From f0fade31cce919e2741b9e1b0fa37988ad9f3e9a Mon Sep 17 00:00:00 2001
From: Zeb Piasecki <zeb@cloudflare.com>
Date: Fri, 15 May 2026 09:25:18 -0400
Subject: [PATCH 1/2] fix(miniflare): dedupe workflow compatibility flags

---
 .changeset/silent-workflows-tap.md            |  8 +++++
 .../miniflare/src/plugins/workflows/index.ts  |  7 ++---
 .../test/plugins/workflows/index.spec.ts      | 30 +++++++++++++++++++
 3 files changed, 41 insertions(+), 4 deletions(-)
 create mode 100644 .changeset/silent-workflows-tap.md

diff --git a/.changeset/silent-workflows-tap.md b/.changeset/silent-workflows-tap.md
new file mode 100644
index 0000000000..4244690cd3
--- /dev/null
+++ b/.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.
diff --git a/packages/miniflare/src/plugins/workflows/index.ts b/packages/miniflare/src/plugins/workflows/index.ts
index 37372e8cf8..2f2de132dc 100644
--- a/packages/miniflare/src/plugins/workflows/index.ts
+++ b/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",
diff --git a/packages/miniflare/test/plugins/workflows/index.spec.ts b/packages/miniflare/test/plugins/workflows/index.spec.ts
index 33b1f0ee2c..7d0414a801 100644
--- a/packages/miniflare/test/plugins/workflows/index.spec.ts
+++ b/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",
+		compatibilityFlags: [
+			"nodejs_compat",
+			"experimental",
+			"enhanced_error_serialization",
+		],
+		modules: true,
+		script: WORKFLOW_SCRIPT(),
+		workflows: {
+			MY_WORKFLOW: {
+				className: "MyWorkflow",
+				name: "MY_WORKFLOW",
+			},
+		},
+		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,
 }) => {

From df736bfbb0fd902043bfe0dd63e688826cd67bf3 Mon Sep 17 00:00:00 2001
From: Zeb Piasecki <zeb@cloudflare.com>
Date: Fri, 15 May 2026 09:42:47 -0400
Subject: [PATCH 2/2] test(miniflare): cover workflow flag deduplication

---
 .../miniflare/test/plugins/workflows/index.spec.ts     | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/packages/miniflare/test/plugins/workflows/index.spec.ts b/packages/miniflare/test/plugins/workflows/index.spec.ts
index 7d0414a801..56061c1fb7 100644
--- a/packages/miniflare/test/plugins/workflows/index.spec.ts
+++ b/packages/miniflare/test/plugins/workflows/index.spec.ts
@@ -29,17 +29,17 @@ test("starts Workflows with user-provided experimental compatibility flag", asyn
 	const mf = new Miniflare({
 		name: "workflow-compatibility-flags-worker",
 		compatibilityDate: "2024-11-20",
-		compatibilityFlags: [
-			"nodejs_compat",
-			"experimental",
-			"enhanced_error_serialization",
-		],
 		modules: true,
 		script: WORKFLOW_SCRIPT(),
 		workflows: {
 			MY_WORKFLOW: {
 				className: "MyWorkflow",
 				name: "MY_WORKFLOW",
+				compatibilityFlags: [
+					"nodejs_compat",
+					"experimental",
+					"enhanced_error_serialization",
+				],
 			},
 		},
 		workflowsPersist: tmp,