The decentralised social web is split across protocols. Your friends might be on Mastodon (ActivityPub), Nostr, or somewhere else entirely. Mostr Bridge is one of the more serious attempts to connect those worlds without forcing everyone to move or keep duplicate accounts. This article explains how it works, where it falls short, and what to check before relying on it.
Connecting ActivityPub and Nostr
Mostr Bridge links two protocols that do not share much at all: ActivityPub (used by Mastodon, Pixelfed, Lemmy, and the broader fediverse) and Nostr, short for Notes and Other Stuff Transmitted by Relays. They have almost no common infrastructure, which is why bridging them is genuinely difficult rather than merely awkward.
The bridge works by creating proxy accounts that stand in for users from one protocol on the other. When a Nostr user posts, the bridge generates an ActivityPub version of that post so Mastodon users can see it and interact with it. The reverse happens too. It sounds tidy on paper. In practice, the translation layer loses enough that it is never quite a one-to-one handover.
Technically, the Nostr-to-Mastodon direction watches Nostr relays for new posts, then creates ActivityPub activities and delivers them to Mastodon users who follow that Nostr user’s bridge account. The other direction signs Mastodon content with a Nostr key and publishes it to Nostr relays. Replies, boosts, and likes are translated where a sensible equivalent exists. Not every interaction has one.
Setting Up the Bridge
Following Nostr Users from Mastodon
Start with the Nostr user’s public key, or npub. From your Mastodon client, search for their bridge handle, which usually looks like npub...@mostr.pub or something close to that. Follow the bridge account and their Nostr posts will appear in your Mastodon home timeline with everything else.
Following Mastodon Users from Nostr
This direction takes a little more work. Add the Mostr Bridge relay address to your Nostr client’s relay list, then search for the Mastodon user’s bridge identity and follow them as you would any other Nostr account.
Bridging Your Own Account
If you want your posts to flow both ways, you need accounts on both platforms. Register with the Mostr Bridge service, link the accounts through its verification process, and choose which content you want bridged. In practice, that usually means public posts only. Keep that in mind before you start.
What Actually Crosses Over, and What Gets Dropped
Text posts move between protocols reliably enough. Basic cross-protocol following works for public content, and the bridge does genuinely make it possible to discover and follow people on a different network without creating a native account there. That is its strongest case.
The limitations are more awkward. Image handling is inconsistent. Reply threading often breaks or shows up badly on one side. Protocol-specific features simply do not carry over: Nostr zaps, Mastodon content warnings, and similar native interactions have no equivalent on the other side, so they disappear rather than degrade gracefully. Delays can also build under load, which makes bridges a poor fit for anything time-sensitive.
Identity verification deserves a separate mention. Confirming that a bridge account really represents the person it claims to means checking the bridge’s own verification system, which most users will not bother with. Direct messages should never be bridged between protocols. The security implications are serious enough that this needs saying plainly.
Privacy and Consent
Bridging raises questions that are easy to ignore until something goes wrong. When a Mastodon post is bridged to Nostr, it lands on relays with different data retention norms and different privacy expectations. The reverse is true as well. You may not expect where your content ends up, or how long it stays there.
The fediverse community has strong views about consent in bridging. Many users want explicit opt-in before their content is bridged to another protocol. That is not a fringe position. Bridging someone’s public posts without their knowledge is technically possible, but socially contentious in these communities.
Metadata exposure is subtler. Timestamps, interaction patterns, and other signals can travel across protocol boundaries in ways you probably did not account for when you posted. Blocking is also imperfect: blocking a bridged account on Mastodon does not block the underlying Nostr account, and the bridge handles block propagation inconsistently.
The practical norms are simple. Only bridge your own content. Respect users who have said they do not want their posts bridged. Put a note in your profile bio if your account is bridged, so followers on either side know what they are looking at.
Other Bridges Worth Knowing About
Mostr Bridge is not the only option. Experimental bridges exist for connecting ActivityPub and the AT Protocol (Bluesky), though they run into similar limitations. Some projects bridge fediverse content to email newsletters through ActivityPub-to-email approaches. RSS bridges are simpler still: they publish Mastodon feeds as RSS so the content can be read in any feed reader. Less powerful, yes. Much more predictable too.
Each bridge sits at a different point on the maturity curve, with different feature sets and different failure modes. The ecosystem is still moving quickly enough that a bridge that works well today may change quite a bit within a year.
For more on cross-protocol considerations, see our developer notes.
When a Bridge Makes Sense, and When It Does Not
Bridges suit a small number of specific use cases: following a few people on a different protocol, publishing your content to a wider audience across networks, or trying out a new protocol without fully committing to it. They also work well enough if you want some presence on multiple networks without much admin overhead.
They are a poor fit for anyone expecting a consistent, feature-complete multi-protocol experience. Bridging private or sensitive content is a bad idea. Replacing native platform presence entirely with a bridge will probably frustrate you eventually. High-volume, real-time cross-protocol interaction is where bridges crack first - latency and dropped interactions pile up quickly at scale.
Common Mistakes
The biggest mistake is assuming bridging is transparent. It is not. Bridged content often loses formatting, drops media quality, or arrives stripped of the context that made it readable on the original platform.
A related error is bridging without understanding the consent norms on the receiving side. The fediverse, in particular, has a culture around this that is worth understanding before routing other people’s posts through it. Relying on bridges for important communications is risky too: bridges go down, and they introduce delays, so anything time-critical should go through native platform access. Another common miss is not disclosing bridge use in your bio. And expecting protocol-specific features to survive the crossing is just the wrong assumption - they will not.
Frequently Asked Questions
Is Mostr Bridge official? Mostr Bridge is a community project with no official affiliation with Mastodon or Nostr. It operates independently.
Can I bridge my Mastodon account without anyone knowing? Technically yes. But transparency is a strong community norm, and disclosing bridge use in your bio is the expected approach.
Does bridging cost anything? Most bridges are free to use, usually funded by donations or the operator’s own resources.
What happens if the bridge goes down? Cross-protocol interactions stop until the bridge is restored. Your native accounts on each platform continue to work normally. Check our tools page for bridge status resources.
Can I bridge to multiple protocols at once? In theory, yes - you could bridge to Nostr and Bluesky at the same time. In practice, each bridge is separate and has to be configured independently, so the admin overhead adds up.
Should I use a bridge or just create accounts on both platforms? For casual cross-protocol following, a bridge is convenient. For active participation in a community, a native account gives you a noticeably better experience. Many users end up doing both for different purposes. See our articles hub for platform comparison guides.