diff --git a/src/libcmd/installables.cc b/src/libcmd/installables.cc
index 635ce19b698..00a3e89b04a 100644
--- a/src/libcmd/installables.cc
+++ b/src/libcmd/installables.cc
@@ -599,14 +599,17 @@ InstallableFlake::InstallableFlake(
     Strings prefixes,
     const flake::LockFlags & lockFlags)
     : InstallableValue(state),
+      cmd(cmd),
       flakeRef(flakeRef),
       attrPaths(fragment == "" ? attrPaths : Strings{(std::string) fragment}),
       prefixes(fragment == "" ? Strings{} : prefixes),
       outputsSpec(std::move(outputsSpec)),
       lockFlags(lockFlags)
 {
+    #if 0
     if (cmd && cmd->getAutoArgs(*state)->size())
         throw UsageError("'--arg' and '--argstr' are incompatible with flakes");
+    #endif
 }
 
 std::tuple<std::string, FlakeRef, InstallableValue::DerivationInfo> InstallableFlake::toDerivation()
@@ -615,42 +618,118 @@ std::tuple<std::string, FlakeRef, InstallableValue::DerivationInfo> InstallableF
 
     auto attrPath = attr->getAttrPathStr();
 
-    if (!attr->isDerivation())
-        throw Error("flake output attribute '%s' is not a derivation", attrPath);
+    auto type = attr->maybeGetStringAttr(state->sType).value_or("");
 
-    auto drvPath = attr->forceDerivation();
+    if (type == "derivation") {
 
-    std::set<std::string> outputsToInstall;
-    std::optional<NixInt> priority;
+        auto drvPath = attr->forceDerivation();
 
-    if (auto aMeta = attr->maybeGetAttr(state->sMeta)) {
-        if (auto aOutputsToInstall = aMeta->maybeGetAttr("outputsToInstall"))
-            for (auto & s : aOutputsToInstall->getListOfStrings())
-                outputsToInstall.insert(s);
-        if (auto aPriority = aMeta->maybeGetAttr("priority"))
-            priority = aPriority->getInt();
-    }
+        std::set<std::string> outputsToInstall;
+        std::optional<NixInt> priority;
+
+        if (auto aMeta = attr->maybeGetAttr(state->sMeta)) {
+            if (auto aOutputsToInstall = aMeta->maybeGetAttr("outputsToInstall"))
+                for (auto & s : aOutputsToInstall->getListOfStrings())
+                    outputsToInstall.insert(s);
+            if (auto aPriority = aMeta->maybeGetAttr("priority"))
+                priority = aPriority->getInt();
+        }
 
-    if (outputsToInstall.empty() || std::get_if<AllOutputs>(&outputsSpec)) {
-        outputsToInstall.clear();
-        if (auto aOutputs = attr->maybeGetAttr(state->sOutputs))
-            for (auto & s : aOutputs->getListOfStrings())
-                outputsToInstall.insert(s);
+        if (outputsToInstall.empty() || std::get_if<AllOutputs>(&outputsSpec)) {
+            outputsToInstall.clear();
+            if (auto aOutputs = attr->maybeGetAttr(state->sOutputs))
+                for (auto & s : aOutputs->getListOfStrings())
+                    outputsToInstall.insert(s);
+        }
+
+        if (outputsToInstall.empty())
+            outputsToInstall.insert("out");
+
+        if (auto outputNames = std::get_if<OutputNames>(&outputsSpec))
+            outputsToInstall = *outputNames;
+
+        auto drvInfo = DerivationInfo {
+            .drvPath = std::move(drvPath),
+            .outputsToInstall = std::move(outputsToInstall),
+            .priority = priority,
+        };
+
+        return {attrPath, getLockedFlake()->flake.lockedRef, std::move(drvInfo)};
     }
 
-    if (outputsToInstall.empty())
-        outputsToInstall.insert("out");
+    else if (type == "configurable") {
 
-    if (auto outputNames = std::get_if<OutputNames>(&outputsSpec))
-        outputsToInstall = *outputNames;
+        // FIXME: use eval cache
 
-    auto drvInfo = DerivationInfo {
-        .drvPath = std::move(drvPath),
-        .outputsToInstall = std::move(outputsToInstall),
-        .priority = priority,
-    };
+        auto & build = attr->getAttr("build")->forceValue();
 
-    return {attrPath, getLockedFlake()->flake.lockedRef, std::move(drvInfo)};
+        struct ValueTree
+        {
+            std::map<std::string, std::variant<Value *, ValueTree>> children;
+        };
+
+        ValueTree tree;
+
+        if (cmd) {
+            auto autoArgs = cmd->getAutoArgs(*state);
+
+            for (auto & attr : *autoArgs) {
+                auto ss = tokenizeString<std::vector<std::string>>(state->symbols[attr.name], ".");
+                auto * pos = &tree;
+                for (const auto & [n, s] : enumerate(ss)) {
+                    if (n + 1 == ss.size()) {
+                        if (pos->children.find(s) != pos->children.end())
+                            throw Error("definition of '%s' clashes with a previous definition", state->symbols[attr.name]);
+                        pos->children.emplace(s, attr.value);
+                    } else {
+                        auto i = pos->children.emplace(s, ValueTree()).first;
+                        if (auto pos2 = std::get_if<ValueTree>(&i->second))
+                            pos = pos2;
+                        else
+                            throw Error("definition of '%s' clashes with a previous definition", state->symbols[attr.name]);
+                    }
+                }
+            }
+        }
+
+        std::function<Value * (const ValueTree & tree)> buildAttrs;
+        buildAttrs = [&](const ValueTree & tree)
+        {
+            auto attrs = state->buildBindings(tree.children.size());
+            for (auto & [name, child] : tree.children) {
+                if (auto tree2 = std::get_if<ValueTree>(&child))
+                    attrs.insert(state->symbols.create(name), buildAttrs(*tree2));
+                else if (auto v = std::get_if<Value *>(&child))
+                    attrs.insert(state->symbols.create(name), *v);
+            }
+            auto vAttrs = state->allocValue();
+            vAttrs->mkAttrs(attrs);
+            return vAttrs;
+        };
+
+        auto vArgs = buildAttrs(tree);
+
+        auto vRes = state->allocValue();
+        state->callFunction(build, *vArgs, *vRes, noPos);
+
+        state->forceAttrs(*vRes, noPos);
+
+        auto aDrvPath = vRes->attrs->get(state->sDrvPath);
+        assert(aDrvPath);
+        PathSet context;
+        auto drvPath = state->coerceToStorePath(aDrvPath->pos, *aDrvPath->value, context);
+
+        auto drvInfo = DerivationInfo {
+            .drvPath = std::move(drvPath),
+            .outputsToInstall = {"out"},
+            //.priority = priority,
+        };
+
+        return {attrPath, getLockedFlake()->flake.lockedRef, std::move(drvInfo)};
+    }
+
+    else
+        throw Error("flake output attribute '%s' is not a derivation", attrPath);
 }
 
 std::vector<InstallableValue::DerivationInfo> InstallableFlake::toDerivations()
diff --git a/src/libcmd/installables.hh b/src/libcmd/installables.hh
index 5d715210e92..fdfcbb5060e 100644
--- a/src/libcmd/installables.hh
+++ b/src/libcmd/installables.hh
@@ -154,6 +154,7 @@ struct InstallableValue : Installable
 
 struct InstallableFlake : InstallableValue
 {
+    SourceExprCommand * cmd;
     FlakeRef flakeRef;
     Strings attrPaths;
     Strings prefixes;
diff --git a/src/libexpr/eval-cache.cc b/src/libexpr/eval-cache.cc
index d77b25898bf..b1dac47b7c1 100644
--- a/src/libexpr/eval-cache.cc
+++ b/src/libexpr/eval-cache.cc
@@ -552,6 +552,13 @@ ref<AttrCursor> AttrCursor::getAttr(std::string_view name)
     return getAttr(root->state.symbols.create(name));
 }
 
+std::optional<std::string> AttrCursor::maybeGetStringAttr(Symbol name)
+{
+    auto cursor = maybeGetAttr(name);
+    if (!cursor) return std::nullopt;
+    return cursor->getString();
+}
+
 OrSuggestions<ref<AttrCursor>> AttrCursor::findAlongAttrPath(const std::vector<Symbol> & attrPath, bool force)
 {
     auto res = shared_from_this();
diff --git a/src/libexpr/eval-cache.hh b/src/libexpr/eval-cache.hh
index c93e55b93d0..6880573b054 100644
--- a/src/libexpr/eval-cache.hh
+++ b/src/libexpr/eval-cache.hh
@@ -109,6 +109,8 @@ public:
 
     ref<AttrCursor> getAttr(std::string_view name);
 
+    std::optional<std::string> maybeGetStringAttr(Symbol name);
+
     /* Get an attribute along a chain of attrsets. Note that this does
        not auto-call functors or functions. */
     OrSuggestions<ref<AttrCursor>> findAlongAttrPath(const std::vector<Symbol> & attrPath, bool force = false);
diff --git a/src/libexpr/get-drvs.cc b/src/libexpr/get-drvs.cc
index d616b392191..e6956e801d1 100644
--- a/src/libexpr/get-drvs.cc
+++ b/src/libexpr/get-drvs.cc
@@ -70,12 +70,11 @@ std::string DrvInfo::querySystem() const
 std::optional<StorePath> DrvInfo::queryDrvPath() const
 {
     if (!drvPath && attrs) {
-        Bindings::iterator i = attrs->find(state->sDrvPath);
-        PathSet context;
-        if (i == attrs->end())
-            drvPath = {std::nullopt};
-        else
+        if (auto i = attrs->get(state->sDrvPath)) {
+            PathSet context;
             drvPath = {state->coerceToStorePath(i->pos, *i->value, context)};
+        } else
+            drvPath = {std::nullopt};
     }
     return drvPath.value_or(std::nullopt);
 }
diff --git a/src/libutil/error.cc b/src/libutil/error.cc
index 9172f67a65d..d9d8d5a4abb 100644
--- a/src/libutil/error.cc
+++ b/src/libutil/error.cc
@@ -198,7 +198,7 @@ void printAtPos(const ErrPos & pos, std::ostream & out)
     }
 }
 
-static std::string indent(std::string_view indentFirst, std::string_view indentRest, std::string_view s)
+std::string indent(std::string_view indentFirst, std::string_view indentRest, std::string_view s)
 {
     std::string res;
     bool first = true;
diff --git a/src/libutil/error.hh b/src/libutil/error.hh
index a53e9802e01..79f675751d6 100644
--- a/src/libutil/error.hh
+++ b/src/libutil/error.hh
@@ -107,6 +107,8 @@ void printCodeLines(std::ostream & out,
 
 void printAtPos(const ErrPos & pos, std::ostream & out);
 
+std::string indent(std::string_view indentFirst, std::string_view indentRest, std::string_view s);
+
 struct Trace {
     std::optional<ErrPos> pos;
     hintformat hint;
diff --git a/src/libutil/util.hh b/src/libutil/util.hh
index 09ccfa591f9..16fa6c54c88 100644
--- a/src/libutil/util.hh
+++ b/src/libutil/util.hh
@@ -700,4 +700,19 @@ template<class... Ts> overloaded(Ts...) -> overloaded<Ts...>;
 std::string showBytes(uint64_t bytes);
 
 
+/* Provide an addition operator between strings and string_views
+   inexplicably omitted from the standard library. */
+inline std::string operator + (const std::string & s1, std::string_view s2)
+{
+    auto s = s1;
+    s.append(s2);
+    return s;
+}
+
+inline std::string operator + (std::string && s, std::string_view s2)
+{
+    s.append(s2);
+    return s;
+}
+
 }
diff --git a/src/nix/app.cc b/src/nix/app.cc
index 821964f8625..1a55043d9fb 100644
--- a/src/nix/app.cc
+++ b/src/nix/app.cc
@@ -64,7 +64,7 @@ UnresolvedApp Installable::toApp(EvalState & state)
     auto cursor = getCursor(state);
     auto attrPath = cursor->getAttrPath();
 
-    auto type = cursor->getAttr("type")->getString();
+    auto type = cursor->getAttr(state.sType)->getString();
 
     std::string expected = !attrPath.empty() && state.symbols[attrPath[0]] == "apps" ? "app" : "derivation";
     if (type != expected)
diff --git a/src/nix/describe.cc b/src/nix/describe.cc
new file mode 100644
index 00000000000..3d81474ee29
--- /dev/null
+++ b/src/nix/describe.cc
@@ -0,0 +1,85 @@
+#include "command.hh"
+#include "eval-cache.hh"
+#include "shared.hh"
+#include "markdown.hh"
+
+#include <regex>
+
+using namespace nix;
+
+struct CmdDescribe : InstallableCommand
+{
+    std::optional<std::string> filter;
+
+    CmdDescribe()
+    {
+        addFlag({
+            .longName = "filter",
+            .description = "Only show options that match this regular expression.",
+            .labels = {"regex"},
+            .handler = {&filter},
+        });
+    }
+
+    std::string description() override
+    {
+        return "show information about a configurable derivation";
+    }
+
+    Category category() override { return catSecondary; }
+
+    // FIXME: add help
+
+    void run(ref<Store> store) override
+    {
+        std::optional<std::regex> filterRegex;
+        if (filter)
+            filterRegex = std::regex(*filter, std::regex::extended | std::regex::icase);
+
+        auto state = getEvalState();
+
+        auto cursor = installable->getCursor(*state);
+
+        auto type = cursor->getAttr(state->sType)->getString();
+
+        if (type != "configurable")
+            throw Error("'%s' is not a configurable derivation", installable);
+
+        std::ostringstream str;
+
+        std::function<void(const ref<eval_cache::AttrCursor> &, std::string_view)> recurse;
+        recurse = [&](const ref<eval_cache::AttrCursor> & cursor, std::string_view attrPath)
+        {
+            auto type = cursor->getAttr(state->sType)->getString();
+
+            if (type == "optionSet") {
+                for (auto & attr : cursor->getAttrs()) {
+                    if (attr == state->sType) continue;
+                    recurse(cursor->getAttr(attr),
+                        attrPath.empty()
+                        ? state->symbols[attr]
+                        : std::string(attrPath) + "." + state->symbols[attr]);
+                }
+            }
+
+            else if (type == "option") {
+                if (filterRegex && !std::regex_search(std::string(attrPath), *filterRegex)) return;
+                auto typeId = trim(stripIndentation(cursor->getAttr("typeId")->getString()));
+                str << fmt("* `%s` (*%s*)\n", attrPath, typeId);
+                str << "\n";
+                auto description = trim(stripIndentation(cursor->getAttr(state->sDescription)->getString()));
+                str << indent("  ", "  ", description) << "\n\n";
+            }
+
+            else
+                throw Error("unexpected type '%s'", type);
+        };
+
+        recurse(cursor->getAttr("options"), "");
+
+        RunPager pager;
+        std::cout << renderMarkdownToTerminal(str.str()) << "\n";
+    }
+};
+
+static auto rCmdDescribe = registerCommand<CmdDescribe>("describe");