diff --git a/src/libexpr/flake/flake.cc b/src/libexpr/flake/flake.cc
index 21d98089f..c40a903b9 100644
--- a/src/libexpr/flake/flake.cc
+++ b/src/libexpr/flake/flake.cc
@@ -68,7 +68,7 @@ static FlakeInput parseFlakeInput(EvalState & state,
     expectType(state, tAttrs, *value, pos);
 
     FlakeInput input {
-        .ref = parseFlakeRef(inputName)
+        .ref = FlakeRef::fromAttrs({{"type", "indirect"}, {"id", inputName}})
     };
 
     auto sInputs = state.symbols.create("inputs");
diff --git a/src/libstore/fetchers/git.cc b/src/libstore/fetchers/git.cc
index 2a7ce5432..5d0448777 100644
--- a/src/libstore/fetchers/git.cc
+++ b/src/libstore/fetchers/git.cc
@@ -370,32 +370,39 @@ struct GitInputScheme : InputScheme
             url.scheme != "git+ssh" &&
             url.scheme != "git+file") return nullptr;
 
-        auto input = std::make_unique<GitInput>(url);
+        auto url2(url);
+        // FIXME: strip git+
+        url2.query.clear();
 
-        input->url.query.clear();
+        Input::Attrs attrs;
+        attrs.emplace("type", "git");
 
         for (auto &[name, value] : url.query) {
-            if (name == "rev") {
-                if (!std::regex_match(value, revRegex))
-                    throw BadURL("Git URL '%s' contains an invalid commit hash", url.url);
-                input->rev = Hash(value, htSHA1);
-            }
-            else if (name == "ref") {
-                if (!std::regex_match(value, refRegex))
-                    throw BadURL("Git URL '%s' contains an invalid branch/tag name", url.url);
-                input->ref = value;
-            }
-            else input->url.query.insert_or_assign(name, value);
+            if (name == "rev" || name == "ref")
+                attrs.emplace(name, value);
+            else
+                url2.query.emplace(name, value);
         }
 
-        return input;
+        attrs.emplace("url", url2.to_string());
+
+        return inputFromAttrs(attrs);
     }
 
     std::unique_ptr<Input> inputFromAttrs(const Input::Attrs & attrs) override
     {
         if (maybeGetStrAttr(attrs, "type") != "git") return {};
+
+        for (auto & [name, value] : attrs)
+            if (name != "type" && name != "url" && name != "ref" && name != "rev")
+                throw Error("unsupported Git input attribute '%s'", name);
+
         auto input = std::make_unique<GitInput>(parseURL(getStrAttr(attrs, "url")));
-        input->ref = maybeGetStrAttr(attrs, "ref");
+        if (auto ref = maybeGetStrAttr(attrs, "ref")) {
+            if (!std::regex_match(*ref, refRegex))
+                throw BadURL("invalid Git branch/tag name '%s'", *ref);
+            input->ref = *ref;
+        }
         if (auto rev = maybeGetStrAttr(attrs, "rev"))
             input->rev = Hash(*rev, htSHA1);
         return input;
diff --git a/src/libstore/fetchers/github.cc b/src/libstore/fetchers/github.cc
index 13ac4e2f1..c8746b723 100644
--- a/src/libstore/fetchers/github.cc
+++ b/src/libstore/fetchers/github.cc
@@ -166,8 +166,6 @@ struct GitHubInputScheme : InputScheme
 
         for (auto &[name, value] : url.query) {
             if (name == "rev") {
-                if (!std::regex_match(value, revRegex))
-                    throw BadURL("GitHub URL '%s' contains an invalid commit hash", url.url);
                 if (input->rev)
                     throw BadURL("GitHub URL '%s' contains multiple commit hashes", url.url);
                 input->rev = Hash(value, htSHA1);
@@ -193,6 +191,11 @@ struct GitHubInputScheme : InputScheme
     std::unique_ptr<Input> inputFromAttrs(const Input::Attrs & attrs) override
     {
         if (maybeGetStrAttr(attrs, "type") != "github") return {};
+
+        for (auto & [name, value] : attrs)
+            if (name != "type" && name != "owner" && name != "repo" && name != "ref" && name != "rev")
+                throw Error("unsupported GitHub input attribute '%s'", name);
+
         auto input = std::make_unique<GitHubInput>();
         input->owner = getStrAttr(attrs, "owner");
         input->repo = getStrAttr(attrs, "repo");
diff --git a/src/libstore/fetchers/indirect.cc b/src/libstore/fetchers/indirect.cc
index d079b3ad3..016f5fb39 100644
--- a/src/libstore/fetchers/indirect.cc
+++ b/src/libstore/fetchers/indirect.cc
@@ -123,6 +123,11 @@ struct IndirectInputScheme : InputScheme
     std::unique_ptr<Input> inputFromAttrs(const Input::Attrs & attrs) override
     {
         if (maybeGetStrAttr(attrs, "type") != "indirect") return {};
+
+        for (auto & [name, value] : attrs)
+            if (name != "type" && name != "id" && name != "ref" && name != "rev")
+                throw Error("unsupported indirect input attribute '%s'", name);
+
         auto input = std::make_unique<IndirectInput>();
         input->id = getStrAttr(attrs, "id");
         input->ref = maybeGetStrAttr(attrs, "ref");
diff --git a/src/libstore/fetchers/mercurial.cc b/src/libstore/fetchers/mercurial.cc
index f0135d512..1bdab1dbf 100644
--- a/src/libstore/fetchers/mercurial.cc
+++ b/src/libstore/fetchers/mercurial.cc
@@ -264,32 +264,39 @@ struct MercurialInputScheme : InputScheme
             url.scheme != "hg+ssh" &&
             url.scheme != "hg+file") return nullptr;
 
-        auto input = std::make_unique<MercurialInput>(url);
+        auto url2(url);
+        // FIXME: strip hg+
+        url2.query.clear();
 
-        input->url.query.clear();
+        Input::Attrs attrs;
+        attrs.emplace("type", "hg");
 
         for (auto &[name, value] : url.query) {
-            if (name == "rev") {
-                if (!std::regex_match(value, revRegex))
-                    throw BadURL("Mercurial URL '%s' contains an invalid commit hash", url.url);
-                input->rev = Hash(value, htSHA1);
-            }
-            else if (name == "ref") {
-                if (!std::regex_match(value, refRegex))
-                    throw BadURL("Mercurial URL '%s' contains an invalid branch/tag name", url.url);
-                input->ref = value;
-            }
-            else input->url.query.insert_or_assign(name, value);
+            if (name == "rev" || name == "ref")
+                attrs.emplace(name, value);
+            else
+                url2.query.emplace(name, value);
         }
 
-        return input;
+        attrs.emplace("url", url2.to_string());
+
+        return inputFromAttrs(attrs);
     }
 
     std::unique_ptr<Input> inputFromAttrs(const Input::Attrs & attrs) override
     {
         if (maybeGetStrAttr(attrs, "type") != "hg") return {};
+
+        for (auto & [name, value] : attrs)
+            if (name != "type" && name != "url" && name != "ref" && name != "rev")
+                throw Error("unsupported Mercurial input attribute '%s'", name);
+
         auto input = std::make_unique<MercurialInput>(parseURL(getStrAttr(attrs, "url")));
-        input->ref = maybeGetStrAttr(attrs, "ref");
+        if (auto ref = maybeGetStrAttr(attrs, "ref")) {
+            if (!std::regex_match(*ref, refRegex))
+                throw BadURL("invalid Mercurial branch/tag name '%s'", *ref);
+            input->ref = *ref;
+        }
         if (auto rev = maybeGetStrAttr(attrs, "rev"))
             input->rev = Hash(*rev, htSHA1);
         return input;
diff --git a/src/libstore/fetchers/tarball.cc b/src/libstore/fetchers/tarball.cc
index 21c785ada..1302299b3 100644
--- a/src/libstore/fetchers/tarball.cc
+++ b/src/libstore/fetchers/tarball.cc
@@ -110,6 +110,11 @@ struct TarballInputScheme : InputScheme
     std::unique_ptr<Input> inputFromAttrs(const Input::Attrs & attrs) override
     {
         if (maybeGetStrAttr(attrs, "type") != "tarball") return {};
+
+        for (auto & [name, value] : attrs)
+            if (name != "type" && name != "url" && name != "hash" && name != "narHash")
+                throw Error("unsupported tarball input attribute '%s'", name);
+
         auto input = std::make_unique<TarballInput>(parseURL(getStrAttr(attrs, "url")));
         if (auto hash = maybeGetStrAttr(attrs, "hash"))
             // FIXME: require SRI hash.
diff --git a/src/nix/installables.cc b/src/nix/installables.cc
index 77a43ef2a..cf91be51b 100644
--- a/src/nix/installables.cc
+++ b/src/nix/installables.cc
@@ -450,7 +450,8 @@ std::vector<std::shared_ptr<Installable>> SourceExprCommand::parseInstallables(
             if (hasPrefix(s, "nixpkgs.")) {
                 bool static warned;
                 warnOnce(warned, "the syntax 'nixpkgs.<attr>' is deprecated; use 'nixpkgs:<attr>' instead");
-                result.push_back(std::make_shared<InstallableFlake>(*this, parseFlakeRef("flake:nixpkgs"),
+                result.push_back(std::make_shared<InstallableFlake>(*this,
+                        FlakeRef::fromAttrs({{"type", "indirect"}, {"id", "nixpkgs"}}),
                         Strings{"legacyPackages." + settings.thisSystem.get() + "." + std::string(s, 8)}, Strings{}));
             }