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
index c5bb8bf..2945014 100644
--- a/.lua-format
+++ b/.lua-format
@@ -1,32 +1,10 @@
-align_args: true
-align_parameter: true
-align_table_field: true
-break_after_functioncall_lp: false
-break_after_functiondef_lp: false
-break_after_operator: true
-break_after_table_lb: true
-break_before_functioncall_rp: false
-break_before_functiondef_rp: false
-break_before_table_rb: true
-chop_down_kv_table: true
-chop_down_parameter: false
-chop_down_table: false
-column_limit: 120
-column_table_limit: 120
-continuation_indent_width: 2
-double_quote_to_single_quote: false
-extra_sep_at_table_end: false
+# 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
-keep_simple_control_block_one_line: true
-keep_simple_function_one_line: true
-line_breaks_after_function_body: 1
-line_separator: input
-single_quote_to_double_quote: false
-spaces_around_equals_in_field: true
-spaces_before_call: 1
-spaces_inside_functioncall_parens: false
-spaces_inside_functiondef_parens: false
-spaces_inside_table_braces: false
-tab_width: 2
-table_sep: ","
 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/src/Data/Number.purs b/src/Data/Number.purs
index 926b2e5..e15b97c 100644
--- a/src/Data/Number.purs
+++ b/src/Data/Number.purs
@@ -18,7 +18,8 @@ module Data.Number
   , max
   , min
   , pow
-  , remainder, (%)
+  , remainder
+  , (%)
   , round
   , sign
   , sin
@@ -112,7 +113,9 @@ foreign import isFinite :: Number -> Boolean
 fromString :: String -> Maybe Number
 fromString str = runFn4 fromStringImpl str isFinite Just Nothing
 
-foreign import fromStringImpl :: Fn4 String (Number -> Boolean) (forall a. a -> Maybe a) (forall a. Maybe a) (Maybe Number)
+foreign import fromStringImpl
+  :: Fn4 String (Number -> Boolean) (forall a. a -> Maybe a) (forall a. Maybe a)
+       (Maybe Number)
 
 -- | Returns the absolute value of the argument.
 -- | ```purs
diff --git a/src/Data/Number/Approximate.purs b/src/Data/Number/Approximate.purs
index b8afcf5..0b747dc 100644
--- a/src/Data/Number/Approximate.purs
+++ b/src/Data/Number/Approximate.purs
@@ -42,9 +42,9 @@ newtype Fraction = Fraction Number
 -- | true
 -- | ```
 eqRelative :: Fraction -> Number -> Number -> Boolean
-eqRelative (Fraction frac) 0.0   y =       abs y <= frac
-eqRelative (Fraction frac)   x 0.0 =       abs x <= frac
-eqRelative (Fraction frac)   x   y = abs (x - y) <= frac * abs (x + y) / 2.0
+eqRelative (Fraction frac) 0.0 y = abs y <= frac
+eqRelative (Fraction frac) x 0.0 = abs x <= frac
+eqRelative (Fraction frac) x y = abs (x - y) <= frac * abs (x + y) / 2.0
 
 -- | Test if two numbers are approximately equal, up to a relative difference
 -- | of one part in a million:
@@ -63,8 +63,8 @@ eqRelative (Fraction frac)   x   y = abs (x - y) <= frac * abs (x + y) / 2.0
 eqApproximate :: Number -> Number -> Boolean
 eqApproximate = eqRelative onePPM
   where
-    onePPM :: Fraction
-    onePPM = Fraction 1.0e-6
+  onePPM :: Fraction
+  onePPM = Fraction 1.0e-6
 
 infix 4 eqApproximate as ~=
 infix 4 eqApproximate as ≅
diff --git a/src/Data/Number/Format.purs b/src/Data/Number/Format.purs
index 6cf4080..fa6d406 100644
--- a/src/Data/Number/Format.purs
+++ b/src/Data/Number/Format.purs
@@ -30,8 +30,8 @@ module Data.Number.Format
 
 import Prelude
 
-foreign import toPrecisionNative ::   Int -> Number -> String
-foreign import toFixedNative ::       Int -> Number -> String
+foreign import toPrecisionNative :: Int -> Number -> String
+foreign import toFixedNative :: Int -> Number -> String
 foreign import toExponentialNative :: Int -> Number -> String
 
 -- | The `Format` data type specifies how a number will be formatted.
@@ -57,8 +57,8 @@ exponential = Exponential <<< clamp 0 20
 
 -- | Convert a number to a string with a given format.
 toStringWith :: Format -> Number -> String
-toStringWith (Precision p)   = toPrecisionNative p
-toStringWith (Fixed p)       = toFixedNative p
+toStringWith (Precision p) = toPrecisionNative p
+toStringWith (Fixed p) = toFixedNative p
 toStringWith (Exponential p) = toExponentialNative p
 
 -- | Convert a number to a string via JavaScript's toString method.
diff --git a/test/Test/Main.purs b/test/Test/Main.purs
index 98e466d..0f5c874 100644
--- a/test/Test/Main.purs
+++ b/test/Test/Main.purs
@@ -60,20 +60,27 @@ infix 1 eqAbsolute' as =~=
 numbersTestCode :: Effect Unit
 numbersTestCode = do
 
-
   log "Data.Number.fromString"
   log "\tvalid number string"
-  assertTrue' "integer strings are coerced" $
-    fromMaybe false $ map (_ == 123.0) $ fromString "123"
-
-  assertTrue' "decimals are coerced" $
-    fromMaybe false $ map (_ == 12.34) $ fromString "12.34"
-
-  assertTrue' "exponents are coerced" $
-    fromMaybe false $ map (_ == 1e4) $ fromString "1e4"
-
-  assertTrue' "decimals exponents are coerced" $
-    fromMaybe false $ map (_ == 1.2e4) $ fromString "1.2e4"
+  assertTrue' "integer strings are coerced"
+    $ fromMaybe false
+    $ map (_ == 123.0)
+    $ fromString "123"
+
+  assertTrue' "decimals are coerced"
+    $ fromMaybe false
+    $ map (_ == 12.34)
+    $ fromString "12.34"
+
+  assertTrue' "exponents are coerced"
+    $ fromMaybe false
+    $ map (_ == 1e4)
+    $ fromString "1e4"
+
+  assertTrue' "decimals exponents are coerced"
+    $ fromMaybe false
+    $ map (_ == 1.2e4)
+    $ fromString "1.2e4"
 
   log "\tinvalid number string"
   assertTrue' "invalid strings are not coerced" $
@@ -215,7 +222,7 @@ numbersTestCode = do
   assertFalse' "should return false for differences larger than 10%" $
     0.000000000001 ~= 0.00000000000111
 
-      -- assertFalse
+  -- assertFalse
   assertFalse' "should return false for differences larger than 10%" $
     0.000000000001 ~= 0.0000000000009
 
@@ -259,7 +266,6 @@ numbersTestCode = do
   assertTrue' "should work for 'fractions' larger than one" $
     eqRelative (Fraction 3.0) 10.0 29.5
 
-
   log "Data.Number.Approximate.eqApproximate"
   log "\t0.1 + 0.2 ≅ 0.3"
   assertTrue' "0.1 + 0.2 should be approximately equal to 0.3" $
@@ -268,7 +274,6 @@ numbersTestCode = do
   assertTrue' "0.1 + 0.200001 should not be approximately equal to 0.3" $
     0.1 + 0.200001 ≇ 0.3
 
-
   log "Data.Number.Approximate.eqAbsolute"
   log "\teqAbsolute"
   assertTrue' "should succeed for differences smaller than the tolerance" $
diff --git a/test/regression/format.lua b/test/regression/format.lua
index 0fb2d6d..8e93aa4 100644
--- a/test/regression/format.lua
+++ b/test/regression/format.lua
@@ -3,6 +3,6 @@
 local M = assert(dofile("src/Data/Number/Format.lua"))
 
 assert(type(M.toPrecisionNative(6)(1234.56789)) == "string",
-  "toPrecisionNative must format its bound argument, not error on a nil global")
+       "toPrecisionNative must format its bound argument, not error on a nil global")
 
 print("OK Format.lua uses the bound argument n")
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"
+  ];
+}