> ## Documentation Index
> Fetch the complete documentation index at: https://test-8862363a-tembo-docs-tembo-nix-loading-and-troubleshoot.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

# Custom Dependencies

> Add tools that are not pre-installed in the sandbox, starting with a project setup script or using tembo.nix if you have beta access.

## Start with a setup script

Use a project setup script as the starting point for custom dependencies. Tembo runs the script after cloning your repositories while it builds the project's prepared environments. Anything the script installs or writes to disk becomes part of those environments.

Add your script under **Advanced setup** when you create or edit a project. See [Setup script](/features/projects#setup-script) for details.

<Note>
  `tembo.nix` is currently in beta and will be available to everyone very soon. If you have beta access, use the sections below to declare system packages and toolchains in your repository.
</Note>

## Prerequisites

* You have a repository connected to Tembo.
* You know which system packages or language toolchains your project needs.

## Create tembo.nix

Create `tembo.nix` in your repository root with a default dev shell. Tembo uses `devShells.x86_64-linux.default` in the sandbox.

```nix theme={null}
{
  description = "Tembo Cloud VM Dependencies";

  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
    flake-utils.url = "github:numtide/flake-utils";
  };

  outputs = { self, nixpkgs, flake-utils }:
    flake-utils.lib.eachDefaultSystem (system:
      let
        pkgs = nixpkgs.legacyPackages.${system};
      in {
        devShells.default = pkgs.mkShell {
          packages = with pkgs; [
            go
            rustc
            cargo
            jdk
          ];
        };
      }
    );
}
```

After you commit the file, new Tembo sessions use the dev shell automatically. Agents can then run commands that depend on those packages, such as `go test`, `cargo test`, or Java build tools.

## Add packages

Add packages to the `packages` list. For example, this dev shell adds PostgreSQL client tools and `pkg-config` for projects that compile native dependencies:

```nix theme={null}
{
  description = "Tembo Cloud VM Dependencies";

  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
    flake-utils.url = "github:numtide/flake-utils";
  };

  outputs = { self, nixpkgs, flake-utils }:
    flake-utils.lib.eachDefaultSystem (system:
      let
        pkgs = nixpkgs.legacyPackages.${system};
      in {
        devShells.default = pkgs.mkShell {
          packages = with pkgs; [
            postgresql
            pkg-config
            openssl
          ];
        };
      }
    );
}
```

## Configure the shell

Use `shellHook` when the sandbox needs environment variables for local commands:

```nix theme={null}
{
  description = "Tembo Cloud VM Dependencies";

  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
    flake-utils.url = "github:numtide/flake-utils";
  };

  outputs = { self, nixpkgs, flake-utils }:
    flake-utils.lib.eachDefaultSystem (system:
      let
        pkgs = nixpkgs.legacyPackages.${system};
      in {
        devShells.default = pkgs.mkShell {
          packages = with pkgs; [
            nodejs_22
            pnpm
          ];

          shellHook = ''
            export NODE_ENV=development
          '';
        };
      }
    );
}
```

Keep secrets out of `tembo.nix`. Add secrets through your sandbox [environment variables](/features/sandbox/environment-variables) instead.

## How Tembo loads tembo.nix

When Tembo needs your dev shell, it temporarily replaces `flake.nix` in the repository with the contents of `tembo.nix`, evaluates the default dev shell, and then restores the original `flake.nix`.

This has a few consequences worth knowing:

* `tembo.nix` must be a complete, self-contained flake. Tembo does not merge it with a `flake.nix` that your repository already has.
* A `flake.nix` in your repository is left untouched, and Tembo does not use it to build the dev shell. Put everything the sandbox needs in `tembo.nix`.
* Tembo never writes a `flake.lock` for `tembo.nix`. Pin `nixpkgs` to a specific revision in `inputs` if you need dependency versions to stay stable over time.
* Tembo only reads `tembo.nix` from the root of each cloned repository. Files in subdirectories are ignored.
* Unfree packages are allowed, so you can add them without extra configuration.

## Dependencies in projects

When a [project](/features/projects) builds with repository dependency installation enabled, Tembo evaluates `tembo.nix` once during the build and caches the resulting toolchain in the prepared environment. Sessions started from that project reuse the cached toolchain instead of evaluating the flake again, which is part of why they start faster.

Because the toolchain is captured at build time, edits to `tembo.nix` do not reach project sessions until the environment is rebuilt. Rebuild the project or wait for its daily refresh to pick up the change.

## Troubleshooting

**Dependencies are missing even though the project build succeeded.** If `tembo.nix` fails to evaluate during a project build, Tembo records the failure and finishes the build without the dependencies rather than failing the whole build. Open the project's build log and look for the `nix develop` step. A syntax error, an input that cannot be fetched, or a package name that does not exist in the pinned `nixpkgs` are the usual causes.

**Changes to `tembo.nix` have no effect.** Confirm the file is committed and pushed, then rebuild the project environment so the new toolchain is captured.

## Tips

* Keep `tembo.nix` focused on system packages and toolchains that your project needs.
* Commit the file so Tembo can load it in every new session.
* Use [projects](/features/projects) if installing dependencies still takes meaningful time at the start of each session.