diff --git a/.changeset/allow-header-405.md b/.changeset/allow-header-405.md
new file mode 100644
index 00000000..bd0c54af
--- /dev/null
+++ b/.changeset/allow-header-405.md
@@ -0,0 +1,5 @@
+---
+"@chatcops/server": patch
+---
+
+405 responses now include `Allow: POST, OPTIONS`.
diff --git a/packages/server/src/adapters/express.ts b/packages/server/src/adapters/express.ts
index 95814e3a..9622e277 100644
--- a/packages/server/src/adapters/express.ts
+++ b/packages/server/src/adapters/express.ts
@@ -42,6 +42,7 @@ export function chatcopsMiddleware(config: ChatCopsServerConfig) {
     }
 
     if (req.method !== 'POST') {
+      res.setHeader('Allow', 'POST, OPTIONS');
       sendError(405, 'Method not allowed');
       return;
     }
diff --git a/packages/server/src/http.ts b/packages/server/src/http.ts
index 84315b53..bfc4374d 100644
--- a/packages/server/src/http.ts
+++ b/packages/server/src/http.ts
@@ -257,7 +257,7 @@ export async function fetchChatResponse(
   if (request.method === 'OPTIONS') {
     return new Response(null, { status: 204, headers: origins.preflightHeaders(origin) });
   }
-  if (request.method !== 'POST') return json(405, { error: 'Method not allowed' });
+  if (request.method !== 'POST') return json(405, { error: 'Method not allowed' }, { Allow: 'POST, OPTIONS' });
   if (!origins.allows(origin)) return json(403, { error: 'origin_not_allowed' });
 
   if (isSessionPath(new URL(request.url).pathname)) {
diff --git a/packages/server/tests/adapters/cloudflare.test.ts b/packages/server/tests/adapters/cloudflare.test.ts
index 94749412..ba7abe43 100644
--- a/packages/server/tests/adapters/cloudflare.test.ts
+++ b/packages/server/tests/adapters/cloudflare.test.ts
@@ -25,6 +25,7 @@ describe('chatcopsCloudflareHandler', () => {
     const res = await handler(req);
     expect(res.status).toBe(405);
     expect(res.headers.get('Access-Control-Allow-Origin')).toBe('*');
+    expect(res.headers.get('Allow')).toBe('POST, OPTIONS');
     const body = await res.json();
     expect(body.error).toBe('Method not allowed');
   });
diff --git a/packages/server/tests/adapters/express.test.ts b/packages/server/tests/adapters/express.test.ts
index 4c08160a..e5f46dc9 100644
--- a/packages/server/tests/adapters/express.test.ts
+++ b/packages/server/tests/adapters/express.test.ts
@@ -254,6 +254,7 @@ describe('express analytics sharing', () => {
 
     expect(fake.statusCode).toBe(405);
     expect(fake.headers['Access-Control-Allow-Origin']).toBe('*');
+    expect(fake.headers.Allow).toBe('POST, OPTIONS');
     expect(fake.body).toEqual({ error: 'Method not allowed' });
     expect(fake.chunks).toEqual([]);
     expect(fake.ended).toBe(false);
diff --git a/packages/server/tests/adapters/vercel.test.ts b/packages/server/tests/adapters/vercel.test.ts
index 6d996e96..d2521206 100644
--- a/packages/server/tests/adapters/vercel.test.ts
+++ b/packages/server/tests/adapters/vercel.test.ts
@@ -25,6 +25,7 @@ describe('chatcopsVercelHandler', () => {
     const res = await handler(req);
     expect(res.status).toBe(405);
     expect(res.headers.get('Access-Control-Allow-Origin')).toBe('*');
+    expect(res.headers.get('Allow')).toBe('POST, OPTIONS');
     const body = await res.json();
     expect(body.error).toBe('Method not allowed');
   });
diff --git a/website/src/content/docs/server/protocol.mdx b/website/src/content/docs/server/protocol.mdx
index 52e13f91..b85035df 100644
--- a/website/src/content/docs/server/protocol.mdx
+++ b/website/src/content/docs/server/protocol.mdx
@@ -345,7 +345,7 @@ Access-Control-Expose-Headers: Retry-After, X-ChatCops-Protocol
 | `401` | `conversation_mismatch` | The session token is valid but was issued for a different conversation. | Obtain a token for the current conversation from `<chat url>/session`. |
 | `401` | `turnstile_failed` | `/session` endpoint only: Cloudflare Turnstile verification failed or was missing. | Returned when bot protection challenge fails. |
 | `403` | `origin_not_allowed` | The `Origin` header is not permitted by `allowedOrigins`. | Answered before the request body is parsed. Returns legacy format `{"error":"origin_not_allowed"}`. |
-| `405` | `Method not allowed` | An HTTP method other than `POST` or `OPTIONS` was used on the chat endpoint. | Answered before the request body is parsed. Returns legacy format `{"error":"Method not allowed"}`. |
+| `405` | `Method not allowed` | An HTTP method other than `POST` or `OPTIONS` was used on the chat endpoint. | Answered before the request body is parsed, with `Allow: POST, OPTIONS`. Returns legacy format `{"error":"Method not allowed"}`. |
 | `409` | `conversation_busy` | Another reply is currently being generated for this `conversationId`. | The server locks conversation turns to prevent state races. |
 | `413` | `payload_too_large` | The request body exceeds the maximum permitted size (default 256 KB). | Prevents excessive memory allocation. |
 | `429` | `rate_limited` | The client IP has exceeded the configured request rate limit. | Includes `retryAfterMs` in body and `Retry-After` in HTTP headers. |