diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index d624d2e..4dc2d20 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -24,4 +24,7 @@ jobs:
         run: if [ -f scripts/test ]; then nix develop -c bash ./scripts/test; fi
 
       - name: Luacheck
-        run: nix develop -c luacheck --quiet --std lua51 --no-unused-args src/
+        run: nix develop -c luacheck --quiet --std lua51 --no-unused-args --max-line-length 130 src/
+
+      - name: Format check
+        run: nix fmt && git diff --exit-code
diff --git a/.gitignore b/.gitignore
index db67e9a..e070528 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,4 +1,6 @@
 /.*
 !/.gitignore
 !/.github/
+!/.tidyrc.json
+!/.lua-format
 /output/
diff --git a/.lua-format b/.lua-format
new file mode 100644
index 0000000..2945014
--- /dev/null
+++ b/.lua-format
@@ -0,0 +1,10 @@
+# LuaFormatter config for the hand-written FFI under src/.
+# 2-space indent. Keep simple functions on one line; column_limit sits a few
+# columns under luacheck's 130 limit because lua-format under-counts the leading
+# indent and trailing comma, so this keeps every emitted line within 130.
+indent_width: 2
+use_tab: false
+column_limit: 126
+continuation_indent_width: 2
+keep_simple_function_one_line: true
+keep_simple_control_block_one_line: true
diff --git a/.tidyrc.json b/.tidyrc.json
new file mode 100644
index 0000000..8636af8
--- /dev/null
+++ b/.tidyrc.json
@@ -0,0 +1,10 @@
+{
+  "importSort": "source",
+  "importWrap": "source",
+  "indent": 2,
+  "operatorsFile": null,
+  "ribbon": 1,
+  "typeArrowPlacement": "first",
+  "unicode": "source",
+  "width": 80
+}
diff --git a/AGENTS.md b/AGENTS.md
index dde531a..a312901 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -4,11 +4,21 @@ A PureScript→Lua FFI fork in the [`purescript-lua`](https://github.com/purescr
 
 ## Commands
 
-All commands run inside the nix dev shell:
-
 - Build: `nix develop -c ./scripts/build`
 - Test (only if the fork has `scripts/test`): `nix develop -c bash ./scripts/test`
-- Lint: `nix develop -c luacheck --quiet --std lua51 --no-unused-args src/`
+- Lint: `nix develop -c luacheck --quiet --std lua51 --no-unused-args --max-line-length 130 src/`
+- Format: `nix fmt` (check: `nix fmt && git diff --exit-code`)
+
+## Formatting
+
+`nix fmt` runs treefmt (`treefmt.nix`): nixfmt for Nix, `dhall format`, purs-tidy
+for `*.purs` (config in `.tidyrc.json`), and LuaFormatter for the `*.lua` FFI
+(config in `.lua-format`). LuaFormatter is used over StyLua because it keeps the
+parentheses pslua's foreign-file parser requires. The Lua line budget is 130
+columns, matching the `luacheck --max-line-length` above. The check is
+content-based (`nix fmt && git diff --exit-code`) rather than `treefmt --ci`,
+since the in-place formatters bump mtime even when content is unchanged, which
+trips treefmt's `--fail-on-change`. CI and the pre-commit hook use it.
 
 ## Lua 5.1 target
 
diff --git a/flake.lock b/flake.lock
index 6b6c417..c47b792 100644
--- a/flake.lock
+++ b/flake.lock
@@ -740,7 +740,8 @@
         "flake-utils": "flake-utils",
         "nixpkgs": "nixpkgs",
         "pslua": "pslua",
-        "purescript-overlay": "purescript-overlay"
+        "purescript-overlay": "purescript-overlay",
+        "treefmt-nix": "treefmt-nix"
       }
     },
     "stackage": {
@@ -803,6 +804,26 @@
         "repo": "default",
         "type": "github"
       }
+    },
+    "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"
+      }
     }
   },
   "root": "root",
diff --git a/flake.nix b/flake.nix
index 5198865..4d86c4b 100644
--- a/flake.nix
+++ b/flake.nix
@@ -9,16 +9,33 @@
       inputs.nixpkgs.follows = "nixpkgs";
     };
     pslua.url = "github:purescript-lua/purescript-lua";
+    treefmt-nix = {
+      url = "github:numtide/treefmt-nix";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
   };
 
-  outputs = { self, nixpkgs, flake-utils, purescript-overlay, pslua }:
-    flake-utils.lib.eachDefaultSystem (system:
+  outputs =
+    {
+      self,
+      nixpkgs,
+      flake-utils,
+      purescript-overlay,
+      pslua,
+      treefmt-nix,
+    }:
+    flake-utils.lib.eachDefaultSystem (
+      system:
       let
         pkgs = import nixpkgs {
           inherit system;
           overlays = [ purescript-overlay.overlays.default ];
         };
-      in {
+        treefmtEval = treefmt-nix.lib.evalModule pkgs ./treefmt.nix;
+      in
+      {
+        formatter = treefmtEval.config.build.wrapper;
+        checks.formatting = treefmtEval.config.build.check self;
         devShell = pkgs.mkShell {
           buildInputs = with pkgs; [
             dhall
@@ -31,8 +48,26 @@
             spago-bin.spago-0_21_0
             treefmt
           ];
+          # Install a content-based pre-commit hook. It compares the working
+          # tree diff before and after `nix fmt`, so it only objects to changes
+          # the formatter itself introduces (not the developer's existing
+          # unstaged work) and is not fooled by formatters that only bump mtime.
+          # Rewritten each shell entry to stay in sync with this flake.
+          shellHook = ''
+            hook=.git/hooks/pre-commit
+            if [ -d .git ]; then
+              printf '%s\n' \
+                '#!/usr/bin/env bash' \
+                'before=$(git diff)' \
+                'nix fmt >/dev/null 2>&1 || exit 0' \
+                '[ "$before" = "$(git diff)" ] || { echo "nix fmt changed files; re-stage them, then commit." >&2; exit 1; }' \
+                > "$hook"
+              chmod +x "$hook"
+            fi
+          '';
         };
-      });
+      }
+    );
 
   # --- Flake Local Nix Configuration ----------------------------
   nixConfig = {
diff --git a/spago.dhall b/spago.dhall
index 171d407..a940a2f 100644
--- a/spago.dhall
+++ b/spago.dhall
@@ -1,11 +1,6 @@
 { name = "purescript-lua-unfoldable"
 , dependencies =
-  [ "foldable-traversable"
-  , "maybe"
-  , "partial"
-  , "prelude"
-  , "tuples"
-  ]
+  [ "foldable-traversable", "maybe", "partial", "prelude", "tuples" ]
 , packages = ./packages.dhall
 , sources = [ "src/**/*.purs" ]
 , backend =
diff --git a/src/Data/Unfoldable.lua b/src/Data/Unfoldable.lua
index 85ce757..444a388 100644
--- a/src/Data/Unfoldable.lua
+++ b/src/Data/Unfoldable.lua
@@ -9,9 +9,7 @@ return {
               local value = b
               while true do
                 local maybe = f(value)
-                if isNothing(maybe) then
-                  return result
-                end
+                if isNothing(maybe) then return result end
                 local tuple = fromJust(maybe)
                 table.insert(result, fst(tuple))
                 value = snd(tuple)
diff --git a/src/Data/Unfoldable.purs b/src/Data/Unfoldable.purs
index fd115c1..5ac0364 100644
--- a/src/Data/Unfoldable.purs
+++ b/src/Data/Unfoldable.purs
@@ -5,7 +5,8 @@
 -- | sequences, etc.
 
 module Data.Unfoldable
-  ( class Unfoldable, unfoldr
+  ( class Unfoldable
+  , unfoldr
   , replicate
   , replicateA
   , none
@@ -63,10 +64,10 @@ foreign import unfoldrArrayImpl
 replicate :: forall f a. Unfoldable f => Int -> a -> f a
 replicate n v = unfoldr step n
   where
-    step :: Int -> Maybe (Tuple a Int)
-    step i =
-      if i <= 0 then Nothing
-      else Just (Tuple v (i - 1))
+  step :: Int -> Maybe (Tuple a Int)
+  step i =
+    if i <= 0 then Nothing
+    else Just (Tuple v (i - 1))
 
 -- | Perform an Applicative action `n` times, and accumulate all the results.
 -- |
diff --git a/src/Data/Unfoldable1.purs b/src/Data/Unfoldable1.purs
index 2eb285b..a487569 100644
--- a/src/Data/Unfoldable1.purs
+++ b/src/Data/Unfoldable1.purs
@@ -1,5 +1,6 @@
 module Data.Unfoldable1
-  ( class Unfoldable1, unfoldr1
+  ( class Unfoldable1
+  , unfoldr1
   , replicate1
   , replicate1A
   , singleton
@@ -65,10 +66,10 @@ foreign import unfoldr1ArrayImpl
 replicate1 :: forall f a. Unfoldable1 f => Int -> a -> f a
 replicate1 n v = unfoldr1 step (n - 1)
   where
-    step :: Int -> Tuple a (Maybe Int)
-    step i
-      | i <= 0 = Tuple v Nothing
-      | otherwise = Tuple v (Just (i - 1))
+  step :: Int -> Tuple a (Maybe Int)
+  step i
+    | i <= 0 = Tuple v Nothing
+    | otherwise = Tuple v (Just (i - 1))
 
 -- | Perform an `Apply` action `n` times (at least once, so values `n` less
 -- | than 1 will be treated as 1), and accumulate the results.
@@ -109,9 +110,11 @@ range :: forall f. Unfoldable1 f => Int -> Int -> f Int
 range start end =
   let delta = if end >= start then 1 else -1 in unfoldr1 (go delta) start
   where
-    go delta i =
-      let i' = i + delta
-      in Tuple i (if i == end then Nothing else Just i')
+  go delta i =
+    let
+      i' = i + delta
+    in
+      Tuple i (if i == end then Nothing else Just i')
 
 -- | Create an `Unfoldable1` by repeated application of a function to a seed value.
 -- | For example:
diff --git a/treefmt.nix b/treefmt.nix
new file mode 100644
index 0000000..0f57573
--- /dev/null
+++ b/treefmt.nix
@@ -0,0 +1,43 @@
+{ pkgs, ... }:
+{
+  projectRootFile = "flake.nix";
+
+  # Nix — RFC 166 formatter.
+  programs.nixfmt.enable = true;
+
+  # Dhall — spago.dhall / packages.dhall layout.
+  programs.dhall.enable = true;
+
+  # PureScript — purs-tidy is not a first-class treefmt program, so wire it via
+  # the generic mechanism. It picks up `.tidyrc.json` from the project root.
+  settings.formatter.purs-tidy = {
+    command = "${pkgs.purs-tidy}/bin/purs-tidy";
+    options = [ "format-in-place" ];
+    includes = [ "*.purs" ];
+  };
+
+  # Lua FFI — LuaFormatter keeps the parentheses pslua's foreign-file parser
+  # requires (unlike StyLua, which strips them). Config in `.lua-format`.
+  settings.formatter.lua-format = {
+    command = "${pkgs.luaformatter}/bin/lua-format";
+    options = [
+      "-i"
+      "-c"
+      ".lua-format"
+    ];
+    includes = [ "*.lua" ];
+  };
+
+  # Never format generated output or vendored trees.
+  settings.global.excludes = [
+    "dist/*"
+    "output/*"
+    ".spago/*"
+    "node_modules/*"
+    "*.lock"
+    "flake.lock"
+    "spago.lock"
+    ".tidyrc.json"
+    ".lua-format"
+  ];
+}