diff --git a/functions/_middleware.test.ts b/functions/_middleware.test.ts
new file mode 100644
index 0000000..f98e879
--- /dev/null
+++ b/functions/_middleware.test.ts
@@ -0,0 +1,138 @@
+import { describe, expect, it, vi } from "vitest"
+import { onRequest } from "./_middleware"
+
+function createContext(
+  url: string,
+  options: { method?: string; headers?: Record<string, string> } = {},
+) {
+  const request = new Request(url, {
+    method: options.method ?? "GET",
+    headers: options.headers ?? {},
+  })
+  const next = vi.fn(() => Promise.resolve(new Response("original")))
+  return { request, next }
+}
+
+describe("markdown content negotiation middleware", () => {
+  it("redirects GET with Accept: text/markdown to {path}.md", async () => {
+    const context = createContext("https://example.com/contributing", {
+      headers: { Accept: "text/markdown" },
+    })
+
+    const response = await onRequest(context)
+
+    expect(response.status).toBe(302)
+    expect(response.headers.get("Location")).toBe("/contributing.md")
+    expect(context.next).not.toHaveBeenCalled()
+  })
+
+  it("redirects GET on root path to /index.md", async () => {
+    const context = createContext("https://example.com/", {
+      headers: { Accept: "text/markdown" },
+    })
+
+    const response = await onRequest(context)
+
+    expect(response.status).toBe(302)
+    expect(response.headers.get("Location")).toBe("/index.md")
+    expect(context.next).not.toHaveBeenCalled()
+  })
+
+  it("passes through when Accept does not include text/markdown", async () => {
+    const context = createContext("https://example.com/contributing", {
+      headers: { Accept: "text/html" },
+    })
+
+    const response = await onRequest(context)
+
+    expect(context.next).toHaveBeenCalled()
+    expect(response).toBe(await context.next.mock.results[0].value)
+  })
+
+  it("passes through for non-GET/HEAD methods", async () => {
+    const context = createContext("https://example.com/contributing", {
+      method: "POST",
+      headers: { Accept: "text/markdown" },
+    })
+
+    await onRequest(context)
+
+    expect(context.next).toHaveBeenCalled()
+  })
+
+  it("passes through when URL path has a file extension", async () => {
+    const context = createContext("https://example.com/styles/main.css", {
+      headers: { Accept: "text/markdown" },
+    })
+
+    await onRequest(context)
+
+    expect(context.next).toHaveBeenCalled()
+  })
+
+  it("redirects HEAD with Accept: text/markdown", async () => {
+    const context = createContext("https://example.com/contributing", {
+      method: "HEAD",
+      headers: { Accept: "text/markdown" },
+    })
+
+    const response = await onRequest(context)
+
+    expect(response.status).toBe(302)
+    expect(response.headers.get("Location")).toBe("/contributing.md")
+  })
+
+  it("redirects when Accept contains text/markdown among other types", async () => {
+    const context = createContext("https://example.com/contributing", {
+      headers: { Accept: "text/markdown, text/html;q=0.9" },
+    })
+
+    const response = await onRequest(context)
+
+    expect(response.status).toBe(302)
+    expect(response.headers.get("Location")).toBe("/contributing.md")
+  })
+
+  it("preserves query string in redirect URL", async () => {
+    const context = createContext(
+      "https://example.com/contributing?ref=footer",
+      {
+        headers: { Accept: "text/markdown" },
+      },
+    )
+
+    const response = await onRequest(context)
+
+    expect(response.status).toBe(302)
+    expect(response.headers.get("Location")).toBe("/contributing.md?ref=footer")
+  })
+
+  it("strips trailing slash before redirecting", async () => {
+    const context = createContext("https://example.com/contributing/", {
+      headers: { Accept: "text/markdown" },
+    })
+
+    const response = await onRequest(context)
+
+    expect(response.status).toBe(302)
+    expect(response.headers.get("Location")).toBe("/contributing.md")
+  })
+
+  it("passes through when URL with trailing slash has a file extension", async () => {
+    const context = createContext("https://example.com/styles/main.css/", {
+      headers: { Accept: "text/markdown" },
+    })
+
+    await onRequest(context)
+
+    expect(context.next).toHaveBeenCalled()
+  })
+
+  it("passes through when no Accept header is present", async () => {
+    const context = createContext("https://example.com/contributing")
+
+    await onRequest(context)
+
+    expect(context.next).toHaveBeenCalled()
+  })
+})
diff --git a/functions/_middleware.ts b/functions/_middleware.ts
new file mode 100644
index 0000000..6a73db3
--- /dev/null
+++ b/functions/_middleware.ts
@@ -0,0 +1,45 @@
+interface Context {
+  request: Request
+  next: () => Promise<Response>
+}
+
+export async function onRequest(context: Context): Promise<Response> {
+  const { request } = context
+  const method = request.method
+  if (method !== "GET" && method !== "HEAD") {
+    return context.next()
+  }
+
+  const accept = request.headers.get("Accept") ?? ""
+  if (!accept.includes("text/markdown")) {
+    return context.next()
+  }
+
+  const url = new URL(request.url)
+  const pathname = cutSuffix(url.pathname, "/")
+  const lastSegment = cutEnd(pathname, "/")
+  if (lastSegment.includes(".")) {
+    return context.next()
+  }
+
+  const path = pathname === "" ? "/index" : pathname
+  return new Response(null, {
+    status: 302,
+    headers: { Location: `${path}.md${url.search}` },
+  })
+}
+
+function cutSuffix(path: string, suffix: string): string {
+  if (path.endsWith(suffix)) {
+    return path.slice(0, -suffix.length)
+  }
+  return path
+}
+
+function cutEnd(str: string, sep: string): string {
+  const index = str.lastIndexOf(sep)
+  if (index === -1) {
+    return str
+  }
+  return str.slice(index)
+}