diff --git a/src/libexpr/primops/flake.cc b/src/libexpr/primops/flake.cc
index 9d1da84f1..df0845c24 100644
--- a/src/libexpr/primops/flake.cc
+++ b/src/libexpr/primops/flake.cc
@@ -3,7 +3,9 @@
 #include "eval-inline.hh"
 #include "fetchGit.hh"
 #include "download.hh"
+#include "args.hh"
 
+#include <iostream>
 #include <queue>
 #include <regex>
 #include <nlohmann/json.hpp>
@@ -32,10 +34,10 @@ static std::unique_ptr<FlakeRegistry> readRegistry(const Path & path)
 }
 
 /* Write the registry or lock file to a file. */
-static void writeRegistry(FlakeRegistry registry, Path path = "./flake.lock")
+void writeRegistry(FlakeRegistry registry, Path path)
 {
     nlohmann::json json = {};
-    json["value"] = 0; // Not sure whether this should be 0.
+    json["version"] = 1;
     json["flakes"] = {};
     for (auto elem : registry.entries) {
         json["flakes"][elem.first] = elem.second.ref.to_string();
@@ -107,9 +109,21 @@ struct FlakeSourceInfo
 
 static FlakeSourceInfo fetchFlake(EvalState & state, const FlakeRef & flakeRef)
 {
-    assert(flakeRef.isDirect());
+    FlakeRef directFlakeRef = FlakeRef(flakeRef);
+    if (!flakeRef.isDirect())
+    {
+        std::vector<const FlakeRegistry *> registries;
+        // 'pureEval' is a setting which cannot be changed in `nix flake`,
+        // but without flagging it off, we can't use any FlakeIds.
+        // if (!evalSettings.pureEval) {
+            registries.push_back(&state.getFlakeRegistry());
+        // }
+        directFlakeRef = lookupFlake(state, flakeRef, registries);
+    }
+    assert(directFlakeRef.isDirect());
+    // NOTE FROM NICK: I don't see why one wouldn't fetch FlakeId flakes..
 
-    if (auto refData = std::get_if<FlakeRef::IsGitHub>(&flakeRef.data)) {
+    if (auto refData = std::get_if<FlakeRef::IsGitHub>(&directFlakeRef.data)) {
         // FIXME: require hash in pure mode.
 
         // FIXME: use regular /archive URLs instead? api.github.com
@@ -141,7 +155,7 @@ static FlakeSourceInfo fetchFlake(EvalState & state, const FlakeRef & flakeRef)
         return info;
     }
 
-    else if (auto refData = std::get_if<FlakeRef::IsGit>(&flakeRef.data)) {
+    else if (auto refData = std::get_if<FlakeRef::IsGit>(&directFlakeRef.data)) {
         auto gitInfo = exportGit(state.store, refData->uri, refData->ref,
             refData->rev ? refData->rev->to_string(Base16, false) : "", "source");
         FlakeSourceInfo info;
@@ -165,7 +179,16 @@ Flake getFlake(EvalState & state, const FlakeRef & flakeRef)
     if (state.allowedPaths)
         state.allowedPaths->insert(flakePath);
 
-    Flake flake;
+    FlakeRef newFlakeRef(flakeRef);
+    if (std::get_if<FlakeRef::IsGitHub>(&newFlakeRef.data)) {
+        FlakeSourceInfo srcInfo = fetchFlake(state, newFlakeRef);
+        if (srcInfo.rev) {
+            std::string uri = flakeRef.to_string();
+            newFlakeRef = FlakeRef(uri + "/" + srcInfo.rev->to_string());
+        }
+    }
+
+    Flake flake(newFlakeRef);
 
     Value vInfo;
     state.evalFile(flakePath + "/flake.nix", vInfo); // FIXME: symlink attack
@@ -258,6 +281,35 @@ static std::tuple<FlakeId, std::map<FlakeId, Flake>> resolveFlake(EvalState & st
     return {*topFlakeId, std::move(done)};
 }
 
+FlakeRegistry updateLockFile(EvalState & evalState, FlakeRef & flakeRef)
+{
+    FlakeRegistry newLockFile;
+    std::map<FlakeId, Flake> myDependencyMap = get<1>(resolveFlake(evalState, flakeRef, false));
+    // Nick assumed that "topRefPure" means that the Flake for flakeRef can be
+    // fetched purely.
+    for (auto const& require : myDependencyMap) {
+        FlakeRegistry::Entry entry = FlakeRegistry::Entry(require.second.ref);
+        // The FlakeRefs are immutable because they come out of the Flake objects,
+        // not from the requires.
+        newLockFile.entries.insert(std::pair<FlakeId, FlakeRegistry::Entry>(require.first, entry));
+    }
+    return newLockFile;
+}
+
+void updateLockFile(EvalState & state, std::string path)
+{
+    // 'path' is the path to the local flake repo.
+    FlakeRef flakeRef = FlakeRef(path);
+    if (std::get_if<FlakeRef::IsGit>(&flakeRef.data)) {
+        FlakeRegistry newLockFile = updateLockFile(state, flakeRef);
+        writeRegistry(newLockFile, path + "/flake.lock");
+    } else if (std::get_if<FlakeRef::IsGitHub>(&flakeRef.data)) {
+        throw UsageError("You can only update local flakes, not flakes on GitHub.");
+    } else {
+       throw UsageError("You can only update local flakes, not flakes through their FlakeId.");
+    }
+}
+
 Value * makeFlakeValue(EvalState & state, std::string flakeUri, Value & v)
 {
     // FIXME: temporary hack to make the default installation source
diff --git a/src/libexpr/primops/flake.hh b/src/libexpr/primops/flake.hh
index 90c6bc7d2..b3a755311 100644
--- a/src/libexpr/primops/flake.hh
+++ b/src/libexpr/primops/flake.hh
@@ -13,6 +13,7 @@ struct FlakeRegistry
     struct Entry
     {
         FlakeRef ref;
+        Entry(const FlakeRef & flakeRef) : ref(flakeRef) {};
     };
     std::map<FlakeId, Entry> entries;
 };
@@ -21,9 +22,12 @@ Value * makeFlakeRegistryValue(EvalState & state);
 
 Value * makeFlakeValue(EvalState & state, std::string flakeUri, Value & v);
 
+void writeRegistry(FlakeRegistry, Path);
+
 struct Flake
 {
     FlakeId id;
+    FlakeRef ref;
     std::string description;
     Path path;
     std::vector<FlakeRef> requires;
@@ -32,9 +36,12 @@ struct Flake
     // commit hash
     // date
     // content hash
+    Flake(FlakeRef & flakeRef) : ref(flakeRef) {};
 };
 
-Flake getFlake(EvalState & state, const FlakeRef & flakeRef);
+Flake getFlake(EvalState &, const FlakeRef &);
 
-void writeRegistry(FlakeRegistry);
+FlakeRegistry updateLockFile(EvalState &, Flake &);
+
+void updateLockFile(EvalState &, std::string);
 }
diff --git a/src/libexpr/primops/flakeref.hh b/src/libexpr/primops/flakeref.hh
index ad0cf8630..4d1756b49 100644
--- a/src/libexpr/primops/flakeref.hh
+++ b/src/libexpr/primops/flakeref.hh
@@ -129,6 +129,9 @@ struct FlakeRef
     // Parse a flake URI.
     FlakeRef(const std::string & uri);
 
+    // Default constructor
+    FlakeRef(const FlakeRef & flakeRef) : data(flakeRef.data) {};
+
     /* Unify two flake references so that the resulting reference
        combines the information from both. For example,
        "nixpkgs/<hash>" and "github:NixOS/nixpkgs" unifies to
diff --git a/src/libutil/util.cc b/src/libutil/util.cc
index e3dcd246c..b0a2b853e 100644
--- a/src/libutil/util.cc
+++ b/src/libutil/util.cc
@@ -344,7 +344,6 @@ void writeFile(const Path & path, Source & source, mode_t mode)
     }
 }
 
-
 string readLine(int fd)
 {
     string s;
diff --git a/src/nix/build.cc b/src/nix/build.cc
index b329ac38a..12ef08679 100644
--- a/src/nix/build.cc
+++ b/src/nix/build.cc
@@ -1,3 +1,5 @@
+#include "primops/flake.hh"
+#include "eval.hh"
 #include "command.hh"
 #include "common-args.hh"
 #include "shared.hh"
@@ -9,6 +11,8 @@ struct CmdBuild : MixDryRun, InstallablesCommand
 {
     Path outLink = "result";
 
+    std::optional<std::string> flakeUri = std::nullopt;
+
     CmdBuild()
     {
         mkFlag()
@@ -22,6 +26,11 @@ struct CmdBuild : MixDryRun, InstallablesCommand
             .longName("no-link")
             .description("do not create a symlink to the build result")
             .set(&outLink, Path(""));
+
+        mkFlag()
+            .longName("flake")
+            .description("update lock file of given flake")
+            .dest(&flakeUri);
     }
 
     std::string name() override
@@ -52,6 +61,8 @@ struct CmdBuild : MixDryRun, InstallablesCommand
     {
         auto buildables = build(store, dryRun ? DryRun : Build, installables);
 
+        auto evalState = std::make_shared<EvalState>(searchPath, store);
+
         if (dryRun) return;
 
         for (size_t i = 0; i < buildables.size(); ++i) {
@@ -66,6 +77,10 @@ struct CmdBuild : MixDryRun, InstallablesCommand
                         store2->addPermRoot(output.second, absPath(symlink), true);
                     }
         }
+
+        if (flakeUri) {
+            updateLockFile(*evalState, *flakeUri);
+        }
     }
 };
 
diff --git a/src/nix/command.cc b/src/nix/command.cc
index e1e32aaae..5967ab36c 100644
--- a/src/nix/command.cc
+++ b/src/nix/command.cc
@@ -27,15 +27,6 @@ void StoreCommand::run()
     run(getStore());
 }
 
-JsonFormattable::JsonFormattable()
-{
-    mkFlag()
-        .longName("json-formattable")
-        .shortName('j')
-        .description("output will be printed as json")
-        .handler([&]() { jsonFormatting = true; });
-}
-
 StorePathsCommand::StorePathsCommand(bool recursive)
     : recursive(recursive)
 {
diff --git a/src/nix/command.hh b/src/nix/command.hh
index 5c2f8c304..b3248222e 100644
--- a/src/nix/command.hh
+++ b/src/nix/command.hh
@@ -26,13 +26,6 @@ private:
     std::shared_ptr<Store> _store;
 };
 
-struct JsonFormattable : virtual Command
-{
-    bool jsonFormatting = false;;
-
-    JsonFormattable();
-};
-
 struct Buildable
 {
     Path drvPath; // may be empty
diff --git a/src/nix/flake.cc b/src/nix/flake.cc
index 22e5b297c..6cef38936 100644
--- a/src/nix/flake.cc
+++ b/src/nix/flake.cc
@@ -34,7 +34,28 @@ struct CmdFlakeList : StoreCommand, MixEvalArgs
     }
 };
 
-struct CmdFlakeInfo : FlakeCommand, JsonFormattable
+struct CmdFlakeUpdate : FlakeCommand
+{
+    std::string name() override
+    {
+        return "update";
+    }
+
+    std::string description() override
+    {
+        return "update flake lock file";
+    }
+
+    void run(nix::ref<nix::Store> store) override
+    {
+        auto evalState = std::make_shared<EvalState>(searchPath, store);
+
+        if (flakeUri == "") flakeUri = absPath("./flake.nix");
+        updateLockFile(*evalState, flakeUri);
+    }
+};
+
+struct CmdFlakeInfo : FlakeCommand, MixJSON
 {
     std::string name() override
     {
@@ -50,7 +71,7 @@ struct CmdFlakeInfo : FlakeCommand, JsonFormattable
     {
         auto evalState = std::make_shared<EvalState>(searchPath, store);
         nix::Flake flake = nix::getFlake(*evalState, FlakeRef(flakeUri));
-        if (jsonFormatting) {
+        if (json) {
             nlohmann::json j;
             j["location"] = flake.path;
             j["description"] = flake.description;
@@ -65,7 +86,9 @@ struct CmdFlakeInfo : FlakeCommand, JsonFormattable
 struct CmdFlake : virtual MultiCommand, virtual Command
 {
     CmdFlake()
-        : MultiCommand({make_ref<CmdFlakeList>(), make_ref<CmdFlakeInfo>()})
+        : MultiCommand({make_ref<CmdFlakeList>()
+            , make_ref<CmdFlakeInfo>()
+            , make_ref<CmdFlakeUpdate>()})
     {
     }