diff --git a/build.nix b/build.nix
index 2c814c1..c9c259e 100644
--- a/build.nix
+++ b/build.nix
@@ -62,6 +62,7 @@
 , writeText
 , runCommandLocal
 , remarshal
+, formats
 , crateDependencies
 , zstd
 , fetchurl
@@ -72,7 +73,7 @@
 let
   builtinz =
     builtins // import ./builtins
-      { inherit lib writeText remarshal runCommandLocal; };
+      { inherit lib writeText remarshal runCommandLocal formats; };
 
   drvAttrs = {
     name = "${pname}-${version}";
@@ -87,7 +88,7 @@ let
 
     # The cargo config with source replacement. Replaces both crates.io crates
     # and git dependencies.
-    cargoconfig = builtinz.toTOML {
+    cargoconfig = builtinz.writeTOML "config" {
       source = {
         crates-io = { replace-with = "nix-sources"; };
         nix-sources = {
@@ -235,7 +236,7 @@ let
       export CARGO_HOME=''${CARGO_HOME:-$PWD/.cargo-home}
       mkdir -p $CARGO_HOME
 
-      echo "$cargoconfig" > $CARGO_HOME/config
+      cp "$cargoconfig" $CARGO_HOME/config
 
       runHook postConfigure
     '';
diff --git a/builtins/default.nix b/builtins/default.nix
index 755647a..b1cf2fa 100644
--- a/builtins/default.nix
+++ b/builtins/default.nix
@@ -3,12 +3,12 @@
 , writeText
 , runCommandLocal
 , remarshal
+, formats
 }:
 
 rec
 {
-  toTOML = import ./to-toml.nix { inherit lib; };
-  writeTOML = name: attrs: writeText name (toTOML attrs);
+  writeTOML = (formats.toml { }).generate;
 
   readTOML = usePure: f:
     if usePure then
diff --git a/builtins/to-toml.nix b/builtins/to-toml.nix
deleted file mode 100644
index 407569e..0000000
--- a/builtins/to-toml.nix
+++ /dev/null
@@ -1,139 +0,0 @@
-{ lib }:
-let
-  inherit (lib)
-    length
-    elemAt
-    concatMap
-    all
-    concatLists
-    concatStringsSep
-    concatMapStringsSep
-    mapAttrsToList
-    ;
-
-  inherit (builtins)
-    abort
-    match
-    toJSON
-    typeOf
-    ;
-
-  quoteKey = k:
-    if match "[a-zA-Z]+" k == []
-    then k
-    else quoteString k;
-
-  quoteString = builtins.toJSON;
-
-  outputValInner = v:
-    let
-      ty = tomlTy v;
-    in
-      if ty == "set" then
-        let
-          vals = mapAttrsToList
-            (k': v': "${quoteKey k'} = ${outputValInner v'}") v;
-          valsStr = concatStringsSep ", " vals;
-        in
-          "{ ${valsStr} }" else
-        outputVal v;
-
-  outputVal = v:
-    let
-      ty = tomlTy v;
-    in
-      if (ty == "bool" || ty == "int") then
-        builtins.toJSON v
-      else
-        if ty == "string" then
-          quoteString v
-        else
-          if ty == "list" || ty == "list_of_attrs" then
-            let
-              vals = map quoteString v;
-              valsStr = concatStringsSep ", " vals;
-            in
-              "[ ${valsStr} ]"
-          else
-            if ty == "set" then
-              abort "unsupported set for not-inner value"
-            else abort "Not implemented: type ${ty}";
-
-  outputKeyValInner = k: v:
-    let
-      ty = tomlTy v;
-    in
-      if ty == "set" then
-        let
-          vals = mapAttrsToList
-            (k': v': "${quoteKey k'} = ${outputValInner v'}") v;
-          valsStr = concatStringsSep ", " vals;
-        in
-          [ "${quoteKey k} = { ${valsStr} }" ] else
-        outputKeyVal k v;
-
-  # Returns a list of strings; one string per line
-  outputKeyVal = k: v:
-    let
-      ty = tomlTy v;
-    in
-      if ty == "bool" || ty == "int" then
-        [ "${quoteKey k} = ${outputValInner v}" ]
-      else
-        if ty == "string" then
-          [ "${quoteKey k} = ${quoteString v}" ]
-        else
-          if ty == "list_of_attrs" then
-            concatMap (
-              inner:
-                [ "[[${k}]]" ] ++ (concatLists (mapAttrsToList outputKeyValInner inner))
-            ) v
-          else
-            if ty == "list" then
-              let
-                vals = map quoteString v;
-                valsStr = concatStringsSep ", " vals;
-              in
-                [ "${quoteKey k} = [ ${valsStr} ]" ] else
-              if ty == "set" then
-                [ "[${k}]" ] ++ (concatLists (mapAttrsToList outputKeyValInner v))
-              else abort "Not implemented: type ${ty} for key ${k}";
-
-  tomlTy = x:
-    if typeOf x == "string" then "string" else
-      if typeOf x == "bool" then "bool" else
-        if typeOf x == "int" then "int" else
-          if typeOf x == "float" then "float" else
-            if typeOf x == "set" then
-              if lib.isDerivation x then "string" else "set" else
-              if typeOf x == "list" then
-                if length x == 0 then "list"
-                else
-                  let
-                    ty = typeOf (elemAt x 0);
-                  in
-                    #assert (all (v: typeOf v == ty) x);
-                    if ty == "set" then "list_of_attrs" else "list"
-              else abort "Not implemented: toml type for ${typeOf x}";
-
-  toTOML = attrs:
-    assert (typeOf attrs == "set");
-    let
-      byTy = lib.foldl
-        (
-          acc: x:
-            let
-              ty = tomlTy x.v;
-            in
-              acc // { "${ty}" = (acc.${ty} or []) ++ [ x ]; }
-        )
-        {} (mapAttrsToList (k: v: { inherit k v; }) attrs);
-    in
-      concatMapStringsSep "\n"
-        (kv: concatStringsSep "\n" (outputKeyVal kv.k kv.v))
-        (
-          (byTy.string or []) ++ (byTy.int or []) ++ (byTy.float or []) ++ (byTy.list or []) ++ (byTy.list_of_attrs or []) ++ (byTy.set or [])
-        )
-  ;
-in
-toTOML
diff --git a/default.nix b/default.nix
index 35de254..61b44ec 100644
--- a/default.nix
+++ b/default.nix
@@ -5,6 +5,7 @@
 , lib
 , lndir
 , remarshal
+, formats
 , rsync
 , runCommandLocal
 , rustc
@@ -15,10 +16,10 @@
 }@defaultBuildAttrs:
 
 let
-  libb = import ./lib.nix { inherit lib writeText runCommandLocal remarshal; };
+  libb = import ./lib.nix { inherit lib writeText runCommandLocal remarshal formats; };
 
   builtinz = builtins // import ./builtins
-    { inherit lib writeText remarshal runCommandLocal; };
+    { inherit lib writeText remarshal runCommandLocal formats; };
 
   mkConfig = arg:
     import ./config.nix { inherit lib arg libb builtinz; };
diff --git a/lib.nix b/lib.nix
index a615bf2..33a5875 100644
--- a/lib.nix
+++ b/lib.nix
@@ -1,8 +1,8 @@
-{ lib, writeText, runCommandLocal, remarshal }:
+{ lib, writeText, runCommandLocal, remarshal, formats }:
 let
   builtinz =
     builtins // import ./builtins
-      { inherit lib writeText remarshal runCommandLocal; };
+      { inherit lib writeText remarshal runCommandLocal formats; };
 in
 rec
 {
@@ -121,14 +121,13 @@ rec
 
   # A very minimal 'src' which makes cargo happy nonetheless
   dummySrc =
-    { cargoconfig # string
+    { cargoconfig # path
     , cargotomls # list
     , cargolock # attrset
     , copySources # list of paths that should be copied to the output
     , copySourcesFrom # path from which to copy ${copySources}
     }:
       let
-        config = writeText "config" cargoconfig;
         cargolock' = builtinz.writeTOML "Cargo.lock" cargolock;
 
         fixupCargoToml = cargotoml:
@@ -154,7 +153,7 @@ rec
           { inherit copySources copySourcesFrom cargotomlss; }
           ''
             mkdir -p $out/.cargo
-            ${lib.optionalString (! isNull cargoconfig) "cp ${config} $out/.cargo/config"}
+            ${lib.optionalString (! isNull cargoconfig) "cp ${cargoconfig} $out/.cargo/config"}
             cp ${cargolock'} $out/Cargo.lock
 
             for tuple in $cargotomlss; do
diff --git a/nix/sources.json b/nix/sources.json
index ecf2b7d..0fa6a3e 100644
--- a/nix/sources.json
+++ b/nix/sources.json
@@ -1,14 +1,14 @@
 {
     "agent-rs": {
-        "branch": "paulliu/add-cargo-lock",
+        "branch": "main",
         "description": "A collection of libraries and tools for building software around the Internet Computer, in Rust.",
-        "homepage": "https://sdk.dfinity.org/",
-        "owner": "ninegua",
+        "homepage": "",
+        "owner": "dfinity",
         "repo": "agent-rs",
-        "rev": "4a22e590516bc79ec3c75a320f7941e7762ea098",
-        "sha256": "0sacddc34nlfgldqghlwchgzjki177h5dsgpmdv70cm8hfy0sg7l",
+        "rev": "6d923cbc918852ef5bfaeb08e63c86580aa80ffe",
+        "sha256": "12y28nmv9af8sj55s998wydy08h32gvxr9dbhfxhfilxdairh0a2",
         "type": "tarball",
-        "url": "https://github.com/ninegua/agent-rs/archive/4a22e590516bc79ec3c75a320f7941e7762ea098.tar.gz",
+        "url": "https://github.com/dfinity/agent-rs/archive/6d923cbc918852ef5bfaeb08e63c86580aa80ffe.tar.gz",
         "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
     },
     "fenix": {
@@ -65,10 +65,10 @@
         "homepage": null,
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "6c2318e451fc0eaf338fb461d9bfcc99869de758",
-        "sha256": "1djwkvjnn26v0xw1swwh7dgmi0yl0b1kb8kansrdwk0jhhqbl7zq",
+        "rev": "1df733d83081fe79c109b066c90badece6b8d8b1",
+        "sha256": "1m3s4xv35jilrmbv8hj9gzpbcqgb8vb0iqhzcm1261x8b1hiylvj",
         "type": "tarball",
-        "url": "https://github.com/NixOS/nixpkgs/archive/6c2318e451fc0eaf338fb461d9bfcc99869de758.tar.gz",
+        "url": "https://github.com/NixOS/nixpkgs/archive/1df733d83081fe79c109b066c90badece6b8d8b1.tar.gz",
         "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
     },
     "nixpkgs-21.05": {
@@ -119,6 +119,18 @@
         "url": "https://github.com/nushell/nushell/archive/85bfdca578157072e51e6972d370cfe63b0fda77.tar.gz",
         "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
     },
+    "probe-rs": {
+        "branch": "master",
+        "description": "A debugging toolset and library for debugging embedded ARM and RISC-V targets on a separate host",
+        "homepage": "https://probe.rs",
+        "owner": "probe-rs",
+        "repo": "probe-rs",
+        "rev": "51fa324aef9f7c413988a3d18052b1bbc278a4c5",
+        "sha256": "0zb9s80hsz83ngjngs9cllp7gf8xq9jz0m3lwdhf08x3cp3bj6fd",
+        "type": "tarball",
+        "url": "https://github.com/probe-rs/probe-rs/archive/51fa324aef9f7c413988a3d18052b1bbc278a4c5.tar.gz",
+        "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
+    },
     "ripgrep-all": {
         "branch": "master",
         "description": "rga: ripgrep, but also search in PDFs, E-Books, Office documents, zip, tar.gz, etc.",
diff --git a/test/default.nix b/test/default.nix
index 75097f6..ede5b1c 100644
--- a/test/default.nix
+++ b/test/default.nix
@@ -2,9 +2,25 @@
 let
   sources = import ../nix/sources.nix;
 
-  pkgs = import ../nix {
-    inherit system nixpkgs;
-  };
+  pkgs =
+    let
+      pkgs' = import ../nix {
+        inherit system nixpkgs;
+      };
+
+      older-pkgs = import ../nix {
+        inherit system;
+
+        nixpkgs = "nixpkgs-21.05";
+      };
+
+    in
+    pkgs' // {
+      # Some of our tests use dynamically-built Git repositories that fail extra
+      # security checks introduced in newer Git versions - so for the time being
+      # let's pin our test-Git to an older version.
+      git = older-pkgs.git;
+    };
 
   naersk = pkgs.callPackage ../default.nix {
     inherit (pkgs.rustPackages) cargo rustc;
diff --git a/test/slow/default.nix b/test/slow/default.nix
index d58dff1..81151d3 100644
--- a/test/slow/default.nix
+++ b/test/slow/default.nix
@@ -2,6 +2,7 @@ args: {
   agent-rs = import ./agent-rs args;
   lorri = import ./lorri args;
   nushell = import ./nushell args;
+  probe-rs = import ./probe-rs args;
   ripgrep-all = import ./ripgrep-all args;
   rustlings = import ./rustlings args;
   talent-plan = import ./talent-plan args;
diff --git a/test/slow/probe-rs/default.nix b/test/slow/probe-rs/default.nix
new file mode 100644
index 0000000..64f8310
--- /dev/null
+++ b/test/slow/probe-rs/default.nix
@@ -0,0 +1,33 @@
+{ sources, pkgs, ... }:
+let
+  fenix = import sources.fenix { };
+
+  toolchain = (fenix.toolchainOf {
+    channel = "nightly";
+    date = "2023-07-01";
+    sha256 = "sha256-pWd4tAHP4QWGC3CKWZzDjzYANxATC6CGRmKuP2ZZv5k=";
+  }).toolchain;
+
+  naersk = pkgs.callPackage ../../../default.nix {
+    cargo = toolchain;
+    rustc = toolchain;
+  };
+
+  app = naersk.buildPackage {
+    src = sources.probe-rs;
+
+    buildInputs = with pkgs; [
+      pkg-config
+      libusb1
+      openssl
+    ] ++ lib.optionals stdenv.isDarwin [
+      darwin.DarwinTools
+      darwin.apple_sdk.frameworks.AppKit
+    ];
+  };
+
+in
+pkgs.runCommand "probe-rs-test"
+{
+  buildInputs = [ app ];
+} "rtthost --help && touch $out"