diff --git a/frontend/src/expr/ExpressionField.test.tsx b/frontend/src/expr/ExpressionField.test.tsx
new file mode 100644
index 0000000..c3c3968
--- /dev/null
+++ b/frontend/src/expr/ExpressionField.test.tsx
@@ -0,0 +1,120 @@
+import { describe, it, expect, vi } from "vitest";
+import { useRef, useState } from "react";
+import { render, screen } from "@testing-library/react";
+import userEvent from "@testing-library/user-event";
+import { ExpressionField, type ExpressionFieldHandle } from "./ExpressionField";
+
+describe("ExpressionField", () => {
+  it("emits the raw text on change", async () => {
+    const onChange = vi.fn();
+    function Harness() {
+      const [v, setV] = useState("");
+      return (
+        <ExpressionField
+          value={v}
+          onChange={(next) => {
+            onChange(next);
+            setV(next);
+          }}
+        />
+      );
+    }
+    render(<Harness />);
+    await userEvent.type(screen.getByRole("textbox"), "hi");
+    expect(onChange).toHaveBeenLastCalledWith("hi");
+  });
+
+  it("shows the live preview against previewInput", () => {
+    render(
+      <ExpressionField
+        value="hi {input.name}!"
+        onChange={() => {}}
+        previewInput={{ name: "alice" }}
+      />,
+    );
+    expect(screen.getByTestId("expr-preview")).toHaveTextContent("hi alice!");
+  });
+
+  it("omits the preview line when previewInput is undefined", () => {
+    render(<ExpressionField value="hi {input.name}!" onChange={() => {}} />);
+    expect(screen.queryByTestId("expr-preview")).not.toBeInTheDocument();
+  });
+
+  it("shows a schema-validity warning for a bad integer literal", () => {
+    render(
+      <ExpressionField
+        value="abc"
+        onChange={() => {}}
+        schemaType="integer"
+        previewInput={{}}
+      />,
+    );
+    expect(screen.getByTestId("expr-preview")).toHaveTextContent(/expected integer/i);
+  });
+
+  it("does not warn on integer literals that are valid", () => {
+    render(
+      <ExpressionField
+        value="200"
+        onChange={() => {}}
+        schemaType="integer"
+        previewInput={{}}
+      />,
+    );
+    expect(screen.getByTestId("expr-preview")).toHaveTextContent("200");
+  });
+
+  it("does not warn on integer fields when the value contains a template", () => {
+    render(
+      <ExpressionField
+        value="{input.code}"
+        onChange={() => {}}
+        schemaType="integer"
+        previewInput={{ code: 200 }}
+      />,
+    );
+    expect(screen.getByTestId("expr-preview")).toHaveTextContent("200");
+  });
+
+  it("insertAtCaret splices text at the current caret position", async () => {
+    function Harness() {
+      const ref = useRef<ExpressionFieldHandle>(null);
+      const [v, setV] = useState("hello ");
+      return (
+        <>
+          <ExpressionField ref={ref} value={v} onChange={setV} />
+          <button onClick={() => ref.current?.insertAtCaret("{input.name}")}>insert</button>
+        </>
+      );
+    }
+    render(<Harness />);
+    const ta = screen.getByRole("textbox") as HTMLTextAreaElement;
+    ta.focus();
+    ta.setSelectionRange(ta.value.length, ta.value.length);
+    await userEvent.click(screen.getByRole("button", { name: /insert/i }));
+    expect(ta.value).toBe("hello {input.name}");
+  });
+
+  it("single-line variant blurs on Enter instead of inserting a newline", async () => {
+    const onChange = vi.fn();
+    function Harness() {
+      const [v, setV] = useState("");
+      return (
+        <ExpressionField
+          value={v}
+          onChange={(next) => {
+            onChange(next);
+            setV(next);
+          }}
+          singleLine
+        />
+      );
+    }
+    render(<Harness />);
+    const ta = screen.getByRole("textbox");
+    await userEvent.type(ta, "hi{Enter}");
+    // The Enter keydown was captured — no newline reached the value.
+    expect(onChange).toHaveBeenLastCalledWith("hi");
+    expect(document.activeElement).not.toBe(ta);
+  });
+});
diff --git a/frontend/src/expr/ExpressionField.tsx b/frontend/src/expr/ExpressionField.tsx
new file mode 100644
index 0000000..4ade9ee
--- /dev/null
+++ b/frontend/src/expr/ExpressionField.tsx
@@ -0,0 +1,179 @@
+import { forwardRef, useEffect, useImperativeHandle, useMemo, useRef, useState } from "react";
+import { render as renderTemplate } from "./templateEngine";
+
+export interface ExpressionFieldHandle {
+  insertAtCaret(text: string): void;
+  focus(): void;
+}
+
+export type ExpressionSchemaType = "string" | "integer" | "number" | "boolean";
+
+interface Props {
+  value: string;
+  onChange: (v: string) => void;
+  onFocus?: () => void;
+  previewInput?: unknown;
+  schemaType?: ExpressionSchemaType;
+  singleLine?: boolean;
+  codeBlock?: boolean;      // format:"code" fields — 10-row textarea, no Enter-blur
+  placeholder?: string;
+  id?: string;
+}
+
+export const ExpressionField = forwardRef<ExpressionFieldHandle, Props>(function ExpressionField(
+  { value, onChange, onFocus, previewInput, schemaType = "string", singleLine, codeBlock, placeholder, id },
+  ref,
+) {
+  const taRef = useRef<HTMLTextAreaElement | null>(null);
+  const overlayRef = useRef<HTMLDivElement | null>(null);
+  const [overlayScroll, setOverlayScroll] = useState({ top: 0, left: 0 });
+
+  useImperativeHandle(ref, () => ({
+    focus() {
+      taRef.current?.focus();
+    },
+    insertAtCaret(text: string) {
+      const ta = taRef.current;
+      if (!ta) return;
+      const start = ta.selectionStart ?? ta.value.length;
+      const end = ta.selectionEnd ?? ta.value.length;
+      const next = ta.value.slice(0, start) + text + ta.value.slice(end);
+      onChange(next);
+      // Restore caret position after React re-renders the value.
+      requestAnimationFrame(() => {
+        ta.focus();
+        const caret = start + text.length;
+        ta.setSelectionRange(caret, caret);
+      });
+    },
+  }), [onChange]);
+
+  const rows = codeBlock ? 10 : singleLine ? 1 : 3;
+
+  const tokens = useMemo(() => tokenize(value), [value]);
+  const preview = useMemo(() => {
+    if (previewInput === undefined) return null;
+    if (schemaType !== "string" && value !== "" && value.indexOf("{") < 0) {
+      if (!isValidLiteral(value, schemaType)) {
+        return { kind: "warn" as const, text: `⚠ expected ${schemaType}` };
+      }
+    }
+    const rendered = renderTemplate(value, previewInput);
+    const truncated = rendered.length > 200 ? rendered.slice(0, 200) + "…" : rendered;
+    return { kind: "ok" as const, text: truncated };
+  }, [value, previewInput, schemaType]);
+
+  function syncScroll() {
+    const ta = taRef.current;
+    if (!ta) return;
+    setOverlayScroll({ top: ta.scrollTop, left: ta.scrollLeft });
+  }
+
+  useEffect(() => {
+    if (overlayRef.current) {
+      overlayRef.current.scrollTop = overlayScroll.top;
+      overlayRef.current.scrollLeft = overlayScroll.left;
+    }
+  }, [overlayScroll]);
+
+  return (
+    <div className="w-full">
+      <div className="relative">
+        <div
+          ref={overlayRef}
+          aria-hidden
+          className={
+            "absolute inset-0 pointer-events-none whitespace-pre-wrap break-words font-mono text-sm " +
+            "px-3 py-2 leading-5 text-slate-100 overflow-hidden"
+          }
+        >
+          {tokens.map((t, i) =>
+            t.kind === "expr"
+              ? <span key={i} className="text-amber-300">{t.text}</span>
+              : <span key={i}>{t.text}</span>,
+          )}
+          {/* Trailing newline safeguard: keeps overlay height equal to the textarea when
+              the value ends in a newline. */}
+          {value.endsWith("\n") && <span> </span>}
+        </div>
+        <textarea
+          ref={taRef}
+          id={id}
+          value={value}
+          onChange={(e) => onChange(e.target.value)}
+          onFocus={onFocus}
+          onScroll={syncScroll}
+          onKeyDown={(e) => {
+            if (singleLine && e.key === "Enter") {
+              e.preventDefault();
+              (e.target as HTMLTextAreaElement).blur();
+            }
+          }}
+          rows={rows}
+          placeholder={placeholder}
+          spellCheck={false}
+          className={
+            "relative w-full rounded bg-slate-950 border border-slate-800 " +
+            "px-3 py-2 outline-none focus:border-brand-500 " +
+            "font-mono text-sm leading-5 resize-none " +
+            "text-transparent caret-slate-100 selection:text-slate-100 selection:bg-brand-600/40"
+          }
+          style={{ WebkitTextFillColor: "transparent" }}
+        />
+      </div>
+      {preview && (
+        <div
+          data-testid="expr-preview"
+          className={
+            "mt-1 text-xs font-mono truncate " +
+            (preview.kind === "warn" ? "text-amber-400" : "text-slate-500")
+          }
+        >
+          {preview.kind === "warn" ? preview.text : `→ ${preview.text}`}
+        </div>
+      )}
+    </div>
+  );
+});
+
+type Token = { kind: "text" | "expr"; text: string };
+
+// Tokenizer for the overlay. Matches the executor's parsing shape: `{{` and
+// `}}` are literal braces (still rendered as `{` / `}` in the "text" class),
+// unclosed `{` is rendered as text, and `{...}` runs are the "expr" class.
+function tokenize(s: string): Token[] {
+  const out: Token[] = [];
+  let i = 0;
+  let buf = "";
+  const flush = () => { if (buf) { out.push({ kind: "text", text: buf }); buf = ""; } };
+  while (i < s.length) {
+    const c = s[i];
+    if (c === "{") {
+      if (s[i + 1] === "{") { buf += "{"; i += 2; continue; }
+      const end = s.indexOf("}", i + 1);
+      if (end < 0) { buf += s.slice(i); break; }
+      flush();
+      out.push({ kind: "expr", text: s.slice(i, end + 1) });
+      i = end + 1;
+      continue;
+    }
+    if (c === "}" && s[i + 1] === "}") { buf += "}"; i += 2; continue; }
+    buf += c;
+    i++;
+  }
+  flush();
+  return out;
+}
+
+function isValidLiteral(v: string, t: ExpressionSchemaType): boolean {
+  if (t === "integer") {
+    if (!/^-?\d+$/.test(v)) return false;
+    return Number.isInteger(Number(v));
+  }
+  if (t === "number") {
+    const n = Number(v);
+    return Number.isFinite(n);
+  }
+  if (t === "boolean") return v === "true" || v === "false";
+  return true;
+}