diff --git a/.changeset/locals-key-dual-package-fix.md b/.changeset/locals-key-dual-package-fix.md
new file mode 100644
index 00000000000..38d42e19dfb
--- /dev/null
+++ b/.changeset/locals-key-dual-package-fix.md
@@ -0,0 +1,5 @@
+---
+"@trigger.dev/core": patch
+---
+
+Fix `LocalsKey<T>` type incompatibility across dual-package builds. The phantom value-type brand no longer uses a module-level `unique symbol`, so a single TypeScript compilation that resolves the type from both the ESM and CJS outputs (which can happen under certain pnpm hoisting layouts) no longer sees two structurally-incompatible variants of the same type.
diff --git a/packages/core/src/v3/locals/manager.ts b/packages/core/src/v3/locals/manager.ts
index 6984d9f4146..3cd0f80d84a 100644
--- a/packages/core/src/v3/locals/manager.ts
+++ b/packages/core/src/v3/locals/manager.ts
@@ -5,7 +5,7 @@ export class NoopLocalsManager implements LocalsManager {
     return {
       __type: Symbol(),
       id,
-    } as unknown as LocalsKey<T>;
+    };
   }
 
   getLocal<T>(key: LocalsKey<T>): T | undefined {
@@ -23,7 +23,7 @@ export class StandardLocalsManager implements LocalsManager {
     return {
       __type: key,
       id,
-    } as unknown as LocalsKey<T>;
+    };
   }
 
   getLocal<T>(key: LocalsKey<T>): T | undefined {
diff --git a/packages/core/src/v3/locals/types.ts b/packages/core/src/v3/locals/types.ts
index aab683df091..84abc4c70f5 100644
--- a/packages/core/src/v3/locals/types.ts
+++ b/packages/core/src/v3/locals/types.ts
@@ -1,10 +1,22 @@
-declare const __local: unique symbol;
-type BrandLocal<T> = { [__local]: T };
-
-// Create a type-safe store for your locals
-export type LocalsKey<T> = BrandLocal<T> & {
+/**
+ * A type-safe key for `locals`. Carries the value type `T` as a phantom
+ * marker on the optional `__valueType` field so two keys with different
+ * value types are distinguishable at the type level.
+ *
+ * The phantom field is intentionally not anchored to a `unique symbol`:
+ * dual-package builds (`tshy`) emit separate `.d.ts` files for ESM and
+ * CJS outputs, and each `unique symbol` declaration in a `.d.ts` is its
+ * own nominal type. If a single compilation ever resolves `LocalsKey`
+ * from both the ESM and CJS paths — which happens under certain pnpm
+ * hoisting layouts — `unique symbol` brands produce structurally
+ * incompatible variants of the same type. A plain string brand avoids
+ * the hazard.
+ */
+export type LocalsKey<T> = {
   readonly id: string;
-  readonly __type: unique symbol;
+  readonly __type: symbol;
+  /** Phantom carrier for the value type — never read at runtime. */
+  readonly __valueType?: T;
 };
 
 export interface LocalsManager {