dodopayments · thepushkaraj · Aug 23, 2026 · Aug 23, 2026
diff --git a/.changeset/html-content-type-case-insensitive.md b/.changeset/html-content-type-case-insensitive.md
@@ -0,0 +1,11 @@
+---
+"@dualmark/cloudflare": patch
+"@dualmark/deno": patch
+"@dualmark/fastly": patch
+"@dualmark/netlify": patch
+"@dualmark/vercel": patch
+---
+
+Match the HTML `Content-Type` case-insensitively when deciding to inject `Vary: Accept` and the `Link` alternate header.
+
+HTTP media types are case-insensitive (RFC 7231 §3.1.1.1), but the cloudflare, deno, fastly, netlify and vercel adapters gated markdown-twin advertisement on a case-sensitive `contentType.includes("text/html")`. An upstream that emitted `Content-Type: Text/HTML` (or any non-lowercase spelling) skipped the block, so the negotiable HTML response was returned without `Vary: Accept` — risking a shared cache serving HTML to a markdown client — and without the `Link rel="alternate"` twin. The check now lowercases the header first, matching the astro, nuxt and sveltekit adapters which already did this.
diff --git a/packages/cloudflare/src/worker.ts b/packages/cloudflare/src/worker.ts
@@ -284,7 +284,7 @@ export function createAEOWorker<Env extends MinimalEnv = MinimalEnv>(
       if (
         !shouldSkip(pathname, skipPrefixes, skipExtensions) &&
         !pathname.endsWith(".md") &&
-        upstreamResponse.headers.get("content-type")?.includes("text/html")
+        upstreamResponse.headers.get("content-type")?.toLowerCase().includes("text/html")
       ) {
         const newHeaders = new Headers(upstreamResponse.headers);
         const vary = newHeaders.get("Vary");

diff --git a/packages/cloudflare/test/worker.test.ts b/packages/cloudflare/test/worker.test.ts
@@ -479,4 +479,22 @@ describe("createAEOWorker — Link header injection", () => {
     expect((res.headers.get("vary") ?? "").toLowerCase()).toContain("accept");
     expect(res.headers.get("link")).toBeNull();
   });
+
+  it("injects Link and Vary on HTML with a mixed-case Content-Type", async () => {
+    const env: TestEnv = { ASSETS: makeAssets({}) };
+    const worker = createAEOWorker({
+      upstream: makeUpstream(
+        () => new Response("<html></html>", { headers: { "Content-Type": "Text/HTML; charset=UTF-8" } }),
+      ),
+    });
+    const req = new Request("https://acme.test/page", {
+      headers: {
+        "user-agent": "Mozilla/5.0 Chrome/130",
+        accept: "text/html,*/*;q=0.8",
+      },
+    });
+    const res = await worker.fetch(req, env, makeCtx());
+    expect(res.headers.get("link")).toContain('rel="alternate"');
+    expect((res.headers.get("vary") ?? "").toLowerCase()).toContain("accept");
+  });
 });
diff --git a/packages/deno/src/handler.ts b/packages/deno/src/handler.ts
@@ -275,7 +275,7 @@ export function createAEOHandler(options: CreateAEOHandlerOptions): AEODenoHandl
     if (
       !shouldSkip(pathname, skipPrefixes, skipExtensions) &&
       !pathname.endsWith(".md") &&
-      upstreamResponse.headers.get("content-type")?.includes("text/html")
+      upstreamResponse.headers.get("content-type")?.toLowerCase().includes("text/html")
     ) {
       const newHeaders = new Headers(upstreamResponse.headers);
       const vary = newHeaders.get("Vary");

diff --git a/packages/deno/test/handler.test.ts b/packages/deno/test/handler.test.ts
@@ -292,6 +292,15 @@ describe("createAEOHandler — Link header injection", () => {
     const res = await handler(new Request("https://acme.test/data"), makeInfo());
     expect(res.headers.get("link")).toBeNull();
   });
+
+  it("injects Link and Vary on HTML with a mixed-case Content-Type", async () => {
+    const upstream: DenoUpstreamHandler = async () =>
+      new Response("<html></html>", { headers: { "Content-Type": "Text/HTML; charset=UTF-8" } });
+    const handler = createAEOHandler({ upstream });
+    const res = await handler(new Request("https://acme.test/page"), makeInfo());
+    expect(res.headers.get("link")).toContain('rel="alternate"');
+    expect((res.headers.get("vary") ?? "").toLowerCase()).toContain("accept");
+  });
 });
 
 describe("createAEOHandler — direct .md handling", () => {

diff --git a/packages/fastly/src/handler.ts b/packages/fastly/src/handler.ts
@@ -274,7 +274,7 @@ export function createAEORequestHandler(options: CreateAEOFastlyOptions): AEOFas
     if (
       !shouldSkip(pathname, skipPrefixes, skipExtensions) &&
       !pathname.endsWith(".md") &&
-      upstreamResponse.headers.get("content-type")?.includes("text/html")
+      upstreamResponse.headers.get("content-type")?.toLowerCase().includes("text/html")
     ) {
       const newHeaders = new Headers(upstreamResponse.headers);
       const vary = newHeaders.get("Vary");

diff --git a/packages/fastly/test/handler.test.ts b/packages/fastly/test/handler.test.ts
@@ -158,6 +158,30 @@ describe("Fastly Adapter", () => {
       expect(fetch).toHaveBeenCalledWith(req, { backend: "default_backend" });
     });
 
+    it("injects Link and Vary on HTML with a mixed-case Content-Type", async () => {
+      vi.stubGlobal(
+        "fetch",
+        vi.fn(
+          async () =>
+            new Response("<html></html>", {
+              status: 200,
+              headers: { "Content-Type": "Text/HTML; charset=UTF-8" },
+            }),
+        ),
+      );
+      const handler = createAEORequestHandler({ backend: "default_backend" });
+      const req = new Request("https://acme.test/page", {
+        headers: {
+          "user-agent": "Mozilla/5.0 Chrome/130",
+          accept: "text/html,*/*;q=0.8",
+        },
+      });
+      const res = await handler(req);
+      expect(res.status).toBe(200);
+      expect(res.headers.get("link")).toContain('rel="alternate"');
+      expect(res.headers.get("vary")).toContain("Accept");
+    });
+
     it("handles missing .md (cache miss) for bot — falls to upstream HTML", async () => {
       const handler = createAEORequestHandler({
         backend: "default_backend",

diff --git a/packages/netlify/src/worker.ts b/packages/netlify/src/worker.ts
@@ -269,7 +269,7 @@ export function createAEOWorker(
     if (
       !shouldSkip(pathname, skipPrefixes, skipExtensions) &&
       !pathname.endsWith(".md") &&
-      originResponse.headers.get("content-type")?.includes("text/html")
+      originResponse.headers.get("content-type")?.toLowerCase().includes("text/html")
     ) {
       const newHeaders = new Headers(originResponse.headers);
       const vary = newHeaders.get("Vary");

diff --git a/packages/netlify/test/worker.test.ts b/packages/netlify/test/worker.test.ts
@@ -493,4 +493,23 @@ describe("createAEOWorker — default options", () => {
     expect(res.status).toBe(301);
     expect(res.headers.get("location")).toBe("https://acme.test/blog?ref=twitter");
   });
+
+  it("injects Link and Vary on HTML with a mixed-case Content-Type", async () => {
+    const assets = makeAssets({});
+    const worker = createAEOWorker({ assets });
+    const req = new Request("https://acme.test/page", {
+      headers: {
+        "user-agent": "Mozilla/5.0 Chrome/130",
+        accept: "text/html,*/*;q=0.8",
+      },
+    });
+    const res = await worker(
+      req,
+      makeContext(
+        () => new Response("<html></html>", { headers: { "Content-Type": "Text/HTML; charset=UTF-8" } }),
+      ),
+    );
+    expect(res.headers.get("link")).toContain('rel="alternate"');
+    expect((res.headers.get("vary") ?? "").toLowerCase()).toContain("accept");
+  });
 });
diff --git a/packages/vercel/src/middleware.ts b/packages/vercel/src/middleware.ts
@@ -295,7 +295,7 @@ export function createAEOMiddleware(
       const ct = upstreamResponse.headers.get("content-type");
       // Passthrough responses (e.g. NextResponse.next()) have no content-type yet —
       // always inject. For concrete responses, only inject on text/html.
-      if (!ct || ct.includes("text/html")) {
+      if (!ct || ct.toLowerCase().includes("text/html")) {
         const mdPath = toMarkdownPath(pathname);
         try {
           // Fast path: mutate headers in-place (works for NextResponse.next() and

diff --git a/packages/vercel/test/middleware.test.ts b/packages/vercel/test/middleware.test.ts
@@ -446,6 +446,27 @@ describe("createAEOMiddleware — Link header injection", () => {
     expect((res.headers.get("vary") ?? "").toLowerCase()).toContain("accept");
     expect(res.headers.get("link")).toBeNull();
   });
+
+  it("injects Link and Vary on HTML with a mixed-case Content-Type", async () => {
+    const middleware = createAEOMiddleware({
+      upstream: makeUpstream(
+        () =>
+          new Response("<html></html>", {
+            headers: { "Content-Type": "Text/HTML; charset=UTF-8" },
+          }),
+      ),
+      fetchAsset,
+    });
+    const req = new Request("https://acme.test/page", {
+      headers: {
+        "user-agent": "Mozilla/5.0 Chrome/130",
+        accept: "text/html,*/*;q=0.8",
+      },
+    });
+    const res = await middleware(req, makeCtx());
+    expect(res.headers.get("link")).toContain('rel="alternate"');
+    expect((res.headers.get("vary") ?? "").toLowerCase()).toContain("accept");
+  });
 });
 
 describe("createAEOMiddleware — edge cases", () => {