diff --git a/core/mcp/inspectorClient.ts b/core/mcp/inspectorClient.ts
index a51b0e2e..ecec24ef 100644
--- a/core/mcp/inspectorClient.ts
+++ b/core/mcp/inspectorClient.ts
@@ -1091,12 +1091,12 @@ export class InspectorClient extends InspectorClientEventTarget {
    * The `progressToken` a caller stamped on its request metadata, when it is
    * one the SDK will accept.
    *
-   * `_meta` values are arbitrary JSON (#1910), but `progressToken` is a
-   * reserved key the spec types as `string | number`. Anything else — an
-   * object, an array, `null`, a boolean — is not a token the SDK can correlate
-   * a `notifications/progress` back to, so it is read as "no token" rather than
-   * forwarded. The key still reaches the wire inside `_meta`; this only governs
-   * whether the client wires up a progress callback for it.
+   * This governs only **callback correlation** — whether the client wires up an
+   * `onprogress` for the request. Keeping an invalid value off the **wire** is
+   * a separate concern handled in {@link mergeMeta}, which deletes the member
+   * from the outgoing `_meta` so a conforming server is never sent something
+   * `ProgressTokenSchema` rejects. The two use the same {@link isProgressToken}
+   * predicate, so they cannot disagree about what counts as valid.
    */
   private progressTokenOf(
     metadata: RequestMetadata | undefined,
diff --git a/test-servers/src/test-server-http.ts b/test-servers/src/test-server-http.ts
index bbbb47ce..b8a5007f 100644
--- a/test-servers/src/test-server-http.ts
+++ b/test-servers/src/test-server-http.ts
@@ -83,7 +83,18 @@ export interface RecordedRequest {
   method: string;
   params?: Record<string, unknown>;
   headers?: Record<string, string>;
-  metadata?: Record<string, string>;
+  /**
+   * The request's `_meta`, recorded verbatim. Values are arbitrary JSON —
+   * objects, arrays, numbers, booleans, `null` — not just strings (#1910), so
+   * a test asserting on a structured value gets a contract that matches what
+   * the recorder actually holds.
+   *
+   * Typed as `unknown` values rather than `@inspector/core`'s
+   * `RequestMetadata`: this package compiles standalone (`tsc -p test-servers`,
+   * NodeNext, no `paths`), so it has no `@inspector/core` alias to import
+   * through.
+   */
+  metadata?: Record<string, unknown>;
   response: unknown;
   timestamp: number;
 }