fix(auth): tag unparseable introspection bodies

An introspection body that failed to parse, or that parsed to null,
threw past every tagged error in introspectToken. It reached the client
as a 401 with WWW-Authenticate and the raw parser message in
error_description, so an upstream fault pushed a working session into
reauthorization and no reason was logged.

Parse defensively and treat a missing boolean `active` as
introspect_malformed_body, matching the guard this file already uses on
other upstream JSON reads. Also scope the telemetry doc comment: it
covers rejections raised while authenticating, not the outbound client
tags raised during tool execution.

Author: Max Loffgren

src/index.ts

@@ -473,8 +473,14 @@ async function introspectToken(
       status: response.status,
     });
   }
-  const data = (await response.json()) as OAuthIntrospectionResponse;
-  if (typeof data.active !== 'boolean') {
+  // A body that fails to parse, or that parses to something without a boolean
+  // `active`, is an unusable answer rather than a verdict on the credential.
+  // Reading `active` off `null` would throw past every tagged error here and
+  // reach the client as an OAuth challenge carrying raw parser text.
+  const data = (await response.json().catch(() => null)) as
+    | OAuthIntrospectionResponse
+    | null;
+  if (!data || typeof data.active !== 'boolean') {
     throw new CredentialValidationUnavailableError({
       elapsedMs: elapsedMs(),
       reason: 'introspect_malformed_body',
@@ -748,6 +754,10 @@ function emitLegacyKeyPathTelemetry(
  * introspection outage, a missing secret, and a credential that introspected
  * cleanly but cannot be used here are indistinguishable after the fact.
  *
+ * Scope is rejections raised while authenticating a request. The outbound client
+ * tags are raised during tool execution and surface as tool errors, so they do
+ * not reach this hook.
+ *
  * Intentionally low cardinality. Never add the token, the resolved API key, the
  * upstream response body, request URLs, user agents, or hashes of any of them.
  */

tests/mcp-smoke.test.mjs

@@ -2037,6 +2037,19 @@ test('each credential validation failure logs its own reason and status', async
       reason: 'introspect_malformed_body',
       respond: () => ({ body: JSON.stringify({ active: 'yes' }), status: 200 }),
     },
+    {
+      // A JSON null would throw on the `active` read and escape as an OAuth
+      // challenge carrying the raw parser message.
+      expectedStatus: 200,
+      reason: 'introspect_malformed_body',
+      respond: () => ({ body: 'null', status: 200 }),
+    },
+    {
+      // Truncated body under a JSON content type: same escape route.
+      expectedStatus: 200,
+      reason: 'introspect_malformed_body',
+      respond: () => ({ body: '{"active":true', status: 200 }),
+    },
     {
       // Introspection answered cleanly and the credential it described cannot
       // be used on this resource. Tagged apart from an outage.