diff --git a/packages/azure-functions-durable/README.md b/packages/azure-functions-durable/README.md
index 49adb637..d45ea14d 100644
--- a/packages/azure-functions-durable/README.md
+++ b/packages/azure-functions-durable/README.md
@@ -52,6 +52,14 @@ changed:
   (testing utilities), `ManagedIdentityTokenSource`, and the entity-lock types above. `TaskFailedError`
   is re-exported from the core SDK (aggregate failures surface as JS-native `AggregateError`); use the
   core `TestOrchestrationWorker` / `TestOrchestrationClient` for orchestration unit tests.
+- **A plain non-generator classic orchestrator is no longer supported.** A classic v3 orchestrator
+  written as a *synchronous, single-argument, non-generator* function `(context) => context.df.*`
+  (one that never `yield`s) is now treated as a **core-native** orchestrator and receives the core
+  `OrchestrationContext`, which has no `.df`. This resolves
+  [#321](https://github.com/microsoft/durabletask-js/issues/321), where a core-native
+  `(ctx) => ctx.instanceId` was mis-routed to the classic context. Standard classic orchestrators —
+  sync **generators** (`function*`) using `context.df.*` — are unaffected; convert any non-generator
+  classic orchestrator to generator form, or to the core-native `ctx.*` API.
 
 ## Getting started
 
diff --git a/packages/azure-functions-durable/src/orchestration-context.ts b/packages/azure-functions-durable/src/orchestration-context.ts
index 14339102..e839d41d 100644
--- a/packages/azure-functions-durable/src/orchestration-context.ts
+++ b/packages/azure-functions-durable/src/orchestration-context.ts
@@ -218,8 +218,12 @@ export type ClassicOrchestrator = (
  *   generator to `yield` durable tasks), so `async` unambiguously means core-native.
  * - `function*` (classic v3 sync generator) is wrapped; the engine cannot drive a sync generator, so
  *   the wrapper delegates to it via `yield*`.
- * - A plain SYNC (non-generator) function is ambiguous, so fall back to arity: a lone `context`
- *   parameter is treated as classic, `(ctx, input)` as core-native.
+ * - A plain SYNC (non-generator) function is CORE-NATIVE regardless of arity and passes through
+ *   unchanged, receiving the core {@link OrchestrationContext}. This fixes #321 (a native
+ *   `(ctx) => ctx.instanceId` previously mis-routed by arity to the classic context). BREAKING: a
+ *   classic v3 orchestrator written as a plain non-generator using `context.df.*` is no longer
+ *   supported — convert it to a `function*` generator (the standard classic shape, unaffected) or
+ *   to the core-native `ctx.*` API.
  */
 export function wrapOrchestrator(handler: TOrchestrator | ClassicOrchestrator): TOrchestrator {
   if (typeof handler === "function" && isClassicOrchestrator(handler)) {
@@ -274,9 +278,11 @@ function isClassicOrchestrator(handler: TOrchestrator | ClassicOrchestrator): bo
   if (kind === "GeneratorFunction") {
     return true; // classic v3: a sync generator the engine can't drive on its own.
   }
-  // Plain SYNC (non-generator) function: kind is ambiguous, so fall back to arity. A lone
-  // `context` parameter is the classic v3 shape; `(ctx, input)` is core-native.
-  return handler.length <= 1;
+  // Plain SYNC (non-generator) function is CORE-NATIVE regardless of arity: it passes through
+  // unchanged and receives the core OrchestrationContext. Only a sync generator (`function*`) is
+  // classic (handled above). A classic v3 orchestrator written as a plain non-generator using
+  // `context.df.*` is no longer supported (breaking) — see #321 and the package README/CHANGELOG.
+  return false;
 }
 
 /**
diff --git a/packages/azure-functions-durable/test/unit/orchestration-context.spec.ts b/packages/azure-functions-durable/test/unit/orchestration-context.spec.ts
index e1d39137..add2a1bf 100644
--- a/packages/azure-functions-durable/test/unit/orchestration-context.spec.ts
+++ b/packages/azure-functions-durable/test/unit/orchestration-context.spec.ts
@@ -272,9 +272,10 @@ describe("wrapOrchestrator", () => {
   });
 
   it("exposes a replay-safe logger as context.log/error on the classic context", async () => {
-    // A classic orchestrator may be a plain (non-generator) function that returns a value; the
-    // wrapper still invokes it with the full classic context, so log wiring is exercised here.
-    const classic = (context: ClassicOrchestrationContext): string => {
+    // A classic orchestrator is a sync generator (`function*`); the wrapper invokes it with the full
+    // classic context, so the replay-safe log wiring is exercised here.
+    // eslint-disable-next-line require-yield
+    const classic = function* (context: ClassicOrchestrationContext): Generator<Task<unknown>, string, unknown> {
       context.log("hi");
       context.error("boom");
       return "logged";
@@ -292,6 +293,22 @@ describe("wrapOrchestrator", () => {
     expect(replaySafeLogger.info).toHaveBeenCalledWith("hi");
     expect(replaySafeLogger.error).toHaveBeenCalledWith("boom");
   });
+
+  it("returns a plain sync single-argument core-native orchestrator unchanged (#321)", () => {
+    // #321: a plain sync, single-argument, non-generator `(ctx) => value` is core-native. It must
+    // pass through wrapOrchestrator UNCHANGED (identity) and receive the core OrchestrationContext.
+    // Previously arity-based detection mis-routed it to the classic `{ df, log }` context, so
+    // `ctx.instanceId` was undefined and core members like `ctx.newGuid()` threw.
+    const native = (ctx: OrchestrationContext): string => `native:${ctx.instanceId}`;
+
+    const wrapped = wrapOrchestrator(native as unknown as TOrchestrator);
+    expect(wrapped).toBe(native as unknown as TOrchestrator); // identity passthrough, not wrapped
+
+    const { ctx } = createFakeCoreContext();
+    const out = (wrapped as unknown as (c: OrchestrationContext) => string)(ctx);
+    expect(out.startsWith("native:")).toBe(true);
+    expect(out).not.toContain("undefined");
+  });
 });
 
 describe("wrapOrchestrator end-to-end (real core executor)", () => {