fix(applicability): match all proxy Anthropic message routes

shouldTransformAnthropicMessages() gated the path with
!path.endsWith('/v1/messages'), but createProxy() routes three exact paths:
/v1/messages, /anthropic/v1/messages, and /anthropic/messages. So an SDK/host
using the public helper to pre-gate got unsupported_path on /anthropic/messages
— a request the proxy actually transforms — while endsWith would also have
wrongly accepted an unrelated /foo/v1/messages.

- lift isAnthropicMessagesPath() into applicability.ts (proxy.ts already imports
  from it; no cycle) as the single source of truth
- proxy.ts imports it instead of its own copy
- shouldTransformAnthropicMessages() uses it, so helper and router agree
- public-api tests cover /anthropic/v1/messages and /anthropic/messages
  (eligible) plus /v1/messages/count_tokens (still unsupported_path)

src/core/applicability.ts

@@ -90,13 +90,25 @@ export function isPxpipeSupportedGptModel(model: string | null | undefined): boo
   return isAllowed(model);
 }
 
+/** Canonical set of Anthropic Messages routes pxpipe transforms. Shared with
+ *  createProxy (src/core/proxy.ts) so the public applicability helper and the
+ *  proxy router can never disagree on which paths are eligible — they did: the
+ *  proxy accepts /anthropic/messages, but the helper's old `endsWith` check
+ *  rejected it (and would have wrongly accepted /foo/v1/messages). Exact matches
+ *  only, so /v1/messages/count_tokens stays unsupported. */
+export function isAnthropicMessagesPath(pathname: string): boolean {
+  return pathname === '/v1/messages'
+    || pathname === '/anthropic/v1/messages'
+    || pathname === '/anthropic/messages';
+}
+
 export function shouldTransformAnthropicMessages(
   input: PxpipeApplicabilityInput,
 ): { eligible: boolean; reason: PxpipeApplicabilityReason } {
   if (input.method !== undefined && input.method !== null && input.method.toUpperCase() !== 'POST') {
     return { eligible: false, reason: 'unsupported_method' };
   }
-  if (input.path !== undefined && input.path !== null && !input.path.endsWith('/v1/messages')) {
+  if (input.path !== undefined && input.path !== null && !isAnthropicMessagesPath(input.path)) {
     return { eligible: false, reason: 'unsupported_path' };
   }
   if (input.bodyBytes !== undefined && input.bodyBytes !== null && input.bodyBytes <= 0) {

src/core/proxy.ts

@@ -5,7 +5,7 @@
 
 import { transformRequest, type TransformOptions, type TransformInfo } from './transform.js';
 import { transformOpenAIChatCompletions, transformOpenAIResponses } from './openai.js';
-import { isPxpipeSupportedGptModel, isPxpipeSupportedModel } from './applicability.js';
+import { isAnthropicMessagesPath, isPxpipeSupportedGptModel, isPxpipeSupportedModel } from './applicability.js';
 import {
   buildBaselineCountTokensBody,
   buildCacheablePrefixCountTokensBody,
@@ -508,12 +508,6 @@ function isProviderPrefixedPath(pathname: string): boolean {
   return PASSTHROUGH_PREFIXES.some((prefix) => pathname.startsWith(prefix));
 }
 
-function isAnthropicMessagesPath(pathname: string): boolean {
-  return pathname === '/v1/messages'
-    || pathname === '/anthropic/v1/messages'
-    || pathname === '/anthropic/messages';
-}
-
 function isOpenAIChatPath(pathname: string): boolean {
   return pathname === '/v1/chat/completions' || pathname === '/openai/v1/chat/completions';
 }

tests/public-api.test.ts

@@ -143,6 +143,16 @@ describe('public library API', () => {
       path: '/v1/messages',
       bodyBytes: 10,
     }).reason).toBe('unsupported_method');
+    // Provider-prefixed routes createProxy() also transforms must be eligible
+    // here too — the old endsWith('/v1/messages') check rejected /anthropic/messages.
+    for (const path of ['/anthropic/v1/messages', '/anthropic/messages']) {
+      expect(shouldTransformAnthropicMessages({
+        model: 'claude-fable-5',
+        method: 'POST',
+        path,
+        bodyBytes: 10,
+      })).toEqual({ eligible: true, reason: 'eligible' });
+    }
     expect(shouldTransformAnthropicMessages({
       model: 'claude-fable-5',
       method: 'POST',