Artifact db capability: reads/writes silently fail (no data persists) across networks, browsers, devices

### Preflight Checklist

- [x] I have searched [existing issues](https://github.com/anthropics/claude-code/issues?q=is%3Aissue%20state%3Aopen%20label%3Abug) and this hasn't been reported yet
- [x] This is a single bug report (please file separate reports for different bugs)
- [x] I am using the latest version of Claude Code

### What's Wrong?

When using a Claude Artifact with the `db` capability declared, data does not persist and reads always return empty — even though the artifact tool's own read_db/write_db confirm the documents exist server-side.

Symptoms:
- Adding an item via the app's own UI appears to save (optimistic UI update), but disappears after a page refresh.
- All read queries return empty results, with no error banner shown.
- Reproduced on a brand-new artifact with default (no custom) db capability settings — not specific to one artifact.
- Reproduced on: normal Chrome tab, Incognito mode, home WiFi, and mobile data (cellular) — ruling out network/extension causes.
- Browser console shows this error on every page load:
Unsafe attempt to load URL http://160.79.104.10:6080/php/urlblock.php?args=... from frame with URL https://claude.ai/code/artifact/... . Domains, protocols and ports must match.

This looks like a platform-side issue establishing the db capability's backend connection for my account, not an artifact code bug.

Affected artifact: https://claude.ai/code/artifact/8daf8fda-ea31-4372-ad40-e5985d9eab4c

### What Should Happen?

Data entered or read through an Artifact's `db` capability should persist to the server and be readable again after a page refresh, on any network/browser/device — matching what the artifact tool's own read_db/write_db actions show is actually stored.

### Error Messages/Logs

```shell

```

### Steps to Reproduce

1. Publish a Claude Artifact with capabilities: {db: {}} declared.
2. Open the published artifact URL in a normal Chrome tab (also reproduced in Incognito mode, on home WiFi, and on mobile cellular data).
3. Use the artifact's own UI to add/save an item (it appears immediately via optimistic UI update).
4. Refresh the page.
5. The item is gone, and any db read (e.g. window.claude.use('db').then(db => db.collection(...).get())) returns 0 documents — even though the artifact tool's own read_db/write_db actions (used from the Claude session that built the artifact) confirm the documents exist server-side.
6. Reproduced this on a brand-new, separate artifact with no custom db rules — not specific to one artifact.
7. Browser console shows this error on every load: "Unsafe attempt to load URL http://160.79.104.10:6080/php/urlblock.php?args=... from frame with URL https://claude.ai/code/artifact/... . Domains, protocols and ports must match."

### Claude Model

None

### Is this a regression?

No, this never worked

### Last Working Version

_No response_

### Claude Code Version

2.1.260 (Claude Code)

### Platform

Anthropic API

### Operating System

Windows

### Terminal/Shell

Terminal.app (macOS)

### Additional Information

_No response_

No repository file or test entry point is named in the report. Start by reproducing the published artifact's db-capability steps at the linked artifact URL, then inspect the browser console and compare the artifact UI's reads and writes with the tool-confirmed server data. Done means persisted data is readable after refresh across the reported browsers, networks, and devices.

索引モデルが issue の本文から書いたものです。