diff --git a/clients/web/package-lock.json b/clients/web/package-lock.json
index b92eff57..e4233a49 100644
--- a/clients/web/package-lock.json
+++ b/clients/web/package-lock.json
@@ -16,6 +16,7 @@
         "@mantine/hooks": "^8.3.17",
         "@mantine/notifications": "^8.3.17",
         "@napi-rs/keyring": "^1.3.0",
+        "ace-builds": "^1.44.0",
         "ajv": "^8.17.1",
         "atomically": "^2.1.1",
         "chokidar": "^4.0.3",
@@ -25,6 +26,7 @@
         "papaparse": "^5.5.4",
         "pino": "^9.14.0",
         "react": "^19.2.4",
+        "react-ace": "^15.0.0",
         "react-dom": "^19.2.4",
         "react-icons": "^5.6.0",
         "react-markdown": "^10.1.0",
@@ -4383,6 +4385,12 @@
       "dev": true,
       "license": "MIT"
     },
+    "node_modules/ace-builds": {
+      "version": "1.44.0",
+      "resolved": "https://registry.npmjs.org/ace-builds/-/ace-builds-1.44.0.tgz",
+      "integrity": "sha512-PFNMSYqFdEUkul2Ntud0HvA09AgY+F1ag0UYdpMH60wNI/qOA8cB8tlTgoALMEwIdUPJK2CjrIQ7OnbiSS/ugQ==",
+      "license": "BSD-3-Clause"
+    },
     "node_modules/acorn": {
       "version": "8.18.0",
       "resolved": "https://registry.npmjs.org/acorn/-/acorn-8.18.0.tgz",
@@ -5061,6 +5069,12 @@
         "url": "https://github.com/sponsors/wooorm"
       }
     },
+    "node_modules/diff-match-patch": {
+      "version": "1.0.5",
+      "resolved": "https://registry.npmjs.org/diff-match-patch/-/diff-match-patch-1.0.5.tgz",
+      "integrity": "sha512-IayShXAgj/QMXgB0IWmKx+rOPuGMhqm5w6jvFxmVenXKIzRqTAAsbBPT3kWQeGANj3jGgvcvv4yK6SxqYmikgw==",
+      "license": "Apache-2.0"
+    },
     "node_modules/doctrine": {
       "version": "3.0.0",
       "resolved": "https://registry.npmjs.org/doctrine/-/doctrine-3.0.0.tgz",
@@ -5487,6 +5501,15 @@
       "integrity": "sha512-f3qQ9oQy9j2AhBe/H9VC91wLmKBCCU/gDOnKNAYG5hswO7BLKj09Hc5HYNz9cGI++xlpDCIgDaitVs03ATR84Q==",
       "license": "MIT"
     },
+    "node_modules/fast-equals": {
+      "version": "5.4.1",
+      "resolved": "https://registry.npmjs.org/fast-equals/-/fast-equals-5.4.1.tgz",
+      "integrity": "sha512-DjlFSM5Pk9cGcL0q5QXl66eGzx0N6szNgaswwc5ZphlBohjTVJSnGgI+rJVOgOi65qUoQnDZN4nDqi33udtydQ==",
+      "license": "MIT",
+      "engines": {
+        "node": ">=6.0.0"
+      }
+    },
     "node_modules/fast-json-stable-stringify": {
       "version": "2.1.0",
       "resolved": "https://registry.npmjs.org/fast-json-stable-stringify/-/fast-json-stable-stringify-2.1.0.tgz",
@@ -8294,6 +8317,22 @@
         "node": ">=0.10.0"
       }
     },
+    "node_modules/react-ace": {
+      "version": "15.0.0",
+      "resolved": "https://registry.npmjs.org/react-ace/-/react-ace-15.0.0.tgz",
+      "integrity": "sha512-gdmS5ftF0hsbkcrAjZQqYbXwFg5JrHuxjki8mP6Bn9kfa6lHKfZk9sU9EfS0ifQ1CpGCmRxF/VC7GRvlJMBuZw==",
+      "license": "MIT",
+      "dependencies": {
+        "ace-builds": "^1.36.3",
+        "diff-match-patch": "^1.0.5",
+        "fast-equals": "^5.3.3",
+        "prop-types": "^15.8.1"
+      },
+      "peerDependencies": {
+        "react": "^0.13.0 || ^0.14.0 || ^15.0.1 || ^16.0.0 || ^17.0.0 || ^18.0.0 || ^19.0.0",
+        "react-dom": "^0.13.0 || ^0.14.0 || ^15.0.1 || ^16.0.0 || ^17.0.0 || ^18.0.0 || ^19.0.0"
+      }
+    },
     "node_modules/react-docgen": {
       "version": "8.0.3",
       "resolved": "https://registry.npmjs.org/react-docgen/-/react-docgen-8.0.3.tgz",
diff --git a/clients/web/package.json b/clients/web/package.json
index 451c0767..8696fb25 100644
--- a/clients/web/package.json
+++ b/clients/web/package.json
@@ -42,6 +42,7 @@
     "@mantine/hooks": "^8.3.17",
     "@mantine/notifications": "^8.3.17",
     "@napi-rs/keyring": "^1.3.0",
+    "ace-builds": "^1.44.0",
     "ajv": "^8.17.1",
     "atomically": "^2.1.1",
     "chokidar": "^4.0.3",
@@ -51,6 +52,7 @@
     "papaparse": "^5.5.4",
     "pino": "^9.14.0",
     "react": "^19.2.4",
+    "react-ace": "^15.0.0",
     "react-dom": "^19.2.4",
     "react-icons": "^5.6.0",
     "react-markdown": "^10.1.0",
diff --git a/clients/web/src/App.css b/clients/web/src/App.css
index b208285c..a32c6435 100644
--- a/clients/web/src/App.css
+++ b/clients/web/src/App.css
@@ -73,6 +73,16 @@
      ScrollArea would break flex sizing, e.g. the Resources accordion) blend in. */
   --inspector-scrollbar-thumb: rgba(0, 0, 0, 0.4);
 
+  /* ── Code editor (Ace) ─────────────────────────────────── */
+  /* Ace's own `github` themes fail WCAG AA on the gutter: `#AAA` on the light
+     gutter's `#e8e8e8` is 1.89:1, and `#7388b5` on the dark gutter's `#24292e`
+     is 4.13:1 — both under 4.5:1 at the 12px the line numbers render. The
+     numbers are `aria-hidden`, which exempts nothing here: contrast is a
+     concern for sighted users, and axe flags them accordingly.
+     `gray-7` is 5.82:1 on the light gutter; the dark block lifts it to
+     `gray-5`, 4.88:1 on the dark one. */
+  --inspector-code-gutter-text: var(--mantine-color-gray-7);
+
   /* ── Connection status ─────────────────────────────────── */
   --inspector-status-connected: var(--mantine-color-green-6);
   --inspector-status-connecting: var(--mantine-color-yellow-6);
@@ -161,6 +171,9 @@
      schemes, which sits too dark on charcoal — lift them one to two steps so
      they stay legible in dark mode. Light-mode `:root` keeps the `*-6` values. */
 
+  /* Code editor (Ace) — see the note in `:root`. */
+  --inspector-code-gutter-text: var(--mantine-color-gray-5);
+
   /* Status */
   --inspector-status-connected: var(--mantine-color-green-4);
   --inspector-status-connecting: var(--mantine-color-yellow-4);
@@ -593,3 +606,12 @@ body.resizing-col {
   user-select: none;
   cursor: col-resize;
 }
+
+/* Ace renders its own DOM, so its gutter can only be reached by selector — it
+   is not a Mantine component with a theme entry. The doubled class raises
+   specificity above Ace's own `.ace-github .ace_gutter`, which is injected into
+   <head> at runtime and therefore wins on source order otherwise. */
+.ace_editor.ace-github .ace_gutter,
+.ace_editor.ace-github-dark .ace_gutter {
+  color: var(--inspector-code-gutter-text);
+}
diff --git a/clients/web/src/components/elements/JsonObjectInput/JsonObjectInput.stories.tsx b/clients/web/src/components/elements/JsonObjectInput/JsonObjectInput.stories.tsx
index 51405384..cdbcfc0f 100644
--- a/clients/web/src/components/elements/JsonObjectInput/JsonObjectInput.stories.tsx
+++ b/clients/web/src/components/elements/JsonObjectInput/JsonObjectInput.stories.tsx
@@ -4,17 +4,23 @@ import { expect, userEvent, within } from "storybook/test";
 import type { JsonObject } from "@inspector/core/json/jsonUtils.js";
 import { JsonObjectInput } from "./JsonObjectInput";
 
+const LABEL = "Request metadata JSON";
+
 /**
  * The component is controlled, so every story drives it through a small
- * stateful wrapper — otherwise the box would not accept a keystroke and the
+ * stateful wrapper — otherwise the editor would not accept a keystroke and the
  * play functions could not exercise the draft behavior at all.
+ *
+ * These stories run in real Chromium, which is what makes them the *only*
+ * place Ace's keyboard behavior can be tested: the unit suite runs in
+ * happy-dom, where Ace's hidden-textarea input path does not work at all.
  */
 function ControlledJsonObjectInput({ initial }: { initial: JsonObject }) {
   const [value, setValue] = useState<JsonObject>(initial);
   return (
     <JsonObjectInput
       label="Request metadata"
-      aria-label="Request metadata JSON"
+      ariaLabel={LABEL}
       value={value}
       onChange={setValue}
     />
@@ -29,6 +35,57 @@ const meta: Meta<typeof ControlledJsonObjectInput> = {
 export default meta;
 type Story = StoryObj<typeof ControlledJsonObjectInput>;
 
+/** The live editor instance, which Ace hangs off its container as `env.editor`. */
+function aceEditor(canvasElement: HTMLElement): {
+  focus(): void;
+  selectAll(): void;
+  getValue(): string;
+} {
+  const container = canvasElement.querySelector(".ace_editor");
+  const editor = (
+    container as
+      | (HTMLElement & {
+          env?: {
+            editor?: {
+              focus(): void;
+              selectAll(): void;
+              getValue(): string;
+            };
+          };
+        })
+      | null
+  )?.env?.editor;
+  if (!editor) throw new Error("Ace editor not mounted");
+  return editor;
+}
+
+/**
+ * Focus the editor and empty it, so the next keystrokes are the whole document
+ * and the typing itself — the thing under test — is left to `userEvent`.
+ *
+ * Emptied rather than merely selected: typing an opening brace or quote with a
+ * selection makes Ace **wrap** the selection instead of replacing it, so typing
+ * `{` over a selected `{}` yields `{{}}` and the assertion stops being about
+ * auto-closing at all.
+ *
+ * Focus and select go through Ace's API on purpose. `userEvent.click` does not
+ * focus the editor — Ace's input is a 1px offscreen textarea, so the click
+ * lands nowhere and the keystrokes reach the body instead. And select-all is
+ * not `Control+A` everywhere (Ace binds `Cmd-A` on mac), so pressing it would
+ * make these stories pass or fail by host OS.
+ */
+async function focusAndClear(canvasElement: HTMLElement): Promise<void> {
+  const editor = aceEditor(canvasElement);
+  editor.focus();
+  editor.selectAll();
+  await userEvent.keyboard("{Backspace}");
+}
+
+/** Ace renders each token in its own element, so read the document, not text. */
+function editorText(canvasElement: HTMLElement): string {
+  return aceEditor(canvasElement).getValue();
+}
+
 /** A structured payload renders as formatted JSON, nesting intact. */
 export const Populated: Story = {
   args: {
@@ -39,9 +96,7 @@ export const Populated: Story = {
     },
   },
   play: async ({ canvasElement }) => {
-    const canvas = within(canvasElement);
-    const box = await canvas.findByLabelText("Request metadata JSON");
-    await expect(JSON.parse((box as HTMLTextAreaElement).value)).toEqual({
+    await expect(JSON.parse(editorText(canvasElement))).toEqual({
       tenant: "acme",
       trace: { id: "abc123", sampled: true },
       features: ["apps", "tasks"],
@@ -54,8 +109,36 @@ export const Empty: Story = {
   args: { initial: {} },
   play: async ({ canvasElement }) => {
     const canvas = within(canvasElement);
-    const box = await canvas.findByLabelText("Request metadata JSON");
-    await expect(box).toHaveValue("{}");
+    await expect(editorText(canvasElement)).toBe("{}");
+    await expect(canvas.queryByText(/Not valid JSON/)).toBeNull();
+  },
+};
+
+/**
+ * Opening a brace closes it — one of the two reasons this is an editor rather
+ * than a textarea, and the thing that makes hand-writing a nested `_meta`
+ * payload bearable.
+ *
+ * Deliberately one keystroke: Ace also auto-pairs quotes and skips over a
+ * closing character you type yourself, so a longer sequence asserts the sum of
+ * several behaviours and stops being a readable statement about this one.
+ */
+export const ClosesBracesWhileTyping: Story = {
+  args: { initial: {} },
+  play: async ({ canvasElement }) => {
+    const canvas = within(canvasElement);
+    await focusAndClear(canvasElement);
+    // `{{` is userEvent's escape for a literal `{`.
+    await userEvent.keyboard("{{");
+    // Ace does not close it on the keystroke — an opening brace at the end of a
+    // line is often the start of a block the user is about to fill in.
+    await expect(editorText(canvasElement)).toBe("{");
+    // Enter is what completes the pair, leaving the cursor indented between
+    // them. This is the behaviour that makes writing nested `_meta` by hand
+    // bearable, and the reason this is an editor rather than a textarea.
+    await userEvent.keyboard("{Enter}");
+    await expect(editorText(canvasElement)).toBe("{\n  \n}");
+    // The result parses, so nothing is flagged.
     await expect(canvas.queryByText(/Not valid JSON/)).toBeNull();
   },
 };
@@ -69,22 +152,31 @@ export const InvalidDraft: Story = {
   args: { initial: {} },
   play: async ({ canvasElement }) => {
     const canvas = within(canvasElement);
-    const box = await canvas.findByLabelText("Request metadata JSON");
-    await userEvent.clear(box);
-    await userEvent.type(box, '{{"a":');
-    await expect(box).toHaveValue('{"a":');
+    await focusAndClear(canvasElement);
+    // Genuinely half-typed — the state a user is in partway through writing an
+    // entry, rather than a contrived bad string.
+    await userEvent.keyboard('{{"a":');
+    // Shown exactly as typed. The old textarea re-escaped its own contents on
+    // every keystroke here, which is what made it unusable (#1928).
+    await expect(editorText(canvasElement)).toBe('{"a":');
     await expect(await canvas.findByText(/Not valid JSON/)).toBeVisible();
   },
 };
 
-/** Valid JSON that is not an object is rejected with its own message. */
+/**
+ * Valid JSON that is not an object is rejected with its own message, distinct
+ * from unparseable text.
+ *
+ * A bare number rather than an array: brackets are auto-paired too, so typing
+ * one would test the pairing as much as the rejection.
+ */
 export const NotAnObject: Story = {
   args: { initial: {} },
   play: async ({ canvasElement }) => {
     const canvas = within(canvasElement);
-    const box = await canvas.findByLabelText("Request metadata JSON");
-    await userEvent.clear(box);
-    await userEvent.type(box, "[[1,2]");
+    await focusAndClear(canvasElement);
+    await userEvent.keyboard("42");
+    await expect(editorText(canvasElement)).toBe("42");
     await expect(
       await canvas.findByText(/Must be a JSON object/),
     ).toBeVisible();
diff --git a/clients/web/src/components/elements/JsonObjectInput/JsonObjectInput.test.tsx b/clients/web/src/components/elements/JsonObjectInput/JsonObjectInput.test.tsx
index b1da0ffb..e805e8d4 100644
--- a/clients/web/src/components/elements/JsonObjectInput/JsonObjectInput.test.tsx
+++ b/clients/web/src/components/elements/JsonObjectInput/JsonObjectInput.test.tsx
@@ -1,9 +1,9 @@
 import { useState } from "react";
 import { describe, it, expect, vi } from "vitest";
-import { screen } from "@testing-library/react";
-import userEvent from "@testing-library/user-event";
+import { act, screen } from "@testing-library/react";
 import type { JsonObject } from "@inspector/core/json/jsonUtils.js";
 import { renderWithMantine } from "../../../test/renderWithMantine";
+import { aceLabel, getAceText, setAceText } from "../../../test/aceEditor";
 import { JsonObjectInput } from "./JsonObjectInput";
 
 const LABEL = "Payload JSON";
@@ -20,7 +20,7 @@ function Harness({
   return (
     <>
       <JsonObjectInput
-        aria-label={LABEL}
+        ariaLabel={LABEL}
         value={value}
         onChange={(next) => {
           setValue(next);
@@ -32,38 +32,60 @@ function Harness({
   );
 }
 
-function box() {
-  return screen.getByLabelText(LABEL) as HTMLTextAreaElement;
-}
-
 describe("JsonObjectInput", () => {
+  it("names the editor for assistive tech", () => {
+    renderWithMantine(<Harness initial={{}} />);
+    // Ace composes "<label>, Cursor at row N", so the match is by substring —
+    // the position readout is Ace's, the name is ours.
+    expect(screen.getByLabelText(aceLabel(LABEL))).toBeInTheDocument();
+  });
+
+  it("uses Ace's dark theme under a dark color scheme", () => {
+    // The editor picks its Ace theme from the computed scheme rather than
+    // inheriting Mantine tokens, so the dark branch is only exercised by
+    // actually rendering dark.
+    renderWithMantine(<Harness initial={{}} />, { colorScheme: "dark" });
+    // `ace-github-dark`, hyphenated — Ace's cssClass, not the module name
+    // (`theme-github_dark`). The App.css gutter-contrast override keys off this
+    // same class, so getting it wrong silently drops the fix in dark mode.
+    expect(document.querySelector(".ace-github-dark")).not.toBeNull();
+    expect(document.querySelector(".ace-github")).toBeNull();
+  });
+
+  it("uses Ace's light theme under a light color scheme", () => {
+    renderWithMantine(<Harness initial={{}} />, { colorScheme: "light" });
+    expect(document.querySelector(".ace-github")).not.toBeNull();
+    expect(document.querySelector(".ace-github_dark")).toBeNull();
+  });
+
   it("opens with the value serialized two-space, nesting intact", () => {
     renderWithMantine(<Harness initial={{ a: { b: [1, 2] } }} />);
-    expect(box().value).toBe(
+    expect(getAceText()).toBe(
       '{\n  "a": {\n    "b": [\n      1,\n      2\n    ]\n  }\n}',
     );
   });
 
   it("emits the parsed object — a structured value stays structured", async () => {
-    const user = userEvent.setup();
     const onChange = vi.fn();
     renderWithMantine(<Harness initial={{}} onChange={onChange} />);
-    await user.clear(box());
-    await user.type(box(), '{{"trace":{{"id":"x"},"n":3}');
+    await setAceText('{"trace":{"id":"x"},"n":3}');
     expect(onChange).toHaveBeenLastCalledWith({ trace: { id: "x" }, n: 3 });
   });
 
+  // Also the regression test for the paired-event bug `handleChange` coalesces:
+  // `setValue` is a remove followed by an insert, and acting on the remove
+  // emitted `{}` and wiped `keep` before the insert had landed.
   it("shows the text as typed while it is invalid and does not emit", async () => {
-    const user = userEvent.setup();
     const onChange = vi.fn();
     renderWithMantine(<Harness initial={{ keep: "me" }} onChange={onChange} />);
-    // Append a stray character rather than clearing: clearing is itself a
-    // valid edit (it means `{}`) and would emit, which is not what is under
-    // test here.
-    await user.click(box());
-    await user.keyboard("{End}x");
+    await setAceText('{"a":');
+    console.log("[dbg] emitted:", screen.getByTestId("emitted").textContent);
+    console.log(
+      "[dbg] body has error?",
+      document.body.textContent?.includes("Not valid JSON"),
+    );
     // Displayed verbatim — not re-escaped back through JSON.stringify.
-    expect(box().value.endsWith("}x")).toBe(true);
+    expect(getAceText()).toBe('{"a":');
     expect(
       screen.getByText("Not valid JSON — changes are not applied"),
     ).toBeInTheDocument();
@@ -73,36 +95,30 @@ describe("JsonObjectInput", () => {
   });
 
   it("flags valid JSON that is not an object without emitting", async () => {
-    const user = userEvent.setup();
     const onChange = vi.fn();
     renderWithMantine(<Harness initial={{ keep: "me" }} onChange={onChange} />);
-    await user.clear(box());
-    // The clear is a legitimate edit to `{}`; only what follows is under test.
-    expect(onChange).toHaveBeenLastCalledWith({});
-    onChange.mockClear();
-    await user.type(box(), "[[1]");
+    await setAceText("[1]");
     expect(screen.getByText(/Must be a JSON object/)).toBeInTheDocument();
     expect(onChange).not.toHaveBeenCalled();
   });
 
-  it("treats a cleared box as the empty object", async () => {
-    const user = userEvent.setup();
+  it("treats a cleared editor as the empty object", async () => {
     const onChange = vi.fn();
     renderWithMantine(<Harness initial={{ a: 1 }} onChange={onChange} />);
-    await user.clear(box());
+    await setAceText("");
     expect(onChange).toHaveBeenLastCalledWith({});
     expect(screen.queryByText(/Not valid JSON/)).toBeNull();
   });
 
   it("re-syncs when the parent's value diverges from the draft", async () => {
     // An external reset (switching servers, a reloaded catalog) must reach the
-    // box; an in-progress edit must not be overwritten by the value it emitted.
+    // editor; an in-progress edit must not be overwritten by what it emitted.
     function Externally() {
       const [value, setValue] = useState<JsonObject>({ a: 1 });
       return (
         <>
           <JsonObjectInput
-            aria-label={LABEL}
+            ariaLabel={LABEL}
             value={value}
             onChange={setValue}
           />
@@ -110,19 +126,18 @@ describe("JsonObjectInput", () => {
         </>
       );
     }
-    const user = userEvent.setup();
     renderWithMantine(<Externally />);
-    await user.click(screen.getByRole("button", { name: "reset" }));
-    expect(JSON.parse(box().value)).toEqual({ b: 2 });
+    await act(async () => {
+      screen.getByRole("button", { name: "reset" }).click();
+    });
+    expect(JSON.parse(getAceText())).toEqual({ b: 2 });
   });
 
   it("leaves an in-progress edit alone when the parent echoes it back", async () => {
-    const user = userEvent.setup();
     renderWithMantine(<Harness initial={{}} />);
-    await user.clear(box());
-    // Typed without the closing brace auto-pair: the text is what the user has
-    // so far, and the echo of the parsed value must not reformat it mid-edit.
-    await user.type(box(), '{{"a":1}');
-    expect(box().value).toBe('{"a":1}');
+    // The parent re-renders with the object this text parsed to; the draft must
+    // not be reformatted out from under the cursor.
+    await setAceText('{"a":1}');
+    expect(getAceText()).toBe('{"a":1}');
   });
 });
diff --git a/clients/web/src/components/elements/JsonObjectInput/JsonObjectInput.tsx b/clients/web/src/components/elements/JsonObjectInput/JsonObjectInput.tsx
index fbec5ee4..8368b291 100644
--- a/clients/web/src/components/elements/JsonObjectInput/JsonObjectInput.tsx
+++ b/clients/web/src/components/elements/JsonObjectInput/JsonObjectInput.tsx
@@ -1,15 +1,21 @@
-import { useState } from "react";
-import { JsonInput } from "@mantine/core";
+import { useRef, useState } from "react";
+import { Input, useComputedColorScheme } from "@mantine/core";
+import AceEditor from "react-ace";
+import ace from "ace-builds/src-noconflict/ace";
+import "ace-builds/src-noconflict/mode-json";
+import "ace-builds/src-noconflict/theme-github";
+import "ace-builds/src-noconflict/theme-github_dark";
+// `?url` so Vite emits the worker as its own asset and hands back a real URL.
+// Without it Ace fetches `worker-json.js` from a path that does not exist in a
+// bundled app, silently loses its annotations, and logs a fetch error.
+import jsonWorkerUrl from "ace-builds/src-noconflict/worker-json.js?url";
 import type { JsonObject } from "@inspector/core/json/jsonUtils.js";
 import { useValueChange } from "../../../hooks/useValueChange";
 import { parseJsonObjectDraft } from "../../../utils/jsonObjectDraft";
 
-const ObjectJsonInput = JsonInput.withProps({
-  formatOnBlur: true,
-  autosize: true,
-  minRows: 4,
-  spellCheck: false,
-});
+// Module scope: registering the worker URL is global to Ace and idempotent, so
+// it must not be repeated per mount.
+ace.config.setModuleUrl("ace/mode/json_worker", jsonWorkerUrl);
 
 /** Two-space canonical form — the same shape the catalog file is written in. */
 function serialize(value: JsonObject): string {
@@ -21,9 +27,16 @@ export interface JsonObjectInputProps {
   onChange: (value: JsonObject) => void;
   label?: string;
   description?: string;
-  placeholder?: string;
-  "aria-label"?: string;
-  "data-testid"?: string;
+  /**
+   * Accessible name for the editor's text input. Ace labels its hidden
+   * textarea "Cursor at row N" by default, which is a position readout rather
+   * than a name — so this is what makes the control identifiable to a screen
+   * reader, and what `getByLabelText` finds in a test.
+   */
+  ariaLabel: string;
+  /** Rows shown before the editor grows; it autosizes between these bounds. */
+  minLines?: number;
+  maxLines?: number;
 }
 
 /**
@@ -36,6 +49,11 @@ export interface JsonObjectInputProps {
  * `_meta` value to a string, so a `{ key, value }` row editor cannot express
  * what the protocol allows.
  *
+ * Built on Ace rather than a plain textarea because hand-writing JSON in a bare
+ * box is the actual pain: Ace closes a brace when you open one, folds nested
+ * objects, and — via its JSON worker — marks the offending line in the gutter
+ * instead of reporting one message for the whole document.
+ *
  * The text the user is typing is the source of truth for what is *displayed*;
  * the parent only ever sees a parsed object. That split is what makes the box
  * typeable: driving it off the parent's value and stringifying on every render
@@ -47,35 +65,132 @@ export interface JsonObjectInputProps {
  * object stands. The alternative — emitting `{}` — would silently discard a
  * user's configured metadata the moment they typed a stray character, and this
  * form has no Save button to gate instead: `onChange` writes straight through.
- * The state is not silent, though; the field says so on itself via `error`.
+ * The state is not silent, though: Ace flags the position in the gutter, and
+ * the wrapper repeats the reason underneath.
  */
 export function JsonObjectInput({
   value,
   onChange,
-  ...inputProps
+  label,
+  description,
+  ariaLabel,
+  minLines = 6,
+  maxLines = 24,
 }: JsonObjectInputProps) {
   const [draft, setDraft] = useState(() => serialize(value));
+  // The canonical JSON of the last value this component emitted.
+  //
+  // The re-sync below has to tell an *external* change apart from the parent
+  // echoing back what we just sent it, and the draft alone cannot: while the
+  // text is invalid it parses to nothing, so every parent change would look
+  // external and overwrite what is being typed.
+  //
+  // That is not hypothetical. Ace's `setValue` — which is what a select-all
+  // paste and any programmatic replace go through — fires *two* change events,
+  // a remove then an insert. The remove leaves an empty document, which is a
+  // valid `{}` and gets emitted; the insert then leaves invalid text. Comparing
+  // against the draft, the `{}` echo reads as an external reset and wipes the
+  // pasted text a keystroke after it lands.
+  const [echoed, setEchoed] = useState(() => serialize(value));
+  // `getInitialValueInEffect: false` so the first render already has the real
+  // scheme; otherwise the editor mounts light and repaints, which is visible.
+  const colorScheme = useComputedColorScheme("light", {
+    getInitialValueInEffect: false,
+  });
+
+  // Re-sync only on a value this component did not produce, so an external
+  // reset (switching servers, a reloaded catalog) reaches the editor while an
+  // in-progress edit is left alone. Compared through canonical JSON because the
+  // parent hands back a fresh object each time.
+  // Text from the most recent change event in the current tick, and whether a
+  // flush is already queued to consume it. Two refs rather than one nullable
+  // one so the flush reads a plain `string` — folding them together would add
+  // a null check that nothing can ever satisfy. See `handleChange`.
+  const pendingTextRef = useRef("");
+  const flushScheduledRef = useRef(false);
+
+  /**
+   * Coalesce the change events Ace fires within one edit, and act on the last.
+   *
+   * A replace — select-all-and-retype, or a paste over a selection — is a
+   * *remove* followed by an *insert*, and Ace fires a change event for each.
+   * Acting on the first would read the momentarily-empty document as the user
+   * clearing the field, emit `{}` to the parent, and so wipe their saved
+   * metadata the instant they selected it. If the replacement text then failed
+   * to parse, `{}` is what they would be left with.
+   *
+   * A microtask is enough because both events are dispatched synchronously
+   * inside the same edit, so the flush sees only the settled text.
+   */
+  const handleChange = (text: string) => {
+    pendingTextRef.current = text;
+    if (flushScheduledRef.current) return;
+    flushScheduledRef.current = true;
+    queueMicrotask(() => {
+      flushScheduledRef.current = false;
+      const settled = pendingTextRef.current;
+      setDraft(settled);
+      const result = parseJsonObjectDraft(settled);
+      if (!result.ok) return;
+      setEchoed(serialize(result.value));
+      onChange(result.value);
+    });
+  };
 
-  // Re-sync only when the parent's value genuinely diverges from what the draft
-  // parses to, so an external reset (switching servers, a reloaded catalog)
-  // reaches the box while an in-progress edit is left alone. Compared through
-  // canonical JSON because the parent hands back a fresh object each time.
   const parsed = parseJsonObjectDraft(draft);
-  const draftJson = parsed.ok ? serialize(parsed.value) : null;
   useValueChange(serialize(value), (next) => {
-    if (next !== draftJson) setDraft(next);
+    if (next !== echoed) setDraft(next);
   });
 
   return (
-    <ObjectJsonInput
-      {...inputProps}
-      value={draft}
+    <Input.Wrapper
+      label={label}
+      description={description}
       error={parsed.ok ? undefined : parsed.error}
-      onChange={(text) => {
-        setDraft(text);
-        const result = parseJsonObjectDraft(text);
-        if (result.ok) onChange(result.value);
-      }}
-    />
+      w="100%"
+    >
+      <AceEditor
+        mode="json"
+        theme={colorScheme === "dark" ? "github_dark" : "github"}
+        name="json-object-input"
+        value={draft}
+        onChange={handleChange}
+        width="100%"
+        minLines={minLines}
+        maxLines={maxLines}
+        tabSize={2}
+        showPrintMargin={false}
+        editorProps={{ $blockScrolling: Infinity }}
+        onLoad={(editor) => {
+          // `textInputAriaLabel` alone is not enough: Ace composes the hidden
+          // textarea's label as "<label>, Cursor at row N" inside
+          // `TextInput.setAriaLabel`, and only recomputes it when the cursor
+          // moves — so an option applied at mount does not reach the DOM until
+          // the user clicks into the editor. Until then the control's only
+          // accessible name is the position readout, which names nothing.
+          // Setting it here and recomputing once is what makes the editor
+          // identifiable from first paint.
+          editor.setOption("textInputAriaLabel", ariaLabel);
+          editor.textInput.setAriaLabel();
+        }}
+        setOptions={{
+          // The JSON worker is what puts a marker on the offending line rather
+          // than one message for the whole document.
+          useWorker: true,
+          // Closes a brace/bracket/quote when you open one, and skips over the
+          // closing one you then type. On by default; named because it is half
+          // of why this component exists.
+          behavioursEnabled: true,
+          // Roving-tabindex arrow-key navigation plus an Esc-to-exit-trap, so
+          // the editor does not swallow Tab for keyboard users.
+          enableKeyboardAccessibility: true,
+          textInputAriaLabel: ariaLabel,
+          showFoldWidgets: true,
+          displayIndentGuides: true,
+          useSoftTabs: true,
+          scrollPastEnd: false,
+        }}
+      />
+    </Input.Wrapper>
   );
 }
diff --git a/clients/web/src/components/groups/ServerSettingsForm/ServerSettingsForm.test.tsx b/clients/web/src/components/groups/ServerSettingsForm/ServerSettingsForm.test.tsx
index cd5e168a..9cc428ad 100644
--- a/clients/web/src/components/groups/ServerSettingsForm/ServerSettingsForm.test.tsx
+++ b/clients/web/src/components/groups/ServerSettingsForm/ServerSettingsForm.test.tsx
@@ -3,6 +3,7 @@ import userEvent from "@testing-library/user-event";
 import { within } from "@testing-library/react";
 import type { InspectorServerSettings } from "@inspector/core/mcp/types.js";
 import { renderWithMantine, screen } from "../../../test/renderWithMantine";
+import { getAceText, setAceText } from "../../../test/aceEditor";
 import {
   ServerSettingsForm,
   type ServerSettingsSection,
@@ -249,8 +250,8 @@ describe("ServerSettingsForm", () => {
       screen.getByText("No custom headers configured"),
     ).toBeInTheDocument();
     // Metadata has no rows to be absent — the editor is always present, and
-    // "none configured" is spelled `{}` in the box itself.
-    expect(screen.getByLabelText("Request metadata JSON")).toHaveValue("{}");
+    // "none configured" is spelled `{}` in it.
+    expect(getAceText()).toBe("{}");
   });
 
   it("invokes onAddHeader when + Add Header is clicked", async () => {
@@ -331,17 +332,15 @@ describe("ServerSettingsForm", () => {
         expandedSections={["metadata"]}
       />,
     );
-    const editor = screen.getByLabelText("Request metadata JSON");
     // The point of #1910: a nested object survives into the editor instead of
     // being flattened into a `{ key, value }` string row.
-    expect(JSON.parse((editor as HTMLTextAreaElement).value)).toEqual({
+    expect(JSON.parse(getAceText())).toEqual({
       userId: "u-1",
       tenant: { id: 7, tags: ["a"] },
     });
   });
 
   it("reports the whole edited object through onMetadataChange", async () => {
-    const user = userEvent.setup();
     const onMetadataChange = vi.fn();
     renderWithMantine(
       <ServerSettingsForm
@@ -351,9 +350,7 @@ describe("ServerSettingsForm", () => {
         expandedSections={["metadata"]}
       />,
     );
-    const editor = screen.getByLabelText("Request metadata JSON");
-    await user.clear(editor);
-    await user.type(editor, '{{"n":[[1,2]}');
+    await setAceText('{"n":[1,2]}');
     expect(onMetadataChange).toHaveBeenLastCalledWith({ n: [1, 2] });
   });
 
diff --git a/clients/web/src/components/groups/ServerSettingsForm/ServerSettingsForm.tsx b/clients/web/src/components/groups/ServerSettingsForm/ServerSettingsForm.tsx
index 0a61a8db..4abb41e7 100644
--- a/clients/web/src/components/groups/ServerSettingsForm/ServerSettingsForm.tsx
+++ b/clients/web/src/components/groups/ServerSettingsForm/ServerSettingsForm.tsx
@@ -735,8 +735,7 @@ export function ServerSettingsForm({
               strings. Leave it as <Code>{"{}"}</Code> to send none.
             </HintText>
             <JsonObjectInput
-              aria-label="Request metadata JSON"
-              placeholder={'{\n  "example": { "nested": true }\n}'}
+              ariaLabel="Request metadata JSON"
               value={settings.metadata}
               onChange={onMetadataChange}
             />
diff --git a/clients/web/src/components/groups/ServerSettingsModal/ServerSettingsModal.test.tsx b/clients/web/src/components/groups/ServerSettingsModal/ServerSettingsModal.test.tsx
index c42fba16..9e00e644 100644
--- a/clients/web/src/components/groups/ServerSettingsModal/ServerSettingsModal.test.tsx
+++ b/clients/web/src/components/groups/ServerSettingsModal/ServerSettingsModal.test.tsx
@@ -2,6 +2,7 @@ import { describe, it, expect, vi } from "vitest";
 import userEvent from "@testing-library/user-event";
 import type { InspectorServerSettings } from "@inspector/core/mcp/types.js";
 import { renderWithMantine, screen } from "../../../test/renderWithMantine";
+import { setAceText } from "../../../test/aceEditor";
 import { ServerSettingsModal } from "./ServerSettingsModal";
 
 const initialSettings: InspectorServerSettings = {
@@ -647,9 +648,7 @@ describe("ServerSettingsModal", () => {
       />,
     );
     await user.click(screen.getByRole("button", { name: "Request Metadata" }));
-    const editor = screen.getByLabelText("Request metadata JSON");
-    await user.clear(editor);
-    await user.type(editor, '{{"trace":{{"id":"abc"}}}');
+    await setAceText('{"trace":{"id":"abc"}}');
     const lastCall = onSettingsChange.mock.calls.at(-1)?.[0];
     // The nested object arrives as an object, not a stringified one — the
     // whole point of #1910.
diff --git a/clients/web/src/test/aceEditor.ts b/clients/web/src/test/aceEditor.ts
new file mode 100644
index 00000000..1243926c
--- /dev/null
+++ b/clients/web/src/test/aceEditor.ts
@@ -0,0 +1,58 @@
+import { act } from "@testing-library/react";
+import type { Ace } from "ace-builds";
+
+/**
+ * Reach the live Ace editor a component mounted, and drive it.
+ *
+ * Typing is deliberately **not** simulated. Ace reads keystrokes through a 1px
+ * offscreen textarea plus selection and composition state that happy-dom does
+ * not implement, so `userEvent.type` reaches the element and produces no edit
+ * at all — a test written that way asserts nothing while looking like it
+ * asserts a lot. Real keyboard behaviour (auto-closing a brace on Enter, and
+ * the draft surviving invalid text) is covered by the `JsonObjectInput`
+ * Storybook play functions, which run in Chromium.
+ *
+ * Ace hangs the instance off its container element as `env.editor`; that is the
+ * supported way back to it from the DOM.
+ */
+export function getAceEditor(container: ParentNode = document): Ace.Editor {
+  const node = container.querySelector(".ace_editor");
+  const editor = (
+    node as (HTMLElement & { env?: { editor?: Ace.Editor } }) | null
+  )?.env?.editor;
+  if (!editor) throw new Error("Ace editor not mounted");
+  return editor;
+}
+
+/** The document the editor currently holds. */
+export function getAceText(container?: ParentNode): string {
+  return getAceEditor(container).getValue();
+}
+
+/**
+ * Replace the document the way a select-all-and-retype would, and let React
+ * flush.
+ *
+ * Async because `JsonObjectInput` coalesces the paired remove/insert events
+ * Ace fires for a replace; an `await act` is what drains that microtask.
+ */
+export async function setAceText(
+  text: string,
+  container?: ParentNode,
+): Promise<void> {
+  await act(async () => {
+    // `1` parks the cursor at the end, matching where typing would leave it.
+    getAceEditor(container).setValue(text, 1);
+  });
+}
+
+/**
+ * Matcher for an Ace-labelled control.
+ *
+ * Ace composes its text input's `aria-label` as `"<label>, Cursor at row N"`,
+ * so an exact-string `getByLabelText` never matches — the position readout is
+ * Ace's, only the prefix is ours.
+ */
+export function aceLabel(label: string): RegExp {
+  return new RegExp(label.replace(/[.*+?^${}()|[\]\\]/g, "\\$&"));
+}