diff --git a/playground/knot/src/Editor/Chrome.tsx b/playground/knot/src/Editor/Chrome.tsx
index 6f4420c05c..2884762d30 100644
--- a/playground/knot/src/Editor/Chrome.tsx
+++ b/playground/knot/src/Editor/Chrome.tsx
@@ -13,7 +13,7 @@ import {
   Theme,
   VerticalResizeHandle,
 } from "shared";
-import type { Diagnostic, Workspace } from "red_knot_wasm";
+import type { Workspace } from "red_knot_wasm";
 import { Panel, PanelGroup } from "react-resizable-panels";
 import { Files, isPythonFile } from "./Files";
 import SecondarySideBar from "./SecondarySideBar";
@@ -21,7 +21,7 @@ import SecondaryPanel, {
   SecondaryPanelResult,
   SecondaryTool,
 } from "./SecondaryPanel";
-import Diagnostics from "./Diagnostics";
+import Diagnostics, { Diagnostic } from "./Diagnostics";
 import { FileId, ReadonlyFiles } from "../Playground";
 import type { editor } from "monaco-editor";
 import type { Monaco } from "@monaco-editor/react";
@@ -168,7 +168,7 @@ export default function Chrome({
                     workspace={workspace}
                     onMount={handleEditorMount}
                     onChange={(content) => onFileChanged(workspace, content)}
-                    onOpenFile={onFileSelected}
+                    onFileOpened={onFileSelected}
                   />
                   {checkResult.error ? (
                     <div
@@ -192,7 +192,6 @@ export default function Chrome({
                 >
                   <Diagnostics
                     diagnostics={checkResult.diagnostics}
-                    workspace={workspace}
                     onGoTo={handleGoTo}
                     theme={theme}
                   />
@@ -290,8 +289,18 @@ function useCheckResult(
         };
       }
 
+      // Eagerly convert the diagnostic to avoid out of bound errors
+      // when the diagnostics are "deferred".
+      const serializedDiagnostics = diagnostics.map((diagnostic) => ({
+        id: diagnostic.id(),
+        message: diagnostic.message(),
+        severity: diagnostic.severity(),
+        range: diagnostic.toRange(workspace) ?? null,
+        textRange: diagnostic.textRange() ?? null,
+      }));
+
       return {
-        diagnostics,
+        diagnostics: serializedDiagnostics,
         error: null,
         secondary,
       };
diff --git a/playground/knot/src/Editor/Diagnostics.tsx b/playground/knot/src/Editor/Diagnostics.tsx
index e9ae8c21ba..2a087c33ed 100644
--- a/playground/knot/src/Editor/Diagnostics.tsx
+++ b/playground/knot/src/Editor/Diagnostics.tsx
@@ -1,11 +1,10 @@
-import { Diagnostic, Workspace } from "red_knot_wasm";
+import type { Severity, Range, TextRange } from "red_knot_wasm";
 import classNames from "classnames";
 import { Theme } from "shared";
 import { useMemo } from "react";
 
 interface Props {
   diagnostics: Diagnostic[];
-  workspace: Workspace;
   theme: Theme;
 
   onGoTo(line: number, column: number): void;
@@ -13,14 +12,13 @@ interface Props {
 
 export default function Diagnostics({
   diagnostics: unsorted,
-  workspace,
   theme,
   onGoTo,
 }: Props) {
   const diagnostics = useMemo(() => {
     const sorted = [...unsorted];
     sorted.sort((a, b) => {
-      return (a.textRange()?.start ?? 0) - (b.textRange()?.start ?? 0);
+      return (a.textRange?.start ?? 0) - (b.textRange?.start ?? 0);
     });
 
     return sorted;
@@ -43,11 +41,7 @@ export default function Diagnostics({
       </div>
 
       <div className="flex grow p-2 overflow-hidden">
-        <Items
-          diagnostics={diagnostics}
-          onGoTo={onGoTo}
-          workspace={workspace}
-        />
+        <Items diagnostics={diagnostics} onGoTo={onGoTo} />
       </div>
     </div>
   );
@@ -56,10 +50,8 @@ export default function Diagnostics({
 function Items({
   diagnostics,
   onGoTo,
-  workspace,
 }: {
   diagnostics: Array<Diagnostic>;
-  workspace: Workspace;
   onGoTo(line: number, column: number): void;
 }) {
   if (diagnostics.length === 0) {
@@ -73,9 +65,9 @@ function Items({
   return (
     <ul className="space-y-0.5 grow overflow-y-scroll">
       {diagnostics.map((diagnostic, index) => {
-        const position = diagnostic.toRange(workspace);
+        const position = diagnostic.range;
         const start = position?.start;
-        const id = diagnostic.id();
+        const id = diagnostic.id;
 
         const startLine = start?.line ?? 1;
         const startColumn = start?.column ?? 1;
@@ -86,7 +78,7 @@ function Items({
               onClick={() => onGoTo(startLine, startColumn)}
               className="w-full text-start cursor-pointer select-text"
             >
-              {diagnostic.message()}
+              {diagnostic.message}
               <span className="text-gray-500">
                 {id != null && ` (${id})`} [Ln {startLine}, Col {startColumn}]
               </span>
@@ -97,3 +89,11 @@ function Items({
     </ul>
   );
 }
+
+export interface Diagnostic {
+  id: string;
+  message: string;
+  severity: Severity;
+  range: Range | null;
+  textRange: TextRange | null;
+}
diff --git a/playground/knot/src/Editor/Editor.tsx b/playground/knot/src/Editor/Editor.tsx
index c6dced6631..e03da98a00 100644
--- a/playground/knot/src/Editor/Editor.tsx
+++ b/playground/knot/src/Editor/Editor.tsx
@@ -17,9 +17,8 @@ import {
 import { RefObject, useCallback, useEffect, useRef } from "react";
 import { Theme } from "shared";
 import {
-  Diagnostic,
   Severity,
-  Workspace,
+  type Workspace,
   Position as KnotPosition,
   type Range as KnotRange,
 } from "red_knot_wasm";
@@ -27,6 +26,7 @@ import {
 import IStandaloneCodeEditor = editor.IStandaloneCodeEditor;
 import { FileId, ReadonlyFiles } from "../Playground";
 import { isPythonFile } from "./Files";
+import { Diagnostic } from "./Diagnostics";
 
 type Props = {
   visible: boolean;
@@ -38,7 +38,7 @@ type Props = {
   workspace: Workspace;
   onChange(content: string): void;
   onMount(editor: IStandaloneCodeEditor, monaco: Monaco): void;
-  onOpenFile(file: FileId): void;
+  onFileOpened(file: FileId): void;
 };
 
 export default function Editor({
@@ -51,7 +51,7 @@ export default function Editor({
   workspace,
   onChange,
   onMount,
-  onOpenFile,
+  onFileOpened,
 }: Props) {
   const disposable = useRef<{
     typeDefinition: IDisposable;
@@ -61,14 +61,14 @@ export default function Editor({
     monaco: null,
     files,
     workspace,
-    onOpenFile,
+    onFileOpened,
   });
 
   playgroundState.current = {
     monaco: playgroundState.current.monaco,
     files,
     workspace,
-    onOpenFile,
+    onFileOpened,
   };
 
   // Update the diagnostics in the editor.
@@ -79,8 +79,8 @@ export default function Editor({
       return;
     }
 
-    updateMarkers(monaco, workspace, diagnostics);
-  }, [workspace, diagnostics]);
+    updateMarkers(monaco, diagnostics);
+  }, [diagnostics]);
 
   const handleChange = useCallback(
     (value: string | undefined) => {
@@ -98,7 +98,7 @@ export default function Editor({
 
   const handleMount: OnMount = useCallback(
     (editor, instance) => {
-      updateMarkers(instance, workspace, diagnostics);
+      updateMarkers(instance, diagnostics);
 
       const server = new PlaygroundServer(playgroundState);
       const typeDefinitionDisposable =
@@ -116,7 +116,7 @@ export default function Editor({
       onMount(editor, instance);
     },
 
-    [onMount, workspace, diagnostics],
+    [onMount, diagnostics],
   );
 
   return (
@@ -141,11 +141,7 @@ export default function Editor({
   );
 }
 
-function updateMarkers(
-  monaco: Monaco,
-  workspace: Workspace,
-  diagnostics: Array<Diagnostic>,
-) {
+function updateMarkers(monaco: Monaco, diagnostics: Array<Diagnostic>) {
   const editor = monaco.editor;
   const model = editor?.getModels()[0];
 
@@ -170,16 +166,16 @@ function updateMarkers(
         }
       };
 
-      const range = diagnostic.toRange(workspace);
+      const range = diagnostic.range;
 
       return {
-        code: diagnostic.id(),
+        code: diagnostic.id,
         startLineNumber: range?.start?.line ?? 0,
         startColumn: range?.start?.column ?? 0,
         endLineNumber: range?.end?.line ?? 0,
         endColumn: range?.end?.column ?? 0,
-        message: diagnostic.message(),
-        severity: mapSeverity(diagnostic.severity()),
+        message: diagnostic.message,
+        severity: mapSeverity(diagnostic.severity),
         tags: [],
       };
     }),
@@ -191,7 +187,7 @@ interface PlaygroundServerProps {
   workspace: Workspace;
   files: ReadonlyFiles;
 
-  onOpenFile: (file: FileId) => void;
+  onFileOpened: (file: FileId) => void;
 }
 
 class PlaygroundServer
@@ -223,26 +219,29 @@ class PlaygroundServer
       new KnotPosition(position.lineNumber, position.column),
     );
 
-    const locations = links.map((link) => {
-      const targetSelection =
-        link.selection_range == null
-          ? undefined
-          : knotRangeToIRange(link.selection_range);
+    return (
+      links
+        .map((link) => {
+          const targetSelection =
+            link.selection_range == null
+              ? undefined
+              : knotRangeToIRange(link.selection_range);
 
-      const originSelection =
-        link.origin_selection_range == null
-          ? undefined
-          : knotRangeToIRange(link.origin_selection_range);
+          const originSelection =
+            link.origin_selection_range == null
+              ? undefined
+              : knotRangeToIRange(link.origin_selection_range);
 
-      return {
-        uri: Uri.parse(link.path),
-        range: knotRangeToIRange(link.full_range),
-        targetSelectionRange: targetSelection,
-        originSelectionRange: originSelection,
-      } as languages.LocationLink;
-    });
-
-    return locations;
+          return {
+            uri: Uri.parse(link.path),
+            range: knotRangeToIRange(link.full_range),
+            targetSelectionRange: targetSelection,
+            originSelectionRange: originSelection,
+          } as languages.LocationLink;
+        })
+        // Filter out vendored files because they aren't open in the editor.
+        .filter((link) => link.uri.scheme !== "vendored")
+    );
   }
 
   openCodeEditor(
@@ -284,7 +283,7 @@ class PlaygroundServer
     if (files.selected !== fileId) {
       source.setModel(model);
 
-      this.props.current.onOpenFile(fileId);
+      this.props.current.onFileOpened(fileId);
     }
 
     if (selectionOrPosition != null) {