diff --git a/hugr-core/Cargo.toml b/hugr-core/Cargo.toml
index 8de77a49c..ce5218e27 100644
--- a/hugr-core/Cargo.toml
+++ b/hugr-core/Cargo.toml
@@ -45,6 +45,7 @@ delegate = { workspace = true }
 paste = { workspace = true }
 strum = { workspace = true }
 strum_macros = { workspace = true }
+semver = { version = "1.0.23", features = ["serde"] }
 
 [dev-dependencies]
 rstest = { workspace = true }
diff --git a/hugr-core/src/extension.rs b/hugr-core/src/extension.rs
index 65e74aae9..9d8dd0570 100644
--- a/hugr-core/src/extension.rs
+++ b/hugr-core/src/extension.rs
@@ -3,6 +3,7 @@
 //! TODO: YAML declaration and parsing. This should be similar to a plugin
 //! system (outside the `types` module), which also parses nested [`OpDef`]s.
 
+pub use semver::Version;
 use std::collections::btree_map;
 use std::collections::hash_map;
 use std::collections::{BTreeMap, BTreeSet, HashMap};
@@ -55,21 +56,17 @@ impl ExtensionRegistry {
     pub fn try_new(
         value: impl IntoIterator<Item = Extension>,
     ) -> Result<Self, ExtensionRegistryError> {
-        let mut exts = BTreeMap::new();
+        let mut res = ExtensionRegistry(BTreeMap::new());
+
         for ext in value.into_iter() {
-            let prev = exts.insert(ext.name.clone(), ext);
-            if let Some(prev) = prev {
-                return Err(ExtensionRegistryError::AlreadyRegistered(
-                    prev.name().clone(),
-                ));
-            };
+            res.register(ext)?;
         }
+
         // Note this potentially asks extensions to validate themselves against other extensions that
         // may *not* be valid themselves yet. It'd be better to order these respecting dependencies,
         // or at least to validate the types first - which we don't do at all yet:
         // TODO https://github.com/CQCL/hugr/issues/624. However, parametrized types could be
         // cyclically dependent, so there is no perfect solution, and this is at least simple.
-        let res = ExtensionRegistry(exts);
         for ext in res.0.values() {
             ext.validate(&res)
                 .map_err(|e| ExtensionRegistryError::InvalidSignature(ext.name().clone(), e))?;
@@ -82,13 +79,35 @@ impl ExtensionRegistry {
     /// Returns a reference to the registered extension if successful.
     pub fn register(&mut self, extension: Extension) -> Result<&Extension, ExtensionRegistryError> {
         match self.0.entry(extension.name().clone()) {
-            btree_map::Entry::Occupied(_) => Err(ExtensionRegistryError::AlreadyRegistered(
+            btree_map::Entry::Occupied(prev) => Err(ExtensionRegistryError::AlreadyRegistered(
                 extension.name().clone(),
+                prev.get().version().clone(),
+                extension.version().clone(),
             )),
             btree_map::Entry::Vacant(ve) => Ok(ve.insert(extension)),
         }
     }
 
+    /// Registers a new extension to the registry, keeping most up to date if extension exists.
+    ///
+    /// If extension IDs match, the extension with the higher version is kept.
+    /// If versions match, the original extension is kept.
+    /// Returns a reference to the registered extension if successful.
+    pub fn register_updated(
+        &mut self,
+        extension: Extension,
+    ) -> Result<&Extension, ExtensionRegistryError> {
+        match self.0.entry(extension.name().clone()) {
+            btree_map::Entry::Occupied(mut prev) => {
+                if prev.get().version() < extension.version() {
+                    *prev.get_mut() = extension;
+                }
+                Ok(prev.into_mut())
+            }
+            btree_map::Entry::Vacant(ve) => Ok(ve.insert(extension)),
+        }
+    }
+
     /// Returns the number of extensions in the registry.
     pub fn len(&self) -> usize {
         self.0.len()
@@ -103,6 +122,11 @@ impl ExtensionRegistry {
     pub fn iter(&self) -> impl Iterator<Item = (&ExtensionId, &Extension)> {
         self.0.iter()
     }
+
+    /// Delete an extension from the registry and return it if it was present.
+    pub fn remove_extension(&mut self, name: &ExtensionId) -> Option<Extension> {
+        self.0.remove(name)
+    }
 }
 
 impl IntoIterator for ExtensionRegistry {
@@ -264,6 +288,8 @@ pub type ExtensionId = IdentList;
 /// A extension is a set of capabilities required to execute a graph.
 #[derive(Clone, Debug, serde::Serialize, serde::Deserialize)]
 pub struct Extension {
+    /// Extension version, follows semver.
+    pub version: Version,
     /// Unique identifier for the extension.
     pub name: ExtensionId,
     /// Other extensions defining types used by this extension.
@@ -286,21 +312,25 @@ pub struct Extension {
 
 impl Extension {
     /// Creates a new extension with the given name.
-    pub fn new(name: ExtensionId) -> Self {
-        Self::new_with_reqs(name, ExtensionSet::default())
-    }
-
-    /// Creates a new extension with the given name and requirements.
-    pub fn new_with_reqs(name: ExtensionId, extension_reqs: impl Into<ExtensionSet>) -> Self {
+    pub fn new(name: ExtensionId, version: Version) -> Self {
         Self {
             name,
-            extension_reqs: extension_reqs.into(),
+            version,
+            extension_reqs: Default::default(),
             types: Default::default(),
             values: Default::default(),
             operations: Default::default(),
         }
     }
 
+    /// Extend the requirements of this extension with another set of extensions.
+    pub fn with_reqs(self, extension_reqs: impl Into<ExtensionSet>) -> Self {
+        Self {
+            extension_reqs: self.extension_reqs.union(extension_reqs.into()),
+            ..self
+        }
+    }
+
     /// Allows read-only access to the operations in this Extension
     pub fn get_op(&self, name: &OpNameRef) -> Option<&Arc<op_def::OpDef>> {
         self.operations.get(name)
@@ -321,6 +351,11 @@ impl Extension {
         &self.name
     }
 
+    /// Returns the version of the extension.
+    pub fn version(&self) -> &Version {
+        &self.version
+    }
+
     /// Iterator over the operations of this [`Extension`].
     pub fn operations(&self) -> impl Iterator<Item = (&OpName, &Arc<OpDef>)> {
         self.operations.iter()
@@ -382,8 +417,8 @@ impl PartialEq for Extension {
 #[derive(Debug, Clone, Error, PartialEq, Eq)]
 pub enum ExtensionRegistryError {
     /// Extension already defined.
-    #[error("The registry already contains an extension with id {0}.")]
-    AlreadyRegistered(ExtensionId),
+    #[error("The registry already contains an extension with id {0} and version {1}. New extension has version {2}.")]
+    AlreadyRegistered(ExtensionId, Version, Version),
     /// A registered extension has invalid signatures.
     #[error("The extension {0} contains an invalid signature, {1}.")]
     InvalidSignature(ExtensionId, #[source] SignatureError),
@@ -544,6 +579,53 @@ pub mod test {
     // We re-export this here because mod op_def is private.
     pub use super::op_def::test::SimpleOpDef;
 
+    use super::*;
+
+    impl Extension {
+        /// Create a new extension for testing, with a 0 version.
+        pub(crate) fn new_test(name: ExtensionId) -> Self {
+            Self::new(name, Version::new(0, 0, 0))
+        }
+    }
+
+    #[test]
+    fn test_register_update() {
+        let mut reg = ExtensionRegistry::try_new([]).unwrap();
+        let ext_1_id = ExtensionId::new("ext1").unwrap();
+        let ext_2_id = ExtensionId::new("ext2").unwrap();
+        let ext1 = Extension::new(ext_1_id.clone(), Version::new(1, 0, 0));
+        let ext1_1 = Extension::new(ext_1_id.clone(), Version::new(1, 1, 0));
+        let ext1_2 = Extension::new(ext_1_id.clone(), Version::new(0, 2, 0));
+        let ext2 = Extension::new(ext_2_id, Version::new(1, 0, 0));
+
+        reg.register(ext1.clone()).unwrap();
+        assert_eq!(reg.get("ext1").unwrap().version(), &Version::new(1, 0, 0));
+
+        // normal registration fails
+        assert_eq!(
+            reg.register(ext1_1.clone()),
+            Err(ExtensionRegistryError::AlreadyRegistered(
+                ext_1_id.clone(),
+                Version::new(1, 0, 0),
+                Version::new(1, 1, 0)
+            ))
+        );
+
+        // register with update works
+        reg.register_updated(ext1_1.clone()).unwrap();
+        assert_eq!(reg.get("ext1").unwrap().version(), &Version::new(1, 1, 0));
+
+        // register with lower version does not change version
+        reg.register_updated(ext1_2.clone()).unwrap();
+        assert_eq!(reg.get("ext1").unwrap().version(), &Version::new(1, 1, 0));
+
+        reg.register(ext2.clone()).unwrap();
+        assert_eq!(reg.get("ext2").unwrap().version(), &Version::new(1, 0, 0));
+        assert_eq!(reg.len(), 2);
+
+        assert!(reg.remove_extension(&ext_1_id).unwrap().version() == &Version::new(1, 1, 0));
+        assert_eq!(reg.len(), 1);
+    }
     mod proptest {
 
         use ::proptest::{collection::hash_set, prelude::*};
diff --git a/hugr-core/src/extension/declarative.rs b/hugr-core/src/extension/declarative.rs
index 5db8999cb..cb98e8215 100644
--- a/hugr-core/src/extension/declarative.rs
+++ b/hugr-core/src/extension/declarative.rs
@@ -89,6 +89,7 @@ struct ExtensionSetDeclaration {
 /// A declarative extension definition.
 #[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq)]
 struct ExtensionDeclaration {
+    // TODO add version
     /// The name of the extension.
     name: ExtensionId,
     /// A list of types that this extension provides.
@@ -150,7 +151,8 @@ impl ExtensionDeclaration {
         imports: &ExtensionSet,
         ctx: DeclarationContext<'_>,
     ) -> Result<Extension, ExtensionDeclarationError> {
-        let mut ext = Extension::new_with_reqs(self.name.clone(), imports.clone());
+        let mut ext = Extension::new(self.name.clone(), crate::extension::Version::new(0, 0, 0))
+            .with_reqs(imports.clone());
 
         for t in &self.types {
             t.register(&mut ext, ctx)?;
diff --git a/hugr-core/src/extension/op_def.rs b/hugr-core/src/extension/op_def.rs
index 06114a582..2b628d820 100644
--- a/hugr-core/src/extension/op_def.rs
+++ b/hugr-core/src/extension/op_def.rs
@@ -591,7 +591,7 @@ pub(super) mod test {
     #[test]
     fn op_def_with_type_scheme() -> Result<(), Box<dyn std::error::Error>> {
         let list_def = EXTENSION.get_type(&LIST_TYPENAME).unwrap();
-        let mut e = Extension::new(EXT_ID);
+        let mut e = Extension::new_test(EXT_ID);
         const TP: TypeParam = TypeParam::Type { b: TypeBound::Any };
         let list_of_var =
             Type::new_extension(list_def.instantiate(vec![TypeArg::new_var_use(0, TP)])?);
@@ -658,7 +658,7 @@ pub(super) mod test {
                 MAX_NAT
             }
         }
-        let mut e = Extension::new(EXT_ID);
+        let mut e = Extension::new_test(EXT_ID);
         let def: &mut crate::extension::OpDef =
             e.add_op("MyOp".into(), "".to_string(), SigFun())?;
 
@@ -720,7 +720,7 @@ pub(super) mod test {
     fn type_scheme_instantiate_var() -> Result<(), Box<dyn std::error::Error>> {
         // Check that we can instantiate a PolyFuncTypeRV-scheme with an (external)
         // type variable
-        let mut e = Extension::new(EXT_ID);
+        let mut e = Extension::new_test(EXT_ID);
         let def = e.add_op(
             "SimpleOp".into(),
             "".into(),
@@ -755,7 +755,7 @@ pub(super) mod test {
     fn instantiate_extension_delta() -> Result<(), Box<dyn std::error::Error>> {
         use crate::extension::prelude::{BOOL_T, PRELUDE_REGISTRY};
 
-        let mut e = Extension::new(EXT_ID);
+        let mut e = Extension::new_test(EXT_ID);
 
         let params: Vec<TypeParam> = vec![TypeParam::Extensions];
         let db_set = ExtensionSet::type_var(0);
diff --git a/hugr-core/src/extension/prelude.rs b/hugr-core/src/extension/prelude.rs
index c220618fa..739caad16 100644
--- a/hugr-core/src/extension/prelude.rs
+++ b/hugr-core/src/extension/prelude.rs
@@ -85,9 +85,11 @@ impl SignatureFromArgs for GenericOpCustom {
 
 /// Name of prelude extension.
 pub const PRELUDE_ID: ExtensionId = ExtensionId::new_unchecked("prelude");
+/// Extension version.
+pub const VERSION: semver::Version = semver::Version::new(0, 1, 0);
 lazy_static! {
     static ref PRELUDE_DEF: Extension = {
-        let mut prelude = Extension::new(PRELUDE_ID);
+        let mut prelude = Extension::new(PRELUDE_ID, VERSION);
         prelude
             .add_type(
                 TypeName::new_inline("usize"),
diff --git a/hugr-core/src/extension/simple_op.rs b/hugr-core/src/extension/simple_op.rs
index 877282a52..d8dc4ec18 100644
--- a/hugr-core/src/extension/simple_op.rs
+++ b/hugr-core/src/extension/simple_op.rs
@@ -319,7 +319,7 @@ mod test {
 
     lazy_static! {
         static ref EXT: Extension = {
-            let mut e = Extension::new(EXT_ID.clone());
+            let mut e = Extension::new_test(EXT_ID.clone());
             DummyEnum::Dumb.add_to_extension(&mut e).unwrap();
             e
         };
diff --git a/hugr-core/src/hugr/validate/test.rs b/hugr-core/src/hugr/validate/test.rs
index 4fc752419..76ccec8b4 100644
--- a/hugr-core/src/hugr/validate/test.rs
+++ b/hugr-core/src/hugr/validate/test.rs
@@ -365,7 +365,7 @@ const_extension_ids! {
 }
 #[test]
 fn invalid_types() {
-    let mut e = Extension::new(EXT_ID);
+    let mut e = Extension::new_test(EXT_ID);
     e.add_type(
         "MyContainer".into(),
         vec![TypeBound::Copyable.into()],
@@ -570,7 +570,7 @@ fn no_polymorphic_consts() -> Result<(), Box<dyn std::error::Error>> {
 
 pub(crate) fn extension_with_eval_parallel() -> Extension {
     let rowp = TypeParam::new_list(TypeBound::Any);
-    let mut e = Extension::new(EXT_ID);
+    let mut e = Extension::new_test(EXT_ID);
 
     let inputs = TypeRV::new_row_var_use(0, TypeBound::Any);
     let outputs = TypeRV::new_row_var_use(1, TypeBound::Any);
@@ -671,7 +671,7 @@ fn row_variables() -> Result<(), Box<dyn std::error::Error>> {
 
 #[test]
 fn test_polymorphic_call() -> Result<(), Box<dyn std::error::Error>> {
-    let mut e = Extension::new(EXT_ID);
+    let mut e = Extension::new_test(EXT_ID);
 
     let params: Vec<TypeParam> = vec![
         TypeBound::Any.into(),
diff --git a/hugr-core/src/std_extensions/arithmetic/conversions.rs b/hugr-core/src/std_extensions/arithmetic/conversions.rs
index 8f7110807..06a06abed 100644
--- a/hugr-core/src/std_extensions/arithmetic/conversions.rs
+++ b/hugr-core/src/std_extensions/arithmetic/conversions.rs
@@ -22,6 +22,8 @@ use lazy_static::lazy_static;
 mod const_fold;
 /// The extension identifier.
 pub const EXTENSION_ID: ExtensionId = ExtensionId::new_unchecked("arithmetic.conversions");
+/// Extension version.
+pub const VERSION: semver::Version = semver::Version::new(0, 1, 0);
 
 /// Extension for conversions between floats and integers.
 #[derive(Clone, Copy, Debug, Hash, PartialEq, Eq, EnumIter, IntoStaticStr, EnumString)]
@@ -121,8 +123,9 @@ impl MakeExtensionOp for ConvertOpType {
 lazy_static! {
     /// Extension for conversions between integers and floats.
     pub static ref EXTENSION: Extension = {
-        let mut extension = Extension::new_with_reqs(
+        let mut extension = Extension::new(
             EXTENSION_ID,
+            VERSION).with_reqs(
             ExtensionSet::from_iter(vec![
                 super::int_types::EXTENSION_ID,
                 super::float_types::EXTENSION_ID,
diff --git a/hugr-core/src/std_extensions/arithmetic/float_ops.rs b/hugr-core/src/std_extensions/arithmetic/float_ops.rs
index f701da233..9fe86ee1f 100644
--- a/hugr-core/src/std_extensions/arithmetic/float_ops.rs
+++ b/hugr-core/src/std_extensions/arithmetic/float_ops.rs
@@ -17,6 +17,8 @@ use lazy_static::lazy_static;
 mod const_fold;
 /// The extension identifier.
 pub const EXTENSION_ID: ExtensionId = ExtensionId::new_unchecked("arithmetic.float");
+/// Extension version.
+pub const VERSION: semver::Version = semver::Version::new(0, 1, 0);
 
 /// Integer extension operation definitions.
 #[derive(Clone, Copy, Debug, Hash, PartialEq, Eq, EnumIter, IntoStaticStr, EnumString)]
@@ -99,8 +101,9 @@ impl MakeOpDef for FloatOps {
 lazy_static! {
     /// Extension for basic float operations.
     pub static ref EXTENSION: Extension = {
-        let mut extension = Extension::new_with_reqs(
+        let mut extension = Extension::new(
             EXTENSION_ID,
+            VERSION).with_reqs(
             ExtensionSet::singleton(&super::int_types::EXTENSION_ID),
         );
 
diff --git a/hugr-core/src/std_extensions/arithmetic/float_types.rs b/hugr-core/src/std_extensions/arithmetic/float_types.rs
index 41ffbe45c..335009e92 100644
--- a/hugr-core/src/std_extensions/arithmetic/float_types.rs
+++ b/hugr-core/src/std_extensions/arithmetic/float_types.rs
@@ -12,6 +12,8 @@ use lazy_static::lazy_static;
 
 /// The extension identifier.
 pub const EXTENSION_ID: ExtensionId = ExtensionId::new_unchecked("arithmetic.float.types");
+/// Extension version.
+pub const VERSION: semver::Version = semver::Version::new(0, 1, 0);
 
 /// Identifier for the 64-bit IEEE 754-2019 floating-point type.
 const FLOAT_TYPE_ID: TypeName = TypeName::new_inline("float64");
@@ -76,7 +78,7 @@ impl CustomConst for ConstF64 {
 lazy_static! {
     /// Extension defining the float type.
     pub static ref EXTENSION: Extension = {
-        let mut extension = Extension::new(EXTENSION_ID);
+        let mut extension = Extension::new(EXTENSION_ID, VERSION);
 
         extension
             .add_type(
diff --git a/hugr-core/src/std_extensions/arithmetic/int_ops.rs b/hugr-core/src/std_extensions/arithmetic/int_ops.rs
index ca3094f07..e9bef39b6 100644
--- a/hugr-core/src/std_extensions/arithmetic/int_ops.rs
+++ b/hugr-core/src/std_extensions/arithmetic/int_ops.rs
@@ -28,6 +28,8 @@ mod const_fold;
 
 /// The extension identifier.
 pub const EXTENSION_ID: ExtensionId = ExtensionId::new_unchecked("arithmetic.int");
+/// Extension version.
+pub const VERSION: semver::Version = semver::Version::new(0, 1, 0);
 
 struct IOValidator {
     // whether the first type argument should be greater than or equal to the second
@@ -261,9 +263,10 @@ fn iunop_sig() -> PolyFuncTypeRV {
 lazy_static! {
     /// Extension for basic integer operations.
     pub static ref EXTENSION: Extension = {
-        let mut extension = Extension::new_with_reqs(
+        let mut extension = Extension::new(
             EXTENSION_ID,
-            ExtensionSet::singleton(&super::int_types::EXTENSION_ID),
+            VERSION).with_reqs(
+            ExtensionSet::singleton(&super::int_types::EXTENSION_ID)
         );
 
         IntOpDef::load_all_ops(&mut extension).unwrap();
diff --git a/hugr-core/src/std_extensions/arithmetic/int_types.rs b/hugr-core/src/std_extensions/arithmetic/int_types.rs
index 49a4cb868..6ec3b7724 100644
--- a/hugr-core/src/std_extensions/arithmetic/int_types.rs
+++ b/hugr-core/src/std_extensions/arithmetic/int_types.rs
@@ -16,6 +16,8 @@ use crate::{
 use lazy_static::lazy_static;
 /// The extension identifier.
 pub const EXTENSION_ID: ExtensionId = ExtensionId::new_unchecked("arithmetic.int.types");
+/// Extension version.
+pub const VERSION: semver::Version = semver::Version::new(0, 1, 0);
 
 /// Identifier for the integer type.
 pub const INT_TYPE_ID: TypeName = TypeName::new_inline("int");
@@ -185,7 +187,7 @@ impl CustomConst for ConstInt {
 
 /// Extension for basic integer types.
 pub fn extension() -> Extension {
-    let mut extension = Extension::new(EXTENSION_ID);
+    let mut extension = Extension::new(EXTENSION_ID, VERSION);
 
     extension
         .add_type(
diff --git a/hugr-core/src/std_extensions/collections.rs b/hugr-core/src/std_extensions/collections.rs
index 0fb809d16..c182543ca 100644
--- a/hugr-core/src/std_extensions/collections.rs
+++ b/hugr-core/src/std_extensions/collections.rs
@@ -31,6 +31,8 @@ pub const POP_NAME: OpName = OpName::new_inline("pop");
 pub const PUSH_NAME: OpName = OpName::new_inline("push");
 /// Reported unique name of the extension
 pub const EXTENSION_NAME: ExtensionId = ExtensionId::new_unchecked("Collections");
+/// Extension version.
+pub const VERSION: semver::Version = semver::Version::new(0, 1, 0);
 
 #[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
 /// Dynamically sized list of values, all of the same type.
@@ -138,7 +140,7 @@ impl ConstFold for PushFold {
 const TP: TypeParam = TypeParam::Type { b: TypeBound::Any };
 
 fn extension() -> Extension {
-    let mut extension = Extension::new(EXTENSION_NAME);
+    let mut extension = Extension::new(EXTENSION_NAME, VERSION);
 
     extension
         .add_type(
diff --git a/hugr-core/src/std_extensions/logic.rs b/hugr-core/src/std_extensions/logic.rs
index 8ad183ef5..de58a5e31 100644
--- a/hugr-core/src/std_extensions/logic.rs
+++ b/hugr-core/src/std_extensions/logic.rs
@@ -168,6 +168,8 @@ impl MakeOpDef for NotOp {
 }
 /// The extension identifier.
 pub const EXTENSION_ID: ExtensionId = ExtensionId::new_unchecked("logic");
+/// Extension version.
+pub const VERSION: semver::Version = semver::Version::new(0, 1, 0);
 
 fn logic_op_sig() -> impl SignatureFromArgs {
     struct LogicOpCustom;
@@ -194,7 +196,7 @@ fn logic_op_sig() -> impl SignatureFromArgs {
 }
 /// Extension for basic logical operations.
 fn extension() -> Extension {
-    let mut extension = Extension::new(EXTENSION_ID);
+    let mut extension = Extension::new(EXTENSION_ID, VERSION);
     NaryLogic::load_all_ops(&mut extension).unwrap();
     NotOp.add_to_extension(&mut extension).unwrap();
 
diff --git a/hugr-core/src/std_extensions/ptr.rs b/hugr-core/src/std_extensions/ptr.rs
index 774cd6376..72c53a97f 100644
--- a/hugr-core/src/std_extensions/ptr.rs
+++ b/hugr-core/src/std_extensions/ptr.rs
@@ -80,9 +80,12 @@ pub const PTR_TYPE_ID: TypeName = TypeName::new_inline("ptr");
 const TYPE_PARAMS: [TypeParam; 1] = [TypeParam::Type {
     b: TypeBound::Copyable,
 }];
+/// Extension version.
+pub const VERSION: semver::Version = semver::Version::new(0, 1, 0);
+
 /// Extension for pointer operations.
 fn extension() -> Extension {
-    let mut extension = Extension::new(EXTENSION_ID);
+    let mut extension = Extension::new(EXTENSION_ID, VERSION);
     extension
         .add_type(
             PTR_TYPE_ID,
diff --git a/hugr-core/src/types/poly_func.rs b/hugr-core/src/types/poly_func.rs
index 4f6df034b..40a92b2ea 100644
--- a/hugr-core/src/types/poly_func.rs
+++ b/hugr-core/src/types/poly_func.rs
@@ -313,7 +313,7 @@ pub(crate) mod test {
         const EXT_ID: ExtensionId = ExtensionId::new_unchecked("my_ext");
         const TYPE_NAME: TypeName = TypeName::new_inline("MyType");
 
-        let mut e = Extension::new(EXT_ID);
+        let mut e = Extension::new_test(EXT_ID);
         e.add_type(
             TYPE_NAME,
             vec![bound.clone()],
diff --git a/hugr-core/src/utils.rs b/hugr-core/src/utils.rs
index 4e2d15e99..885aebf50 100644
--- a/hugr-core/src/utils.rs
+++ b/hugr-core/src/utils.rs
@@ -129,7 +129,7 @@ pub(crate) mod test_quantum_extension {
     /// The extension identifier.
     pub const EXTENSION_ID: ExtensionId = ExtensionId::new_unchecked("test.quantum");
     fn extension() -> Extension {
-        let mut extension = Extension::new(EXTENSION_ID);
+        let mut extension = Extension::new_test(EXTENSION_ID);
 
         extension
             .add_op(OpName::new_inline("H"), "Hadamard".into(), one_qb_func())
diff --git a/hugr-passes/src/merge_bbs.rs b/hugr-passes/src/merge_bbs.rs
index fae6b061f..65ae5ec99 100644
--- a/hugr-passes/src/merge_bbs.rs
+++ b/hugr-passes/src/merge_bbs.rs
@@ -178,7 +178,7 @@ mod test {
     }
 
     fn extension() -> Extension {
-        let mut e = Extension::new(EXT_ID);
+        let mut e = Extension::new(EXT_ID, hugr_core::extension::Version::new(0, 1, 0));
         e.add_op(
             "Test".into(),
             String::new(),
diff --git a/hugr/src/lib.rs b/hugr/src/lib.rs
index f142cffc8..ef9c01a0b 100644
--- a/hugr/src/lib.rs
+++ b/hugr/src/lib.rs
@@ -39,7 +39,7 @@
 //!     use hugr::{
 //!         extension::{
 //!             prelude::{BOOL_T, QB_T},
-//!             ExtensionId, ExtensionRegistry, PRELUDE,
+//!             ExtensionId, ExtensionRegistry, PRELUDE, Version,
 //!         },
 //!         ops::{CustomOp, OpName},
 //!         type_row,
@@ -58,8 +58,9 @@
 //!     }
 //!     /// The extension identifier.
 //!     pub const EXTENSION_ID: ExtensionId = ExtensionId::new_unchecked("mini.quantum");
+//!     pub const VERSION: Version = Version::new(0, 1, 0);
 //!     fn extension() -> Extension {
-//!         let mut extension = Extension::new(EXTENSION_ID);
+//!         let mut extension = Extension::new(EXTENSION_ID, VERSION);
 //!
 //!         extension
 //!             .add_op(OpName::new_inline("H"), "Hadamard".into(), one_qb_func())