I built a local-first tab manager and gave it an MCP server so Claude can organize my tabs

My browser is a war crime I'm a designer and developer, and a hopeless tab hoarder. On a normal day I've got 70+ tabs open across a couple of windows, every favicon crushed into an unreadable sliver, and my "system" for finding anything is to just… open another tab. Bookmarks? A write-only graveyard I never revisit. I bounced between tab managers for a while. Toby was the one that stuck - until it capped the free plan at 60 saved tabs and started asking for an account. For a tab hoarder, a 60-tab cap is like a notes app that locks after 60 words. That was the nudge. So I did the reasonable thing and spent way too long building my own. It's called Mos Tab. This is the build story - the parts I'm proud of, the part that nearly broke me (multiple windows, I'm looking at you), and the weirdly fun part at the end where I let Claude reorganize my tabs. What I actually wanted Three non-negotiables: - Local-first. No account required. Data lives in the browser, works offline. I was tired of "sign in to see your own bookmarks." - No limits on the free stuff. Saving a tab locally costs me nothing to run - so there's no reason to cap it. - A calm dashboard, not a list. I stare at my new tab all day; it should feel like a desk, not a void. The shape of it Mos Tab overrides chrome://newtab (MV3). The data model is a boring normalized tree - Space → Collection → Section → Tab - records plus ID arrays for ordering. It's a single Zustand store with a persist middleware writing to chrome.storage.local . Your currently-open tabs show in a live side panel (chrome.tabs ); one click saves them into a collection, drag to organize, and a cmdk palette searches everything you've saved. Stack, for the curious: React 18 + TypeScript + Vite + @crxjs/vite-plugin , Tailwind, dnd-kit, zod, i18next (5 languages). Since the new tab is prime real estate, I threw in the stuff I open a tab for anyway - calendar, pomodoro, to-dos, sticky notes. All free. So far, so normal. Then I wanted sync. The hard part: sync (or, how multiple windows tried to kill me) Local-first is lovely until you have two machines. I wanted optional cloud sync (the paid tier - more on money later) without turning this into a database migration. The backend is deliberately dumb: a Cloudflare Worker + R2, one JSON blob per user (state.json ). No per-row DB. To save, you PUT the blob back with an ETag conditional request (If-Match ). On a 412 (someone wrote first), you pull, 3-way merge on the client, and push again. The 3-way merge (base + local + remote) handles the real cases: both sides added a tab → keep both; something in base vanished on one side → it was deleted, respect it; field conflicts → local wins. Sticky-note text conflicts resolve to a symmetric content-addressed copy, so two machines converge instead of endlessly duplicating. The part that actually broke me wasn't cross-device - it was multiple windows on the same machine. Every window wanted to push, each invalidating the others' ETag → 412 → merge → push → 412 → forever. A beautiful infinite loop. The fix: only the focused window pushes. Every window can pull (reads are idempotent), but writes are gated to the focused one, with a flush-on-blur so the last edit isn't lost when you switch away. Add adaptive polling backoff (15s active → 2min idle) and an "away mode" that stops polling after 20 min of no activity, and it stops hammering the Worker. The fun part: teaching Claude to use my tabs (MCP) Here's the part I'm actually excited about. I gave Mos Tab a remote MCP server so you can connect Claude or ChatGPT to it. Once connected, the assistant can search and summarize everything you've saved (tabs, notes, to-dos) and - the fun bit - save new things it finds into a brand-new space, add notes, or make a public share link. All from the chat. Concrete example. I told Claude: "Find the top-rated budget strollers on Amazon, save them to my tabs, and make a share link." A few seconds later there was a new "Stroller shortlist" space on my dashboard, organized into categories, and a public link I sent straight to my wife - she opened it in her browser, nothing to install. Two design decisions I'm happy with: - It's add-only. The assistant can create spaces, notes, and shares - but it can't touch or reorganize your existing spaces. No "the AI nuked my setup" risk. That made me trust it way more. - The implementation is boring on purpose. Streamable HTTP JSON-RPC on the same plain- fetch Worker - no Durable Objects, no McpAgent. Auth is a long-lived scoped token you mint in settings (JWTs expire, which is bad for a persistent connection). Reads filter the same per-user blob in memory; writes are read-modify-write withIf-Match / 412 retry - so an AI-created space merges through the exact same 3-way sync as a manual edit. That last bit is the thing I like most: AI writes go through the same door as everything else. No special path, no separate store. Money, honestly Because someone always asks: the whole local app is free - unlimited tabs, spaces, all the widgets, import/export. Local storage costs me nothing to run, so there's no reason to cap it. The paid tier is only the stuff that actually hits a server: multi-device sync, public sharing, and the AI/MCP integration. I can't give the server stuff away free - kind of wish I could. That's the whole pricing philosophy, and it keeps me honest. What I learned - "Local-first" is a great default and a real constraint. Deciding exactly what syncs vs. what stays on-device (your data and notes sync; theme and zoom are per-device) took more thought than the merge algorithm. - The boring backend was the right call. One blob + conditional PUT scales further than I expected - the 5MB blob cap only bites around 15-30k saved tabs, and by then I'd happily move to per-entity storage (D1). - MCP is more fun than it has any right to be. Giving an assistant a small, safe, add-only surface into your own data feels like the actual point of these protocols. Try it Live on the Chrome Web Store (Chrome + Chromium - Edge, Brave, Arc, etc.): - Chrome Web Store: https://chromewebstore.google.com/detail/jmahojbjdibdhfniieiljgildbpcefoc - Site: https://mos-tab.com - A live shared space (the actual "stroller shortlist" Claude built): https://mos-tab.com/s/10yyW9pbA_R69a2quKLM392V I'm a solo dev, so I read everything - feedback on the sync merge model or the MCP design especially welcome. Top comments (0)