diff --git a/docs/decisions/0010-worker-http-negotiation.md b/docs/decisions/0010-worker-http-negotiation.md
new file mode 100644
index 0000000..0c4e64a
--- /dev/null
+++ b/docs/decisions/0010-worker-http-negotiation.md
@@ -0,0 +1,27 @@
+# ADR 0010: Worker HTTP representation negotiation
+
+## Status
+
+Accepted for PR #38.
+
+## Context
+
+The optional Cloudflare Worker serves pre-generated HTML and Markdown representations from the same public export. The route must select a representation from the request `Accept` header without treating a mere substring match as authoritative, and method responses must advertise only methods actually implemented by the route class.
+
+## Decision
+
+- HTML remains the default representation when `Accept` is absent, unsupported, tied, or explicitly rejects every representation currently offered.
+- `q=0` makes that matching media range unacceptable; it is never preferred over a positive-quality alternative.
+- Media-range parameters that appear before `q` constrain matching. A range such as `text/html;level=1` does not match the plain HTML representation. Parameters after `q` are accept extensions and do not constrain representation matching.
+- Both generated text representations are UTF-8, so `charset=utf-8` is a matching media parameter.
+- Specific media types outrank type wildcards, which outrank `*/*`; quality and original header order break ties within the same specificity.
+- API routes advertise `GET, HEAD, OPTIONS`; non-API page routes advertise `GET, HEAD` and reject `OPTIONS` with 405.
+- Negotiated page responses retain `Vary: Accept`.
+
+## Consequences
+
+The Worker performs deterministic local header parsing only. It does not inspect R2 contents to decide representation and does not add a 406 response path in this change. The fallback-to-HTML behavior is therefore an explicit compatibility contract rather than an inference from malformed or unsupported `Accept` values.
+
+## Verification
+
+The behavior is covered by helper tests and Worker integration tests, including quality values, wildcards, `q=0`, pre-`q` media parameters, post-`q` accept extensions, UTF-8 charset matching, negotiated content type, and route-specific `Allow` headers. Measured hosted-CI evidence is recorded in `docs/verification/2026-08-22-worker-http-contract.md`.
diff --git a/docs/verification/2026-08-22-worker-http-contract.md b/docs/verification/2026-08-22-worker-http-contract.md
new file mode 100644
index 0000000..b34f3f1
--- /dev/null
+++ b/docs/verification/2026-08-22-worker-http-contract.md
@@ -0,0 +1,39 @@
+# Worker HTTP contract verification — 2026-08-22
+
+対象PR: #38
+
+## 検証対象
+
+- `Accept`のmedia range、wildcard、quality valueを解釈する。
+- `q=0`を選択対象から除外する。
+- `q`より前のmedia parameterを表現選択条件として照合する。
+- `q`より後のaccept extensionを表現選択条件にしない。
+- UTF-8 charset parameterを生成表現と照合する。
+- 同一media typeでは、matching parameter数が多いrangeをよりspecificとして扱う。
+- APIと非API routeで実装済みmethodだけを`Allow`に返す。
+- helper testがWorkerの通常verifyに実際に含まれる。
+
+## 期待する回帰条件
+
+- `text/html;level=1;q=1, text/markdown;q=0.5`は、`level=1`を持たないHTMLではなくMarkdownを選ぶ。
+- `text/html;charset=UTF-8;q=1, text/markdown;q=0.5`はHTMLを選ぶ。
+- `text/html;q=0.8, text/html;charset=utf-8;q=0.4, text/markdown;q=0.6`は、よりspecificなHTML rangeの`q=0.4`を採用しMarkdownを選ぶ。
+- `text/html;q=0;level=1, text/markdown;q=0.5`では`level=1`をaccept extensionとして扱いMarkdownを選ぶ。
+- HTMLとMarkdownがともに`q=0`、または対応外media typeだけの場合は互換契約としてHTMLへfallbackする。
+- API routeの`Allow`は`GET, HEAD, OPTIONS`、通常page routeは`GET, HEAD`とする。
+
+## Hosted CI evidence
+
+実装・回帰test・ADRを含むcommit `f6b0b18bb4df1a05c9490a92a1436d0dce99f612`をGitHub Actions CI run `32566260740`（run #250）で検証した。
+
+- Cloudflare Worker on Node 22: success。
+- Worker Vitest: `2 passed` test files、`46 passed` tests。内訳は`http.spec.ts` 15件、`worker.spec.ts` 31件。
+- WebMCP Vitest: `1 passed` test file、`3 passed` tests。
+- Worker type generation check、TypeScript typecheck、oxlintが成功。lintは0 warnings / 0 errors。
+- Wrangler dry-run buildが成功。
+- `npm audit --audit-level=high`: 0 vulnerabilities。
+- Python 3.12 / 3.14のUbuntu、Windows、macOS、installed-wheel 3 OS、synthetic determinism 3 OSとcross-OS compareを含むCI全jobが成功。
+- CI failure: 0。
+- Worker test skip: 0。
+
+この後のcommitは上記測定結果を固定する文書変更のみであり、Worker production codeと回帰testは変更しない。最終マージ前に、この文書変更を含むheadでも必須CIを再実行する。
diff --git a/worker/src/http.ts b/worker/src/http.ts
new file mode 100644
index 0000000..836c75e
--- /dev/null
+++ b/worker/src/http.ts
@@ -0,0 +1,124 @@
+export type PageFormat = "html" | "markdown";
+
+interface MediaRange {
+  type: string;
+  subtype: string;
+  parameters: Readonly<Record<string, string>>;
+  quality: number;
+  order: number;
+}
+
+interface Representation {
+  type: string;
+  subtype: string;
+  parameters: Readonly<Record<string, string>>;
+}
+
+const REPRESENTATIONS: Readonly<Record<PageFormat, Representation>> = {
+  html: { type: "text", subtype: "html", parameters: { charset: "utf-8" } },
+  markdown: { type: "text", subtype: "markdown", parameters: { charset: "utf-8" } },
+};
+
+function unquote(raw: string): string {
+  const value = raw.trim();
+  return value.length >= 2 && value.startsWith('"') && value.endsWith('"')
+    ? value.slice(1, -1)
+    : value;
+}
+
+function parseQuality(raw: string): number {
+  const parsed = Number(unquote(raw));
+  return Number.isFinite(parsed) && parsed >= 0 && parsed <= 1 ? parsed : 0;
+}
+
+function parseAccept(value: string): MediaRange[] {
+  const ranges: MediaRange[] = [];
+  for (const [order, rawRange] of value.split(",").entries()) {
+    const [rawMediaType, ...rawParameters] = rawRange.split(";");
+    const [rawType, rawSubtype, ...extra] = (rawMediaType ?? "")
+      .trim()
+      .toLowerCase()
+      .split("/");
+    if (rawType === undefined || rawSubtype === undefined || extra.length > 0) {
+      continue;
+    }
+
+    const parameters: Record<string, string> = {};
+    let quality = 1;
+    let sawQuality = false;
+    for (const rawParameter of rawParameters) {
+      const separator = rawParameter.indexOf("=");
+      const rawName = separator < 0 ? rawParameter : rawParameter.slice(0, separator);
+      const rawValue = separator < 0 ? "" : rawParameter.slice(separator + 1);
+      const name = rawName.trim().toLowerCase();
+      if (name === "") continue;
+      if (!sawQuality && name === "q") {
+        quality = rawValue === "" ? 0 : parseQuality(rawValue);
+        sawQuality = true;
+        continue;
+      }
+      if (!sawQuality) {
+        const parameterValue = unquote(rawValue);
+        parameters[name] = name === "charset" ? parameterValue.toLowerCase() : parameterValue;
+      }
+      // Parameters after q are accept extensions and do not constrain the media range.
+    }
+    ranges.push({ type: rawType, subtype: rawSubtype, parameters, quality, order });
+  }
+  return ranges;
+}
+
+function parametersMatch(range: MediaRange, representation: Representation): boolean {
+  return Object.entries(range.parameters).every(
+    ([name, value]) => representation.parameters[name] === value,
+  );
+}
+
+function matchSpecificity(range: MediaRange, representation: Representation): number {
+  if (!parametersMatch(range, representation)) return -1;
+  const parameterSpecificity = Object.keys(range.parameters).length;
+  if (range.type === representation.type && range.subtype === representation.subtype) {
+    return 2000 + parameterSpecificity;
+  }
+  if (range.type === representation.type && range.subtype === "*") {
+    return 1000 + parameterSpecificity;
+  }
+  if (range.type === "*" && range.subtype === "*") {
+    return parameterSpecificity;
+  }
+  return -1;
+}
+
+function representationQuality(ranges: MediaRange[], format: PageFormat): number {
+  const representation = REPRESENTATIONS[format];
+  let selected: MediaRange | null = null;
+  let selectedSpecificity = -1;
+  for (const range of ranges) {
+    const specificity = matchSpecificity(range, representation);
+    if (specificity < 0) continue;
+    if (
+      selected === null ||
+      specificity > selectedSpecificity ||
+      (specificity === selectedSpecificity && range.quality > selected.quality) ||
+      (specificity === selectedSpecificity &&
+        range.quality === selected.quality &&
+        range.order < selected.order)
+    ) {
+      selected = range;
+      selectedSpecificity = specificity;
+    }
+  }
+  return selected?.quality ?? 0;
+}
+
+export function selectPageFormat(accept: string): PageFormat {
+  if (accept.trim() === "") return "html";
+  const ranges = parseAccept(accept);
+  const htmlQuality = representationQuality(ranges, "html");
+  const markdownQuality = representationQuality(ranges, "markdown");
+  return markdownQuality > htmlQuality ? "markdown" : "html";
+}
+
+export function allowedMethods(api: boolean): string {
+  return api ? "GET, HEAD, OPTIONS" : "GET, HEAD";
+}
diff --git a/worker/src/responses.ts b/worker/src/responses.ts
index cfa60b6..f808e73 100644
--- a/worker/src/responses.ts
+++ b/worker/src/responses.ts
@@ -1,4 +1,5 @@
 import { PublicError } from "./errors";
+import { allowedMethods } from "./http";
 
 const CONTENT_SIGNAL = "search=yes, ai-input=yes, ai-train=no";
 const CSP = [
@@ -80,7 +81,7 @@ export function errorResponse(error: PublicError, r2Reads = 0): Response {
 
 export function methodNotAllowed(api: boolean): Response {
   const headers = new Headers({
-    Allow: "GET, HEAD, OPTIONS",
+    Allow: allowedMethods(api),
     "Content-Type": api ? "application/json; charset=utf-8" : "text/plain; charset=utf-8",
   });
   applyOptions(headers, { api, cache: "none" });
diff --git a/worker/src/routes.ts b/worker/src/routes.ts
index e5902de..8664cf0 100644
--- a/worker/src/routes.ts
+++ b/worker/src/routes.ts
@@ -1,5 +1,6 @@
 import { getDocument, lookupClassification } from "./api";
 import { PublicError, unavailable } from "./errors";
+import { selectPageFormat } from "./http";
 import {
   cleanEdition,
   decodePathSegment,
@@ -73,7 +74,7 @@ function pageObjectRoute(pathname: string, env: Env, accept: string): ResolvedRo
     return null;
   }
   const kind = segments[1];
-  const wantsMarkdown = accept.toLowerCase().includes("text/markdown");
+  const wantsMarkdown = selectPageFormat(accept) === "markdown";
   const suffix = wantsMarkdown ? "md" : "html";
   let objectSuffix: string;
   let chunk: string;
diff --git a/worker/test/http.spec.ts b/worker/test/http.spec.ts
new file mode 100644
index 0000000..46d28a6
--- /dev/null
+++ b/worker/test/http.spec.ts
@@ -0,0 +1,29 @@
+import { describe, expect, it } from "vitest";
+
+import { allowedMethods, selectPageFormat } from "../src/http";
+
+describe("HTTP representation helpers", () => {
+  it.each([
+    ["", "html"],
+    ["*/*", "html"],
+    ["text/*", "html"],
+    ["text/markdown", "markdown"],
+    ["TEXT/MARKDOWN ; Q=1", "markdown"],
+    ["text/html;q=1, text/markdown;q=0", "html"],
+    ["text/html;q=0.5, text/markdown;q=1", "markdown"],
+    ["text/*;q=1, text/markdown;q=0", "html"],
+    ["text/html;level=1;q=1, text/markdown;q=0.5", "markdown"],
+    ["text/html;charset=UTF-8;q=1, text/markdown;q=0.5", "html"],
+    ["text/html;q=0.8, text/html;charset=utf-8;q=0.4, text/markdown;q=0.6", "markdown"],
+    ["text/html;q=0;level=1, text/markdown;q=0.5", "markdown"],
+    ["text/html;q=0, text/markdown;q=0", "html"],
+    ["application/json", "html"],
+  ] as const)("selects %s as %s", (accept, expected) => {
+    expect(selectPageFormat(accept)).toBe(expected);
+  });
+
+  it("reports only methods implemented for each route class", () => {
+    expect(allowedMethods(true)).toBe("GET, HEAD, OPTIONS");
+    expect(allowedMethods(false)).toBe("GET, HEAD");
+  });
+});
diff --git a/worker/test/worker.spec.ts b/worker/test/worker.spec.ts
index 2dae1ec..c2c266b 100644
--- a/worker/test/worker.spec.ts
+++ b/worker/test/worker.spec.ts
@@ -225,12 +225,20 @@ describe("static and negotiated routes", () => {
     const markdown = await request("/ja/classification/G06F3", {
       headers: { Accept: "text/markdown" },
     });
+    const rejectedMarkdown = await request("/ja/classification/G06F3", {
+      headers: { Accept: "text/html;q=1, text/markdown;q=0" },
+    });
+    const preferredMarkdown = await request("/ja/classification/G06F3", {
+      headers: { Accept: "text/html;q=0.5, text/markdown;q=1" },
+    });
 
     expect(html.status).toBe(200);
     expect(html.headers.get("Content-Type")).toContain("text/html");
     expect(html.headers.get("Vary")).toContain("Accept");
     expect(await html.text()).toContain("FI interaction 公式定義");
     expect(markdown.headers.get("Content-Type")).toContain("text/markdown");
+    expect(rejectedMarkdown.headers.get("Content-Type")).toContain("text/html");
+    expect(preferredMarkdown.headers.get("Content-Type")).toContain("text/markdown");
     const markdownBody = await markdown.text();
     expect(markdownBody).toContain("# FI G06F3/048");
     expect(markdownBody).toContain("合成テストデータを変換して作成しています");
@@ -291,11 +299,14 @@ describe("HTTP policy", () => {
   it("handles preflight, methods, CORS, and security headers", async () => {
     const options = await request("/api/v1/lookup", { method: "OPTIONS" });
     const post = await request("/api/v1/lookup", { method: "POST" });
+    const staticOptions = await request("/ja/classification/G06F3", { method: "OPTIONS" });
 
     expect(options.status).toBe(204);
     expect(options.headers.get("Access-Control-Allow-Origin")).toBe("*");
     expect(post.status).toBe(405);
-    expect(post.headers.get("Allow")).toContain("GET");
+    expect(post.headers.get("Allow")).toBe("GET, HEAD, OPTIONS");
+    expect(staticOptions.status).toBe(405);
+    expect(staticOptions.headers.get("Allow")).toBe("GET, HEAD");
     expect(post.headers.get("Content-Security-Policy")).toContain("default-src 'none'");
     expect(post.headers.get("Strict-Transport-Security")).toContain("max-age=31536000");
     expect(post.headers.get("X-Content-Type-Options")).toBe("nosniff");
diff --git a/worker/vitest.config.ts b/worker/vitest.config.ts
index 60b4b67..af3ff19 100644
--- a/worker/vitest.config.ts
+++ b/worker/vitest.config.ts
@@ -15,6 +15,6 @@ export default defineConfig({
     }),
   ],
   test: {
-    include: ["test/worker.spec.ts"],
+    include: ["test/worker.spec.ts", "test/http.spec.ts"],
   },
 });