diff --git a/rust/candid/src/parser/grammar.lalrpop b/rust/candid/src/parser/grammar.lalrpop
index 850079c7..6f0e2356 100644
--- a/rust/candid/src/parser/grammar.lalrpop
+++ b/rust/candid/src/parser/grammar.lalrpop
@@ -1,7 +1,7 @@
 use crate::types::value::{IDLField, IDLValue, IDLArgs, VariantValue};
 use crate::types::{TypeEnv, FuncMode};
 use crate::utils::check_unique;
-use super::types::{IDLType, PrimType, TypeField, FuncType, Binding, Dec, IDLProg, IDLTypes};
+use super::types::{IDLType, PrimType, TypeField, FuncType, Binding, Dec, IDLProg, IDLTypes, IDLInitArgs};
 use super::test::{Assert, Input, Test};
 use super::token::{Token, error2, LexicalError, Span};
 use crate::{Principal, types::Label};
@@ -272,6 +272,10 @@ pub IDLProg: IDLProg = {
     <decs:SepBy<Def, ";">> <actor:MainActor?> => IDLProg { decs, actor }
 }
 
+pub IDLInitArgs: IDLInitArgs = {
+    <decs:SepBy<Def, ";">> <args:TupTyp> => IDLInitArgs { decs, args }
+}
+
 // Test file
 
 Input: Input = {
diff --git a/rust/candid/src/parser/types.rs b/rust/candid/src/parser/types.rs
index 738c719e..5c9794f6 100644
--- a/rust/candid/src/parser/types.rs
+++ b/rust/candid/src/parser/types.rs
@@ -91,6 +91,12 @@ pub struct IDLProg {
     pub actor: Option<IDLType>,
 }
 
+#[derive(Debug)]
+pub struct IDLInitArgs {
+    pub decs: Vec<Dec>,
+    pub args: Vec<IDLType>,
+}
+
 impl std::str::FromStr for IDLProg {
     type Err = crate::Error;
     fn from_str(str: &str) -> Result<Self> {
@@ -98,6 +104,13 @@ impl std::str::FromStr for IDLProg {
         Ok(super::grammar::IDLProgParser::new().parse(lexer)?)
     }
 }
+impl std::str::FromStr for IDLInitArgs {
+    type Err = crate::Error;
+    fn from_str(str: &str) -> Result<Self> {
+        let lexer = super::token::Tokenizer::new(str);
+        Ok(super::grammar::IDLInitArgsParser::new().parse(lexer)?)
+    }
+}
 
 impl std::str::FromStr for IDLType {
     type Err = crate::Error;
diff --git a/rust/candid/src/parser/typing.rs b/rust/candid/src/parser/typing.rs
index 2d6f0983..ec2d831d 100644
--- a/rust/candid/src/parser/typing.rs
+++ b/rust/candid/src/parser/typing.rs
@@ -241,12 +241,23 @@ fn load_imports(
 }
 
 /// Type check IDLProg and adds bindings to type environment. Returns
-/// a hash map for the serivce method signatures. This function ignores the imports.
+/// the main actor if present. This function ignores the imports.
 pub fn check_prog(te: &mut TypeEnv, prog: &IDLProg) -> Result<Option<Type>> {
     let mut env = Env { te, pre: false };
     check_decs(&mut env, &prog.decs)?;
     check_actor(&env, &prog.actor)
 }
+/// Type check init args extracted from canister metadata candid:args.
+pub fn check_init_args(te: &mut TypeEnv, main_env: &TypeEnv, prog: &IDLInitArgs) -> Result<Vec<Type>> {
+    let mut env = Env { te, pre: false };
+    check_decs(&mut env, &prog.decs)?;
+    env.te.merge(main_env)?;
+    let mut args = Vec::new();
+    for arg in prog.args.iter() {
+        args.push(check_type(&env, arg)?);
+    }
+    Ok(args)
+}
 
 fn check_file_(file: &Path, is_pretty: bool) -> Result<(TypeEnv, Option<Type>)> {
     let base = if file.is_absolute() {
diff --git a/rust/candid/src/utils.rs b/rust/candid/src/utils.rs
index c8f275e3..c251be65 100644
--- a/rust/candid/src/utils.rs
+++ b/rust/candid/src/utils.rs
@@ -82,7 +82,7 @@ pub fn service_equal(left: CandidSource, right: CandidSource) -> Result<()> {
 #[cfg_attr(docsrs, doc(cfg(feature = "parser")))]
 #[cfg(feature = "parser")]
 /// Take a did file and outputs the init args and the service type (without init args).
-/// If the original did file contains imports, the output flatens the type definitions.
+/// If the original did file contains imports, the output flattens the type definitions.
 /// For now, the comments from the original did file is omitted.
 pub fn instantiate_candid(candid: CandidSource) -> Result<(Vec<Type>, (TypeEnv, Type))> {
     use crate::types::TypeInner;
@@ -95,6 +95,28 @@ pub fn instantiate_candid(candid: CandidSource) -> Result<(Vec<Type>, (TypeEnv,
     })
 }
 
+/// Merge canister metadata candid:args and candid:service into a service constructor.
+/// If candid:service already contains init args, returns the original did file.
+#[cfg_attr(docsrs, doc(cfg(feature = "parser")))]
+#[cfg(feature = "parser")]
+pub fn merge_init_args(candid: &str, init: &str) -> Result<(TypeEnv, Type)> {
+    use crate::parser::{types::IDLInitArgs, typing::check_init_args};
+    use crate::types::TypeInner;
+    let candid = CandidSource::Text(candid);
+    let (env, serv) = candid.load()?;
+    let serv = serv.ok_or_else(|| Error::msg("the Candid interface has no main service type"))?;
+    match serv.as_ref() {
+        TypeInner::Class(_, _) => Ok((env, serv)),
+        TypeInner::Service(_) => {
+            let prog = init.parse::<IDLInitArgs>()?;
+            let mut env2 = TypeEnv::new();
+            let args = check_init_args(&mut env2, &env, &prog)?;
+            Ok((env2, TypeInner::Class(args, serv).into()))
+        }
+        _ => unreachable!(),
+    }
+}
+
 /// Encode sequence of Rust values into Candid message of type `candid::Result<Vec<u8>>`.
 #[macro_export]
 macro_rules! Encode {
diff --git a/tools/ui/Cargo.lock b/tools/ui/Cargo.lock
index 014b3103..47229acc 100644
--- a/tools/ui/Cargo.lock
+++ b/tools/ui/Cargo.lock
@@ -111,15 +111,14 @@ checksum = "14c189c53d098945499cdfa7ecc63567cf3886b3332b312a5b4585d8d3a6a610"
 
 [[package]]
 name = "candid"
-version = "0.9.5"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "762aa04e3a889d47a1773b74ee3b13438a9bc895954fff79ebf7f308c3744a6c"
+version = "0.9.6"
 dependencies = [
  "anyhow",
  "binread",
  "byteorder",
  "candid_derive",
  "codespan-reporting",
+ "convert_case",
  "crc32fast",
  "data-encoding",
  "hex",
@@ -141,14 +140,12 @@ dependencies = [
 
 [[package]]
 name = "candid_derive"
-version = "0.6.2"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "810b3bd60244f282090652ffc7c30a9d23892e72dfe443e46ee55569044f7dd5"
+version = "0.6.3"
 dependencies = [
  "lazy_static",
  "proc-macro2",
  "quote",
- "syn 2.0.29",
+ "syn 2.0.31",
 ]
 
 [[package]]
@@ -176,6 +173,15 @@ dependencies = [
  "unicode-width",
 ]
 
+[[package]]
+name = "convert_case"
+version = "0.6.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ec182b0ca2f35d8fc196cf3404988fd8b8c739a4d270ff118a398feb0cbec1ca"
+dependencies = [
+ "unicode-segmentation",
+]
+
 [[package]]
 name = "cpufeatures"
 version = "0.2.9"
@@ -511,7 +517,7 @@ dependencies = [
  "proc-macro2",
  "quote",
  "regex-syntax 0.6.29",
- "syn 2.0.29",
+ "syn 2.0.31",
 ]
 
 [[package]]
@@ -525,9 +531,9 @@ dependencies = [
 
 [[package]]
 name = "memchr"
-version = "2.6.2"
+version = "2.6.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "5486aed0026218e61b8a01d5fbd5a0a134649abb71a0e53b7bc088529dced86e"
+checksum = "8f232d6ef707e1956a43342693d2a31e72989554d58299d7a88738cc95b0d35c"
 
 [[package]]
 name = "new_debug_unreachable"
@@ -584,7 +590,7 @@ dependencies = [
  "proc-macro-crate",
  "proc-macro2",
  "quote",
- "syn 2.0.29",
+ "syn 2.0.31",
 ]
 
 [[package]]
@@ -732,9 +738,9 @@ dependencies = [
 
 [[package]]
 name = "regex"
-version = "1.9.4"
+version = "1.9.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "12de2eff854e5fa4b1295edd650e227e9d8fb0c9e90b12e7f36d6a6811791a29"
+checksum = "697061221ea1b4a94a624f67d0ae2bfe4e22b8a17b6a192afb11046542cc8c47"
 dependencies = [
  "aho-corasick",
  "memchr",
@@ -744,9 +750,9 @@ dependencies = [
 
 [[package]]
 name = "regex-automata"
-version = "0.3.7"
+version = "0.3.8"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "49530408a136e16e5b486e883fbb6ba058e8e4e8ae6621a77b048b314336e629"
+checksum = "c2f401f4955220693b56f8ec66ee9c78abffd8d1c4f23dc41a23839eb88f0795"
 dependencies = [
  "aho-corasick",
  "memchr",
@@ -767,9 +773,9 @@ checksum = "dbb5fb1acd8a1a18b3dd5be62d25485eb770e05afb408a9627d14d451bae12da"
 
 [[package]]
 name = "rustix"
-version = "0.38.10"
+version = "0.38.11"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "ed6248e1caa625eb708e266e06159f135e8c26f2bb7ceb72dc4b2766d0340964"
+checksum = "c0c3dde1fc030af041adc40e79c0e7fbcf431dd24870053d187d7c66e4b87453"
 dependencies = [
  "bitflags 2.4.0",
  "errno",
@@ -816,7 +822,7 @@ checksum = "4eca7ac642d82aa35b60049a6eccb4be6be75e599bd2e9adb5f875a737654af2"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 2.0.29",
+ "syn 2.0.31",
 ]
 
 [[package]]
@@ -892,9 +898,9 @@ dependencies = [
 
 [[package]]
 name = "syn"
-version = "2.0.29"
+version = "2.0.31"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "c324c494eba9d92503e6f1ef2e6df781e78f6a7705a0202d9801b198807d518a"
+checksum = "718fa2415bcb8d8bd775917a1bf12a7931b6dfa890753378538118181e0cb398"
 dependencies = [
  "proc-macro2",
  "quote",
@@ -923,22 +929,22 @@ dependencies = [
 
 [[package]]
 name = "thiserror"
-version = "1.0.47"
+version = "1.0.48"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "97a802ec30afc17eee47b2855fc72e0c4cd62be9b4efe6591edde0ec5bd68d8f"
+checksum = "9d6d7a740b8a666a7e828dd00da9c0dc290dff53154ea77ac109281de90589b7"
 dependencies = [
  "thiserror-impl",
 ]
 
 [[package]]
 name = "thiserror-impl"
-version = "1.0.47"
+version = "1.0.48"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "6bb623b56e39ab7dcd4b1b98bb6c8f8d907ed255b18de254088016b27a8ee19b"
+checksum = "49922ecae66cc8a249b77e68d1d0623c1b2c514f0060c27cdc68bd62a1219d35"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 2.0.29",
+ "syn 2.0.31",
 ]
 
 [[package]]
@@ -958,9 +964,9 @@ checksum = "7cda73e2f1397b1262d6dfdcef8aafae14d1de7748d66822d3bfeeb6d03e5e4b"
 
 [[package]]
 name = "toml_edit"
-version = "0.19.14"
+version = "0.19.15"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "f8123f27e969974a3dfba720fdb560be359f57b44302d280ba72e76a74480e8a"
+checksum = "1b5bb770da30e5cbfde35a2d7b9b8a2c4b8ef89548a7a6aeab5c9a576e3e7421"
 dependencies = [
  "indexmap",
  "toml_datetime",
diff --git a/tools/ui/Cargo.toml b/tools/ui/Cargo.toml
index 4c9d1a40..4eb9f146 100644
--- a/tools/ui/Cargo.toml
+++ b/tools/ui/Cargo.toml
@@ -5,4 +5,8 @@ members = [
 
 [profile.release]
 lto = true
-opt-level = 'z'
+opt-level = 2
+
+[patch.crates-io.candid]
+path = "../../rust/candid"
+
diff --git a/tools/ui/src/didjs/lib.rs b/tools/ui/src/didjs/lib.rs
index 99f9e2b9..62c5d657 100644
--- a/tools/ui/src/didjs/lib.rs
+++ b/tools/ui/src/didjs/lib.rs
@@ -58,6 +58,13 @@ fn binding(prog: String, lang: String) -> Option<String> {
     Some(res)
 }
 
+#[ic_cdk::query]
+fn merge_init_args(prog: String, init_args: String) -> Option<String> {
+    use candid::bindings;
+    let (env, actor) = candid::utils::merge_init_args(&prog, &init_args).ok()?;
+    Some(bindings::candid::compile(&env, &Some(actor)))
+}
+
 #[ic_cdk::query]
 fn subtype(new: String, old: String) -> Result<(), String> {
     let new = new.parse::<IDLProg>().unwrap();