diff --git a/tvix/eval/src/builtins/mod.rs b/tvix/eval/src/builtins/mod.rs
index 62684b824..4bdce300a 100644
--- a/tvix/eval/src/builtins/mod.rs
+++ b/tvix/eval/src/builtins/mod.rs
@@ -377,6 +377,15 @@ mod pure_builtins {
         toml::from_str(&toml_str).map_err(|err| err.into())
     }
 
+    #[builtin("filterSource")]
+    #[allow(non_snake_case)]
+    async fn builtin_filterSource(_co: GenCo, #[lazy] _e: Value) -> Result<Value, ErrorKind> {
+        // TODO: implement for nixpkgs compatibility
+        Ok(Value::Catchable(CatchableErrorKind::UnimplementedFeature(
+            "filterSource".to_string(),
+        )))
+    }
+
     #[builtin("genericClosure")]
     async fn builtin_generic_closure(co: GenCo, input: Value) -> Result<Value, ErrorKind> {
         let attrs = input.to_attrs()?;
@@ -1084,6 +1093,14 @@ pub fn pure_builtins() -> Vec<(&'static str, Value)> {
         crate::systems::llvm_triple_to_nix_double(CURRENT_PLATFORM).into(),
     ));
 
+    // TODO: implement for nixpkgs compatibility
+    result.push((
+        "__curPos",
+        Value::Catchable(CatchableErrorKind::UnimplementedFeature(
+            "__curPos".to_string(),
+        )),
+    ));
+
     result
 }
 
diff --git a/tvix/eval/src/compiler/mod.rs b/tvix/eval/src/compiler/mod.rs
index f6d6ca7c0..f54dc8ba4 100644
--- a/tvix/eval/src/compiler/mod.rs
+++ b/tvix/eval/src/compiler/mod.rs
@@ -147,6 +147,7 @@ const GLOBAL_BUILTINS: &[&str] = &[
     "scopedImport",
     "throw",
     "toString",
+    "__curPos",
 ];
 
 pub struct Compiler<'observer> {
diff --git a/tvix/eval/src/errors.rs b/tvix/eval/src/errors.rs
index a61d55aa2..23905e438 100644
--- a/tvix/eval/src/errors.rs
+++ b/tvix/eval/src/errors.rs
@@ -42,6 +42,7 @@ use crate::{SourceCode, Value};
 pub enum CatchableErrorKind {
     Throw(String),
     AssertionFailed,
+    UnimplementedFeature(String),
     /// Resolving a user-supplied angle brackets path literal failed in some way.
     NixPathResolution(String),
 }