diff --git a/quantinuum-hugr/src/ops/constant.rs b/quantinuum-hugr/src/ops/constant.rs
index 14b5ef222..c8079e31d 100644
--- a/quantinuum-hugr/src/ops/constant.rs
+++ b/quantinuum-hugr/src/ops/constant.rs
@@ -2,10 +2,10 @@
 
 mod custom;
 
-use super::{OpName, OpTrait, StaticTag};
+use super::{OpName, OpParent, OpTrait, StaticTag};
 use super::{OpTag, OpType};
 use crate::extension::ExtensionSet;
-use crate::types::{CustomType, EdgeKind, SumType, SumTypeError, Type};
+use crate::types::{CustomType, EdgeKind, PolyFuncType, SumType, SumTypeError, Type};
 use crate::{Hugr, HugrView};
 
 use itertools::Itertools;
@@ -100,7 +100,7 @@ impl Const {
             Self::Tuple { vs } => Type::new_tuple(vs.iter().map(Self::const_type).collect_vec()),
             Self::Sum { sum_type, .. } => sum_type.clone().into(),
             Self::Function { hugr } => {
-                let func_type = hugr.get_function_type().unwrap_or_else(|| {
+                let func_type = get_const_function_type(hugr).unwrap_or_else(|| {
                     panic!(
                         "{}",
                         ConstTypeError::FunctionTypeMissing {
@@ -143,7 +143,7 @@ impl Const {
     /// Returns an error if the Hugr root node does not define a function.
     pub fn function(hugr: impl Into<Hugr>) -> Result<Self, ConstTypeError> {
         let hugr = hugr.into();
-        if hugr.get_function_type().is_none() {
+        if get_const_function_type(&hugr).is_none() {
             Err(ConstTypeError::FunctionTypeMissing {
                 hugr_root_type: hugr.get_optype(hugr.root()).clone(),
             })?;
@@ -210,7 +210,7 @@ impl OpName for Const {
         match self {
             Self::Extension { c: e } => format!("const:custom:{}", e.0.name()),
             Self::Function { hugr: h } => {
-                let Some(t) = h.get_function_type() else {
+                let Some(t) = get_const_function_type(h) else {
                     panic!("HUGR root node isn't a valid function parent.");
                 };
                 format!("const:function:[{}]", t)
@@ -254,6 +254,27 @@ impl OpTrait for Const {
     }
 }
 
+/// Returns the function type defined by the HUGR. In contrast to
+/// [`Hugr::get_function_type`], this function also returns function types for
+/// `FuncDecl` and `FuncDefn` operations.
+///
+/// For HUGRs with a [`DataflowParent`][crate::ops::DataflowParent] root
+/// operation, report the signature of the inner dataflow sibling graph.
+///
+/// For HUGRS with a [`FuncDecl`][crate::ops::FuncDecl] or
+/// [`FuncDefn`][crate::ops::FuncDefn] root operation, report the signature of
+/// the function.
+///
+/// Otherwise, returns `None`.
+fn get_const_function_type(hugr: &impl HugrView) -> Option<PolyFuncType> {
+    let op = hugr.get_optype(hugr.root());
+    match op {
+        OpType::FuncDecl(decl) => Some(decl.signature.clone()),
+        OpType::FuncDefn(defn) => Some(defn.signature.clone()),
+        _ => op.inner_function_type().map(PolyFuncType::from),
+    }
+}
+
 // [KnownTypeConst] is guaranteed to be the right type, so can be constructed
 // without initial type check.
 impl<T> From<T> for Const