[miniflare] Deduplicate Workflow compatibility flags

.changeset/silent-workflows-tap.md

@@ -0,0 +1,8 @@
+---
+"miniflare": patch
+"wrangler": patch
+---
+
+Fix local Workflow startup when compatibility flags include `experimental`
+
+Miniflare now deduplicates compatibility flags for the internal Workflow engine service. This prevents `wrangler dev` from failing with `Compatibility flag specified multiple times: experimental` when the user's Worker already enables that flag.

packages/miniflare/src/plugins/workflows/index.ts

@@ -119,10 +119,9 @@ export const WORKFLOWS_PLUGIN: Plugin<
 					),
 					worker: {
 						compatibilityDate: "2024-10-22",
-						compatibilityFlags: [
-							"experimental",
-							...(workflow.compatibilityFlags ?? []),
-						],
+						compatibilityFlags: Array.from(
+							new Set(["experimental", ...(workflow.compatibilityFlags ?? [])])
+						),
 						modules: [
 							{
 								name: "workflows.mjs",

packages/miniflare/test/plugins/workflows/index.spec.ts

@@ -22,6 +22,36 @@ export class MyWorkflow extends WorkflowEntrypoint {
 	},
   };`;
 
+test("starts Workflows with user-provided experimental compatibility flag", async ({
+	expect,
+}) => {
+	const tmp = await useTmp();
+	const mf = new Miniflare({
+		name: "workflow-compatibility-flags-worker",
+		compatibilityDate: "2024-11-20",
+		modules: true,
+		script: WORKFLOW_SCRIPT(),
+		workflows: {
+			MY_WORKFLOW: {
+				className: "MyWorkflow",
+				name: "MY_WORKFLOW",
+				compatibilityFlags: [
+					"nodejs_compat",
+					"experimental",
+					"enhanced_error_serialization",
+				],
+			},
+		},
+		workflowsPersist: tmp,
+	});
+	useDispose(mf);
+
+	const res = await mf.dispatchFetch("http://localhost");
+	expect(await res.text()).toBe(
+		'{"status":"complete","__LOCAL_DEV_STEP_OUTPUTS":["yes you are"],"output":"I\'m a output string"}'
+	);
+});
+
 test("persists Workflow data on file-system between runs", async ({
 	expect,
 }) => {