facebook/react · error · Error

Expected mounted root to have known pseudo key.

Error message

Expected mounted root to have known pseudo key.

What it means

After resolving the root instance for a HostRoot fiber, getPathFrame() requires rootPseudoKeys to contain an entry for that instance's id. The pseudo key is written by setRootPseudoKey at initial flush, filter re-mount, and commit; the throw means a known root instance reached path computation without ever being keyed - registration and keying got out of sync.

Source

Thrown at packages/react-devtools-shared/src/backend/fiber/renderer.js:7853

  }

  function getPathFrame(fiber: Fiber): PathFrame {
    const {key} = fiber;
    let displayName = getDisplayNameForFiber(fiber);
    const index = fiber.index;
    switch (fiber.tag) {
      case HostRoot:
        // Roots don't have a real displayName, index, or key.
        // Instead, we'll use the pseudo key (childDisplayName:indexWithThatName).
        const rootInstance = rootToFiberInstanceMap.get(fiber.stateNode);
        if (rootInstance === undefined) {
          throw new Error(
            'Expected the root instance to exist when computing a path',
          );
        }
        const pseudoKey = rootPseudoKeys.get(rootInstance.id);
        if (pseudoKey === undefined) {
          throw new Error('Expected mounted root to have known pseudo key.');
        }
        displayName = pseudoKey;
        break;
      case HostComponent:
        displayName = fiber.type;
        break;
      default:
        break;
    }
    return {
      displayName,
      key: key === REACT_OPTIMISTIC_KEY ? null : key,
      index,
    };
  }

  function getVirtualPathFrame(virtualInstance: VirtualInstance): PathFrame {
    return {

View on GitHub (pinned to eafeac097b)

Solutions

1. Reload the page with DevTools open to rebuild all root keys.
2. Re-select the element after the tree settles instead of relying on a selection persisted across the mount.
3. Update DevTools; keying coverage fixes land in react-devtools-shared.
4. Report a repro if current versions still throw.

// only persist/restore selection paths after the target root has committed and appears in the tree
const rootVisible = store.getElementByID(rootTreeID) != null;
if (rootVisible) {
  persistSelectionPath(id);
}

When it happens

Trigger: Path computation for a root that was registered in rootToFiberInstanceMap but skipped setRootPseudoKey - roots that appeared mid filter re-mount or during the attach window, or bookkeeping partially rewritten by a re-mount.

Common situations: Selection capture/restore right as a root mounts; toggling filters in multi-root apps; DevTools attach racing root creation.

Related errors

• Expected root pseudo key to be known.
• Expected counter to be known.
• Expected the root instance to exist when computing a path
• Expected to see a frame at the next depth.
• Expected a root instance to exist for this Fiber root

AI-assisted analysis of facebook/react@eafeac097b (2026-08-21). Data as JSON: /api/errors/dc9f7d381bfefaf8. Report an issue: GitHub.