diff --git a/.changeset/red-lamps-tickle.md b/.changeset/red-lamps-tickle.md
new file mode 100644
index 000000000..67b912cfa
--- /dev/null
+++ b/.changeset/red-lamps-tickle.md
@@ -0,0 +1,7 @@
+---
+"@hey-api/openapi-ts": patch
+---
+
+Add explicit type annotations to `Interceptors`
+
+This allows the generated code to work with TypeScript 5.5's new `isolatedDeclarations` configuration.
diff --git a/packages/openapi-ts/src/templates/core/OpenAPI.hbs b/packages/openapi-ts/src/templates/core/OpenAPI.hbs
index 2fcf933bf..9b501bf08 100644
--- a/packages/openapi-ts/src/templates/core/OpenAPI.hbs
+++ b/packages/openapi-ts/src/templates/core/OpenAPI.hbs
@@ -20,14 +20,14 @@ export class Interceptors<T> {
     this._fns = [];
   }
 
-  eject(fn: Middleware<T>) {
+  eject(fn: Middleware<T>): void {
     const index = this._fns.indexOf(fn);
     if (index !== -1) {
       this._fns = [...this._fns.slice(0, index), ...this._fns.slice(index + 1)];
     }
   }
 
-  use(fn: Middleware<T>) {
+  use(fn: Middleware<T>): void {
     this._fns = [...this._fns, fn];
   }
 }
diff --git a/packages/openapi-ts/test/__snapshots__/test/generated/v2/core/OpenAPI.ts.snap b/packages/openapi-ts/test/__snapshots__/test/generated/v2/core/OpenAPI.ts.snap
index 7286660a4..438d879ee 100644
--- a/packages/openapi-ts/test/__snapshots__/test/generated/v2/core/OpenAPI.ts.snap
+++ b/packages/openapi-ts/test/__snapshots__/test/generated/v2/core/OpenAPI.ts.snap
@@ -11,14 +11,14 @@ export class Interceptors<T> {
     this._fns = [];
   }
 
-  eject(fn: Middleware<T>) {
+  eject(fn: Middleware<T>): void {
     const index = this._fns.indexOf(fn);
     if (index !== -1) {
       this._fns = [...this._fns.slice(0, index), ...this._fns.slice(index + 1)];
     }
   }
 
-  use(fn: Middleware<T>) {
+  use(fn: Middleware<T>): void {
     this._fns = [...this._fns, fn];
   }
 }
diff --git a/packages/openapi-ts/test/__snapshots__/test/generated/v3/core/OpenAPI.ts.snap b/packages/openapi-ts/test/__snapshots__/test/generated/v3/core/OpenAPI.ts.snap
index 7286660a4..438d879ee 100644
--- a/packages/openapi-ts/test/__snapshots__/test/generated/v3/core/OpenAPI.ts.snap
+++ b/packages/openapi-ts/test/__snapshots__/test/generated/v3/core/OpenAPI.ts.snap
@@ -11,14 +11,14 @@ export class Interceptors<T> {
     this._fns = [];
   }
 
-  eject(fn: Middleware<T>) {
+  eject(fn: Middleware<T>): void {
     const index = this._fns.indexOf(fn);
     if (index !== -1) {
       this._fns = [...this._fns.slice(0, index), ...this._fns.slice(index + 1)];
     }
   }
 
-  use(fn: Middleware<T>) {
+  use(fn: Middleware<T>): void {
     this._fns = [...this._fns, fn];
   }
 }
diff --git a/packages/openapi-ts/test/__snapshots__/test/generated/v3_angular/core/OpenAPI.ts.snap b/packages/openapi-ts/test/__snapshots__/test/generated/v3_angular/core/OpenAPI.ts.snap
index 5f049af07..eda4c9d4b 100644
--- a/packages/openapi-ts/test/__snapshots__/test/generated/v3_angular/core/OpenAPI.ts.snap
+++ b/packages/openapi-ts/test/__snapshots__/test/generated/v3_angular/core/OpenAPI.ts.snap
@@ -12,14 +12,14 @@ export class Interceptors<T> {
     this._fns = [];
   }
 
-  eject(fn: Middleware<T>) {
+  eject(fn: Middleware<T>): void {
     const index = this._fns.indexOf(fn);
     if (index !== -1) {
       this._fns = [...this._fns.slice(0, index), ...this._fns.slice(index + 1)];
     }
   }
 
-  use(fn: Middleware<T>) {
+  use(fn: Middleware<T>): void {
     this._fns = [...this._fns, fn];
   }
 }
diff --git a/packages/openapi-ts/test/__snapshots__/test/generated/v3_axios/core/OpenAPI.ts.snap b/packages/openapi-ts/test/__snapshots__/test/generated/v3_axios/core/OpenAPI.ts.snap
index f08bd7576..4bbc86a7f 100644
--- a/packages/openapi-ts/test/__snapshots__/test/generated/v3_axios/core/OpenAPI.ts.snap
+++ b/packages/openapi-ts/test/__snapshots__/test/generated/v3_axios/core/OpenAPI.ts.snap
@@ -12,14 +12,14 @@ export class Interceptors<T> {
     this._fns = [];
   }
 
-  eject(fn: Middleware<T>) {
+  eject(fn: Middleware<T>): void {
     const index = this._fns.indexOf(fn);
     if (index !== -1) {
       this._fns = [...this._fns.slice(0, index), ...this._fns.slice(index + 1)];
     }
   }
 
-  use(fn: Middleware<T>) {
+  use(fn: Middleware<T>): void {
     this._fns = [...this._fns, fn];
   }
 }
diff --git a/packages/openapi-ts/test/__snapshots__/test/generated/v3_client/core/OpenAPI.ts.snap b/packages/openapi-ts/test/__snapshots__/test/generated/v3_client/core/OpenAPI.ts.snap
index 7286660a4..438d879ee 100644
--- a/packages/openapi-ts/test/__snapshots__/test/generated/v3_client/core/OpenAPI.ts.snap
+++ b/packages/openapi-ts/test/__snapshots__/test/generated/v3_client/core/OpenAPI.ts.snap
@@ -11,14 +11,14 @@ export class Interceptors<T> {
     this._fns = [];
   }
 
-  eject(fn: Middleware<T>) {
+  eject(fn: Middleware<T>): void {
     const index = this._fns.indexOf(fn);
     if (index !== -1) {
       this._fns = [...this._fns.slice(0, index), ...this._fns.slice(index + 1)];
     }
   }
 
-  use(fn: Middleware<T>) {
+  use(fn: Middleware<T>): void {
     this._fns = [...this._fns, fn];
   }
 }
diff --git a/packages/openapi-ts/test/__snapshots__/test/generated/v3_enums_typescript/core/OpenAPI.ts.snap b/packages/openapi-ts/test/__snapshots__/test/generated/v3_enums_typescript/core/OpenAPI.ts.snap
index 7286660a4..438d879ee 100644
--- a/packages/openapi-ts/test/__snapshots__/test/generated/v3_enums_typescript/core/OpenAPI.ts.snap
+++ b/packages/openapi-ts/test/__snapshots__/test/generated/v3_enums_typescript/core/OpenAPI.ts.snap
@@ -11,14 +11,14 @@ export class Interceptors<T> {
     this._fns = [];
   }
 
-  eject(fn: Middleware<T>) {
+  eject(fn: Middleware<T>): void {
     const index = this._fns.indexOf(fn);
     if (index !== -1) {
       this._fns = [...this._fns.slice(0, index), ...this._fns.slice(index + 1)];
     }
   }
 
-  use(fn: Middleware<T>) {
+  use(fn: Middleware<T>): void {
     this._fns = [...this._fns, fn];
   }
 }
diff --git a/packages/openapi-ts/test/__snapshots__/test/generated/v3_legacy_positional_args/core/OpenAPI.ts.snap b/packages/openapi-ts/test/__snapshots__/test/generated/v3_legacy_positional_args/core/OpenAPI.ts.snap
index 7286660a4..438d879ee 100644
--- a/packages/openapi-ts/test/__snapshots__/test/generated/v3_legacy_positional_args/core/OpenAPI.ts.snap
+++ b/packages/openapi-ts/test/__snapshots__/test/generated/v3_legacy_positional_args/core/OpenAPI.ts.snap
@@ -11,14 +11,14 @@ export class Interceptors<T> {
     this._fns = [];
   }
 
-  eject(fn: Middleware<T>) {
+  eject(fn: Middleware<T>): void {
     const index = this._fns.indexOf(fn);
     if (index !== -1) {
       this._fns = [...this._fns.slice(0, index), ...this._fns.slice(index + 1)];
     }
   }
 
-  use(fn: Middleware<T>) {
+  use(fn: Middleware<T>): void {
     this._fns = [...this._fns, fn];
   }
 }
diff --git a/packages/openapi-ts/test/__snapshots__/test/generated/v3_node/core/OpenAPI.ts.snap b/packages/openapi-ts/test/__snapshots__/test/generated/v3_node/core/OpenAPI.ts.snap
index 091d25a4e..e62d0c4ce 100644
--- a/packages/openapi-ts/test/__snapshots__/test/generated/v3_node/core/OpenAPI.ts.snap
+++ b/packages/openapi-ts/test/__snapshots__/test/generated/v3_node/core/OpenAPI.ts.snap
@@ -12,14 +12,14 @@ export class Interceptors<T> {
     this._fns = [];
   }
 
-  eject(fn: Middleware<T>) {
+  eject(fn: Middleware<T>): void {
     const index = this._fns.indexOf(fn);
     if (index !== -1) {
       this._fns = [...this._fns.slice(0, index), ...this._fns.slice(index + 1)];
     }
   }
 
-  use(fn: Middleware<T>) {
+  use(fn: Middleware<T>): void {
     this._fns = [...this._fns, fn];
   }
 }
diff --git a/packages/openapi-ts/test/__snapshots__/test/generated/v3_options/core/OpenAPI.ts.snap b/packages/openapi-ts/test/__snapshots__/test/generated/v3_options/core/OpenAPI.ts.snap
index 7286660a4..438d879ee 100644
--- a/packages/openapi-ts/test/__snapshots__/test/generated/v3_options/core/OpenAPI.ts.snap
+++ b/packages/openapi-ts/test/__snapshots__/test/generated/v3_options/core/OpenAPI.ts.snap
@@ -11,14 +11,14 @@ export class Interceptors<T> {
     this._fns = [];
   }
 
-  eject(fn: Middleware<T>) {
+  eject(fn: Middleware<T>): void {
     const index = this._fns.indexOf(fn);
     if (index !== -1) {
       this._fns = [...this._fns.slice(0, index), ...this._fns.slice(index + 1)];
     }
   }
 
-  use(fn: Middleware<T>) {
+  use(fn: Middleware<T>): void {
     this._fns = [...this._fns, fn];
   }
 }
diff --git a/packages/openapi-ts/test/__snapshots__/test/generated/v3_xhr/core/OpenAPI.ts.snap b/packages/openapi-ts/test/__snapshots__/test/generated/v3_xhr/core/OpenAPI.ts.snap
index ec476fed8..afe4ec663 100644
--- a/packages/openapi-ts/test/__snapshots__/test/generated/v3_xhr/core/OpenAPI.ts.snap
+++ b/packages/openapi-ts/test/__snapshots__/test/generated/v3_xhr/core/OpenAPI.ts.snap
@@ -11,14 +11,14 @@ export class Interceptors<T> {
     this._fns = [];
   }
 
-  eject(fn: Middleware<T>) {
+  eject(fn: Middleware<T>): void {
     const index = this._fns.indexOf(fn);
     if (index !== -1) {
       this._fns = [...this._fns.slice(0, index), ...this._fns.slice(index + 1)];
     }
   }
 
-  use(fn: Middleware<T>) {
+  use(fn: Middleware<T>): void {
     this._fns = [...this._fns, fn];
   }
 }