diff --git a/compiler/forget/packages/playground/components/Editor/Input.tsx b/compiler/forget/packages/playground/components/Editor/Input.tsx
index cf3e19e0ef..75f67c1558 100644
--- a/compiler/forget/packages/playground/components/Editor/Input.tsx
+++ b/compiler/forget/packages/playground/components/Editor/Input.tsx
@@ -3,16 +3,22 @@
  */
 
 import MonacoEditor, { type Monaco } from "@monaco-editor/react";
+import { CompilerErrorDetail } from "babel-plugin-react-forget";
 import invariant from "invariant";
 import type { editor } from "monaco-editor";
 import { useEffect, useState } from "react";
+import { renderForgetMarkers } from "../../lib/forgetMonacoDiagnostics";
 import { useStore, useStoreDispatch } from "../StoreContext";
 import { monacoOptions } from "./monacoOptions";
 // TODO: Make TS recognize .d.ts files, in addition to loading them with webpack.
 // @ts-ignore
 import React$Types from "../../node_modules/@types/react/index.d.ts";
 
-export default function Input({}: {}) {
+type Props = {
+  errors: CompilerErrorDetail[];
+};
+
+export default function Input({ errors }: Props) {
   const [monaco, setMonaco] = useState<Monaco | null>(null);
   const store = useStore();
   const dispatchStore = useStoreDispatch();
@@ -23,10 +29,11 @@ export default function Input({}: {}) {
     const uri = monaco.Uri.parse(`file:///index.js`);
     const model = monaco.editor.getModel(uri);
     invariant(model, "Model must exist for the selected input file.");
+    renderForgetMarkers({ monaco, model, details: errors });
     // N.B. that `tabSize` is a model property, not an editor property.
     // So, the tab size has to be set per model.
     model.updateOptions({ tabSize: 2 });
-  }, [monaco]);
+  }, [monaco, errors]);
 
   const handleChange = (value: string | undefined) => {
     if (!value) return;
diff --git a/compiler/forget/packages/playground/components/Editor/Output.tsx b/compiler/forget/packages/playground/components/Editor/Output.tsx
index 314d9fe59e..68b1200375 100644
--- a/compiler/forget/packages/playground/components/Editor/Output.tsx
+++ b/compiler/forget/packages/playground/components/Editor/Output.tsx
@@ -6,121 +6,104 @@
  */
 
 import generate from "@babel/generator";
-import { parse } from "@babel/parser";
-import traverse, { NodePath } from "@babel/traverse";
 import * as t from "@babel/types";
 import MonacoEditor from "@monaco-editor/react";
 import {
   printHIR,
   printReactiveFunction,
-  run,
+  type CompilerError,
+  type CompilerPipelineValue,
 } from "babel-plugin-react-forget";
 import prettier from "prettier";
 import prettierParserBabel from "prettier/parser-babel";
 import { memo, useMemo, useState } from "react";
-import type { Store } from "../../lib/stores";
+import { type Store } from "../../lib/stores";
 import TabbedWindow from "../TabbedWindow";
 import { monacoOptions } from "./monacoOptions";
 const MemoizedOutput = memo(Output);
 
 export default MemoizedOutput;
 
+export type CompilerOutput =
+  | { kind: "ok"; results: Map<string, CompilerPipelineValue[]> }
+  | { kind: "err"; error: CompilerError };
+
 type Props = {
   store: Store;
+  compilerOutput: CompilerOutput;
 };
 
-type CompilerError = string;
-
-function parseFunctions(
-  source: string
-): Array<NodePath<t.FunctionDeclaration>> {
-  try {
-    const ast = parse(source, {
-      plugins: ["typescript", "jsx"],
-      sourceType: "module",
-    });
-    const items: Array<NodePath<t.FunctionDeclaration>> = [];
-    traverse(ast, {
-      FunctionDeclaration: {
-        enter(nodePath) {
-          items.push(nodePath);
-        },
-      },
-    });
-    return items;
-  } catch (e) {
-    return [];
-  }
-}
-
-function compile(source: string): Map<string, React.ReactNode> | CompilerError {
-  try {
-    const astFunctions = parseFunctions(source);
-    if (astFunctions.length === 0) {
-      return "";
-    }
-
-    // TODO: Handle multiple functions
-    const func = astFunctions[0];
-    const tabs = new Map<string, React.ReactNode>();
-    let ast: t.Function | null = null;
-    for (const result of run(func)) {
+function tabify(source: string, compilerOutput: CompilerOutput) {
+  const tabs = new Map<string, React.ReactNode>();
+  const reorderedTabs = new Map<string, React.ReactNode>();
+  const concattedResults = new Map<string, string>();
+  let topLevelFnDecls: Array<t.FunctionDeclaration> = [];
+  if (compilerOutput.kind === "err") return reorderedTabs;
+  // Concat all top level function declaration results into a single tab for each pass
+  for (const [passName, results] of compilerOutput.results) {
+    for (const result of results) {
       switch (result.kind) {
-        case "ast": {
-          ast = result.value;
-          break;
-        }
         case "hir": {
-          const text = printHIR(result.value.body);
-          tabs.set(
-            result.name,
-            <TextTabContent output={text}></TextTabContent>
-          );
+          const prev = concattedResults.get(result.name);
+          const next = printHIR(result.value.body);
+          const identName = `function ${result.value.id?.name}`;
+          if (prev != null) {
+            concattedResults.set(passName, `${prev}\n\n${identName}\n${next}`);
+          } else {
+            concattedResults.set(passName, `${identName}\n${next}`);
+          }
           break;
         }
         case "reactive": {
-          const text = printReactiveFunction(result.value);
-          tabs.set(
-            result.name,
-            <TextTabContent output={text}></TextTabContent>
-          );
+          const prev = concattedResults.get(passName);
+          const next = printReactiveFunction(result.value);
+          if (prev != null) {
+            concattedResults.set(passName, `${prev}\n\n${next}`);
+          } else {
+            concattedResults.set(passName, next);
+          }
           break;
         }
+        case "ast":
+          topLevelFnDecls.push(result.value);
+          break;
         default: {
           throw new Error("Unexpected result kind");
         }
       }
     }
-    // Ensure that JS and the JS source map come first
-    const reorderedTabs = new Map();
-    if (ast !== null) {
-      const { code, sourceMapUrl } = codegen(ast, source);
-      reorderedTabs.set("JS", <TextTabContent output={code}></TextTabContent>);
-      if (sourceMapUrl) {
-        reorderedTabs.set(
-          "SourceMap",
-          <>
-            <iframe
-              src={sourceMapUrl}
-              className="w-full h-96"
-              title="Generated Code"
-            />
-          </>
-        );
-      }
-    }
-    tabs.forEach((tab, name) => {
-      reorderedTabs.set(name, tab);
-    });
-    return reorderedTabs;
-  } catch (e: any) {
-    console.error(e);
-    return e.toString();
   }
+  for (const [passName, text] of concattedResults) {
+    tabs.set(passName, <TextTabContent output={text}></TextTabContent>);
+  }
+  // Ensure that JS and the JS source map come first
+  if (topLevelFnDecls.length > 0) {
+    // Make a synthetic Program so we can have a single AST with all the top level
+    // FunctionDeclarations
+    const ast = t.program(topLevelFnDecls);
+    const { code, sourceMapUrl } = codegen(ast, source);
+    reorderedTabs.set("JS", <TextTabContent output={code}></TextTabContent>);
+    if (sourceMapUrl) {
+      reorderedTabs.set(
+        "SourceMap",
+        <>
+          <iframe
+            src={sourceMapUrl}
+            className="w-full h-96"
+            title="Generated Code"
+          />
+        </>
+      );
+    }
+  }
+  tabs.forEach((tab, name) => {
+    reorderedTabs.set(name, tab);
+  });
+  return reorderedTabs;
 }
 
 function codegen(
-  ast: any,
+  ast: t.Program,
   source: string
 ): { code: any; sourceMapUrl: string | null } {
   const generated = generate(
@@ -140,30 +123,6 @@ function codegen(
   return { code: codegenOutput, sourceMapUrl };
 }
 
-// TODO(gsn: Update diagnostics ƒrom HIR output
-function Output({ store }: Props) {
-  const [tabsOpen, setTabsOpen] = useState<Set<string>>(() => new Set());
-  const compilerOutput = useMemo(() => compile(store.source), [store.source]);
-
-  if (typeof compilerOutput === "string") {
-    if (compilerOutput === "") return <></>;
-    return (
-      <pre>
-        <code>${compilerOutput}</code>
-      </pre>
-    );
-  }
-
-  return (
-    <TabbedWindow
-      defaultTab="HIR"
-      setTabsOpen={setTabsOpen}
-      tabsOpen={tabsOpen}
-      tabs={compilerOutput}
-    />
-  );
-}
-
 function utf16ToUTF8(s: string): string {
   return unescape(encodeURIComponent(s));
 }
@@ -176,6 +135,31 @@ function getSourceMapUrl(code: string, map: string): string | null {
   )}`;
 }
 
+function Output({ store, compilerOutput }: Props) {
+  const [tabsOpen, setTabsOpen] = useState<Set<string>>(() => new Set());
+  const tabs = useMemo(
+    () => tabify(store.source, compilerOutput),
+    [store.source, compilerOutput]
+  );
+
+  if (compilerOutput.kind === "err") {
+    return (
+      <pre>
+        <code>{compilerOutput.error.toString()}</code>
+      </pre>
+    );
+  }
+
+  return (
+    <TabbedWindow
+      defaultTab="HIR"
+      setTabsOpen={setTabsOpen}
+      tabsOpen={tabsOpen}
+      tabs={tabs}
+    />
+  );
+}
+
 function TextTabContent({ output }: { output: string }) {
   return (
     // Restrict MonacoEditor's height, since the config autoLayout:true
diff --git a/compiler/forget/packages/playground/components/Editor/index.tsx b/compiler/forget/packages/playground/components/Editor/index.tsx
index a6d826184f..26837ed606 100644
--- a/compiler/forget/packages/playground/components/Editor/index.tsx
+++ b/compiler/forget/packages/playground/components/Editor/index.tsx
@@ -5,10 +5,14 @@
  * LICENSE file in the root directory of this source tree.
  */
 
+import { parse } from "@babel/parser";
+import traverse, { NodePath } from "@babel/traverse";
+import * as t from "@babel/types";
+import { CompilerPipelineValue, run } from "babel-plugin-react-forget";
 import clsx from "clsx";
 import invariant from "invariant";
 import { useSnackbar } from "notistack";
-import { useDeferredValue } from "react";
+import { useDeferredValue, useMemo } from "react";
 import { useMountEffect } from "../../hooks";
 import { defaultStore } from "../../lib/defaultStore";
 import {
@@ -20,13 +24,62 @@ import {
 } from "../../lib/stores";
 import { useStore, useStoreDispatch } from "../StoreContext";
 import Input from "./Input";
-import Output from "./Output";
+import { CompilerOutput, default as Output } from "./Output";
+
+function parseFunctions(
+  source: string
+): Array<NodePath<t.FunctionDeclaration>> {
+  const items: Array<NodePath<t.FunctionDeclaration>> = [];
+  try {
+    const ast = parse(source, {
+      plugins: ["typescript", "jsx"],
+      sourceType: "module",
+    });
+    traverse(ast, {
+      FunctionDeclaration: {
+        enter(nodePath) {
+          items.push(nodePath);
+        },
+      },
+    });
+  } catch (e) {
+    console.error(e);
+  }
+  return items;
+}
+
+function compile(source: string): CompilerOutput {
+  try {
+    const results = new Map<string, CompilerPipelineValue[]>();
+    const upsert = (result: CompilerPipelineValue) => {
+      const entry = results.get(result.name);
+      if (Array.isArray(entry)) {
+        entry.push(result);
+      } else {
+        results.set(result.name, [result]);
+      }
+    };
+    for (const fn of parseFunctions(source)) {
+      for (const result of run(fn)) {
+        upsert(result);
+      }
+    }
+    return { kind: "ok", results };
+  } catch (error: any) {
+    console.error(error);
+    return { kind: "err", error };
+  }
+}
 
 export default function Editor() {
   const store = useStore();
   const deferredStore = useDeferredValue(store);
   const dispatchStore = useStoreDispatch();
   const { enqueueSnackbar } = useSnackbar();
+  const compilerOutput = useMemo(
+    () => compile(deferredStore.source),
+    [deferredStore.source]
+  );
 
   useMountEffect(() => {
     let mountStore: Store;
@@ -57,10 +110,14 @@ export default function Editor() {
           style={{ minWidth: 650 }}
           className={clsx("relative sm:basis-1/4")}
         >
-          <Input />
+          <Input
+            errors={
+              compilerOutput.kind === "err" ? compilerOutput.error.details : []
+            }
+          />
         </div>
         <div className={clsx("flex sm:flex")}>
-          <Output store={deferredStore} />
+          <Output store={deferredStore} compilerOutput={compilerOutput} />
         </div>
       </div>
     </>
diff --git a/compiler/forget/packages/playground/lib/forgetMonacoDiagnostics.ts b/compiler/forget/packages/playground/lib/forgetMonacoDiagnostics.ts
new file mode 100644
index 0000000000..2c3d470b93
--- /dev/null
+++ b/compiler/forget/packages/playground/lib/forgetMonacoDiagnostics.ts
@@ -0,0 +1,84 @@
+/*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ */
+
+import { Monaco } from "@monaco-editor/react";
+import { CompilerErrorDetail, ErrorSeverity } from "babel-plugin-react-forget";
+import { MarkerSeverity, type editor } from "monaco-editor";
+
+function mapForgetSeverityToMonaco(
+  level: ErrorSeverity,
+  monaco: Monaco
+): MarkerSeverity {
+  switch (level) {
+    case ErrorSeverity.Todo:
+    case ErrorSeverity.InvalidInput:
+      return monaco.MarkerSeverity.Error;
+    default:
+      return monaco.MarkerSeverity.Warning;
+  }
+}
+
+function mapForgetDiagnosticToMonacoMarker(
+  detail: CompilerErrorDetail,
+  monaco: Monaco
+): editor.IMarkerData | null {
+  if (detail.nodePath == null || detail.nodePath.node?.loc == null) {
+    return null;
+  }
+  const severity = mapForgetSeverityToMonaco(detail.severity, monaco);
+  let message = detail.errorMessage;
+  return {
+    severity,
+    message,
+    startLineNumber: detail.nodePath.node.loc.start.line,
+    startColumn: detail.nodePath.node.loc.start.column + 1,
+    endLineNumber: detail.nodePath.node.loc.end.line,
+    endColumn: detail.nodePath.node.loc.end.column + 1,
+  };
+}
+
+type ForgetMarkerConfig = {
+  monaco: Monaco;
+  model: editor.ITextModel;
+  details: CompilerErrorDetail[];
+};
+let decorations: string[] = [];
+export function renderForgetMarkers({
+  monaco,
+  model,
+  details,
+}: ForgetMarkerConfig): void {
+  let markers = [];
+  for (const detail of details) {
+    const marker = mapForgetDiagnosticToMonacoMarker(detail, monaco);
+    if (marker == null) {
+      continue;
+    }
+    markers.push(marker);
+  }
+  if (markers.length > 0) {
+    monaco.editor.setModelMarkers(model, "owner", markers);
+    const newDecorations = markers.map((marker) => {
+      return {
+        range: new monaco.Range(
+          marker.startLineNumber,
+          marker.startColumn,
+          marker.endLineNumber,
+          marker.endColumn
+        ),
+        options: {
+          isWholeLine: true,
+          glyphMarginClassName: "bg-red-300",
+        },
+      };
+    });
+    decorations = model.deltaDecorations(decorations, newDecorations);
+  } else {
+    monaco.editor.setModelMarkers(model, "owner", []);
+    decorations = model.deltaDecorations(
+      model.getAllDecorations().map((d) => d.id),
+      []
+    );
+  }
+}
diff --git a/compiler/forget/src/CompilerPipeline.ts b/compiler/forget/src/CompilerPipeline.ts
index 0f70eeaf21..825555eeca 100644
--- a/compiler/forget/src/CompilerPipeline.ts
+++ b/compiler/forget/src/CompilerPipeline.ts
@@ -39,7 +39,7 @@ import { logHIRFunction, logReactiveFunction } from "./Utils/logger";
 import { assertExhaustive } from "./Utils/utils";
 
 export type CompilerPipelineValue =
-  | { kind: "ast"; name: string; value: t.Function }
+  | { kind: "ast"; name: string; value: t.FunctionDeclaration }
   | { kind: "hir"; name: string; value: HIRFunction }
   | { kind: "reactive"; name: string; value: ReactiveFunction };
 
diff --git a/compiler/forget/src/ReactiveScopes/CodegenReactiveFunction.ts b/compiler/forget/src/ReactiveScopes/CodegenReactiveFunction.ts
index 17968a77a5..f0b684aeca 100644
--- a/compiler/forget/src/ReactiveScopes/CodegenReactiveFunction.ts
+++ b/compiler/forget/src/ReactiveScopes/CodegenReactiveFunction.ts
@@ -28,7 +28,9 @@ import {
 import { todoInvariant } from "../Utils/todo";
 import { assertExhaustive } from "../Utils/utils";
 
-export function codegenReactiveFunction(fn: ReactiveFunction): t.Function {
+export function codegenReactiveFunction(
+  fn: ReactiveFunction
+): t.FunctionDeclaration {
   const cx = new Context();
   const params = fn.params.map((param) => convertIdentifier(param.identifier));
   const body = codegenBlock(cx, fn.body);
diff --git a/compiler/forget/src/index.ts b/compiler/forget/src/index.ts
index a2ca50dc0c..4f5e9738dc 100644
--- a/compiler/forget/src/index.ts
+++ b/compiler/forget/src/index.ts
@@ -5,8 +5,12 @@
  * LICENSE file in the root directory of this source tree.
  */
 export { default as BabelPlugin } from "./Babel/BabelPlugin";
-export { ErrorSeverity } from "./CompilerError";
-export { compile, run } from "./CompilerPipeline";
+export {
+  CompilerError,
+  CompilerErrorDetail,
+  ErrorSeverity,
+} from "./CompilerError";
+export { compile, CompilerPipelineValue, run } from "./CompilerPipeline";
 export { printHIR } from "./HIR";
 export { printReactiveFunction } from "./ReactiveScopes";