diff --git a/.gitignore b/.gitignore
index c5edab1..7a6353d 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,2 +1 @@
 .envrc
-.direnv/
diff --git a/flake.lock b/flake.lock
index 3ea27c6..42b5046 100644
--- a/flake.lock
+++ b/flake.lock
@@ -2,11 +2,11 @@
   "nodes": {
     "nixpkgs": {
       "locked": {
-        "lastModified": 1755577059,
-        "narHash": "sha256-5hYhxIpco8xR+IpP3uU56+4+Bw7mf7EMyxS/HqUYHQY=",
+        "lastModified": 1783279667,
+        "narHash": "sha256-/NAkDSsve+GNM0Bt6tleJdCGfsTlK89nPjkVOzZMo0s=",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "97eb7ee0da337d385ab015a23e15022c865be75c",
+        "rev": "f205b5574fd0cb7da5b702a2da51507b7f4fdd1b",
         "type": "github"
       },
       "original": {
@@ -18,7 +18,28 @@
     },
     "root": {
       "inputs": {
-        "nixpkgs": "nixpkgs"
+        "nixpkgs": "nixpkgs",
+        "treefmt-nix": "treefmt-nix"
+      }
+    },
+    "treefmt-nix": {
+      "inputs": {
+        "nixpkgs": [
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1780220602,
+        "narHash": "sha256-eynAfOmbmxJnkp7YewvCEbShNnnYJ9gLLqkzsYtBPeM=",
+        "owner": "numtide",
+        "repo": "treefmt-nix",
+        "rev": "db947814a175b7ca6ded66e21383d938df01c227",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "repo": "treefmt-nix",
+        "type": "github"
       }
     }
   },
diff --git a/flake.nix b/flake.nix
index 24210ba..8e095e0 100644
--- a/flake.nix
+++ b/flake.nix
@@ -1,9 +1,18 @@
 {
   inputs = {
     nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
+    treefmt-nix = {
+      url = "github:numtide/treefmt-nix";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
   };
 
-  outputs = {nixpkgs, ...} @ inputs: let
+  outputs = {
+    self,
+    nixpkgs,
+    treefmt-nix,
+    ...
+  }: let
     lib = nixpkgs.lib;
     supportedSystems = ["x86_64-linux" "aarch64-linux" "x86_64-darwin" "aarch64-darwin"];
     forEachSupportedSystem = f:
@@ -11,27 +20,27 @@
         f {
           pkgs = import nixpkgs {inherit system;};
         });
+    treefmtEval = forEachSupportedSystem ({pkgs, ...}: treefmt-nix.lib.evalModule pkgs ./treefmt.nix);
   in {
-    devShells = forEachSupportedSystem ({pkgs}: {
+    devShells = forEachSupportedSystem ({pkgs, ...}: {
       default = pkgs.mkShell {
         packages = with pkgs; [
           gleam
-          erlang_28
-          beam28Packages.rebar3
-          jq
+          beamMinimal29Packages.erlang
+          beamMinimal29Packages.rebar3
         ];
       };
     });
-    apps = forEachSupportedSystem ({pkgs}: let
+
+    apps = forEachSupportedSystem ({pkgs, ...}: let
       cdInto = service: ''
         rootDir=$(jj --ignore-working-copy root || git rev-parse --show-toplevel) || (echo "error: can't find repo root?"; exit 1)
         cd "$rootDir/${service}"
       '';
-      # TODO: dedup depencency decl.
       runtimeInputs = with pkgs; [
         gleam
-        erlang_28
-        beam28Packages.rebar3
+        beamMinimal29Packages.erlang
+        beamMinimal29Packages.rebar3
       ];
     in {
       # TODO: Check if env vars are already set
@@ -61,31 +70,12 @@
           '';
         })}/bin/run-web";
       };
-      test-api = {
-        type = "app";
-        program = "${(pkgs.writeShellApplication {
-          inherit runtimeInputs;
-          name = "test-api";
-          text = ''
-            ${cdInto "api"}
-            export KATARI_SRD_PATH="$rootDir/srd"
-            ${pkgs.gleam}/bin/gleam test
-          '';
-        })}/bin/test-api";
-      };
-      test-web = {
-        type = "app";
-        program = "${(pkgs.writeShellApplication {
-          inherit runtimeInputs;
-          name = "test-web";
-          text = ''
-            ${cdInto "web"}
-            export KATARI_SRD_PATH="$rootDir/srd"
-            export KATARI_API_DOCS=true
-            ${pkgs.gleam}/bin/gleam test
-          '';
-        })}/bin/test-web";
-      };
+    });
+
+    formatter = forEachSupportedSystem ({pkgs, ...}: treefmtEval.${pkgs.stdenv.hostPlatform.system}.config.build.wrapper);
+    checks = forEachSupportedSystem ({pkgs, ...}: {
+      formatting =
+        treefmtEval.${pkgs.stdenv.hostPlatform.system}.config.build.check self;
     });
   };
 }
diff --git a/treefmt.nix b/treefmt.nix
new file mode 100644
index 0000000..15a8d59
--- /dev/null
+++ b/treefmt.nix
@@ -0,0 +1,5 @@
+{pkgs, ...}: {
+  projectRootFile = "flake.nix";
+  programs.gleam.enable = true;
+  programs.alejandra.enable = true;
+}