");
+        assert!(err.contains("Recommendation: use --platformio"), "{err}");
+    }
+
+    #[test]
+    fn test_resolve_extra_script_overlay_rejects_projenv_in_pre_script() {
+        if find_python().is_none() {
+            return;
+        }
+
+        let temp = write_runtime_project(
+            "pre:pre_projenv_test.py",
+            "pre_projenv_test.py",
+            "\
+Import(\"env\", \"projenv\")
+",
+        );
+        let err = resolve_runtime_error(temp.path());
+        assert!(
+            err.contains("projenv is not available in PRE extra_scripts"),
+            "{err}"
+        );
+        assert!(err.contains("Recommendation: use --platformio"), "{err}");
+    }
+
+    #[test]
+    fn test_resolve_extra_script_overlay_rejects_unsupported_scope_mutation() {
+        if find_python().is_none() {
+            return;
+        }
+
+        let temp = write_runtime_project(
+            "post:unsupported_scope_test.py",
+            "unsupported_scope_test.py",
+            "\
+Import(\"env\")
+env.Append(FOO=[\"x\"])
+",
+        );
+        let err = resolve_runtime_error(temp.path());
+        assert!(
+            err.contains("env.append on unsupported scope 'FOO'"),
+            "{err}"
+        );
+        assert!(err.contains("Recommendation: use --platformio"), "{err}");
+    }
+
+    #[test]
+    fn test_resolve_extra_script_overlay_rejects_unsupported_script_prefix() {
+        if find_python().is_none() {
+            return;
+        }
+
+        let temp = write_runtime_project(
+            "mid:prefix_test.py",
+            "prefix_test.py",
+            "\
+Import(\"env\")
+",
+        );
+        let err = resolve_runtime_error(temp.path());
+        assert!(
+            err.contains("unsupported extra_scripts prefix 'mid'"),
+            "{err}"
+        );
+        assert!(err.contains("Recommendation: use --platformio"), "{err}");
+    }
 }