diff --git a/CHANGELOG.md b/CHANGELOG.md
new file mode 100644
--- /dev/null
+++ b/CHANGELOG.md
@@ -0,0 +1,12 @@
+# Changelog
+
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
+and this project adheres to the [Haskell Package Versioning Policy](https://pvp.haskell.org/).
+
+## [1.0.0] - 2026-10-20
+
+### Added
+
+- Initial release.
diff --git a/LICENCE b/LICENCE
new file mode 100644
--- /dev/null
+++ b/LICENCE
@@ -0,0 +1,287 @@
+                      EUROPEAN UNION PUBLIC LICENCE v. 1.2
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+
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+
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+  residence or registered office inside a European Union Member State.
+
+Appendix
+
+‘Compatible Licences’ according to Article 5 EUPL are:
+
+- GNU General Public License (GPL) v. 2, v. 3
+- GNU Affero General Public License (AGPL) v. 3
+- Open Software License (OSL) v. 2.1, v. 3.0
+- Eclipse Public License (EPL) v. 1.0
+- CeCILL v. 2.0, v. 2.1
+- Mozilla Public Licence (MPL) v. 2
+- GNU Lesser General Public Licence (LGPL) v. 2.1, v. 3
+- Creative Commons Attribution-ShareAlike v. 3.0 Unported (CC BY-SA 3.0) for
+  works other than software
+- European Union Public Licence (EUPL) v. 1.1, v. 1.2
+- Québec Free and Open-Source Licence — Reciprocity (LiLiQ-R) or Strong
+  Reciprocity (LiLiQ-R+).
+
+The European Commission may update this Appendix to later versions of the above
+licences without producing a new version of the EUPL, as long as they provide
+the rights granted in Article 2 of this Licence and protect the covered Source
+Code from exclusive appropriation.
+
+All other changes or additions to this Appendix require the production of a new
+EUPL version.
diff --git a/app/Config.hs b/app/Config.hs
new file mode 100644
--- /dev/null
+++ b/app/Config.hs
@@ -0,0 +1,40 @@
+module Config where
+
+import Data.List.NonEmpty (NonEmpty, some1)
+import Data.Version (showVersion)
+import Options.Applicative
+import Paths_fluent_lint qualified
+import Prelude
+
+-- | What the command line asks for.
+data Config = Config
+    { lenient :: Bool
+    , reference :: Maybe FilePath
+    , files :: NonEmpty FilePath
+    }
+
+-- | The options and arguments of the command line.
+parser :: Parser Config
+parser = do
+    lenient <-
+        switch $
+            long "lenient"
+                <> help "Warn about syntax errors instead of failing"
+    reference <-
+        optional . strOption $
+            long "reference"
+                <> metavar "FILE"
+                <> help "The file whose messages every file must match"
+    files <- some1 . strArgument $ metavar "FILE" <> help "The files to check"
+    pure Config{..}
+
+parserInfo :: ParserInfo Config
+parserInfo =
+    info (helper <*> versioner <*> parser) . header $
+        "Read Fluent files and validate their contents for errors or missing keys."
+  where
+    versioner = simpleVersioner $ "fluent-lint " <> showVersion Paths_fluent_lint.version
+
+-- | Reads the command line.
+get :: IO Config
+get = execParser parserInfo
diff --git a/app/Document.hs b/app/Document.hs
new file mode 100644
--- /dev/null
+++ b/app/Document.hs
@@ -0,0 +1,64 @@
+module Document where
+
+import Data.Functor ((<&>))
+import Data.List ((!?))
+import Data.List qualified as List
+import Data.Map.Strict (Map)
+import Data.Map.Strict qualified as Map
+import Data.Maybe (fromMaybe)
+import Data.Set (Set)
+import Data.Set qualified as Set
+import Data.Text (Text)
+import Data.Text qualified as Text
+import Data.Text.IO qualified as Text
+import Effectful
+import Language.Fluent.AST
+import Language.Fluent.Parser (parseResourceLenient)
+import System.IO (IOMode (ReadMode), hSetEncoding, utf8, withFile)
+import Prelude
+
+-- | A Fluent file, with its text and what was parsed from it.
+data Document = Document
+    { path :: FilePath
+    , source :: Text
+    , messages :: Map Text (Set Text)
+    , errors :: [ParseError]
+    }
+
+-- | Reads and parses a file, which is UTF-8 whatever the locale is.
+read :: (IOE :> es) => FilePath -> Eff es Document
+read path = do
+    source <- liftIO . withFile path ReadMode $ \handle ->
+        hSetEncoding handle utf8 *> Text.hGetContents handle
+    let resource = parseResourceLenient source
+    pure
+        Document
+            { messages =
+                Map.fromList
+                    [ (name, Set.fromList [name | Attribute (Identifier name) _ <- attributes])
+                    | MessageEntry Message{id = Identifier name, attributes} <- resource.entries
+                    ]
+            , errors = [e | JunkEntry _ e <- resource.entries]
+            , ..
+            }
+
+-- | Renders syntax errors, each with its source line and an arrow under the column.
+renderErrors :: Document -> String
+renderErrors Document{source = Text.lines -> source, ..} =
+    List.intercalate "\n\n" $
+        errors <&> \ParseError{..} ->
+            let number = show line
+                shown =
+                    Text.unpack
+                        . Text.dropWhileEnd (== '\r')
+                        . fromMaybe Text.empty
+                        $ source !? (line - 1)
+             in List.intercalate
+                    "\n"
+                    [ path <> ":" <> number <> ":" <> show column <> ": " <> reason
+                    , number <> " | " <> shown
+                    , blank number <> " | " <> blank (take (column - 1) shown) <> "^"
+                    ]
+  where
+    blank :: String -> String
+    blank = fmap \c -> if c == '\t' then c else ' '
diff --git a/app/Lint.hs b/app/Lint.hs
new file mode 100644
--- /dev/null
+++ b/app/Lint.hs
@@ -0,0 +1,40 @@
+module Lint where
+
+import Control.Monad (unless, when)
+import Data.Foldable (for_)
+import Data.Map.Strict qualified as Map
+import Data.Set qualified as Set
+import Document (Document (..))
+import Document qualified
+import Effectful (Eff, (:>))
+import Effectful.Hspec (Hspec, expectationFailure, it, pendingWith, shouldBe)
+import Prelude
+
+-- | A syntax error fails the file, which is then not compared with the reference.
+strict :: (Hspec :> es) => Maybe Document -> Document -> Eff es ()
+strict reference document@Document{errors} = do
+    it "parses" . unless (null errors) . expectationFailure $ Document.renderErrors document
+    when (null errors) . for_ reference $ flip matches document
+
+-- | A syntax error is a warning, and the file is still compared with the reference.
+lenient :: (Hspec :> es) => Maybe Document -> Document -> Eff es ()
+lenient reference document@Document{errors} = do
+    it "parses" . unless (null errors) . pendingWith $ Document.renderErrors document
+    for_ reference $ flip matches document
+
+-- | Checks that the file has the messages and attributes of the reference.
+matches :: (Hspec :> es) => Document -> Document -> Eff es ()
+matches reference document = do
+    it "has every expected message" $
+        Map.keys (reference.messages `Map.difference` document.messages)
+            `shouldBe` []
+    it "has no unexpected messages" $
+        Map.keys (document.messages `Map.difference` reference.messages)
+            `shouldBe` []
+    it "has every expected attribute" $
+        [ name <> "." <> attribute
+        | (name, missing) <-
+            Map.toList $ Map.intersectionWith Set.difference reference.messages document.messages
+        , attribute <- Set.toList missing
+        ]
+            `shouldBe` []
diff --git a/app/Main.hs b/app/Main.hs
new file mode 100644
--- /dev/null
+++ b/app/Main.hs
@@ -0,0 +1,22 @@
+module Main where
+
+import Config (Config (..))
+import Config qualified
+import Data.Containers.ListUtils (nubOrd)
+import Data.Foldable (for_, toList)
+import Document qualified
+import Effectful
+import Effectful.Hspec (describe, runHspec)
+import Lint qualified
+import System.Environment (withArgs)
+import Prelude
+
+main :: IO ()
+main = do
+    Config{..} <- Config.get
+    withArgs [] . runEff . runHspec $ do
+        documents <- mapM Document.read . nubOrd . filter (`notElem` reference) . toList $ files
+        reference <- mapM Document.read reference
+        for_ (toList reference <> documents) \document ->
+            describe document.path $
+                (if lenient then Lint.lenient else Lint.strict) reference document
diff --git a/fluent-lint.cabal b/fluent-lint.cabal
new file mode 100644
--- /dev/null
+++ b/fluent-lint.cabal
@@ -0,0 +1,64 @@
+cabal-version: 3.0
+name: fluent-lint
+version: 1.0.0
+synopsis: Static analysis tool for validating Project Fluent translation files
+description:
+  A command-line tool that parses Project Fluent translation files and checks
+  for syntax errors and consistency issues.
+
+license: EUPL-1.2
+license-file: LICENCE
+author: IDA
+maintainer: IDA
+homepage: https://digital-autonomy.institute
+bug-reports: https://issues.digital-autonomy.institute
+category: Language
+build-type: Simple
+extra-doc-files:
+  CHANGELOG.md
+
+common common
+  default-language: GHC2021
+  ghc-options:
+    -Weverything
+    -Wno-all-missed-specialisations
+    -Wno-missing-export-lists
+    -Wno-missing-import-lists
+    -Wno-missing-kind-signatures
+    -Wno-missing-local-signatures
+    -Wno-missing-safe-haskell-mode
+    -Wno-name-shadowing
+    -Wno-unsafe
+
+  default-extensions:
+    ApplicativeDo
+    BlockArguments
+    DerivingStrategies
+    LambdaCase
+    NoImplicitPrelude
+    OverloadedRecordDot
+    OverloadedStrings
+    RecordWildCards
+    ViewPatterns
+
+executable fluent-lint
+  import: common
+  hs-source-dirs: app
+  main-is: Main.hs
+  autogen-modules:
+    Paths_fluent_lint
+
+  other-modules:
+    Config
+    Document
+    Lint
+    Paths_fluent_lint
+
+  build-depends:
+    base >=4.19 && <5,
+    containers >=0.6 && <0.9,
+    effectful >=2.6 && <2.7,
+    fluent-syntax >=1.1 && <1.2,
+    hspec-effectful >=1.1 && <1.2,
+    optparse-applicative >=0.18 && <0.20,
+    text >=2.1 && <2.3,
