diff --git a/crates/api-macros-impl/src/lib.rs b/crates/api-macros-impl/src/lib.rs
index aabbe99f0..66cd9ab0c 100644
--- a/crates/api-macros-impl/src/lib.rs
+++ b/crates/api-macros-impl/src/lib.rs
@@ -473,7 +473,7 @@ fn normalize_operation(method: &mut TraitItemFn) -> syn::Result<Operation> {
     }
 
     Ok(Operation {
-        marker_ident: format_ident!("{}", to_pascal_case(&method.sig.ident)),
+        marker_ident: operation_marker_ident(&method.sig.ident),
         method_ident: method.sig.ident.clone(),
         operation_id,
         method: http_method,
@@ -926,6 +926,21 @@ fn ident_text(ident: &Ident) -> String {
     value.strip_prefix("r#").unwrap_or(&value).to_owned()
 }
 
+fn operation_marker_ident(method: &Ident) -> Ident {
+    let candidate = to_pascal_case(method);
+    if let Ok(mut marker) = syn::parse_str::<Ident>(&candidate) {
+        marker.set_span(method.span());
+        return marker;
+    }
+
+    let encoded = ident_text(method)
+        .as_bytes()
+        .iter()
+        .map(|byte| format!("{byte:02x}"))
+        .collect::<String>();
+    Ident::new(&format!("Operation{encoded}"), method.span())
+}
+
 fn to_pascal_case(ident: &Ident) -> String {
     ident_text(ident)
         .split('_')
diff --git a/crates/api-macros/src/lib.rs b/crates/api-macros/src/lib.rs
index c7431e3b2..006dd6e4b 100644
--- a/crates/api-macros/src/lib.rs
+++ b/crates/api-macros/src/lib.rs
@@ -51,8 +51,9 @@
 //! For `WidgetApi`, the macro emits `WidgetApiMetadata`, which implements [`ApiContract`],
 //! plus a `widget_api_operations` module. That module contains one marker per Rust method
 //! (`Create`, `Get`, ...), each implementing [`Operation`] and connecting metadata to concrete
-//! body types at compile time. The `ApiContract::OPERATIONS` inventory is sorted by operation
-//! ID, so its ordering is independent of source method order.
+//! body types at compile time. Unusual method names which have no valid PascalCase form use the
+//! deterministic `Operation<utf8-hex>` fallback. The `ApiContract::OPERATIONS` inventory is
+//! sorted by operation ID, so its ordering is independent of source method order.
 //!
 //! Only empty and JSON bodies are accepted by this first contract. [`WireKind`] reserves
 //! explicit variants for future transport work; accepting one requires a deliberate macro and
diff --git a/crates/api-macros/tests/expansion.rs b/crates/api-macros/tests/expansion.rs
index 3f8c027a3..a52ae4a78 100644
--- a/crates/api-macros/tests/expansion.rs
+++ b/crates/api-macros/tests/expansion.rs
@@ -41,6 +41,15 @@ pub trait RawIdentifierApi {
     async fn r#type(&self) -> Widget;
 }
 
+#[api]
+pub trait UnusualIdentifierApi {
+    #[get("/underscores", operation_id = "unusual.underscores")]
+    async fn __(&self) -> Widget;
+
+    #[get("/underscore-digit", operation_id = "unusual.underscore_digit")]
+    async fn _0(&self) -> Widget;
+}
+
 fn assert_operation_types<O>()
 where
     O: Operation<
@@ -60,6 +69,11 @@ fn expansion_exposes_deterministic_metadata_and_type_connections() {
     assert_named_wrapper::<wrapper_api_operations::Search>();
     let raw = <raw_identifier_api_operations::Type as Operation>::METADATA;
     assert_eq!(raw.operation_id, "raw.type");
+    let underscores = <unusual_identifier_api_operations::Operation5f5f as Operation>::METADATA;
+    assert_eq!(underscores.operation_id, "unusual.underscores");
+    let underscore_digit =
+        <unusual_identifier_api_operations::Operation5f30 as Operation>::METADATA;
+    assert_eq!(underscore_digit.operation_id, "unusual.underscore_digit");
 
     let operations = <WidgetApiMetadata as ApiContract>::OPERATIONS;
     assert_eq!(operations.len(), 3);