fix(everything): scope session resources to their server (#4808)

.changeset/everything-session-resources-per-server.md

@@ -0,0 +1,5 @@
+---
+"@modelcontextprotocol/server-everything": patch
+---
+
+Session resources are now tracked per server, so two sessions that create a session resource with the same name (for example via `gzip-file-as-resource`) no longer evict each other's resource (#4808).

.claude/skills/testing/SKILL.md

@@ -238,7 +238,7 @@ above the test (or above its `describe`/class when the whole block pins the
 same bug):
 
 ```ts
-// KNOWN BUG #4808: pins current (wrong) behavior; the fix changes this assertion.
+// KNOWN BUG #<N>: pins current (wrong) behavior; the fix changes this assertion.
 ```
 
 In Python the same text follows `#`. A bug with no issue yet reads

src/everything/__tests__/gzip-file-as-resource.test.ts

@@ -3,9 +3,9 @@
  * (#4854), with its default limits: fetching a `data:` or local `http:` URL,
  * returning the gzipped file as a resource link or an embedded resource,
  * registering it for later reads, and the errors for bad protocols, oversized
- * or empty responses and failed fetches. It also pins #4808: two sessions that
- * use the same file name evict each other's resource. The configurable limits
- * are in `gzip-limits.test.ts`.
+ * or empty responses and failed fetches. It also guards #4808: two sessions
+ * that use the same file name each keep their own resource. The configurable
+ * limits are in `gzip-limits.test.ts`.
  */
 import { createServer as createHttpServer, type Server } from "node:http";
 import { once } from "node:events";
@@ -150,24 +150,21 @@ describe("gzip-file-as-resource", () => {
     expect(gunzipSync(blob).toString()).toBe("second");
   });
 
-  // KNOWN BUG #4808: pins current (wrong) behavior; the fix changes this assertion.
-  it("evicts another session's resource of the same name (#4808)", async () => {
-    // Characterization of #4808: session resources are tracked in one
-    // module-level map keyed by URI, so the second session's registration
-    // removes the first session's resource from the first session's server.
+  it("keeps each session's resource when two sessions use the same name (#4808)", async () => {
+    // Regression guard for #4808: session resources are tracked per server,
+    // so the second session's registration leaves the first session's
+    // resource (and its content) in place.
     const a = await open();
     const b = await open();
+    const second = `data:text/plain;base64,${Buffer.from("from b").toString("base64")}`;
     await gzip(a, { name: "shared.gz", data: DATA_URI });
-    await gzip(b, { name: "shared.gz", data: DATA_URI });
+    await gzip(b, { name: "shared.gz", data: second });
 
-    await expect(
-      a.client.readResource({ uri: "demo://resource/session/shared.gz" }),
-    ).rejects.toThrow("Resource demo://resource/session/shared.gz not found");
-    expect(
-      gunzipSync(
-        await readBlob(b, "demo://resource/session/shared.gz"),
-      ).toString(),
-    ).toBe(TEXT);
+    const uri = "demo://resource/session/shared.gz";
+    expect(gunzipSync(await readBlob(a, uri)).toString()).toBe(TEXT);
+    expect(gunzipSync(await readBlob(b, uri)).toString()).toBe("from b");
+    const { resources } = await a.client.listResources();
+    expect(resources.map((r) => r.uri)).toContain(uri);
   });
 
   it("rejects a URL that is not http, https or data", async () => {

src/everything/resources/session.ts

@@ -5,11 +5,15 @@ import {
 import { Resource, ResourceLink } from "@modelcontextprotocol/sdk/types.js";
 
 /**
- * Tracks registered session resources by URI to allow updating/removing on re-registration.
- * This prevents "Resource already registered" errors when a tool creates a resource
- * with the same URI multiple times during a session.
+ * Tracks registered session resources per server, by URI, to allow updating/removing on
+ * re-registration. This prevents "Resource already registered" errors when a tool creates
+ * a resource with the same URI multiple times during a session, without touching another
+ * session's server that registered the same URI (#4808).
  */
-const registeredResources = new Map<string, RegisteredResource>();
+const registeredResources = new WeakMap<
+  McpServer,
+  Map<string, RegisteredResource>
+>();
 
 /**
  * Generates a session-scoped resource URI string based on the provided resource name.
@@ -57,11 +61,16 @@ export const registerSessionResource = (
           blob: payload,
         };
 
-  // Check if a resource with this URI is already registered and remove it
-  const existingResource = registeredResources.get(uri);
+  const serverResources =
+    registeredResources.get(server) ?? new Map<string, RegisteredResource>();
+  registeredResources.set(server, serverResources);
+
+  // Check if a resource with this URI is already registered on this server and remove it
+  const resourceKey = uri.toString();
+  const existingResource = serverResources.get(resourceKey);
   if (existingResource) {
     existingResource.remove();
-    registeredResources.delete(uri);
+    serverResources.delete(resourceKey);
   }
 
   // Register file resource
@@ -77,7 +86,7 @@ export const registerSessionResource = (
   );
 
   // Track the registered resource for potential future removal
-  registeredResources.set(uri, registeredResource);
+  serverResources.set(resourceKey, registeredResource);
 
   return { type: "resource_link", ...resource };
 };