Tailscale ships Tailcat, its data plane without accounts or admins

Tailscale released Tailcat on August 31st, an open-source WireGuard-based data plane that enables encrypted peer-to-peer connections without accounts, identity controls, or a persistent network. Co-founder Brad Fitzpatrick wrote the first version during a 10-hour flight in September 2023, and the tool was revived after customer demand and use cases involving AI agents. Tailcat is designed for disposable or isolated machines such as development sandboxes, test hosts, and virtual machines controlled by coding agents.

Tailscale ships Tailcat, its data plane without accounts or admins Brad Fitzpatrick built the open-source tool on a 10-hour flight in 2023; customer demand and AI agents brought it back to life. By RuntimeWire Staff /author/runtimewire-staff · Published Primary source: Tailscale https://tailscale.com/blog/tailcat Why it matters Tailcat makes Tailscale's commercial boundary explicit: the encrypted networking layer can be free and self-hosted, while identity, policy, reliable relays and administration remain the products businesses pay for. Tailscale https://tailscale.com/?ref=runtimewire released Tailcat on August 31st, giving developers its WireGuard-based data plane without the accounts, identity controls or persistent network that make up Tailscale's commercial service. The release carries the fingerprints of Brad Fitzpatrick @bradfitz https://x.com/bradfitz?ref=runtimewire , Tailscale's co-founder and chief engineer. Fitzpatrick created LiveJournal in 1999, spent a decade working on Go at Google and joined Tailscale in January 2020 https://tailscale.com/blog/bradfitz-joins?ref=runtimewire with a stated preference for systems that bury necessary complexity behind simple interfaces. Tailcat follows that pattern rather literally: one machine prints a connection token, another machine uses it, and encrypted traffic starts moving. Fitzpatrick wrote the first version, then called derpcat, during a 10-hour flight in September 2023 while catching up on what he described as bad movies. According to Fitzpatrick's launch account https://tailscale.com/blog/tailcat?ref=runtimewire , he presented the tool internally, used it occasionally and mostly forgot about it; the project's repository history says it repeatedly suffered bit rot as Tailscale's internals changed. Tailscale revived it after customers arrived with use cases that fit the tool and, more recently, Fitzpatrick began using it to connect sandboxed coding agents with temporary virtual machines and remote hardware. That path from flight project to public product has precedent in Fitzpatrick's work. Before Tailscale, he created or helped create memcached and OpenID, two infrastructure projects that preceded Tailcat. Tailcat is another small utility extracted from a much larger system, released after its usefulness became clearer than its place on a roadmap. A connection token replaces the control plane Tailcat https://tailscale.com/tailcat?ref=runtimewire is an open-source Go package and command-line tool that moves bidirectional bytes between machines. Its closest conceptual ancestor is netcat, the long-standing networking utility, although Tailcat handles encryption and the unpleasant work of connecting devices behind different network address translators. A server generates a keypair and prints a Tailcat address containing its public key and information about a DERP relay. The user passes that address to the client through another channel. The client and server initially rendezvous through DERP while Tailscale's magicsock networking layer tries to establish a direct, peer-to-peer WireGuard connection over UDP. DERP continues carrying the traffic when hole punching fails. The entire process runs in userspace. Tailcat does not create a TUN device, change the host's routing table or require root access. An embedded TCP/IP stack handles connections inside the Tailcat process, allowing the tool to transfer files, forward ports, open SSH sessions or provide a temporary SOCKS proxy to another program. Those properties make Tailcat suited to machines that should remain disposable or isolated: development sandboxes, test hosts, short-lived cloud instances, edge hardware and virtual machines controlled by coding agents. In the launch post https://tailscale.com/blog/tailcat?ref=runtimewire , Fitzpatrick described using it to give an AI agent access to several generations of Raspberry Pi, let an EC2 sandbox reboot another EC2 instance while he ported Tailscale to a UEFI environment, and create and destroy Hyper-V virtual machines while debugging Go. The public repository https://github.com/tailscale/tailcat?ref=runtimewire showed 38 GitHub stars in an August 26, 2026 snapshot, five days before the public release. Tailscale offers binaries and installation through Go and Nix, and publishes Tailcat under a BSD three-clause license. Simplicity moves responsibility to the user Removing the control plane also removes the layer that normally answers who may connect, which devices belong to whom and which policies apply. Possession of the Tailcat address acts as an unlisted connection capability by default. A server can restrict access to a specified client public key, and saved keypairs can support reusable connections, but users must manage how those tokens and keys are distributed. The repository makes the support boundary equally plain. Tailscale offers no stability promise for Tailcat's command-line interface, Go API or wire format. Public DERP relays are rate-limited, carry no uptime commitment and may be withdrawn. Teams that require dedicated relays or support can pay Tailscale, while anyone prepared to operate the infrastructure can run the open-source DERP server independently. That arrangement gives Tailcat a credible route into commercial work without charging for the utility itself. Tailscale gets more developers exercising magicsock, WireGuard integration and DERP under unusual network conditions. Improvements and bug reports can flow back into the same data plane used by paying customers. Organizations that turn an experiment into production infrastructure become prospects for dedicated relay capacity, support, identity and policy controls. Fitzpatrick stated the incentive directly in the launch post https://tailscale.com/blog/tailcat?ref=runtimewire : Tailscale would prefer developers to use and improve its data plane because that work also improves the broader product. Tailscale is giving away the narrow connection primitive while retaining the administrative machinery businesses pay to avoid operating themselves. Tailscale defines what customers are paying for Tailcat draws a sharper line through Tailscale's architecture than a conventional product announcement would. Tailscale's clients and core data plane are substantially open source. Its hosted control plane handles persistent identity, single sign-on, device administration, access policy and audit functions. Tailcat demonstrates that the networking machinery can operate without any of them. That distinction matters for a business that has raised roughly $275 million across publicly announced rounds, including a $100 million Series B https://tailscale.com/blog/series-b?ref=runtimewire in 2022 and a $160 million Series C https://tailscale.com/blog/series-c?ref=runtimewire in April 2025. Accel led the Series C, with CRV, Insight Partners, Heavybit and Uncork Capital participating. Tailscale says 40,000 businesses use its products https://tailscale.com/company?ref=runtimewire , a company-reported figure that covers the broader platform rather than Tailcat. Open-sourcing Tailcat therefore amounts to a confident statement about where Tailscale expects durable value to sit. WireGuard tunnels and NAT traversal are powerful building blocks. Companies spend money on governable access, reliable relay infrastructure, administration and somebody else carrying the operational burden. That view reaches back to the founding thesis of CEO Avery Pennarun https://stratechery.com/2025/an-interview-with-tailscale-co-founder-and-ceo-avery-pennarun/?ref=runtimewire , David Carney and David Crawshaw. Pennarun, a University of Waterloo graduate who previously co-founded the networking company Net Integration Technologies and later worked at Google, has argued that most software teams inherit internet-scale complexity despite operating at far smaller scales. Tailscale was built to make private networking feel closer to a local network again. Tailcat takes that thesis down to two machines and a string copied between them. It gives Fitzpatrick the small, sharp tool he originally wanted, gives developers another route into Tailscale's networking code, and gives Tailscale a clean demonstration of what remains when every paid management feature is removed. The answer is still useful enough to ship.