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The filesystem API lets you read, write, and manage files inside a sandbox without mounting volumes or SSH. It uses the same channel as command execution, so it doesn’t touch the sandbox’s network. Handy for pushing generated code into a sandbox, pulling back results, or inspecting logs without having to pre-mount a shared directory.

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Write a file

sb.fs().write("/app/config.json", r#"{"debug": true}"#).await?;

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Read a file

let content = sb.fs().read_to_string("/app/config.json").await?;

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List a directory

let entries = sb.fs().list("/app").await?;
for entry in entries {
    println!("{}: {:?}", entry.path, entry.kind);
}

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Stream large files

For files too large to fit in memory, use streaming. Data is transferred in chunks of approximately 3 MiB each. 
let mut stream = sb.fs().read_stream("/app/data.bin").await?;
while let Some(chunk) = stream.recv().await? {
    process(&chunk);
}

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Copy from host

Copy a file from the host machine into the sandbox in a single call. 
sb.fs().copy_from_host("./local-file.txt", "/app/remote-file.txt").await?;
If you need to transfer many files at once, consider using a bind-mounted volume instead. Volumes give the guest direct filesystem access, whereas the filesystem API transfers each file individually. For bulk operations, volumes are significantly faster.