Build 2025: Microsoft open-sources WSL and puts MCP in Windows

Microsoft Build ran this week. Three announcements matter to developers and one of them has been requested since 2016.

WSL is open source#

The Windows Subsystem for Linux is now open source at github.com/microsoft/WSL. Not all of it — some Windows-side components remain closed — but the substantial majority, including the init system, the networking daemons, and the file sharing layer.

This was the single most requested item on the WSL issue tracker for years. The practical effect is that people can finally debug the parts of WSL that go wrong, which historically meant filing an issue and waiting.

The strategic read: WSL won. It is how a very large number of developers run Linux tooling on a Windows machine, it removed most of the reason to dual-boot, and open-sourcing it costs Microsoft nothing now that the position is secure. It is a mature move from a company that spent the nineties doing the opposite.

GitHub Copilot coding agent#

Copilot gets a delegated mode: assign an issue to it, and it opens a pull request. It runs in GitHub Actions, follows repository instructions in .github/copilot-instructions.md, and its pull requests go through normal review and CI.

Notably it cannot approve its own pull requests and its Actions runs require approval. Those are the correct guardrails and it is good that they shipped with them rather than after an incident.

The integration point is the interesting bit. An agent that operates on issues and pull requests slots into a workflow every team already has, rather than requiring a new one. That is a much lower adoption barrier than "install this IDE."

MCP in Windows#

Windows gets native support for the Model Context Protocol: an MCP registry, built-in servers exposing filesystem, windowing, and shell capabilities, and a permission model gating what an agent can reach.

An operating system vendor adopting a protocol that a model company published six months ago is fast by any standard. It also tells you Microsoft has concluded that the agent-to-system interface is going to be standardized and would rather be the platform for it than fight it.

The security model is where this gets interesting and where I would like to see more detail. An OS-level MCP surface means any agent with the right permission can drive the machine. The consent flow, the scoping granularity, and the audit trail are the whole ballgame, and "there is a permission model" is not yet enough information to evaluate it.

also announced#

NLWeb, a project to let websites expose a natural-language query interface with MCP underneath. Positioned as "HTML for the agentic web." Ambitious; adoption is everything and adoption is unknowable.

Edit, a new command-line text editor for Windows, open source, small. A genuine gap — Windows has had no default CLI editor since edit.com — and filling it is a nice piece of housekeeping.

Windows AI Foundry, the rebranded local model runtime story, with Foundry Local for running models on-device.

the pattern#

Microsoft's developer strategy for a decade has been: meet developers where they are, adopt other people's standards, open-source the layers where control is not worth the friction, and monetize the cloud underneath.

It keeps working. Build 2025 was more of it, executed well.