diff --git a/.envrc b/.envrc
new file mode 100644
index 0000000..af3b9a0
--- /dev/null
+++ b/.envrc
@@ -0,0 +1,3 @@
+use flake
+
+source_env_if_exists .envrc.local
diff --git a/.gitignore b/.gitignore
index 5e8862c..d071cd6 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,2 +1,14 @@
 # Nix
 result
+result-*
+
+# Haskell / Cabal
+dist-newstyle/
+/cabal.project
+/cabal.project.*
+/.cabal/
+.ghc.environment.*
+
+# direnv
+.direnv/
+.envrc.local
diff --git a/README.md b/README.md
index e9c8aec..dd58c9e 100644
--- a/README.md
+++ b/README.md
@@ -50,6 +50,7 @@ Available commands:
 ```
 ❯ testgen-hs generate -g1 -n1 ApplyTxErr_Conway
 ```
+
 ```json
 {
   "seed": 1731690796,
@@ -266,11 +267,55 @@ And then easily reproduce it by providing the same seed that was found:
 
 ```
 ❯ ( echo 8182068182028200a0
-    echo 81820681820481581cc0231342a5c66b25d652a7116559d02cbe9515ef890fd698de38d456 
+    echo 81820681820481581cc0231342a5c66b25d652a7116559d02cbe9515ef890fd698de38d456
   ) | testgen-hs deserialize-stream
 ```
 
-``` json
+```json
 {"cbor":"8182068182028200a0","haskellRepr":"ApplyTxError (ConwayCertsFailure (WithdrawalsNotInRewardsCERTS (fromList [])) :| [])","json":{"contents":{"contents":{"contents":{"era":"ShelleyBasedEraConway","error":["ConwayCertsFailure (WithdrawalsNotInRewardsCERTS (fromList []))"],"kind":"ShelleyTxValidationError"},"tag":"TxValidationErrorInCardanoMode"},"tag":"TxCmdTxSubmitValidationError"},"tag":"TxSubmitFail"},"typeTag":"ApplyTxError (ConwayEra StandardCrypto)"}
 {"cbor":"81820681820481581cc0231342a5c66b25d652a7116559d02cbe9515ef890fd698de38d456","haskellRepr":"ApplyTxError (ConwayWdrlNotDelegatedToDRep (KeyHash {unKeyHash = \"c0231342a5c66b25d652a7116559d02cbe9515ef890fd698de38d456\"} :| []) :| [])","json":{"contents":{"contents":{"contents":{"era":"ShelleyBasedEraConway","error":["ConwayWdrlNotDelegatedToDRep (KeyHash {unKeyHash = \"c0231342a5c66b25d652a7116559d02cbe9515ef890fd698de38d456\"} :| [])"],"kind":"ShelleyTxValidationError"},"tag":"TxValidationErrorInCardanoMode"},"tag":"TxCmdTxSubmitValidationError"},"tag":"TxSubmitFail"},"typeTag":"ApplyTxError (ConwayEra StandardCrypto)"}
 ```
+
+## Development – HLS setup
+
+This project provides a Nix devshell with GHC and HLS. To get full IDE
+support you need [`direnv`](https://direnv.net/) with
+[`nix-direnv`](https://github.com/nix-community/nix-direnv) so that the
+`.envrc` is picked up automatically. See their official documentation for
+installation instructions.
+
+The instructions below use VS Code as an example, but any editor with LSP and
+`direnv` support (Neovim, Emacs, etc.) works the same way. The devshell puts
+both `haskell-language-server` and `haskell-language-server-wrapper` on
+`PATH`, so most editors should pick up HLS automatically.
+
+### VS Code example
+
+1. Install the extensions **Haskell** (`haskell.haskell`) and **direnv**
+   (`mkhl.direnv`).
+
+2. Tell the Haskell extension to use the HLS provided by the devshell
+   instead of managing its own. Add this to your VS Code settings:
+
+   ```json
+   { "haskell.manageHLS": "PATH" }
+   ```
+
+   Optionally, enable semantic highlighting (richer colours for types,
+   type variables, etc.):
+
+   ```json
+   { "haskell.plugin.semanticTokens.globalOn": true }
+   ```
+
+3. Allow direnv for this project and restart HLS:
+
+   ```
+   direnv allow
+   ```
+
+   Then in VS Code: `Ctrl+Shift+P` → _Haskell: Restart Haskell LSP Server_.
+
+> **Note:** The first HLS session takes roughly 10 minutes while `hie-bios`
+> builds all dependencies via `cabal v2-repl`. Subsequent loads use the
+> cache at `~/.cache/hie-bios/`.
diff --git a/flake.lock b/flake.lock
index b53c9a6..ae08974 100644
--- a/flake.lock
+++ b/flake.lock
@@ -17,6 +17,26 @@
         "type": "github"
       }
     },
+    "devshell": {
+      "inputs": {
+        "nixpkgs": [
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1768818222,
+        "narHash": "sha256-460jc0+CZfyaO8+w8JNtlClB2n4ui1RbHfPTLkpwhU8=",
+        "owner": "numtide",
+        "repo": "devshell",
+        "rev": "255a2b1725a20d060f566e4755dbf571bbbb5f76",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "repo": "devshell",
+        "type": "github"
+      }
+    },
     "flake-compat": {
       "flake": false,
       "locked": {
@@ -98,6 +118,7 @@
     "root": {
       "inputs": {
         "cardano-node": "cardano-node",
+        "devshell": "devshell",
         "flake-compat": "flake-compat",
         "flake-parts": "flake-parts",
         "nix-bundle-exe": "nix-bundle-exe",
diff --git a/flake.nix b/flake.nix
index 23e2471..6bf1c9d 100644
--- a/flake.nix
+++ b/flake.nix
@@ -2,14 +2,26 @@
   inputs = {
     nixpkgs.url = "github:nixos/nixpkgs/nixpkgs-unstable";
     flake-parts.url = "github:hercules-ci/flake-parts";
-    treefmt-nix.url = "github:numtide/treefmt-nix";
-    treefmt-nix.inputs.nixpkgs.follows = "nixpkgs";
-    flake-compat.url = "github:input-output-hk/flake-compat";
-    flake-compat.flake = false;
-    cardano-node.url = "github:IntersectMBO/cardano-node/10.6.2";
-    cardano-node.flake = false; # otherwise, +2k dependencies we don’t really use
-    nix-bundle-exe.url = "github:3noch/nix-bundle-exe";
-    nix-bundle-exe.flake = false;
+    treefmt-nix = {
+      url = "github:numtide/treefmt-nix";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+    flake-compat = {
+      url = "github:input-output-hk/flake-compat";
+      flake = false;
+    };
+    devshell = {
+      url = "github:numtide/devshell";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+    cardano-node = {
+      url = "github:IntersectMBO/cardano-node/10.6.2";
+      flake = false; # otherwise, +2k dependencies we don’t really use
+    };
+    nix-bundle-exe = {
+      url = "github:3noch/nix-bundle-exe";
+      flake = false;
+    };
   };
 
   outputs = inputs: let
@@ -18,6 +30,7 @@
     inputs.flake-parts.lib.mkFlake {inherit inputs;} ({config, ...}: {
       imports = [
         inputs.treefmt-nix.flakeModule
+        inputs.devshell.flakeModule
       ];
 
       flake.internal =
@@ -29,15 +42,10 @@
         );
 
       systems = ["x86_64-linux" "aarch64-linux" "aarch64-darwin" "x86_64-darwin"];
-      perSystem = {
-        config,
-        system,
-        pkgs,
-        ...
-      }: {
-        packages = let
-          internal = inputs.self.internal.${system};
-        in
+      perSystem = {system, ...}: let
+        internal = inputs.self.internal.${system};
+      in {
+        packages =
           {
             default = internal.defaultPackage;
           }
@@ -45,13 +53,20 @@
             default-x86_64-windows = inputs.self.internal.x86_64-windows.defaultPackage;
           });
 
+        devshells.default = internal.devShell.new;
+
         treefmt = {pkgs, ...}: {
           projectRootFile = "flake.nix";
-          programs.alejandra.enable = true; # Nix
-          programs.ormolu.enable = true; # Haskell
-          programs.cabal-fmt.enable = true;
-          programs.shfmt.enable = true;
-          programs.yamlfmt.enable = pkgs.system != "x86_64-darwin"; # a treefmt-nix+yamlfmt bug on Intel Macs
+          programs = {
+            alejandra.enable = true; # Nix
+            ormolu.enable = true; # Haskell
+            cabal-fmt.enable = true;
+            prettier.enable = true; # Markdown, JSON, …
+            shfmt.enable = true;
+            ruff-check.enable = true; # Python
+            ruff-format.enable = true; # Python
+            yamlfmt.enable = pkgs.system != "x86_64-darwin"; # a treefmt-nix+yamlfmt bug on Intel Macs
+          };
         };
       };
 
@@ -60,7 +75,7 @@
           testgen-hs = lib.genAttrs (config.systems ++ ["x86_64-windows"]) (
             targetSystem: inputs.self.internal.${targetSystem}.hydraPackage
           );
-          inherit (inputs.self) checks;
+          inherit (inputs.self) checks devShells;
         };
       in
         allJobs
diff --git a/nix/internal.nix b/nix/internal.nix
index a9ab471..7ca4b97 100644
--- a/nix/internal.nix
+++ b/nix/internal.nix
@@ -3,44 +3,134 @@
   targetSystem,
 }:
 # For now, let's keep all UNIX definitions together, until they diverge more in the future.
-assert __elem targetSystem ["x86_64-linux" "aarch64-linux" "aarch64-darwin" "x86_64-darwin" "x86_64-windows"]; let
+assert builtins.elem targetSystem ["x86_64-linux" "aarch64-linux" "aarch64-darwin" "x86_64-darwin" "x86_64-windows"]; let
   buildSystem =
     if targetSystem != "x86_64-windows"
     then targetSystem
     else "x86_64-linux";
   pkgs = inputs.nixpkgs.legacyPackages.${buildSystem};
   inherit (pkgs) lib;
-in rec {
-  defaultPackage = testgen-hs;
-
-  cardano-node-flake = let
+  cardano-node-src = let
     unpatched = inputs.cardano-node;
   in
+    if targetSystem != "aarch64-linux"
+    then unpatched
+    else
+      pkgs.runCommand "source" {} ''
+        cp -r ${unpatched} $out
+        chmod -R +w $out
+        cd $out
+        ${lib.optionalString (targetSystem == "aarch64-linux") ''
+          echo ${lib.escapeShellArg (builtins.toJSON [targetSystem])} >$out/nix/supported-systems.nix
+          sed -r 's/"-fexternal-interpreter"//g' -i $out/nix/haskell.nix
+        ''}
+      '';
+  cardano-node-src-for-flake =
+    if targetSystem != "aarch64-linux"
+    then cardano-node-src
+    else {
+      outPath = toString cardano-node-src;
+      inherit (inputs.cardano-node) rev shortRev lastModified lastModifiedDate;
+    };
+  cardano-node-flake' = (import inputs.flake-compat {src = cardano-node-src-for-flake;}).defaultNix;
+  cardano-ledger-src = let
+    dep-id = cardano-node-flake'.project.${buildSystem}.hsPkgs.cardano-ledger-core.identifier;
+    dep-tag = "${dep-id.name}-${dep-id.version}";
+  in
+    pkgs.fetchFromGitHub {
+      name = "cardano-ledger--${dep-tag}";
+      owner = "IntersectMBO";
+      repo = "cardano-ledger";
+      rev = dep-tag;
+      hash = "sha256-RvnNYY76OhRuC/uP5Lr+HLEKWyMHCWxx+10HlPrH6mQ=";
+    };
+  patched-cardano-ledger-src = pkgs.runCommandNoCC "cardano-ledger-src-patched" {} ''
+    cp -r ${cardano-ledger-src} $out
+    chmod -R +w $out
+    patch -p1 -d $out/libs/cardano-ledger-core -i ${./cardano-ledger-core--Arbitrary-PoolMetadata.diff}
+    patch -p1 -d $out/libs/cardano-ledger-test -i ${./cardano-ledger-test--expose-helpers.diff}
+    ${lib.optionalString (targetSystem == "x86_64-windows") ''
+      patch -p1 -d $out/libs/cardano-ledger-test -i ${./cardano-ledger-test--windows-fix.diff}
+    ''}
+  '';
+  constrained-generators-src = patched-cardano-node-flake'.project.${buildSystem}.hsPkgs.constrained-generators.src;
+  patched-constrained-generators-src = constrained-generators-src;
+  cardano-api-src = cardano-node-flake'.project.${buildSystem}.hsPkgs.cardano-api.src;
+  cardano-api-id = cardano-node-flake'.project.${buildSystem}.hsPkgs.cardano-api.identifier;
+  # Extract the latest revised .cabal from the local CHaP index.
+  # CHaP revisions relax version bounds to match the package set;
+  # without this, the on-disk .cabal carries the original tight bounds
+  # and cabal-install's solver rejects valid dependency plans.
+  #
+  # GNU tar overwrites earlier entries with later ones, so a plain
+  # extract of a duplicate member yields the last revision — exactly
+  # the latest CHaP revision we need.
+  cardano-api-revised-cabal = let
+    member = "${cardano-api-id.name}/${cardano-api-id.version}/${cardano-api-id.name}.cabal";
+  in
+    pkgs.runCommandNoCC "cardano-api-revised-cabal" {} ''
+      tar xf ${cardano-node-flake'.inputs.CHaP}/01-index.tar.gz '${member}'
+      cp '${member}' $out
+    '';
+  patched-cardano-api-src = pkgs.applyPatches {
+    name = "cardano-api-src-patched";
+    src = cardano-api-src;
+    prePatch = ''
+      cp ${cardano-api-revised-cabal} cardano-api.cabal
+    '';
+    patches = [./cardano-api--expose-internal.diff];
+  };
+  patched-cardano-node-src = {withOurCode ? true}:
+    pkgs.runCommandNoCC "cardano-node-src-patched" {} ''
+      cp -r ${cardano-node-src} $out
+      chmod -R +w $out
+      cd $out
+      ${lib.optionalString (targetSystem == "aarch64-linux") ''
+        echo ${lib.escapeShellArg (builtins.toJSON [targetSystem])} >$out/nix/supported-systems.nix
+        sed -r 's/"-fexternal-interpreter"//g' -i $out/nix/haskell.nix
+      ''}
+      ${
+        if withOurCode
+        then ''
+          cp -r ${../testgen-hs} ./testgen-hs
+        ''
+        else ''
+          mkdir -p ./testgen-hs
+        ''
+      }
+      sed -r '/^packages:/ a\  testgen-hs' -i cabal.project
+
+      patch -p1 -i ${./cardano-node--apply-patches.diff}
+      cp  ${./cardano-ledger-core--Arbitrary-PoolMetadata.diff} nix/cardano-ledger-core--Arbitrary-PoolMetadata.diff
+      cp  ${./cardano-ledger-test--expose-helpers.diff} nix/cardano-ledger-test--expose-helpers.diff
+      cp  ${
+        if targetSystem == "x86_64-windows"
+        then ./cardano-ledger-test--windows-fix.diff
+        else pkgs.emptyFile
+      } nix/cardano-ledger-test--windows-fix.diff
+      cp  ${./cardano-api--expose-internal.diff} nix/cardano-api--expose-internal.diff
+
+      patch -p1 -i ${./cardano-node--expose-cardano-ledger-test.diff}
+      sed -r 's,CARDANO_LEDGER_SOURCE,${cardano-ledger-src},g' -i nix/haskell.nix
+
+      patch -p1 -i ${./cardano-node--export-cardano-submit-api.diff}
+    '';
+  patched-cardano-node-flake' =
     (import inputs.flake-compat {
-      src =
-        if targetSystem != "aarch64-linux"
-        then unpatched
-        else {
-          outPath = toString (pkgs.runCommand "source" {} ''
-            cp -r ${unpatched} $out
-            chmod -R +w $out
-            cd $out
-            ${lib.optionalString (targetSystem == "aarch64-linux") ''
-              echo ${lib.escapeShellArg (builtins.toJSON [targetSystem])} >$out/nix/supported-systems.nix
-              sed -r 's/"-fexternal-interpreter"//g' -i $out/nix/haskell.nix
-            ''}
-          '');
-          inherit (unpatched) rev shortRev lastModified lastModifiedDate;
-        };
+      src = {
+        outPath = toString (patched-cardano-node-src {withOurCode = true;});
+        inherit (inputs.cardano-node) rev shortRev lastModified lastModifiedDate;
+      };
     })
     .defaultNix;
+in rec {
+  defaultPackage = testgen-hs;
+  cardano-node-flake = cardano-node-flake';
 
   cardano-node-packages =
     {
       x86_64-linux = cardano-node-flake.hydraJobs.x86_64-linux.musl;
-      aarch64-linux = cardano-node-flake.packages.aarch64-linux;
-      x86_64-darwin = cardano-node-flake.packages.x86_64-darwin;
-      aarch64-darwin = cardano-node-flake.packages.aarch64-darwin;
+      inherit (cardano-node-flake.packages) aarch64-linux x86_64-darwin aarch64-darwin;
     }
     .${
       targetSystem
@@ -48,53 +138,198 @@ in rec {
 
   inherit (cardano-node-packages) cardano-node cardano-cli;
 
-  testgen-hs = let
-    patched-flake = let
-      unpatched = inputs.cardano-node;
-      cardano-ledger-src = let
-        dep-id = cardano-node-flake.project.${buildSystem}.hsPkgs.cardano-ledger-core.identifier;
-        dep-tag = "${dep-id.name}-${dep-id.version}";
-      in
-        pkgs.fetchFromGitHub {
-          name = "cardano-ledger--${dep-tag}";
-          owner = "IntersectMBO";
-          repo = "cardano-ledger";
-          rev = dep-tag;
-          hash = "sha256-RvnNYY76OhRuC/uP5Lr+HLEKWyMHCWxx+10HlPrH6mQ=";
-        };
+  devShell = let
+    cardano-node-devshell = cardano-node-flake.devShells.${buildSystem}.default;
+    cardano-node-inputs = lib.filter lib.isDerivation (
+      (cardano-node-devshell.buildInputs or [])
+      ++ (cardano-node-devshell.nativeBuildInputs or [])
+      ++ (cardano-node-devshell.propagatedBuildInputs or [])
+      ++ (cardano-node-devshell.propagatedNativeBuildInputs or [])
+    );
+    cardano-node-env = pkgs.buildEnv {
+      name = "cardano-node-devshell-env";
+      paths = cardano-node-inputs;
+      ignoreCollisions = true;
+    };
+    cardano-node-ghc-libdir = cardano-node-devshell.NIX_GHC_LIBDIR or "";
+
+    # Extract an individual package from the cardano-node devshell inputs
+    # so we can expose it as a devshell command with its own menu entry.
+    findInput = pred: label:
+      lib.findFirst pred (throw "devshell input '${label}' not found") cardano-node-inputs;
+
+    haskell-language-server = findInput (p: (p.pname or "") == "haskell-language-server-exe-haskell-language-server") "haskell-language-server";
+
+    # Shim so that editors looking for haskell-language-server-wrapper find it
+    # on PATH; the devshell already sets the correct GHC.
+    haskell-language-server-wrapper = pkgs.writeShellApplication {
+      name = "haskell-language-server-wrapper";
+      text = ''
+        exec ${lib.getExe haskell-language-server} "$@"
+      '';
+    };
+
+    # numtide/devshell does not run stdenv setup hooks, so env vars that
+    # mkShell would set (e.g. PKG_CONFIG_PATH) are missing.
+    # PKG_CONFIG_PATH is critical: without it the cabal solver cannot
+    # verify pkgconfig-depends and dependency resolution fails.
+    # We extract it by running the same stdenv setup with the same inputs.
+    devshell-pkg-config-path =
+      pkgs.runCommand "devshell-pkg-config-path" {
+        inherit (cardano-node-devshell) buildInputs nativeBuildInputs;
+        propagatedBuildInputs = cardano-node-devshell.propagatedBuildInputs or [];
+        propagatedNativeBuildInputs = cardano-node-devshell.propagatedNativeBuildInputs or [];
+      } ''
+        source $stdenv/setup 2>/dev/null || true
+        mkdir -p $out
+        echo -n "''${PKG_CONFIG_PATH:-}" >$out/PKG_CONFIG_PATH
+      '';
+
+    cabal-project-base = cardano-node-flake.project.${buildSystem}.args.cabalProject;
+    cabal-project-template = pkgs.writeText "cabal.project" cabal-project-base;
+    cabal-project-extra-packages = [
+      "${patched-cardano-api-src}"
+      "${patched-cardano-ledger-src}/libs/cardano-ledger-core"
+      "${patched-cardano-ledger-src}/libs/cardano-ledger-test"
+      "${patched-constrained-generators-src}"
+      "@REPO_ROOT@/testgen-hs"
+    ];
+    cabal-project-extra-packages-json = builtins.toJSON cabal-project-extra-packages;
+    # cardano-ledger-core uses the deprecated PV3.fromGHC (should be
+    # fromHaskellRatio). The upstream cabal.project sets -Werror globally,
+    # which promotes this deprecation warning into a build error. The nix
+    # build is unaffected because haskell.nix doesn't pass -Werror from
+    # the project file, but the devshell's cabal does.
+    cabal-project-extra-suffix = ''
+      package cardano-ledger-core
+        ghc-options: -Wwarn=deprecations
+    '';
+    cabal-project-rewrite-script = pkgs.substituteAll {
+      src = ./rewrite_cabal_project.py;
+      cabal_project_template = toString cabal-project-template;
+      patched_node_src = toString (patched-cardano-node-src {withOurCode = false;});
+      extra_packages_json = cabal-project-extra-packages-json;
+      extra_project_suffix = cabal-project-extra-suffix;
+      local_chap_path = toString cardano-node-flake'.inputs.CHaP;
+      repo_root = "@REPO_ROOT@";
+    };
+
+    # Pre-generate cabal.project at Nix build time so the devshell
+    # startup doesn't need to launch Python (~260 ms saved).
+    # The only remaining placeholder is @REPO_ROOT@, which is
+    # sed-replaced with $PRJ_ROOT at shell entry.
+    cabal-project-generated =
+      pkgs.runCommand "cabal-project-generated" {
+        nativeBuildInputs = [pkgs.python3];
+      } ''
+        python3 ${cabal-project-rewrite-script}
+        cp cabal.project $out
+      '';
+  in {
+    old = let
+      chap-store-path = toString cardano-node-flake'.inputs.CHaP;
     in
-      (import inputs.flake-compat {
-        src = {
-          outPath = toString (pkgs.runCommandNoCC "source" {} ''
-            cp -r ${unpatched} $out
-            chmod -R +w $out
-            cd $out
-            ${lib.optionalString (targetSystem == "aarch64-linux") ''
-              echo ${lib.escapeShellArg (builtins.toJSON [targetSystem])} >$out/nix/supported-systems.nix
-              sed -r 's/"-fexternal-interpreter"//g' -i $out/nix/haskell.nix
-            ''}
-            cp -r ${../testgen-hs} ./testgen-hs
-            sed -r '/^packages:/ a\  testgen-hs' -i cabal.project
-
-            patch -p1 -i ${./cardano-node--apply-patches.diff}
-            cp  ${./cardano-ledger-core--Arbitrary-PoolMetadata.diff} nix/cardano-ledger-core--Arbitrary-PoolMetadata.diff
-            cp  ${./cardano-ledger-test--expose-helpers.diff} nix/cardano-ledger-test--expose-helpers.diff
-            cp  ${
-              if targetSystem == "x86_64-windows"
-              then ./cardano-ledger-test--windows-fix.diff
-              else pkgs.emptyFile
-            } nix/cardano-ledger-test--windows-fix.diff
-            cp  ${./cardano-api--expose-internal.diff} nix/cardano-api--expose-internal.diff
-
-            patch -p1 -i ${./cardano-node--expose-cardano-ledger-test.diff}
-            sed -r 's,CARDANO_LEDGER_SOURCE,${cardano-ledger-src},g' -i nix/haskell.nix
-
-            patch -p1 -i ${./cardano-node--export-cardano-submit-api.diff}
-          '');
-          inherit (unpatched) rev shortRev lastModified lastModifiedDate;
-        };
-      })
-      .defaultNix;
+      pkgs.mkShell {
+        inputsFrom = [cardano-node-devshell];
+        shellHook = ''
+          export CABAL_DIR="$PWD/.cabal"
+          _new=$(sed "s|@REPO_ROOT@|$PWD|g" ${cabal-project-generated})
+          if [ ! -e "$PWD/cabal.project" ] || [ "$(cat "$PWD/cabal.project")" != "$_new" ]; then
+            printf '%s\n' "$_new" > "$PWD/cabal.project"
+          fi
+
+          _chap_marker="$CABAL_DIR/.chap-store-path"
+          if [ ! -e "$_chap_marker" ] || [ "$(cat "$_chap_marker")" != "${chap-store-path}" ]; then
+            mkdir -p "$CABAL_DIR/packages/cardano-haskell-packages"
+            cabal update cardano-haskell-packages 2>/dev/null
+            printf '%s' '${chap-store-path}' >"$_chap_marker"
+          fi
+
+          if [ ! -e "$CABAL_DIR/packages/hackage.haskell.org/01-index.tar" ]; then
+            echo "First-time setup: downloading Hackage package index…"
+            cabal update hackage.haskell.org
+          fi
+        '';
+      };
+    new = {config, ...}: {
+      name = "testgen-hs-devshell";
+      env =
+        lib.optional (cardano-node-ghc-libdir != "") {
+          name = "NIX_GHC_LIBDIR";
+          value = cardano-node-ghc-libdir;
+        }
+        ++ [
+          {
+            name = "CABAL_DIR";
+            eval = "$PRJ_ROOT/.cabal";
+          }
+          {
+            name = "PKG_CONFIG_PATH";
+            eval = "$(cat ${devshell-pkg-config-path}/PKG_CONFIG_PATH)\${PKG_CONFIG_PATH:+:\$PKG_CONFIG_PATH}";
+          }
+        ];
+      commands = [
+        {
+          name = "ghc";
+          package = findInput (p: lib.hasPrefix "ghc-shell-for-packages" (p.name or "")) "ghc";
+          category = "haskell";
+        }
+        {
+          name = "cabal";
+          package = findInput (p: (p.pname or "") == "cabal-install-exe-cabal") "cabal";
+          category = "haskell";
+        }
+        {
+          name = "haskell-language-server";
+          package = haskell-language-server;
+          category = "haskell";
+        }
+      ];
+      devshell = {
+        packages = [
+          cardano-node-env
+          haskell-language-server-wrapper
+        ];
+        startup.rewrite-cabal-project.text = let
+          chap-store-path = toString cardano-node-flake'.inputs.CHaP;
+        in ''
+          _new=$(sed "s|@REPO_ROOT@|$PRJ_ROOT|g" ${cabal-project-generated})
+          if [ ! -e "$PRJ_ROOT/cabal.project" ] || [ "$(cat "$PRJ_ROOT/cabal.project")" != "$_new" ]; then
+            printf '%s\n' "$_new" > "$PRJ_ROOT/cabal.project"
+          fi
+          # Re-index the local CHaP only when the underlying Nix store
+          # path changes (i.e. after a flake.lock update).  A marker
+          # file records the store path that was last indexed.
+          _chap_marker="$CABAL_DIR/.chap-store-path"
+          if [ ! -e "$_chap_marker" ] || [ "$(cat "$_chap_marker")" != "${chap-store-path}" ]; then
+            mkdir -p "$CABAL_DIR/packages/cardano-haskell-packages"
+            cabal update cardano-haskell-packages 2>/dev/null
+            printf '%s' '${chap-store-path}' >"$_chap_marker"
+          fi
+
+          # On first use of this project-local CABAL_DIR, also fetch the
+          # Hackage index so that 'cabal build' works out of the box.
+          if [ ! -e "$CABAL_DIR/packages/hackage.haskell.org/01-index.tar" ]; then
+            echo "First-time setup: downloading Hackage package index…"
+            cabal update hackage.haskell.org
+          fi
+
+        '';
+        motd = ''
+
+          {202}🔨 Welcome to ${config.name}{reset}
+          $(menu)
+
+          You can now run:
+            · {bold}cabal build testgen-hs{reset}
+            · {bold}cabal run testgen-hs -- --help{reset}
+        '';
+      };
+    };
+  };
+
+  testgen-hs = let
+    patched-flake = patched-cardano-node-flake';
   in
     {
       x86_64-linux = patched-flake.hydraJobs.x86_64-linux.musl.testgen-hs;
diff --git a/nix/rewrite_cabal_project.py b/nix/rewrite_cabal_project.py
new file mode 100644
index 0000000..e9e0cae
--- /dev/null
+++ b/nix/rewrite_cabal_project.py
@@ -0,0 +1,91 @@
+"""Rewrite cabal.project packages for the devshell."""
+
+import json
+import re
+from pathlib import Path
+
+
+TEMPLATE_PATH = Path("@cabal_project_template@")
+PATCHED_NODE_SRC = "@patched_node_src@"
+EXTRA_PACKAGES = json.loads(r"""@extra_packages_json@""")
+EXTRA_PROJECT_SUFFIX = r"""@extra_project_suffix@"""
+LOCAL_CHAP_PATH = "@local_chap_path@"
+REPO_ROOT = "@repo_root@"
+
+REPO_NAME = "cardano-haskell-packages"
+
+
+def rewrite_packages(text: str) -> str:
+    """Rewrite the packages section to use patched and extra packages."""
+    lines = text.splitlines()
+    out = []
+    i = 0
+    while i < len(lines):
+        line = lines[i]
+        if line.strip() != "packages:":
+            out.append(line)
+            i += 1
+            continue
+
+        out.append(line)
+        i += 1
+        block_lines = []
+        while i < len(lines):
+            next_line = lines[i]
+            if next_line and not next_line.startswith(" "):
+                break
+            block_lines.append(next_line)
+            i += 1
+
+        trailing_blanks = []
+        while block_lines and block_lines[-1].strip() == "":
+            trailing_blanks.insert(0, block_lines.pop())
+
+        for block_line in block_lines:
+            stripped = block_line.strip()
+            if stripped == "" or stripped.startswith("--"):
+                out.append(block_line)
+                continue
+            entry = stripped
+            if entry.startswith("/"):
+                out.append(f"  {entry}")
+            else:
+                out.append(f"  {PATCHED_NODE_SRC}/{entry}")
+
+        for pkg in EXTRA_PACKAGES:
+            out.append(f"  {pkg.replace('@REPO_ROOT@', REPO_ROOT)}")
+
+        out.extend(trailing_blanks)
+
+    result = "\n".join(out)
+    if text.endswith("\n"):
+        result += "\n"
+    return result
+
+
+def use_local_chap(text: str) -> str:
+    """Point the CHaP repository at a local Nix-store copy.
+
+    This uses the same pinned CHaP snapshot that cardano-node's haskell.nix
+    uses, giving us cabal-file revisions and full reproducibility.
+    """
+    # Replace the "repository cardano-haskell-packages" block (url, secure,
+    # root-keys, and any key-threshold lines) with a local file: URL.
+    text = re.sub(
+        r"(?:--[^\n]*\n)*"  # optional preceding comment lines
+        r"repository cardano-haskell-packages\n"
+        r"(?:  [^\n]+\n)*",  # indented continuation lines
+        f"repository {REPO_NAME}\n  url: file:{LOCAL_CHAP_PATH}\n  secure: True\n",
+        text,
+    )
+
+    return text
+
+
+base_text = TEMPLATE_PATH.read_text(encoding="utf-8")
+result = rewrite_packages(base_text)
+if LOCAL_CHAP_PATH.strip():
+    result = use_local_chap(result)
+if EXTRA_PROJECT_SUFFIX.strip():
+    result = result.rstrip("\n") + "\n\n" + EXTRA_PROJECT_SUFFIX.strip() + "\n"
+Path("cabal.project").write_text(result, encoding="utf-8")
diff --git a/testgen-hs/SynthEvalTx.hs b/testgen-hs/SynthEvalTx.hs
index 0e6145c..1411cf0 100644
--- a/testgen-hs/SynthEvalTx.hs
+++ b/testgen-hs/SynthEvalTx.hs
@@ -42,7 +42,7 @@ import Data.Aeson
 import qualified Data.Aeson as J
 import qualified Data.Aeson.Encoding as AesonEncoding
 import qualified Data.ByteString as BS
-import Data.FileEmbed (embedFile)
+import Data.FileEmbed (embedFile, makeRelativeToProject)
 import qualified Data.Map.Strict as Map
 import qualified Data.Set as Set
 import Data.Text (Text)
@@ -135,7 +135,7 @@ dummySystemStart =
   SystemStart $ UTCTime (fromGregorian 2020 7 29) (secondsToDiffTime 0)
 
 protocolParamsJSON :: BS.ByteString
-protocolParamsJSON = $(embedFile "protocol-params-preview.json")
+protocolParamsJSON = $(embedFile =<< makeRelativeToProject "protocol-params-preview.json")
 
 protocolParams :: PParams (Cardano.Ledger.Api.Era.ConwayEra)
 protocolParams =
diff --git a/testgen-hs/protocol-params-preview.json b/testgen-hs/protocol-params-preview.json
index 744c658..0aa0009 100644
--- a/testgen-hs/protocol-params-preview.json
+++ b/testgen-hs/protocol-params-preview.json
@@ -4,648 +4,58 @@
   "committeeMinSize": 0,
   "costModels": {
     "PlutusV1": [
-      100788,
-      420,
-      1,
-      1,
-      1000,
-      173,
-      0,
-      1,
-      1000,
-      59957,
-      4,
-      1,
-      11183,
-      32,
-      201305,
-      8356,
-      4,
-      16000,
-      100,
-      16000,
-      100,
-      16000,
-      100,
-      16000,
-      100,
-      16000,
-      100,
-      16000,
-      100,
-      100,
-      100,
-      16000,
-      100,
-      94375,
-      32,
-      132994,
-      32,
-      61462,
-      4,
-      72010,
-      178,
-      0,
-      1,
-      22151,
-      32,
-      91189,
-      769,
-      4,
-      2,
-      85848,
-      228465,
-      122,
-      0,
-      1,
-      1,
-      1000,
-      42921,
-      4,
-      2,
-      24548,
-      29498,
-      38,
-      1,
-      898148,
-      27279,
-      1,
-      51775,
-      558,
-      1,
-      39184,
-      1000,
-      60594,
-      1,
-      141895,
-      32,
-      83150,
-      32,
-      15299,
-      32,
-      76049,
-      1,
-      13169,
-      4,
-      22100,
-      10,
-      28999,
-      74,
-      1,
-      28999,
-      74,
-      1,
-      43285,
-      552,
-      1,
-      44749,
-      541,
-      1,
-      33852,
-      32,
-      68246,
-      32,
-      72362,
-      32,
-      7243,
-      32,
-      7391,
-      32,
-      11546,
-      32,
-      85848,
-      228465,
-      122,
-      0,
-      1,
-      1,
-      90434,
-      519,
-      0,
-      1,
-      74433,
-      32,
-      85848,
-      228465,
-      122,
-      0,
-      1,
-      1,
-      85848,
-      228465,
-      122,
-      0,
-      1,
-      1,
-      270652,
-      22588,
-      4,
-      1457325,
-      64566,
-      4,
-      20467,
-      1,
-      4,
-      0,
-      141992,
-      32,
-      100788,
-      420,
-      1,
-      1,
-      81663,
-      32,
-      59498,
-      32,
-      20142,
-      32,
-      24588,
-      32,
-      20744,
-      32,
-      25933,
-      32,
-      24623,
-      32,
-      53384111,
-      14333,
-      10
+      100788, 420, 1, 1, 1000, 173, 0, 1, 1000, 59957, 4, 1, 11183, 32, 201305,
+      8356, 4, 16000, 100, 16000, 100, 16000, 100, 16000, 100, 16000, 100,
+      16000, 100, 100, 100, 16000, 100, 94375, 32, 132994, 32, 61462, 4, 72010,
+      178, 0, 1, 22151, 32, 91189, 769, 4, 2, 85848, 228465, 122, 0, 1, 1, 1000,
+      42921, 4, 2, 24548, 29498, 38, 1, 898148, 27279, 1, 51775, 558, 1, 39184,
+      1000, 60594, 1, 141895, 32, 83150, 32, 15299, 32, 76049, 1, 13169, 4,
+      22100, 10, 28999, 74, 1, 28999, 74, 1, 43285, 552, 1, 44749, 541, 1,
+      33852, 32, 68246, 32, 72362, 32, 7243, 32, 7391, 32, 11546, 32, 85848,
+      228465, 122, 0, 1, 1, 90434, 519, 0, 1, 74433, 32, 85848, 228465, 122, 0,
+      1, 1, 85848, 228465, 122, 0, 1, 1, 270652, 22588, 4, 1457325, 64566, 4,
+      20467, 1, 4, 0, 141992, 32, 100788, 420, 1, 1, 81663, 32, 59498, 32,
+      20142, 32, 24588, 32, 20744, 32, 25933, 32, 24623, 32, 53384111, 14333, 10
     ],
     "PlutusV2": [
-      100788,
-      420,
-      1,
-      1,
-      1000,
-      173,
-      0,
-      1,
-      1000,
-      59957,
-      4,
-      1,
-      11183,
-      32,
-      201305,
-      8356,
-      4,
-      16000,
-      100,
-      16000,
-      100,
-      16000,
-      100,
-      16000,
-      100,
-      16000,
-      100,
-      16000,
-      100,
-      100,
-      100,
-      16000,
-      100,
-      94375,
-      32,
-      132994,
-      32,
-      61462,
-      4,
-      72010,
-      178,
-      0,
-      1,
-      22151,
-      32,
-      91189,
-      769,
-      4,
-      2,
-      85848,
-      228465,
-      122,
-      0,
-      1,
-      1,
-      1000,
-      42921,
-      4,
-      2,
-      24548,
-      29498,
-      38,
-      1,
-      898148,
-      27279,
-      1,
-      51775,
-      558,
-      1,
-      39184,
-      1000,
-      60594,
-      1,
-      141895,
-      32,
-      83150,
-      32,
-      15299,
-      32,
-      76049,
-      1,
-      13169,
-      4,
-      22100,
-      10,
-      28999,
-      74,
-      1,
-      28999,
-      74,
-      1,
-      43285,
-      552,
-      1,
-      44749,
-      541,
-      1,
-      33852,
-      32,
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-      32,
-      72362,
-      32,
-      7243,
-      32,
-      7391,
-      32,
-      11546,
-      32,
-      85848,
-      228465,
-      122,
-      0,
-      1,
-      1,
-      90434,
-      519,
-      0,
-      1,
-      74433,
-      32,
-      85848,
-      228465,
-      122,
-      0,
-      1,
-      1,
-      85848,
-      228465,
-      122,
-      0,
-      1,
-      1,
-      955506,
-      213312,
-      0,
-      2,
-      270652,
-      22588,
-      4,
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-      64566,
-      4,
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-      1,
-      4,
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-      32,
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-      1,
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-      32,
-      59498,
-      32,
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-      32,
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-      32,
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-      32,
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-      32,
-      24623,
-      32,
-      43053543,
-      10,
-      53384111,
-      14333,
-      10,
-      43574283,
-      26308,
-      10
+      100788, 420, 1, 1, 1000, 173, 0, 1, 1000, 59957, 4, 1, 11183, 32, 201305,
+      8356, 4, 16000, 100, 16000, 100, 16000, 100, 16000, 100, 16000, 100,
+      16000, 100, 100, 100, 16000, 100, 94375, 32, 132994, 32, 61462, 4, 72010,
+      178, 0, 1, 22151, 32, 91189, 769, 4, 2, 85848, 228465, 122, 0, 1, 1, 1000,
+      42921, 4, 2, 24548, 29498, 38, 1, 898148, 27279, 1, 51775, 558, 1, 39184,
+      1000, 60594, 1, 141895, 32, 83150, 32, 15299, 32, 76049, 1, 13169, 4,
+      22100, 10, 28999, 74, 1, 28999, 74, 1, 43285, 552, 1, 44749, 541, 1,
+      33852, 32, 68246, 32, 72362, 32, 7243, 32, 7391, 32, 11546, 32, 85848,
+      228465, 122, 0, 1, 1, 90434, 519, 0, 1, 74433, 32, 85848, 228465, 122, 0,
+      1, 1, 85848, 228465, 122, 0, 1, 1, 955506, 213312, 0, 2, 270652, 22588, 4,
+      1457325, 64566, 4, 20467, 1, 4, 0, 141992, 32, 100788, 420, 1, 1, 81663,
+      32, 59498, 32, 20142, 32, 24588, 32, 20744, 32, 25933, 32, 24623, 32,
+      43053543, 10, 53384111, 14333, 10, 43574283, 26308, 10
     ],
     "PlutusV3": [
-      100788,
-      420,
-      1,
-      1,
-      1000,
-      173,
-      0,
-      1,
-      1000,
-      59957,
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@@ -683,7 +93,7 @@
   "maxValueSize": 5000,
   "minFeeRefScriptCostPerByte": 15,
   "minPoolCost": 170000000,
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+  "monetaryExpansion": 0.003,
   "poolPledgeInfluence": 0.3,
   "poolRetireMaxEpoch": 18,
   "poolVotingThresholds": {