diff --git a/frame/contracts/src/chain_extension.rs b/frame/contracts/src/chain_extension.rs
index 23242a2a542c1..57101668f794d 100644
--- a/frame/contracts/src/chain_extension.rs
+++ b/frame/contracts/src/chain_extension.rs
@@ -38,7 +38,7 @@
 //!
 //! However, only extensions implementing [`RegisteredChainExtension`] can be put into a tuple.
 //! This is because the [`RegisteredChainExtension::ID`] is used to decide which of those extensions
-//! should should be used when the contract calls a chain extensions. Extensions which are generally
+//! should be used when the contract calls a chain extensions. Extensions which are generally
 //! useful should claim their `ID` with [the registry](https://github.com/paritytech/chainextension-registry)
 //! so that no collisions with other vendors will occur.
 //!
@@ -215,7 +215,7 @@ where
 	/// It returns the two least significant bytes of the `id` passed by a contract as the other
 	/// two bytes represent the chain extension itself (the code which is calling this function).
 	pub fn func_id(&self) -> u16 {
-		(self.inner.id & 0x00FF) as u16
+		(self.inner.id & 0x0000FFFF) as u16
 	}
 
 	/// The chain extension id within the `id` passed by a contract.
diff --git a/frame/contracts/src/tests.rs b/frame/contracts/src/tests.rs
index 0febfec929b6e..30417d8544489 100644
--- a/frame/contracts/src/tests.rs
+++ b/frame/contracts/src/tests.rs
@@ -158,14 +158,14 @@ impl ChainExtension<Test> for TestExtension {
 		let func_id = env.func_id();
 		let id = env.ext_id() as u32 | func_id as u32;
 		match func_id {
-			0 => {
+			0x8000 => {
 				let mut env = env.buf_in_buf_out();
 				let input = env.read(8)?;
 				env.write(&input, false, None)?;
 				TEST_EXTENSION.with(|e| e.borrow_mut().last_seen_buffer = input);
 				Ok(RetVal::Converging(id))
 			},
-			1 => {
+			0x8001 => {
 				let env = env.only_in();
 				TEST_EXTENSION.with(|e| {
 					e.borrow_mut().last_seen_inputs =
@@ -173,13 +173,13 @@ impl ChainExtension<Test> for TestExtension {
 				});
 				Ok(RetVal::Converging(id))
 			},
-			2 => {
+			0x8002 => {
 				let mut env = env.buf_in_buf_out();
 				let weight = env.read(5)?[4].into();
 				env.charge_weight(weight)?;
 				Ok(RetVal::Converging(id))
 			},
-			3 => Ok(RetVal::Diverging { flags: ReturnFlags::REVERT, data: vec![42, 99] }),
+			0x8003 => Ok(RetVal::Diverging { flags: ReturnFlags::REVERT, data: vec![42, 99] }),
 			_ => {
 				panic!("Passed unknown id to test chain extension: {}", func_id);
 			},
@@ -1646,21 +1646,22 @@ fn chain_extension_works() {
 		),);
 		let addr = Contracts::contract_address(&ALICE, &hash, &[]);
 
-		// 0 = read input buffer and pass it through as output
-		let input: Vec<u8> = ExtensionInput { extension_id: 0, func_id: 0, extra: &[99] }.into();
+		// 0x8000 = read input buffer and pass it through as output
+		let input: Vec<u8> =
+			ExtensionInput { extension_id: 0, func_id: 0x8000, extra: &[99] }.into();
 		let result =
 			Contracts::bare_call(ALICE, addr.clone(), 0, GAS_LIMIT, None, input.clone(), false);
 		assert_eq!(TestExtension::last_seen_buffer(), input);
 		assert_eq!(result.result.unwrap().data, Bytes(input));
 
-		// 1 = treat inputs as integer primitives and store the supplied integers
+		// 0x8001 = treat inputs as integer primitives and store the supplied integers
 		Contracts::bare_call(
 			ALICE,
 			addr.clone(),
 			0,
 			GAS_LIMIT,
 			None,
-			ExtensionInput { extension_id: 0, func_id: 1, extra: &[] }.into(),
+			ExtensionInput { extension_id: 0, func_id: 0x8001, extra: &[] }.into(),
 			false,
 		)
 		.result
@@ -1668,14 +1669,14 @@ fn chain_extension_works() {
 		// those values passed in the fixture
 		assert_eq!(TestExtension::last_seen_inputs(), (4, 4, 16, 12));
 
-		// 2 = charge some extra weight (amount supplied in the fifth byte)
+		// 0x8002 = charge some extra weight (amount supplied in the fifth byte)
 		let result = Contracts::bare_call(
 			ALICE,
 			addr.clone(),
 			0,
 			GAS_LIMIT,
 			None,
-			ExtensionInput { extension_id: 0, func_id: 2, extra: &[0] }.into(),
+			ExtensionInput { extension_id: 0, func_id: 0x8002, extra: &[0] }.into(),
 			false,
 		);
 		assert_ok!(result.result);
@@ -1686,7 +1687,7 @@ fn chain_extension_works() {
 			0,
 			GAS_LIMIT,
 			None,
-			ExtensionInput { extension_id: 0, func_id: 2, extra: &[42] }.into(),
+			ExtensionInput { extension_id: 0, func_id: 0x8002, extra: &[42] }.into(),
 			false,
 		);
 		assert_ok!(result.result);
@@ -1697,20 +1698,20 @@ fn chain_extension_works() {
 			0,
 			GAS_LIMIT,
 			None,
-			ExtensionInput { extension_id: 0, func_id: 2, extra: &[95] }.into(),
+			ExtensionInput { extension_id: 0, func_id: 0x8002, extra: &[95] }.into(),
 			false,
 		);
 		assert_ok!(result.result);
 		assert_eq!(result.gas_consumed, gas_consumed + 95);
 
-		// 3 = diverging chain extension call that sets flags to 0x1 and returns a fixed buffer
+		// 0x8003 = diverging chain extension call that sets flags to 0x1 and returns a fixed buffer
 		let result = Contracts::bare_call(
 			ALICE,
 			addr.clone(),
 			0,
 			GAS_LIMIT,
 			None,
-			ExtensionInput { extension_id: 0, func_id: 3, extra: &[] }.into(),
+			ExtensionInput { extension_id: 0, func_id: 0x8003, extra: &[] }.into(),
 			false,
 		)
 		.result