diff --git a/Cargo.lock b/Cargo.lock
index 6a9ad59..6764dad 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -90,6 +90,12 @@ version = "1.0.93"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "4c95c10ba0b00a02636238b814946408b1322d5ac4760326e6fb8ec956d85775"
 
+[[package]]
+name = "arraydeque"
+version = "0.5.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7d902e3d592a523def97af8f317b08ce16b7ab854c1985a0c671e6f15cebc236"
+
 [[package]]
 name = "autocfg"
 version = "1.3.0"
@@ -498,6 +504,12 @@ dependencies = [
  "serde",
 ]
 
+[[package]]
+name = "bumpalo"
+version = "3.16.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "79296716171880943b8470b5f8d03aa55eb2e645a4874bdbb28adb49162e012c"
+
 [[package]]
 name = "bytes"
 version = "1.6.0"
@@ -676,6 +688,12 @@ version = "1.0.7"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "3f9eec918d3f24069decb9af1554cad7c880e2da24a9afd88aca000531ab82c1"
 
+[[package]]
+name = "foldhash"
+version = "0.1.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f81ec6369c545a7d40e4589b5597581fa1c441fe1cce96dd1de43159910a36a2"
+
 [[package]]
 name = "form_urlencoded"
 version = "1.2.1"
@@ -812,6 +830,26 @@ dependencies = [
  "allocator-api2",
 ]
 
+[[package]]
+name = "hashbrown"
+version = "0.15.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3a9bfc1af68b1726ea47d3d5109de126281def866b33970e10fbab11b5dafab3"
+dependencies = [
+ "allocator-api2",
+ "equivalent",
+ "foldhash",
+]
+
+[[package]]
+name = "hashlink"
+version = "0.8.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e8094feaf31ff591f651a2664fb9cfd92bba7a60ce3197265e9482ebe753c8f7"
+dependencies = [
+ "hashbrown 0.14.5",
+]
+
 [[package]]
 name = "heck"
 version = "0.5.0"
@@ -964,7 +1002,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "168fb715dda47215e360912c096649d23d58bf392ac62f73919e831745e40f26"
 dependencies = [
  "equivalent",
- "hashbrown",
+ "hashbrown 0.14.5",
 ]
 
 [[package]]
@@ -991,12 +1029,6 @@ version = "0.2.155"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "97b3888a4aecf77e811145cadf6eef5901f4782c53886191b2f693f24761847c"
 
-[[package]]
-name = "libyaml-safer"
-version = "0.1.1"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "2cff24caa08828851c9b7ed1bce365b1d2496b9323dd0d4b241c1d10135f5bd6"
-
 [[package]]
 name = "lock_api"
 version = "0.4.12"
@@ -1272,9 +1304,9 @@ dependencies = [
 
 [[package]]
 name = "rsjsonnet-front"
-version = "0.1.1"
+version = "0.3.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "b532a8af07b6781c59a68b7e55745720e26a043ae780c67f5789bf90eaad1c5c"
+checksum = "4c49c7a2d195ba780dd95738ea920ad413636b68572b33167357b608a64da317"
 dependencies = [
  "rsjsonnet-lang",
  "sourceannot",
@@ -1282,13 +1314,15 @@ dependencies = [
 
 [[package]]
 name = "rsjsonnet-lang"
-version = "0.1.1"
+version = "0.3.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "1776c10b94e43de20c39493d947acc339098eb97d2369795238584f76defda66"
+checksum = "e4e1e32cd07aac2d7a0ad2a72982bd133b772d7c8b78e47c448d29aa7c5cac52"
 dependencies = [
- "hashbrown",
- "libyaml-safer",
+ "bumpalo",
+ "foldhash",
+ "hashbrown 0.15.1",
  "md-5",
+ "saphyr-parser",
 ]
 
 [[package]]
@@ -1355,6 +1389,16 @@ version = "1.0.18"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "f3cb5ba0dc43242ce17de99c180e96db90b235b8a9fdc9543c96d2209116bd9f"
 
+[[package]]
+name = "saphyr-parser"
+version = "0.0.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "123f0a6da68f3072c7c761450276d1d444cb391c8be182a757cd26cf684cb77f"
+dependencies = [
+ "arraydeque",
+ "hashlink",
+]
+
 [[package]]
 name = "schannel"
 version = "0.1.23"
@@ -1517,9 +1561,9 @@ dependencies = [
 
 [[package]]
 name = "sourceannot"
-version = "0.2.0"
+version = "0.2.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "806ab42b7a19d82b5f611462f3de3234a1032e16f1837209fc874d7041401d91"
+checksum = "d1266b6cd460a74a33feaca00abc1c85617b20e9790f0effff696deeee75578d"
 dependencies = [
  "unicode-width",
 ]
diff --git a/Cargo.toml b/Cargo.toml
index 7cc0c21..1b09204 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -12,8 +12,8 @@ text_io = "0.1"
 aws-config = { version = "1", features = ["behavior-version-latest"] }
 aws-sdk-scheduler = { version = "1", features = ["behavior-version-latest"] }
 aws-sdk-sfn = { version = "1", features = ["behavior-version-latest"] }
-rsjsonnet-front = "0.1"
-rsjsonnet-lang = "0.1"
+rsjsonnet-front = "0.3"
+rsjsonnet-lang = "0.3"
 similar = "2"
 serde = { version = "1", features = ["derive"] }
 serde_json = "1.0"
diff --git a/src/jsonnet_evaluator.rs b/src/jsonnet_evaluator.rs
index 9c2c5b9..158c440 100644
--- a/src/jsonnet_evaluator.rs
+++ b/src/jsonnet_evaluator.rs
@@ -1,26 +1,36 @@
-use std::path::Path;
+use std::{collections::HashSet, path::Path};
 
 use serde_json::Value;
 
 use crate::cli::StrKeyVal;
 
+use rsjsonnet_front::Session;
 use rsjsonnet_lang::program::Value as JsonnetValue;
 
 pub fn eval(file_path: &str, ext_str: &[StrKeyVal]) -> Result<Value, String> {
     let source_path = Path::new(file_path);
 
-    let mut session = rsjsonnet_front::Session::new();
+    let arena = rsjsonnet_lang::arena::Arena::new();
+    let mut session = Session::new(&arena);
 
-    ext_str.iter().for_each(|ext_str| {
-        let key = session.program().str_interner().intern(&ext_str.var);
-        let val = if let Some(val) = &ext_str.val {
+    let mut ext_names = HashSet::new();
+
+    for arg in ext_str.iter() {
+        let key = session.program().intern_str(&arg.var);
+        if !ext_names.insert(key) {
+            let err_msg = format!("External variable {:?} defined more than once", arg.var);
+            return Err(err_msg);
+        }
+
+        let val = if let Some(val) = &arg.val {
             JsonnetValue::string(val.as_ref())
         } else {
             JsonnetValue::null()
         };
-        let value_thunk = session.program_mut().value_to_thunk(&val);
-        session.program_mut().add_ext_var(key, &value_thunk);
-    });
+
+        let thunk = session.program_mut().value_to_thunk(&val);
+        session.program_mut().add_ext_var(key, &thunk);
+    }
 
     let Some(thunk) = session.load_real_file(source_path) else {
         return Err("Failed to load file".to_string());