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P4 Virtual File Service Documentation | 2025.2

# Optional: Custom development

Perforce provides a virtual file service implementation for end users. P4 VFS is an implementation that makes use of an agent that relies on the `P4ALTSYNC` environment variable to provide a syncing mechanism that is an alternative to the traditional P4 sync mechanism. Hence the name `P4ALTSYNC`.

However, if you are a software developer at an organization that wants to implement a different solution for a virtual file service, you can use this Guide's protocol reference to implement your own agent.

## Operating systems

Although P4 VFS is available solely on Windows because it depends on Windows-specific APIs, the P4ALTSYNC protocol is cross-platform.

## Prerequisites for your custom agent

The custom agent must:

- Be specified in the `P4ALTSYNC` environment variable in one of the following ways:

  - A path to the agent's executable.

  - A named pipe prefixed with `pipe:` on which the agent is listening.

- Use the JSON Protocol commands in the Protocol reference.

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