diff --git a/package.json b/package.json
index 013c1a44f4..9df886d7b7 100644
--- a/package.json
+++ b/package.json
@@ -116,7 +116,9 @@
       "@agentclientprotocol/claude-agent-acp@0.73.0>@anthropic-ai/claude-agent-sdk": "0.3.263",
       "rollup": ">=4.59.0",
       "react": "^19.2.8",
-      "react-dom": "^19.2.8"
+      "react-dom": "^19.2.8",
+      "@codemirror/state": "^6.7.2",
+      "@codemirror/view": "^6.43.11"
     }
   }
 }
diff --git a/pnpm-workspace.yaml b/pnpm-workspace.yaml
index bec2de453b..026c899c97 100644
--- a/pnpm-workspace.yaml
+++ b/pnpm-workspace.yaml
@@ -36,3 +36,10 @@ overrides:
   rollup: ">=4.59.0"
   react: "^19.2.8"
   react-dom: "^19.2.8"
+  # CodeMirror validates extensions with instanceof, so exactly one copy of
+  # these two packages may resolve. Different CodeMirror packages pin
+  # different transitive minors, which resolves two copies and crashes the
+  # editor with "Unrecognized extension value in extension set". A shared
+  # range forces every consumer onto one resolution.
+  "@codemirror/state": "^6.7.2"
+  "@codemirror/view": "^6.43.11"
diff --git a/ui/src/lib/codemirror-single-instance.test.ts b/ui/src/lib/codemirror-single-instance.test.ts
new file mode 100644
index 0000000000..570a6fc7e8
--- /dev/null
+++ b/ui/src/lib/codemirror-single-instance.test.ts
@@ -0,0 +1,129 @@
+import { existsSync, readdirSync, readFileSync, realpathSync } from "node:fs";
+import path from "node:path";
+import { describe, expect, it } from "vitest";
+
+// CodeMirror validates extensions with instanceof, so the installed graph
+// must contain exactly one physical copy of these packages. Two resolved
+// versions ship two module instances and crash the editor at runtime with
+// "Unrecognized extension value in extension set" — a failure no unit test
+// of the editor itself catches, because each test file sees only one copy.
+// The pnpm.overrides entries in the root package.json hold the graph to a
+// single resolution; this file pins that invariant against the graph the
+// current install actually resolved, so it holds wherever the tests run —
+// CI (which installs from the lockfile it regenerates for the PR) and
+// local checkouts alike.
+const SINGLE_INSTANCE_PACKAGES = ["@codemirror/state", "@codemirror/view"];
+
+const repoRoot = path.resolve(__dirname, "../../..");
+const uiRoot = path.resolve(__dirname, "../..");
+const workspaceManifest = readFileSync(
+  path.join(repoRoot, "pnpm-workspace.yaml"),
+  "utf8",
+);
+const rootManifest = JSON.parse(
+  readFileSync(path.join(repoRoot, "package.json"), "utf8"),
+) as { pnpm?: { overrides?: Record<string, string> } };
+
+/**
+ * Every physical copy of `target` reachable from the ui package's
+ * installed graph. Walks the graph the way pnpm lays it out — each
+ * package's dependencies are linked either in its own node_modules
+ * (workspace packages) or beside it in its .pnpm bucket — and realpaths
+ * every link, so each resolved version collapses to one path. Walking
+ * from the ui roots keeps orphaned .pnpm buckets from older installs out
+ * of the census.
+ */
+function reachableCopies(target: string): string[] {
+  const copies = new Set<string>();
+  const visited = new Set<string>();
+  const queue: string[] = [uiRoot];
+
+  const packageEntries = (nodeModulesDir: string): string[] => {
+    const entries: string[] = [];
+    for (const name of readdirSync(nodeModulesDir)) {
+      if (name.startsWith(".")) continue;
+      if (name.startsWith("@")) {
+        const scopeDir = path.join(nodeModulesDir, name);
+        for (const child of readdirSync(scopeDir)) {
+          if (!child.startsWith(".")) entries.push(`${name}/${child}`);
+        }
+      } else {
+        entries.push(name);
+      }
+    }
+    return entries;
+  };
+
+  while (queue.length > 0) {
+    const packageDir = queue.shift()!;
+    if (visited.has(packageDir)) continue;
+    visited.add(packageDir);
+    if (visited.size > 20_000) {
+      throw new Error("dependency walk exceeded its safety bound");
+    }
+
+    // A package's dependencies live in its own node_modules (workspace
+    // packages) and, for store-installed packages, beside it in the
+    // .pnpm bucket's shared node_modules.
+    const dependencyDirs = [path.join(packageDir, "node_modules")];
+    const parent = path.dirname(packageDir);
+    const grandparent = path.dirname(parent);
+    if (path.basename(parent) === "node_modules") {
+      dependencyDirs.push(parent);
+    } else if (path.basename(grandparent) === "node_modules") {
+      dependencyDirs.push(grandparent); // scoped package
+    }
+
+    for (const dependencyDir of dependencyDirs) {
+      if (!existsSync(dependencyDir)) continue;
+      for (const entry of packageEntries(dependencyDir)) {
+        let entryDir: string;
+        try {
+          entryDir = realpathSync(path.join(dependencyDir, entry));
+        } catch {
+          continue; // dangling symlink
+        }
+        if (entry === target) copies.add(entryDir);
+        queue.push(entryDir);
+      }
+    }
+  }
+  return [...copies];
+}
+
+describe("codemirror single-instance invariant", () => {
+  for (const pkg of SINGLE_INSTANCE_PACKAGES) {
+    it(`keeps the ${pkg} override in the root manifest and its workspace mirror`, () => {
+      // Removing the override is the only way a second copy can come
+      // back (an override rewrites every dependent's range), so the
+      // override's presence is the other half of the invariant.
+      expect(
+        rootManifest.pnpm?.overrides?.[pkg],
+        `${pkg} must stay in pnpm.overrides (root package.json); without ` +
+          "it the graph can resolve two copies and instanceof checks " +
+          "inside the editor break.",
+      ).toMatch(/^\^6\./);
+      expect(
+        workspaceManifest,
+        `pnpm-workspace.yaml mirrors the pnpm.overrides block and must ` +
+          `carry the same ${pkg} entry.`,
+      ).toMatch(new RegExp(`^\\s+"${pkg}":`, "m"));
+    });
+
+    it(`installs exactly one physical copy of ${pkg}`, () => {
+      const copies = reachableCopies(pkg);
+      expect(
+        copies.length,
+        `${pkg} is not installed anywhere in the ui graph`,
+      ).toBeGreaterThan(0);
+      expect(
+        copies,
+        `the installed graph carries multiple physical copies of ${pkg}, ` +
+          "which break instanceof checks inside the editor. Reinstall " +
+          "against the current manifests; if the copies persist, fix the " +
+          "pnpm.overrides entry in the root package.json instead of " +
+          "allowing a second copy.",
+      ).toHaveLength(1);
+    });
+  }
+});