fix: surface list-fetch failures instead of swallowing them

A list load that failed — a transport error, or a result the SDK's era
codec rejects as invalid — reported nothing. The connect-time refresh is
fired with no caller to await it (`void this.refresh()`), so its rejection
became an unhandled rejection: the header said "Connected", the panel
rendered an empty list indistinguishable from a server that has none, and
the failed exchange showed in the Protocol panel as a clean success.

Two surfaces now tell the truth:

- ManagedListState records the failure as observable state (`getError` +
  an `errorChange` event) and still re-throws, so App's auth-recovery
  wrapper keeps working. The four `useManaged*` hooks expose it via a
  shared `useManagedListError`, and the Tools/Prompts/Resources sidebars
  render a `ListLoadError` alert with the reason and a Retry.
- A response the client refused is attributed back to its Protocol entry
  (`markResponseRejected` → `responseRejected` → `MessageEntry.clientError`),
  which now renders an Error status and a "Rejected by the Inspector"
  alert rather than the green resultType badge alone. The SDK gives no
  request id with a decode failure, so the id is recovered by correlation;
  see the comment on `markResponseRejected` for why that is exact.

Also stops `listAllTools` from dropping a failing excluded-tools walk on
the floor — it stays non-fatal but is logged.

Closes #1953

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SW1p8E2uiyyLx4RKwrwSrt

clients/web/src/test/core/mcp/state/managedPromptsState.test.ts

@@ -285,6 +285,22 @@ describe("ManagedPromptsState", () => {
     });
   });
 
+  // The base class owns the error plumbing (covered in managedToolsState); this
+  // pins THIS list's method string, which is what attributes a failure to the
+  // right Protocol entry (#1953).
+  it("records a failed load and attributes it to prompts/list", async () => {
+    const boom = new Error("nope");
+    client.setStatus("connected");
+    client.listAllPrompts.mockRejectedValueOnce(boom);
+
+    await expect(state.refresh()).rejects.toThrow(boom);
+    expect(state.getError()).toBe(boom);
+    expect(client.markResponseRejected).toHaveBeenCalledWith(
+      "prompts/list",
+      "nope",
+    );
+  });
+
   it("destroy is idempotent", () => {
     state.destroy();
     expect(() => state.destroy()).not.toThrow();

clients/web/src/test/core/mcp/state/managedResourceTemplatesState.test.ts

@@ -266,6 +266,22 @@ describe("ManagedResourceTemplatesState", () => {
     expect(state.getResourceTemplates()).toEqual([]);
   });
 
+  // The base class owns the error plumbing (covered in managedToolsState); this
+  // pins THIS list's method string, which is what attributes a failure to the
+  // right Protocol entry (#1953).
+  it("records a failed load and attributes it to resources/templates/list", async () => {
+    const boom = new Error("nope");
+    client.setStatus("connected");
+    client.listAllResourceTemplates.mockRejectedValueOnce(boom);
+
+    await expect(state.refresh()).rejects.toThrow(boom);
+    expect(state.getError()).toBe(boom);
+    expect(client.markResponseRejected).toHaveBeenCalledWith(
+      "resources/templates/list",
+      "nope",
+    );
+  });
+
   it("destroy is idempotent", () => {
     state.destroy();
     expect(() => state.destroy()).not.toThrow();

clients/web/src/test/core/mcp/state/managedResourcesState.test.ts

@@ -292,6 +292,22 @@ describe("ManagedResourcesState", () => {
     });
   });
 
+  // The base class owns the error plumbing (covered in managedToolsState); this
+  // pins THIS list's method string, which is what attributes a failure to the
+  // right Protocol entry (#1953).
+  it("records a failed load and attributes it to resources/list", async () => {
+    const boom = new Error("nope");
+    client.setStatus("connected");
+    client.listAllResources.mockRejectedValueOnce(boom);
+
+    await expect(state.refresh()).rejects.toThrow(boom);
+    expect(state.getError()).toBe(boom);
+    expect(client.markResponseRejected).toHaveBeenCalledWith(
+      "resources/list",
+      "nope",
+    );
+  });
+
   it("destroy is idempotent", () => {
     state.destroy();
     expect(() => state.destroy()).not.toThrow();

clients/web/src/test/core/mcp/state/managedToolsState.test.ts

@@ -412,6 +412,110 @@ describe("ManagedToolsState", () => {
     });
   });
 
+  // A failed load used to vanish: the connect-time refresh is fired with no
+  // caller to await it, so its rejection became an unhandled rejection and the
+  // panel just rendered an empty list — indistinguishable from a server with no
+  // tools (#1953).
+  describe("load errors", () => {
+    const boom = new Error("Invalid result for tools/list: ttlMs required");
+
+    function waitForError(state: ManagedToolsState): Promise<Error | null> {
+      return waitForChangeEvent(state, "errorChange");
+    }
+
+    it("starts with no error", () => {
+      expect(state.getError()).toBeNull();
+    });
+
+    it("records a failed refresh as state AND re-throws", async () => {
+      client.setStatus("connected");
+      client.listAllTools.mockRejectedValueOnce(boom);
+
+      // The rejection still propagates: App's auth-recovery wrapper keys off it
+      // to detect a 401 and start a re-authorization.
+      await expect(state.refresh()).rejects.toThrow(boom);
+      expect(state.getError()).toBe(boom);
+    });
+
+    it("dispatches errorChange with the error", async () => {
+      client.setStatus("connected");
+      client.listAllTools.mockRejectedValueOnce(boom);
+      const changed = waitForError(state);
+      await expect(state.refresh()).rejects.toThrow(boom);
+      expect(await changed).toBe(boom);
+    });
+
+    it("wraps a non-Error rejection", async () => {
+      client.setStatus("connected");
+      client.listAllTools.mockRejectedValueOnce("just a string");
+      await expect(state.refresh()).rejects.toBe("just a string");
+      expect(state.getError()).toBeInstanceOf(Error);
+      expect(state.getError()?.message).toBe("just a string");
+    });
+
+    it("attributes the failure to its Protocol entry", async () => {
+      client.setStatus("connected");
+      client.listAllTools.mockRejectedValueOnce(boom);
+      await expect(state.refresh()).rejects.toThrow(boom);
+      expect(client.markResponseRejected).toHaveBeenCalledWith(
+        "tools/list",
+        boom.message,
+      );
+    });
+
+    it("clears the error once a refresh succeeds", async () => {
+      client.setStatus("connected");
+      client.listAllTools.mockRejectedValueOnce(boom);
+      await expect(state.refresh()).rejects.toThrow(boom);
+
+      const cleared = waitForError(state);
+      client.queueToolPages({ tools: [tool("a")] });
+      await state.refresh();
+      expect(await cleared).toBeNull();
+      expect(state.getError()).toBeNull();
+    });
+
+    it("keeps the connect-time failure in state instead of rejecting unobserved", async () => {
+      client.listAllTools.mockRejectedValueOnce(boom);
+      const changed = waitForError(state);
+      await client.connect();
+      expect(await changed).toBe(boom);
+    });
+
+    it("keeps the auto-refresh failure in state instead of rejecting unobserved", async () => {
+      client.setServerSettings(AUTO_REFRESH_SETTINGS);
+      client.setStatus("connected");
+      client.listAllTools.mockRejectedValueOnce(boom);
+      const changed = waitForError(state);
+      client.dispatchTypedEvent("toolsListChanged");
+      expect(await changed).toBe(boom);
+    });
+
+    it("clears the error on disconnect so it can't outlive its session", async () => {
+      client.setStatus("connected");
+      client.listAllTools.mockRejectedValueOnce(boom);
+      await expect(state.refresh()).rejects.toThrow(boom);
+
+      const cleared = waitForError(state);
+      client.setStatus("disconnected");
+      expect(await cleared).toBeNull();
+      expect(state.getError()).toBeNull();
+    });
+
+    it("does not re-dispatch when the same error is recorded twice", async () => {
+      client.setStatus("connected");
+      client.listAllTools.mockRejectedValue(boom);
+      await expect(state.refresh()).rejects.toThrow(boom);
+
+      let fired = 0;
+      state.addEventListener("errorChange", () => {
+        fired++;
+      });
+      await expect(state.refresh()).rejects.toThrow(boom);
+      expect(fired).toBe(0);
+    });
+  });
+
   it("destroy is idempotent", () => {
     state.destroy();
     expect(() => state.destroy()).not.toThrow();

clients/web/src/test/core/mcp/state/messageLogState.test.ts

@@ -234,4 +234,69 @@ describe("MessageLogState", () => {
     state.destroy();
     expect(() => state.destroy()).not.toThrow();
   });
+
+  // A response the SERVER answered successfully but the CLIENT then refused
+  // (the SDK codec rejecting the result). The frame is a valid JSON-RPC
+  // result, so without the annotation the entry renders as a clean success
+  // (#1953).
+  describe("responseRejected", () => {
+    const REASON = "Invalid result for tools/list: ttlMs required";
+
+    it("annotates the request entry the response was folded into", () => {
+      client.dispatchTypedEvent("message", requestEntry(1));
+      client.dispatchTypedEvent("message", responseEntry(1));
+
+      const seen: MessageEntry[] = [];
+      state.addEventListener("message", (e) => seen.push(e.detail));
+      client.dispatchTypedEvent("responseRejected", { id: 1, reason: REASON });
+
+      expect(state.getMessages()[0]?.clientError).toBe(REASON);
+      expect(seen).toHaveLength(1);
+    });
+
+    it("annotates a standalone response entry with no matching request", () => {
+      client.dispatchTypedEvent("message", responseEntry(7));
+      client.dispatchTypedEvent("responseRejected", { id: 7, reason: REASON });
+
+      expect(state.getMessages()[0]?.clientError).toBe(REASON);
+    });
+
+    it("skips a request still awaiting its response", () => {
+      // Same JSON-RPC id reused after the first exchange completed: the
+      // pending one isn't what was rejected.
+      client.dispatchTypedEvent("message", requestEntry(1));
+      client.dispatchTypedEvent("message", responseEntry(1));
+      client.dispatchTypedEvent("message", requestEntry(1));
+
+      client.dispatchTypedEvent("responseRejected", { id: 1, reason: REASON });
+
+      const [completed, pending] = state.getMessages();
+      expect(pending?.clientError).toBeUndefined();
+      expect(completed?.clientError).toBe(REASON);
+    });
+
+    it("ignores an id that matches no entry", () => {
+      client.dispatchTypedEvent("message", requestEntry(1));
+      client.dispatchTypedEvent("message", responseEntry(1));
+
+      let fired = 0;
+      state.addEventListener("messagesChange", () => {
+        fired++;
+      });
+      client.dispatchTypedEvent("responseRejected", { id: 99, reason: REASON });
+
+      expect(fired).toBe(0);
+      expect(state.getMessages()[0]?.clientError).toBeUndefined();
+    });
+
+    it("never annotates a notification", () => {
+      client.dispatchTypedEvent("message", notificationEntry());
+      client.dispatchTypedEvent("responseRejected", {
+        id: 1,
+        reason: REASON,
+      });
+
+      expect(state.getMessages()[0]?.clientError).toBeUndefined();
+    });
+  });
 });

clients/web/src/test/core/react/useManagedListError.test.tsx

@@ -0,0 +1,87 @@
+import { describe, it, expect, beforeEach } from "vitest";
+import { act, renderHook } from "@testing-library/react";
+import { FakeInspectorClient } from "@inspector/core/mcp/__tests__/fakeInspectorClient";
+import { ManagedToolsState } from "@inspector/core/mcp/state/managedToolsState";
+import { useManagedListError } from "@inspector/core/react/useManagedListError";
+
+// The shared subscription behind the four `useManaged*` hooks' `error` field
+// (#1953). Exercised through ManagedToolsState — any managed list would do,
+// since the error lives entirely in the shared base.
+describe("useManagedListError", () => {
+  let client: FakeInspectorClient;
+  let state: ManagedToolsState;
+  const boom = new Error("Invalid result for tools/list: ttlMs required");
+
+  beforeEach(() => {
+    client = new FakeInspectorClient({
+      status: "connected",
+      capabilities: { tools: {} },
+    });
+    state = new ManagedToolsState(client, 0);
+  });
+
+  it("returns null when there is no state", () => {
+    const { result } = renderHook(() => useManagedListError(null));
+    expect(result.current).toBeNull();
+  });
+
+  it("returns null before any load fails", () => {
+    const { result } = renderHook(() => useManagedListError(state));
+    expect(result.current).toBeNull();
+  });
+
+  it("seeds from an error the state already holds", async () => {
+    client.listAllTools.mockRejectedValueOnce(boom);
+    await expect(state.refresh()).rejects.toThrow(boom);
+
+    const { result } = renderHook(() => useManagedListError(state));
+    expect(result.current).toBe(boom);
+  });
+
+  it("updates when the state dispatches errorChange", async () => {
+    const { result } = renderHook(() => useManagedListError(state));
+
+    client.listAllTools.mockRejectedValueOnce(boom);
+    await act(async () => {
+      await expect(state.refresh()).rejects.toThrow(boom);
+    });
+    expect(result.current).toBe(boom);
+  });
+
+  it("clears when a later load succeeds", async () => {
+    client.listAllTools.mockRejectedValueOnce(boom);
+    await expect(state.refresh()).rejects.toThrow(boom);
+    const { result } = renderHook(() => useManagedListError(state));
+    expect(result.current).toBe(boom);
+
+    await act(async () => {
+      await state.refresh();
+    });
+    expect(result.current).toBeNull();
+  });
+
+  it("resets to null when the state becomes null", async () => {
+    client.listAllTools.mockRejectedValueOnce(boom);
+    await expect(state.refresh()).rejects.toThrow(boom);
+
+    const { result, rerender } = renderHook(
+      ({ s }: { s: ManagedToolsState | null }) => useManagedListError(s),
+      { initialProps: { s: state as ManagedToolsState | null } },
+    );
+    expect(result.current).toBe(boom);
+
+    rerender({ s: null });
+    expect(result.current).toBeNull();
+  });
+
+  it("unsubscribes on unmount", async () => {
+    const { unmount } = renderHook(() => useManagedListError(state));
+    unmount();
+
+    client.listAllTools.mockRejectedValueOnce(boom);
+    // No act() wrapper: a listener still attached would warn about an update
+    // outside act, and the assertion below would be the only other signal.
+    await expect(state.refresh()).rejects.toThrow(boom);
+    expect(state.getError()).toBe(boom);
+  });
+});