diff --git a/app/src/index.css b/app/src/index.css
index ea2b657..9c300f6 100644
--- a/app/src/index.css
+++ b/app/src/index.css
@@ -33,6 +33,15 @@
   /* Two radii only: controls and panels. */
   --radius-control: 6px;
   --radius-panel: 8px;
+  /* Syntax colours for the file viewer's editor (viewer/viewerTheme.ts). Same
+     GitHub-dark palette TerminalView.tsx already uses for ANSI, expressed as
+     tokens rather than hard-coded hex per the styling convention above. */
+  --syntax-keyword: #ff7b72;
+  --syntax-string: #a5d6ff;
+  --syntax-number: #79c0ff;
+  --syntax-function: #d2a8ff;
+  --syntax-type: #ffa657;
+  --syntax-property: #7ee787;
   color-scheme: dark;
 }
 
diff --git a/app/src/viewer/CodeEditor.tsx b/app/src/viewer/CodeEditor.tsx
new file mode 100644
index 0000000..fae857b
--- /dev/null
+++ b/app/src/viewer/CodeEditor.tsx
@@ -0,0 +1,125 @@
+import { forwardRef, useEffect, useImperativeHandle, useRef } from "react";
+import { Annotation, EditorState, Compartment, EditorSelection, type Extension } from "@codemirror/state";
+import { EditorView, keymap, lineNumbers, highlightActiveLine, highlightActiveLineGutter, drawSelection, highlightSpecialChars } from "@codemirror/view";
+import { defaultKeymap, history, historyKeymap, indentWithTab } from "@codemirror/commands";
+import { search, searchKeymap } from "@codemirror/search";
+import { bracketMatching, indentOnInput } from "@codemirror/language";
+import type { ViewerLocation } from "../lib/types";
+import { viewerTheme } from "./viewerTheme";
+import { highlightExtension, setHighlight } from "./highlightLine";
+
+export interface CodeEditorHandle {
+  getDoc(): string;
+  /** Replace the whole document, keeping scroll and a clamped cursor. Does not mark dirty. */
+  setDoc(text: string): void;
+  goTo(loc: ViewerLocation): void;
+  focus(): void;
+}
+
+export interface CodeEditorProps {
+  initialDoc: string;
+  readOnly: boolean;
+  language: Extension | null;
+  lineWrapping: boolean;
+  initialLocation: ViewerLocation;
+  onDocChanged(): void;
+  onSave(): void;
+}
+
+/** A `dispatch` from `setDoc` is a reload, not a user edit; the listener must not mark it dirty. */
+const reloadTag = Annotation.define<boolean>();
+
+function readOnlyExt(readOnly: boolean): Extension[] {
+  return [EditorState.readOnly.of(readOnly), EditorView.editable.of(!readOnly)];
+}
+
+export const CodeEditor = forwardRef<CodeEditorHandle, CodeEditorProps>(function CodeEditor(props, ref) {
+  const host = useRef<HTMLDivElement>(null);
+  const view = useRef<EditorView | null>(null);
+  const readOnlyCompartment = useRef(new Compartment());
+  const languageCompartment = useRef(new Compartment());
+  const wrapCompartment = useRef(new Compartment());
+  const callbacks = useRef(props);
+  callbacks.current = props;
+
+  useEffect(() => {
+    if (!host.current) return;
+    const v = new EditorView({
+      parent: host.current,
+      state: EditorState.create({
+        doc: props.initialDoc,
+        extensions: [
+          lineNumbers(),
+          highlightActiveLine(),
+          highlightActiveLineGutter(),
+          highlightSpecialChars(),
+          drawSelection(),
+          history(),
+          bracketMatching(),
+          indentOnInput(),
+          search({ top: true }),
+          highlightExtension(),
+          viewerTheme,
+          keymap.of([
+            { key: "Mod-s", run: () => { callbacks.current.onSave(); return true; } },
+            ...defaultKeymap, ...historyKeymap, ...searchKeymap, indentWithTab,
+          ]),
+          readOnlyCompartment.current.of(readOnlyExt(props.readOnly)),
+          languageCompartment.current.of(props.language ?? []),
+          wrapCompartment.current.of(props.lineWrapping ? EditorView.lineWrapping : []),
+          EditorView.updateListener.of((u) => {
+            if (u.docChanged && !u.transactions.some((tr) => tr.annotation(reloadTag))) callbacks.current.onDocChanged();
+          }),
+        ],
+      }),
+    });
+    view.current = v;
+    goTo(v, props.initialLocation);
+    return () => { v.destroy(); view.current = null; };
+    // The editor is created once per mount; later prop changes go through compartments below.
+    // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, []);
+
+  useEffect(() => {
+    view.current?.dispatch({ effects: readOnlyCompartment.current.reconfigure(readOnlyExt(props.readOnly)) });
+  }, [props.readOnly]);
+  useEffect(() => {
+    view.current?.dispatch({ effects: languageCompartment.current.reconfigure(props.language ?? []) });
+  }, [props.language]);
+  useEffect(() => {
+    view.current?.dispatch({ effects: wrapCompartment.current.reconfigure(props.lineWrapping ? EditorView.lineWrapping : []) });
+  }, [props.lineWrapping]);
+
+  useImperativeHandle(ref, () => ({
+    getDoc: () => view.current?.state.doc.toString() ?? "",
+    setDoc: (text) => {
+      const v = view.current;
+      if (!v) return;
+      const scrollTop = v.scrollDOM.scrollTop;
+      const head = Math.min(v.state.selection.main.head, text.length);
+      v.dispatch({
+        changes: { from: 0, to: v.state.doc.length, insert: text },
+        selection: EditorSelection.single(head),
+        annotations: reloadTag.of(true),
+      });
+      v.scrollDOM.scrollTop = scrollTop;
+    },
+    goTo: (loc) => { if (view.current) goTo(view.current, loc); },
+    focus: () => view.current?.focus(),
+  }));
+
+  return <div ref={host} className="h-full min-h-0" data-testid="code-editor" />;
+});
+
+function goTo(v: EditorView, loc: ViewerLocation): void {
+  if (loc.line === null) return;
+  const from = loc.line;
+  const to = loc.end_line ?? loc.line;
+  const lineNo = Math.min(Math.max(1, from), v.state.doc.lines);
+  const line = v.state.doc.line(lineNo);
+  const pos = Math.min(line.from + Math.max(0, (loc.col ?? 1) - 1), line.to);
+  v.dispatch({
+    selection: EditorSelection.cursor(pos),
+    effects: [setHighlight.of({ from, to }), EditorView.scrollIntoView(pos, { y: "center" })],
+  });
+}
diff --git a/app/src/viewer/highlightLine.test.ts b/app/src/viewer/highlightLine.test.ts
new file mode 100644
index 0000000..04e91d8
--- /dev/null
+++ b/app/src/viewer/highlightLine.test.ts
@@ -0,0 +1,31 @@
+import { describe, expect, it } from "vitest";
+import { EditorState, Text } from "@codemirror/state";
+import { highlightExtension, highlightLineField, lineRangeToPositions, setHighlight } from "./highlightLine";
+
+describe("lineRangeToPositions", () => {
+  const doc = Text.of(["one", "two", "three"]);
+  it("maps 1-based inclusive lines to document offsets", () => {
+    expect(lineRangeToPositions(doc, 2, 2)).toEqual({ from: 4, to: 4 });
+    expect(lineRangeToPositions(doc, 1, 3)).toEqual({ from: 0, to: 8 });
+  });
+  it("clamps past the end and refuses nonsense", () => {
+    expect(lineRangeToPositions(doc, 2, 99)).toEqual({ from: 4, to: 8 });
+    expect(lineRangeToPositions(doc, 99, 100)).toEqual({ from: 8, to: 8 });
+    expect(lineRangeToPositions(doc, 0, 1)).toEqual({ from: 0, to: 0 });
+    expect(lineRangeToPositions(doc, 3, 1)).toEqual({ from: 8, to: 8 });
+  });
+});
+
+describe("highlightLineField", () => {
+  it("decorates every line in the range and clears on null", () => {
+    let state = EditorState.create({ doc: "a\nb\nc\nd", extensions: [highlightExtension()] });
+    state = state.update({ effects: setHighlight.of({ from: 2, to: 3 }) }).state;
+    let count = 0;
+    state.field(highlightLineField).between(0, state.doc.length, () => { count++; });
+    expect(count).toBe(2);
+    state = state.update({ effects: setHighlight.of(null) }).state;
+    count = 0;
+    state.field(highlightLineField).between(0, state.doc.length, () => { count++; });
+    expect(count).toBe(0);
+  });
+});
diff --git a/app/src/viewer/highlightLine.ts b/app/src/viewer/highlightLine.ts
new file mode 100644
index 0000000..937d94f
--- /dev/null
+++ b/app/src/viewer/highlightLine.ts
@@ -0,0 +1,40 @@
+import { StateEffect, StateField, type Extension, type Text } from "@codemirror/state";
+import { Decoration, EditorView, type DecorationSet } from "@codemirror/view";
+
+export const setHighlight = StateEffect.define<{ from: number; to: number } | null>();
+
+const lineMark = Decoration.line({ class: "cm-triple-c-target" });
+
+export function lineRangeToPositions(doc: Text, from: number, to: number): { from: number; to: number } | null {
+  const clamp = (n: number) => Math.min(Math.max(1, Math.floor(n)), doc.lines);
+  const a = clamp(from);
+  const b = Math.max(a, clamp(to));
+  return { from: doc.line(a).from, to: doc.line(b).from };
+}
+
+export const highlightLineField = StateField.define<DecorationSet>({
+  create: () => Decoration.none,
+  update(value, tr) {
+    let next = value.map(tr.changes);
+    for (const e of tr.effects) {
+      if (!e.is(setHighlight)) continue;
+      if (e.value === null) { next = Decoration.none; continue; }
+      const range = lineRangeToPositions(tr.state.doc, e.value.from, e.value.to);
+      if (!range) { next = Decoration.none; continue; }
+      const marks = [];
+      for (let pos = range.from; pos <= range.to; ) {
+        const line = tr.state.doc.lineAt(pos);
+        marks.push(lineMark.range(line.from));
+        if (line.to + 1 > tr.state.doc.length) break;
+        pos = line.to + 1;
+      }
+      next = Decoration.set(marks, true);
+    }
+    return next;
+  },
+  provide: (f) => EditorView.decorations.from(f),
+});
+
+export function highlightExtension(): Extension {
+  return [highlightLineField];
+}
diff --git a/app/src/viewer/languages.test.ts b/app/src/viewer/languages.test.ts
new file mode 100644
index 0000000..9f558e7
--- /dev/null
+++ b/app/src/viewer/languages.test.ts
@@ -0,0 +1,28 @@
+import { describe, expect, it } from "vitest";
+import { languageFor, wrapsLines } from "./languages";
+
+describe("languageFor", () => {
+  it.each(["a.ts", "a.tsx", "a.js", "a.jsx", "a.mjs", "a.rs", "a.py", "a.json", "a.yaml", "a.yml", "a.toml", "a.sh", "a.bash", "a.css", "a.html", "a.md", "Dockerfile", "Cargo.lock", "README"])(
+    "resolves %s without throwing", async (name) => {
+      const result = await languageFor(`/workspace/${name}`);
+      if (name === "README") {
+        expect(result).toBeNull();
+      } else {
+        expect(result).not.toBeNull();
+      }
+    });
+  it("returns null for an unknown extension", async () => {
+    await expect(languageFor("/workspace/x.xyz")).resolves.toBeNull();
+  });
+  it("returns an extension for markdown", async () => {
+    await expect(languageFor("/workspace/x.md")).resolves.not.toBeNull();
+  });
+});
+
+describe("wrapsLines", () => {
+  it("wraps prose, not code", () => {
+    expect(wrapsLines("x.md")).toBe(true);
+    expect(wrapsLines("x.txt")).toBe(true);
+    expect(wrapsLines("x.rs")).toBe(false);
+  });
+});
diff --git a/app/src/viewer/languages.ts b/app/src/viewer/languages.ts
new file mode 100644
index 0000000..4ee1a22
--- /dev/null
+++ b/app/src/viewer/languages.ts
@@ -0,0 +1,59 @@
+/**
+ * Extension → CodeMirror language, loaded on demand so a window only pays for
+ * the grammar it shows. Dynamic `import()` becomes a same-origin chunk, fine
+ * under `script-src 'self'`.
+ */
+import type { Extension } from "@codemirror/state";
+import { extensionOf } from "../components/projects/home/filePreview";
+
+type Loader = () => Promise<Extension>;
+
+const BY_EXTENSION: Record<string, Loader> = {
+  md: () => import("@codemirror/lang-markdown").then((m) => m.markdown()),
+  markdown: () => import("@codemirror/lang-markdown").then((m) => m.markdown()),
+  js: () => import("@codemirror/lang-javascript").then((m) => m.javascript()),
+  mjs: () => import("@codemirror/lang-javascript").then((m) => m.javascript()),
+  cjs: () => import("@codemirror/lang-javascript").then((m) => m.javascript()),
+  jsx: () => import("@codemirror/lang-javascript").then((m) => m.javascript({ jsx: true })),
+  ts: () => import("@codemirror/lang-javascript").then((m) => m.javascript({ typescript: true })),
+  tsx: () => import("@codemirror/lang-javascript").then((m) => m.javascript({ jsx: true, typescript: true })),
+  rs: () => import("@codemirror/lang-rust").then((m) => m.rust()),
+  py: () => import("@codemirror/lang-python").then((m) => m.python()),
+  json: () => import("@codemirror/lang-json").then((m) => m.json()),
+  jsonc: () => import("@codemirror/lang-json").then((m) => m.json()),
+  yaml: () => import("@codemirror/lang-yaml").then((m) => m.yaml()),
+  yml: () => import("@codemirror/lang-yaml").then((m) => m.yaml()),
+  css: () => import("@codemirror/lang-css").then((m) => m.css()),
+  html: () => import("@codemirror/lang-html").then((m) => m.html()),
+  htm: () => import("@codemirror/lang-html").then((m) => m.html()),
+  toml: () => stream("toml"),
+  lock: () => stream("toml"),
+  sh: () => stream("shell"),
+  bash: () => stream("shell"),
+  zsh: () => stream("shell"),
+};
+
+const BY_BASENAME: Record<string, Loader> = {
+  dockerfile: () => stream("shell"),
+  makefile: () => stream("shell"),
+};
+
+async function stream(mode: "toml" | "shell"): Promise<Extension> {
+  const { StreamLanguage } = await import("@codemirror/language");
+  const parser = mode === "toml"
+    ? (await import("@codemirror/legacy-modes/mode/toml")).toml
+    : (await import("@codemirror/legacy-modes/mode/shell")).shell;
+  return StreamLanguage.define(parser);
+}
+
+export function languageFor(path: string): Promise<Extension | null> {
+  const ext = extensionOf(path);
+  const base = path.slice(path.lastIndexOf("/") + 1).toLowerCase();
+  const loader = BY_EXTENSION[ext] ?? BY_BASENAME[base];
+  return loader ? loader() : Promise.resolve(null);
+}
+
+const PROSE = new Set(["md", "markdown", "txt", "rst", "log", ""]);
+export function wrapsLines(path: string): boolean {
+  return PROSE.has(extensionOf(path));
+}
diff --git a/app/src/viewer/viewerTheme.ts b/app/src/viewer/viewerTheme.ts
new file mode 100644
index 0000000..8fb88c2
--- /dev/null
+++ b/app/src/viewer/viewerTheme.ts
@@ -0,0 +1,42 @@
+import { EditorView } from "@codemirror/view";
+import { HighlightStyle, syntaxHighlighting } from "@codemirror/language";
+import { tags as t } from "@lezer/highlight";
+import type { Extension } from "@codemirror/state";
+
+// Syntax colours come from the `--syntax-*` tokens in index.css (P12), not
+// hard-coded hex, even though the values match the GitHub-dark ANSI palette
+// TerminalView.tsx already uses.
+export const viewerTheme: Extension = [
+  EditorView.theme(
+    {
+      "&": { backgroundColor: "var(--bg-primary)", color: "var(--text-primary)", height: "100%", fontSize: "13px" },
+      ".cm-content": { fontFamily: "'JetBrains Mono', 'Fira Code', 'Cascadia Code', Menlo, Monaco, monospace", caretColor: "var(--accent)" },
+      ".cm-scroller": { overflow: "auto" },
+      ".cm-gutters": { backgroundColor: "var(--bg-secondary)", color: "var(--text-secondary)", borderRight: "1px solid var(--border-color)" },
+      ".cm-activeLine": { backgroundColor: "var(--accent-muted)" },
+      ".cm-activeLineGutter": { backgroundColor: "var(--accent-muted)" },
+      ".cm-triple-c-target": { backgroundColor: "var(--warning-muted)", outline: "1px solid var(--warning)" },
+      "&.cm-focused .cm-selectionBackground, .cm-selectionBackground": { backgroundColor: "var(--accent-muted)" },
+      ".cm-panels": { backgroundColor: "var(--bg-secondary)", color: "var(--text-primary)", borderBottom: "1px solid var(--border-color)" },
+      ".cm-searchMatch": { backgroundColor: "var(--warning-muted)", outline: "1px solid var(--warning)" },
+      ".cm-searchMatch.cm-searchMatch-selected": { backgroundColor: "var(--success-muted)" },
+    },
+    { dark: true },
+  ),
+  syntaxHighlighting(
+    HighlightStyle.define([
+      { tag: [t.keyword, t.modifier, t.operatorKeyword], color: "var(--syntax-keyword)" },
+      { tag: [t.string, t.special(t.string)], color: "var(--syntax-string)" },
+      { tag: [t.comment, t.lineComment, t.blockComment], color: "var(--text-secondary)", fontStyle: "italic" },
+      { tag: [t.number, t.bool, t.null, t.atom], color: "var(--syntax-number)" },
+      { tag: [t.function(t.variableName), t.function(t.propertyName)], color: "var(--syntax-function)" },
+      { tag: [t.typeName, t.className, t.namespace], color: "var(--syntax-type)" },
+      { tag: [t.propertyName, t.attributeName], color: "var(--syntax-property)" },
+      { tag: t.heading, fontWeight: "bold", color: "var(--accent)" },
+      { tag: t.emphasis, fontStyle: "italic" },
+      { tag: t.strong, fontWeight: "bold" },
+      { tag: t.link, color: "var(--accent)", textDecoration: "underline" },
+      { tag: t.invalid, color: "var(--syntax-keyword)", textDecoration: "underline wavy" },
+    ]),
+  ),
+];