diff --git a/.hgignore b/.hgignore
index 3a09a70ee8..dbac9f8bad 100644
--- a/.hgignore
+++ b/.hgignore
@@ -12,6 +12,9 @@ rust/passgen/target/
 
 web/quotes/theme/static/
 
+nix/pkgs/factorio-mods/cache/
+nix/pkgs/factorio-mods/.pytest_cache/
+
 py/tumblrcap/dl/
 py/tumblrcap/mylikes/
 
diff --git a/nix/pkgs/default.nix b/nix/pkgs/default.nix
index 5d1ac87deb..462bc80caf 100644
--- a/nix/pkgs/default.nix
+++ b/nix/pkgs/default.nix
@@ -71,4 +71,7 @@
   qrca = pkgs.libsForQt5.callPackage ./qrca { };
 
   terminfos = pkgs.callPackage ./terminfos { };
+
+  factorio-mods = import ./factorio-mods args;
+  libsolv-py = pkgs.callPackage ./libsolv-py.nix { };
 } // (import ./heptapod-runner args)
diff --git a/nix/pkgs/factorio-mods/default.nix b/nix/pkgs/factorio-mods/default.nix
new file mode 100644
index 0000000000..ed8a95ce2d
--- /dev/null
+++ b/nix/pkgs/factorio-mods/default.nix
@@ -0,0 +1,21 @@
+{ depot, pkgs, ... }:
+
+let
+  modData = builtins.fromJSON (builtins.readFile ./mods_lock.json);
+  modDrv = pkgs.factorio-utils.modDrv { allRecommendedMods = false; allOptionalMods = false; };
+  inherit (depot.ops.secrets.factorio) username token;
+
+  testModData = modData.mods.Krastorio2;
+  modData2Drv = d: modDrv rec {
+    inherit (d) name;
+    src = pkgs.fetchurl {
+      name = d.file_name;
+      url = "https://mods.factorio.com${d.download_url}?username=${username}&token=${token}";
+      inherit (d) sha1;
+    };
+  };
+  allMods = builtins.mapAttrs (_: modData2Drv) modData.mods;
+in
+  allMods // {
+    _all = builtins.attrValues allMods;
+  }
diff --git a/nix/pkgs/factorio-mods/lock_mods.py b/nix/pkgs/factorio-mods/lock_mods.py
new file mode 100755
index 0000000000..2ca4ab5291
--- /dev/null
+++ b/nix/pkgs/factorio-mods/lock_mods.py
@@ -0,0 +1,386 @@
+#!/usr/bin/env python3
+
+import json
+import pathlib
+import re
+import sys
+from enum import Enum
+from typing import Dict, Iterator, Optional
+
+import requests
+import solv
+from absl import app, flags
+from attrs import define, field, frozen, asdict
+
+_USERNAME = flags.DEFINE_string("username", "", "Factorio username")
+_TOKEN = flags.DEFINE_string("token", "", "Factorio token")
+_FACTORIO_VERSION = flags.DEFINE_string(
+    "factorio_version", "1.1.77", "Factorio version to filter to"
+)
+
+
+class DependencyType(Enum):
+    INCOMPATIBLE_WITH = "!"
+    OPTIONAL = "?"
+    HIDDEN_OPTIONAL = "(?)"
+    LOAD_ORDER_IGNORED = "~"
+    HARD = ""
+
+    @property
+    def solv_flags(self):
+        return {
+            DependencyType.INCOMPATIBLE_WITH: solv.SOLVABLE_CONFLICTS,
+            DependencyType.OPTIONAL: solv.SOLVABLE_RECOMMENDS,
+            DependencyType.HIDDEN_OPTIONAL: solv.SOLVABLE_SUGGESTS,
+            DependencyType.LOAD_ORDER_IGNORED: solv.SOLVABLE_REQUIRES,
+            DependencyType.HARD: solv.SOLVABLE_REQUIRES,
+        }.get(self)
+
+
+class VersionConstraintType(Enum):
+    NONE = "none"
+    LESS = "<"
+    LESS_EQUAL = "<="
+    EQUAL = "="
+    GREATER_EQUAL = ">="
+    GREATER = ">"
+
+    @property
+    def solv_flags(self):
+        return {
+            VersionConstraintType.LESS: solv.REL_LT,
+            VersionConstraintType.LESS_EQUAL: solv.REL_LT | solv.REL_EQ,
+            VersionConstraintType.GREATER: solv.REL_GT,
+            VersionConstraintType.GREATER_EQUAL: solv.REL_GT | solv.REL_EQ,
+            VersionConstraintType.EQUAL: solv.REL_EQ,
+        }.get(self)
+
+
+_DEPENDENCY_RE = re.compile(
+    r"^(?:(?P<dependency_type>[!?~]|\(\?\))\s*)?(?P<mod_name>[^><=]+?)(?:\s*(?P<version_constraint_type>[<>]=?|=)\s*(?P<version_constraint>[^\s]+))?$"
+)
+
+
+class DependencySpecificationError(ValueError):
+    pass
+
+
+@frozen
+class Dependency:
+    dependency_type: DependencyType
+    dependent_on: str
+    version_constraint_type: VersionConstraintType
+    version_constraint: Optional[str]
+
+    @classmethod
+    def from_str(cls, dep_str):
+        match = _DEPENDENCY_RE.match(dep_str)
+        if not match:
+            raise DependencySpecificationError(dep_str)
+        d = match.groupdict()
+        return Dependency(
+            dependency_type=DependencyType(d["dependency_type"] or ""),
+            dependent_on=d["mod_name"],
+            version_constraint_type=VersionConstraintType(
+                d["version_constraint_type"] or "none"
+            ),
+            version_constraint=d["version_constraint"],
+        )
+
+    def to_solv(self):
+        if self.version_constraint_type == VersionConstraintType.NONE:
+            return self.dependent_on
+        return f"{self.dependent_on} {self.version_constraint_type.value} {self.version_constraint}"
+
+
+@frozen
+class Mod:
+    name: str
+    version: str
+    file_name: str
+    download_url: str
+    sha1: str
+
+
+@define
+class ModFile:
+    mods: Dict[str, Mod]
+
+
+class ModAPI:
+    def __init__(self, sess=None):
+        self.sess = sess or requests.session()
+        self._base = "https://mods.factorio.com/api/mods"
+
+    def _get_json_or_cached(self, cache_key, *args, **kwargs):
+        p = pathlib.Path(f"cache/{cache_key}.json")
+        if p.exists():
+            with open(p, "rt") as f:
+                return json.load(f)
+
+        resp = self.sess.get(*args, **kwargs)
+        resp.raise_for_status()
+        data = resp.json()
+        with open(p, "wt") as f:
+            json.dump(data, f)
+        return data
+
+    def all_mods(self, factorio_version):
+        if factorio_version.count(".") >= 2:
+            factorio_version = ".".join(factorio_version.split(".")[:2])
+        page = 1
+        while True:
+            data = self._get_json_or_cached(
+                f"all_mods.page{page}",
+                self._base,
+                params={
+                    "hide_deprecated": "true",
+                    "page": page,
+                    "page_size": "max",
+                    "version": factorio_version,
+                },
+            )
+            for result in data["results"]:
+                yield result
+            page_count = (data.get("pagination", {}) or {}).get("page_count", 0)
+            if page >= page_count:
+                break
+            page += 1
+
+    def fetch_mod_info(self, name):
+        return self._get_json_or_cached(f"mod.{name}", f"{self._base}/{name}/full")
+
+    def fetch_mod(self, name, version):
+        all_data = self.fetch_mod_info(name)
+        for release in all_data['releases']:
+            if release['version'] == version:
+                return Mod(
+                    name=all_data['name'],
+                    version=release['version'],
+                    file_name=release['file_name'],
+                    download_url=release['download_url'],
+                    sha1=release['sha1'],
+                )
+        raise KeyError(version)
+
+
+class ModAPIRepo:
+    def __init__(self, api, pool):
+        self.api = api
+        self.pool = pool
+        self.handle = pool.add_repo("Factorio ModAPI")
+        self.handle.appdata = self
+        self.handle.priority = 99
+        self.populated = set()
+
+    def _populate_dependencies(self, solvable, dependencies):
+        conflicts = []
+        requires = []
+        suggests = []
+        for dep_obj in dependencies:
+            dep_id = self.pool.str2id(dep_obj.dependent_on)
+            if dep_obj.version_constraint_type != VersionConstraintType.NONE:
+                ver_id = self.pool.str2id(dep_obj.version_constraint)
+                dep_id = self.pool.rel2id(
+                    dep_id, ver_id, dep_obj.version_constraint_type.solv_flags
+                )
+            solvable.add_deparray(dep_obj.dependency_type.solv_flags, dep_id)
+
+    def _populate_release(
+        self, all_data, release, *, load_incompatible=False, load_optional=False
+    ):
+        known_dependencies = set()
+        try:
+            dependencies = [
+                Dependency.from_str(dep)
+                for dep in release["info_json"].get("dependencies", [])
+            ]
+        except DependencySpecificationError:
+            print(
+                f"couldn't parse dependencies for {all_data['name']} {release['version']}"
+            )
+            return set()
+        for dep in dependencies:
+            if (
+                not load_incompatible
+                and dep.dependency_type == DependencyType.INCOMPATIBLE_WITH
+            ):
+                continue
+            if not load_optional and dep.dependency_type in (
+                DependencyType.HIDDEN_OPTIONAL,
+                DependencyType.OPTIONAL,
+            ):
+                continue
+            known_dependencies.add(dep.dependent_on)
+        repodata = self.handle.add_repodata(flags=0)
+        solvable = self.handle.add_solvable()
+        solvable.name = all_data["name"]
+        solvable.evr = release["version"]
+        self._populate_dependencies(solvable, dependencies)
+        solvable.add_deparray(
+            solv.SOLVABLE_PROVIDES,
+            self.pool.rel2id(
+                self.pool.str2id(solvable.name),
+                self.pool.str2id(release["version"]),
+                solv.REL_EQ,
+            ),
+        )
+        solvable.add_deparray(
+            solv.SOLVABLE_REQUIRES,
+            self.pool.rel2id(
+                self.pool.str2id("base"),
+                self.pool.str2id(release["info_json"]["factorio_version"]),
+                solv.REL_GT | solv.REL_EQ,
+            ),
+        )
+        solvable.add_deparray(
+            solv.SOLVABLE_REQUIRES,
+            self.pool.rel2id(
+                self.pool.str2id("base"),
+                self.pool.str2id(
+                    _next_factorio_version(release["info_json"]["factorio_version"])
+                ),
+                solv.REL_LT,
+            ),
+        )
+        return known_dependencies
+
+    def _populate_tree(
+        self,
+        factorio_version,
+        mod_name,
+        *,
+        load_incompatible=False,
+        load_optional=False,
+    ):
+        if mod_name in self.populated:
+            return False
+        self.populated.add(mod_name)
+        known_dependencies_across_all_versions = set()
+        all_data = self.api.fetch_mod_info(mod_name)
+        for release in all_data["releases"]:
+            known_dependencies_across_all_versions |= self._populate_release(
+                all_data,
+                release,
+                load_incompatible=load_incompatible,
+                load_optional=load_optional,
+            )
+        for dep in known_dependencies_across_all_versions:
+            try:
+                self._populate_tree(factorio_version, dep)
+            except requests.exceptions.HTTPError as ex:
+                print(mod_name, "->", dep, ex)
+        return True
+
+    def populate_tree(
+        self,
+        factorio_version,
+        mod_name,
+        *,
+        load_incompatible=False,
+        load_optional=False,
+    ):
+        if self._populate_tree(
+            factorio_version,
+            mod_name,
+            load_incompatible=load_incompatible,
+            load_optional=load_optional,
+        ):
+            self.handle.create_stubs()
+
+    def populate(self, factorio_version):
+        for mod in self.api.all_mods(factorio_version):
+            self.populate_tree(factorio_version, mod["name"])
+
+
+def _next_factorio_version(v):
+    bits = v.split(".")
+    return f"{bits[0]}.{int(bits[1])+1}"
+
+
+class InstalledRepo:
+    def __init__(self, pool):
+        self.pool = pool
+        self.handle = pool.add_repo("Factorio Installed")
+        self.handle.appdata = self
+        self.handle.priority = 99
+        pool.installed = self.handle
+
+    def populate(self, factorio_version):
+        repodata = self.handle.add_repodata()
+        solvable = self.handle.add_solvable()
+        solvable.name = "base"
+        solvable.evr = factorio_version
+        solvable.add_deparray(
+            solv.SOLVABLE_PROVIDES,
+            self.pool.rel2id(
+                self.pool.str2id("base"),
+                self.pool.str2id(factorio_version),
+                solv.REL_EQ,
+            ),
+        )
+        self.handle.create_stubs()
+
+
+def main(args):
+    if len(args) != 2:
+        raise app.UsageError("Requires a path to a mod listfile.")
+
+    pool = solv.Pool()
+    api = ModAPI()
+
+    installed_repo = InstalledRepo(pool)
+    installed_repo.populate(_FACTORIO_VERSION.value)
+
+    repo = ModAPIRepo(api, pool)
+    # repo.populate(_FACTORIO_VERSION.value)
+
+    jobs = []
+    with open(args[1], "rt") as f:
+        print(f"asked to install (for Factorio {_FACTORIO_VERSION.value}):")
+        for ln in f:
+            ln = ln.strip()
+            dep = Dependency.from_str(ln)
+            repo.populate_tree(_FACTORIO_VERSION.value, dep.dependent_on)
+            flags = solv.Selection.SELECTION_NAME | solv.Selection.SELECTION_NOCASE
+            print(f"  {dep.to_solv()}")
+            sel = pool.select(dep.to_solv(), flags)
+            if sel.isempty():
+                print(f'nothing matches "{ln}" (interpreted as: {dep.to_solv()})')
+                sys.exit(1)
+            jobs += sel.jobs(solv.Job.SOLVER_INSTALL)
+
+    solver = pool.Solver()
+    while True:
+        problems = solver.solve(jobs)
+        if problems:
+            print("problems :(")
+            for problem in problems:
+                print(f"Problem {problem.id}/{len(problems)}:")
+                print(f"  {problem}")
+                solutions = problem.solutions()
+                for solution in solutions:
+                    print(f"  Solution {solution.id}:")
+                    for element in solution.elements(True):
+                        print(f"  - {element.str()}")
+                    print("")
+            sys.exit(2)
+        break
+
+    trans = solver.transaction()
+    if trans.isempty():
+        print("nothing to do.")
+        sys.exit(0)
+    print()
+    print("need to install:")
+    mod_file = ModFile(mods={})
+    for p in trans.newsolvables():
+        print(f" - {p.name} {p.evr}")
+        mod = api.fetch_mod(p.name, p.evr)
+        mod_file.mods[mod.name] = mod
+    with open('mods_lock.json', 'wt') as f:
+        json.dump(asdict(mod_file), f)
+
+
+if __name__ == "__main__":
+    app.run(main)
diff --git a/nix/pkgs/factorio-mods/lock_mods_test.py b/nix/pkgs/factorio-mods/lock_mods_test.py
new file mode 100644
index 0000000000..5955f27030
--- /dev/null
+++ b/nix/pkgs/factorio-mods/lock_mods_test.py
@@ -0,0 +1,77 @@
+#!/usr/bin/env python3
+
+from lock_mods import *
+
+
+class TestDependency:
+    def test_from_str_dependency_type(self):
+        assert Dependency.from_str("a") == Dependency(
+            dependency_type=DependencyType.HARD,
+            dependent_on="a",
+            version_constraint_type=VersionConstraintType.NONE,
+            version_constraint=None,
+        )
+        assert Dependency.from_str("! a") == Dependency(
+            dependency_type=DependencyType.INCOMPATIBLE_WITH,
+            dependent_on="a",
+            version_constraint_type=VersionConstraintType.NONE,
+            version_constraint=None,
+        )
+        assert Dependency.from_str("? a") == Dependency(
+            dependency_type=DependencyType.OPTIONAL,
+            dependent_on="a",
+            version_constraint_type=VersionConstraintType.NONE,
+            version_constraint=None,
+        )
+        assert Dependency.from_str("(?) a") == Dependency(
+            dependency_type=DependencyType.HIDDEN_OPTIONAL,
+            dependent_on="a",
+            version_constraint_type=VersionConstraintType.NONE,
+            version_constraint=None,
+        )
+        assert Dependency.from_str("~ a") == Dependency(
+            dependency_type=DependencyType.LOAD_ORDER_IGNORED,
+            dependent_on="a",
+            version_constraint_type=VersionConstraintType.NONE,
+            version_constraint=None,
+        )
+
+    def test_from_str_version_constraint(self):
+        assert Dependency.from_str("hactorio") == Dependency(
+            dependency_type=DependencyType.HARD,
+            dependent_on="hactorio",
+            version_constraint_type=VersionConstraintType.NONE,
+            version_constraint=None,
+        )
+        assert Dependency.from_str("hactorio > 0.1") == Dependency(
+            dependency_type=DependencyType.HARD,
+            dependent_on="hactorio",
+            version_constraint_type=VersionConstraintType.GREATER,
+            version_constraint="0.1",
+        )
+        assert Dependency.from_str("hactorio >= 0.1") == Dependency(
+            dependency_type=DependencyType.HARD,
+            dependent_on="hactorio",
+            version_constraint_type=VersionConstraintType.GREATER_EQUAL,
+            version_constraint="0.1",
+        )
+        assert Dependency.from_str("hactorio < 0.1") == Dependency(
+            dependency_type=DependencyType.HARD,
+            dependent_on="hactorio",
+            version_constraint_type=VersionConstraintType.LESS,
+            version_constraint="0.1",
+        )
+        assert Dependency.from_str("hactorio <= 0.1") == Dependency(
+            dependency_type=DependencyType.HARD,
+            dependent_on="hactorio",
+            version_constraint_type=VersionConstraintType.LESS_EQUAL,
+            version_constraint="0.1",
+        )
+
+    def test_from_str_complex(self):
+        assert Dependency.from_str("(?) hactorio <= 0.1") == Dependency(
+            dependency_type=DependencyType.HIDDEN_OPTIONAL,
+            dependent_on="hactorio",
+            version_constraint_type=VersionConstraintType.LESS_EQUAL,
+            version_constraint="0.1",
+        )
diff --git a/nix/pkgs/factorio-mods/mods.txt b/nix/pkgs/factorio-mods/mods.txt
new file mode 100644
index 0000000000..13907b413b
--- /dev/null
+++ b/nix/pkgs/factorio-mods/mods.txt
@@ -0,0 +1 @@
+Krastorio2
diff --git a/nix/pkgs/factorio-mods/mods_lock.json b/nix/pkgs/factorio-mods/mods_lock.json
new file mode 100644
index 0000000000..8c0c57aa04
--- /dev/null
+++ b/nix/pkgs/factorio-mods/mods_lock.json
@@ -0,0 +1 @@
+{"mods": {"Krastorio2": {"name": "Krastorio2", "version": "1.3.16", "file_name": "Krastorio2_1.3.16.zip", "download_url": "/download/Krastorio2/64057d537d9565d6cc833ca1", "sha1": "6e2c0aea3c86e2d08ed1fd3c57931e87ef08dfaf"}, "Krastorio2Assets": {"name": "Krastorio2Assets", "version": "1.2.1", "file_name": "Krastorio2Assets_1.2.1.zip", "download_url": "/download/Krastorio2Assets/637324c46ea252725429adb7", "sha1": "a7db4435657adf9ed78799c72b0262e56346aa80"}, "flib": {"name": "flib", "version": "0.12.5", "file_name": "flib_0.12.5.zip", "download_url": "/download/flib/640aa1dcbdb9bd7b5bc7602a", "sha1": "91cc9921132132d963202c9e6277676c5e2deced"}}}
\ No newline at end of file
diff --git a/nix/pkgs/factorio-mods/shell.nix b/nix/pkgs/factorio-mods/shell.nix
new file mode 100644
index 0000000000..5c6d6fffc7
--- /dev/null
+++ b/nix/pkgs/factorio-mods/shell.nix
@@ -0,0 +1,16 @@
+{ depot ? import <depot> {} }:
+
+let
+  inherit (depot) pkgs;
+in pkgs.mkShell {
+  buildInputs = with pkgs; [
+    black
+    python3
+    python3.pkgs.isort
+    python3.pkgs.absl-py
+    python3.pkgs.requests
+    python3.pkgs.attrs
+    python3.pkgs.pytest
+    depot.nix.pkgs.libsolv-py
+  ];
+}
diff --git a/nix/pkgs/libsolv-py.nix b/nix/pkgs/libsolv-py.nix
new file mode 100644
index 0000000000..3da278daa5
--- /dev/null
+++ b/nix/pkgs/libsolv-py.nix
@@ -0,0 +1,25 @@
+{ libsolv, python3, swig, ... }:
+
+(libsolv.override {
+  withRpm = false;
+}).overrideAttrs ({
+  cmakeFlags,
+  buildInputs,
+  nativeBuildInputs,
+  postPatch ? "",
+  ... }: {
+  postPatch = ''
+    ${postPatch}
+
+    sed -i 's,DESTINATION ''${PYTHON_INSTALL_DIR},DESTINATION "${placeholder "out"}/${python3.sitePackages}",g' bindings/python/CMakeLists.txt
+  '';
+  cmakeFlags = cmakeFlags ++ [
+    "-DENABLE_PYTHON=true"
+  ];
+  nativeBuildInputs = nativeBuildInputs ++ [
+    swig
+  ];
+  buildInputs = buildInputs ++ [
+    python3
+  ];
+})