diff --git a/lix/libfetchers/fetchers.cc b/lix/libfetchers/fetchers.cc
index 0dc9f5e0c..dd8ec9436 100644
--- a/lix/libfetchers/fetchers.cc
+++ b/lix/libfetchers/fetchers.cc
@@ -304,6 +304,24 @@ std::optional<time_t> Input::getLastModified() const
     return maybeGetIntAttr(attrs, "lastModified");
 }
 
+std::optional<Input> InputScheme::inputFromAttrs(const Attrs & attrs) const
+{
+    if (maybeGetStrAttr(attrs, "type") != schemeType()) return {};
+
+    Attrs finalAttrs = preprocessAttrs(attrs);
+
+    for (auto & [name, value] : finalAttrs)
+        // All attrs need to accept a `type` and `narHash` key, the rest is scheme-specific
+        if (name != "type" && name != "narHash" && !allowedAttrs().contains(name))
+            throw UnsupportedAttributeError("unsupported input attribute '%s' for the '%s' scheme", name, schemeType());
+
+    Input input;
+    input.attrs = finalAttrs;
+    return input;
+}
+
+
+
 ParsedURL InputScheme::toURL(const Input & input) const
 {
     throw Error("don't know how to convert input '%s' to a URL", attrsToJSON(input.attrs));
diff --git a/lix/libfetchers/fetchers.hh b/lix/libfetchers/fetchers.hh
index 420256bd2..77bbc8805 100644
--- a/lix/libfetchers/fetchers.hh
+++ b/lix/libfetchers/fetchers.hh
@@ -2,6 +2,7 @@
 ///@file
 
 #include "lix/libstore/content-address.hh"
+#include "lix/libutil/error.hh"
 #include "lix/libutil/result.hh"
 #include "lix/libutil/types.hh"
 #include "lix/libutil/hash.hh"
@@ -19,6 +20,8 @@ namespace nix { class Store; }
 
 namespace nix::fetchers {
 
+MakeError(UnsupportedAttributeError, Error);
+
 struct Tree
 {
     Path actualPath;
@@ -125,7 +128,6 @@ public:
     std::optional<time_t> getLastModified() const;
 };
 
-
 /**
  * The InputScheme represents a type of fetcher.  Each fetcher
  * registers with nix at startup time.  When processing an input for a
@@ -142,7 +144,12 @@ struct InputScheme
 
     virtual std::optional<Input> inputFromURL(const ParsedURL & url, bool requireTree) const = 0;
 
-    virtual std::optional<Input> inputFromAttrs(const Attrs & attrs) const = 0;
+    virtual Attrs preprocessAttrs(const Attrs & attrs) const = 0;
+
+    // The scheme type, which is used to match attributes to a specific scheme
+    virtual std::string schemeType() const = 0;
+
+    virtual std::optional<Input> inputFromAttrs(const Attrs & attrs) const;
 
     virtual ParsedURL toURL(const Input & input) const;
 
@@ -174,6 +181,9 @@ struct InputScheme
     virtual bool isLockedByRev() const { return true; }
 
 protected:
+    // The set of allowed attributes for this specific fetcher
+    virtual const std::set<std::string> & allowedAttrs() const = 0;
+
     void emplaceURLQueryIntoAttrs(
         const ParsedURL & parsedURL,
         Attrs & attrs,
diff --git a/lix/libfetchers/git.cc b/lix/libfetchers/git.cc
index 21fa1904d..e4b2d51cb 100644
--- a/lix/libfetchers/git.cc
+++ b/lix/libfetchers/git.cc
@@ -288,8 +288,28 @@ static std::optional<Path> resolveRefToCachePath(
     return std::nullopt;
 }
 
+static const std::set<std::string> allowedGitAttrs = {
+    "allRefs",
+    "dirtyRev",
+    "dirtyShortRev",
+    "lastModified",
+    "name",
+    "ref",
+    "rev",
+    "revCount",
+    "shallow",
+    "submodules",
+    "url",
+};
+
 struct GitInputScheme : InputScheme
 {
+    std::string schemeType() const override { return "git"; }
+
+    const std::set<std::string> & allowedAttrs() const override {
+        return allowedGitAttrs;
+    }
+
     std::optional<Input> inputFromURL(const ParsedURL & url, bool requireTree) const override
     {
         if (url.scheme != "git" &&
@@ -316,14 +336,7 @@ struct GitInputScheme : InputScheme
         return inputFromAttrs(attrs);
     }
 
-    std::optional<Input> inputFromAttrs(const Attrs & attrs) const override
-    {
-        if (maybeGetStrAttr(attrs, "type") != "git") return {};
-
-        for (auto & [name, value] : attrs)
-            if (name != "type" && name != "url" && name != "ref" && name != "rev" && name != "shallow" && name != "submodules" && name != "lastModified" && name != "revCount" && name != "narHash" && name != "allRefs" && name != "name" && name != "dirtyRev" && name != "dirtyShortRev")
-                throw Error("unsupported Git input attribute '%s'", name);
-
+    Attrs preprocessAttrs(const Attrs & attrs) const override {
         parseURL(getStrAttr(attrs, "url"));
         maybeGetBoolAttr(attrs, "shallow");
         maybeGetBoolAttr(attrs, "submodules");
@@ -334,9 +347,7 @@ struct GitInputScheme : InputScheme
                 throw BadURL("invalid Git branch/tag name '%s'", *ref);
         }
 
-        Input input;
-        input.attrs = attrs;
-        return input;
+        return attrs;
     }
 
     ParsedURL toURL(const Input & input) const override
diff --git a/lix/libfetchers/github.cc b/lix/libfetchers/github.cc
index dcd1d4632..c8402b80e 100644
--- a/lix/libfetchers/github.cc
+++ b/lix/libfetchers/github.cc
@@ -1,7 +1,5 @@
 #include "lix/libfetchers/attrs.hh"
 #include "lix/libstore/filetransfer.hh"
-#include "lix/libfetchers/cache.hh"
-#include "lix/libstore/globals.hh"
 #include "lix/libfetchers/builtin-fetchers.hh"
 #include "lix/libstore/store-api.hh"
 #include "lix/libutil/async.hh"
@@ -24,19 +22,30 @@ struct DownloadUrl
     Headers headers;
 };
 
+static const std::set<std::string> allowedGitArchiveAttrs = {
+    "host",
+    "lastModified",
+    "owner",
+    "ref",
+    "repo",
+    "rev",
+};
+
 // A github, gitlab, or sourcehut host
 const static std::string hostRegexS = "[a-zA-Z0-9.-]*"; // FIXME: check
 std::regex hostRegex = regex::parse(hostRegexS, std::regex::ECMAScript);
 
 struct GitArchiveInputScheme : InputScheme
 {
-    virtual std::string type() const = 0;
+    const std::set<std::string> & allowedAttrs() const override {
+        return allowedGitArchiveAttrs;
+    }
 
     virtual std::optional<std::pair<std::string, std::string>> accessHeaderFromToken(const std::string & token) const = 0;
 
     std::optional<Input> inputFromURL(const ParsedURL & url, bool requireTree) const override
     {
-        if (url.scheme != type()) return {};
+        if (url.scheme != schemeType()) return {};
 
         auto path = tokenizeString<std::vector<std::string>>(url.path, "/");
 
@@ -63,7 +72,7 @@ struct GitArchiveInputScheme : InputScheme
             throw BadURL("URL '%s' is invalid", url.url);
 
         Attrs attrs;
-        attrs.emplace("type", type());
+        attrs.emplace("type", schemeType());
         attrs.emplace("owner", path[0]);
         attrs.emplace("repo", path[1]);
 
@@ -93,20 +102,18 @@ struct GitArchiveInputScheme : InputScheme
         return inputFromAttrs(attrs);
     }
 
-    std::optional<Input> inputFromAttrs(const Attrs & attrs) const override
+    Attrs preprocessAttrs(const Attrs & attrs) const override
     {
         // Attributes can contain refOrRev and it needs to be figured out
         // which one it is (see inputFromURL for when that may happen).
         // The correct one (ref or rev) will be written into finalAttrs and
         // it needs to be mutable for that.
         Attrs finalAttrs(attrs);
-        auto type_ = maybeGetStrAttr(finalAttrs, "type");
-        if (type_ != type()) return {};
 
         auto owner = getStrAttr(finalAttrs, "owner");
         auto repo = getStrAttr(finalAttrs, "repo");
 
-        auto url = fmt("%s:%s/%s", *type_, owner, repo);
+        auto url = fmt("%s:%s/%s", schemeType(), owner, repo);
         if (auto host = maybeGetStrAttr(finalAttrs, "host")) {
             if (!std::regex_match(*host, hostRegex)) {
                 throw BadURL("URL '%s' contains an invalid instance host", url);
@@ -132,15 +139,7 @@ struct GitArchiveInputScheme : InputScheme
             }
         }
 
-        for (auto & [name, value] : finalAttrs) {
-            if (name != "type" && name != "owner" && name != "repo" && name != "ref" && name != "rev" && name != "narHash" && name != "lastModified" && name != "host") {
-                throw Error("unsupported input attribute '%s'", name);
-            }
-        }
-
-        Input input;
-        input.attrs = finalAttrs;
-        return input;
+        return finalAttrs;
     }
 
     ParsedURL toURL(const Input & input) const override
@@ -154,7 +153,7 @@ struct GitArchiveInputScheme : InputScheme
         if (ref) path += "/" + *ref;
         if (rev) path += "/" + rev->to_string(Base::Base16, false);
         return ParsedURL {
-            .scheme = type(),
+            .scheme = schemeType(),
             .path = path,
         };
     }
@@ -239,7 +238,7 @@ struct GitArchiveInputScheme : InputScheme
 
 struct GitHubInputScheme : GitArchiveInputScheme
 {
-    std::string type() const override { return "github"; }
+    std::string schemeType() const override { return "github"; }
 
     std::optional<std::pair<std::string, std::string>> accessHeaderFromToken(const std::string & token) const override
     {
@@ -329,7 +328,7 @@ struct GitHubInputScheme : GitArchiveInputScheme
 
 struct GitLabInputScheme : GitArchiveInputScheme
 {
-    std::string type() const override { return "gitlab"; }
+    std::string schemeType() const override { return "gitlab"; }
 
     std::optional<std::pair<std::string, std::string>> accessHeaderFromToken(const std::string & token) const override
     {
@@ -405,7 +404,7 @@ struct GitLabInputScheme : GitArchiveInputScheme
 
 struct SourceHutInputScheme : GitArchiveInputScheme
 {
-    std::string type() const override { return "sourcehut"; }
+    std::string schemeType() const override { return "sourcehut"; }
 
     std::optional<std::pair<std::string, std::string>> accessHeaderFromToken(const std::string & token) const override
     {
diff --git a/lix/libfetchers/indirect.cc b/lix/libfetchers/indirect.cc
index 342c8974e..c671adc5c 100644
--- a/lix/libfetchers/indirect.cc
+++ b/lix/libfetchers/indirect.cc
@@ -8,8 +8,20 @@ namespace nix::fetchers {
 
 std::regex flakeRegex = regex::parse("[a-zA-Z][a-zA-Z0-9_-]*", std::regex::ECMAScript);
 
+static const std::set<std::string> allowedIndirectAttrs = {
+    "id",
+    "ref",
+    "rev",
+};
+
 struct IndirectInputScheme : InputScheme
 {
+    std::string schemeType() const override { return "indirect"; }
+
+    const std::set<std::string> & allowedAttrs() const override {
+        return allowedIndirectAttrs;
+    }
+
     std::optional<Input> inputFromURL(const ParsedURL & url, bool requireTree) const override
     {
         if (url.scheme != "flake") return {};
@@ -47,14 +59,7 @@ struct IndirectInputScheme : InputScheme
         return inputFromAttrs(attrs);
     }
 
-    std::optional<Input> inputFromAttrs(const Attrs & attrs) const override
-    {
-        if (maybeGetStrAttr(attrs, "type") != "indirect") return {};
-
-        for (auto & [name, value] : attrs)
-            if (name != "type" && name != "id" && name != "ref" && name != "rev" && name != "narHash")
-                throw Error("unsupported indirect input attribute '%s'", name);
-
+    Attrs preprocessAttrs(const Attrs & attrs) const override {
         auto id = getStrAttr(attrs, "id");
         if (!std::regex_match(id, flakeRegex))
             throw BadURL("'%s' is not a valid flake ID", id);
@@ -71,9 +76,17 @@ struct IndirectInputScheme : InputScheme
             }
         }
 
-        Input input;
-        input.direct = false;
-        input.attrs = attrs;
+        return attrs;
+    }
+
+    std::optional<Input> inputFromAttrs(const Attrs & attrs) const override
+    {
+        std::optional<Input> input = InputScheme::inputFromAttrs(attrs);
+
+        if (input) {
+            input->direct = false;
+        }
+
         return input;
     }
 
diff --git a/lix/libfetchers/mercurial.cc b/lix/libfetchers/mercurial.cc
index 0cd007d8c..72300ca52 100644
--- a/lix/libfetchers/mercurial.cc
+++ b/lix/libfetchers/mercurial.cc
@@ -44,8 +44,22 @@ static std::string runHg(const Strings & args)
     return res.second;
 }
 
+static const std::set<std::string> allowedMercurialAttrs = {
+    "name",
+    "ref",
+    "rev",
+    "revCount",
+    "url",
+};
+
 struct MercurialInputScheme : InputScheme
 {
+    std::string schemeType() const override { return "hg"; }
+
+    const std::set<std::string> & allowedAttrs() const override {
+        return allowedMercurialAttrs;
+    }
+
     std::optional<Input> inputFromURL(const ParsedURL & url, bool requireTree) const override
     {
         if (url.scheme != "hg+http" &&
@@ -67,14 +81,8 @@ struct MercurialInputScheme : InputScheme
         return inputFromAttrs(attrs);
     }
 
-    std::optional<Input> inputFromAttrs(const Attrs & attrs) const override
+    Attrs preprocessAttrs(const Attrs & attrs) const override
     {
-        if (maybeGetStrAttr(attrs, "type") != "hg") return {};
-
-        for (auto & [name, value] : attrs)
-            if (name != "type" && name != "url" && name != "ref" && name != "rev" && name != "revCount" && name != "narHash" && name != "name")
-                throw Error("unsupported Mercurial input attribute '%s'", name);
-
         parseURL(getStrAttr(attrs, "url"));
 
         if (auto ref = maybeGetStrAttr(attrs, "ref")) {
@@ -82,9 +90,7 @@ struct MercurialInputScheme : InputScheme
                 throw BadURL("invalid Mercurial branch/tag name '%s'", *ref);
         }
 
-        Input input;
-        input.attrs = attrs;
-        return input;
+        return attrs;
     }
 
     ParsedURL toURL(const Input & input) const override
diff --git a/lix/libfetchers/path.cc b/lix/libfetchers/path.cc
index dc45db021..c2da221b5 100644
--- a/lix/libfetchers/path.cc
+++ b/lix/libfetchers/path.cc
@@ -6,8 +6,25 @@
 
 namespace nix::fetchers {
 
+/* Allow the user to pass in "fake" tree info
+   attributes. This is useful for making a pinned tree
+   work the same as the repository from which is exported
+   (e.g. path:/nix/store/...-source?lastModified=1585388205&rev=b0c285...). */
+static const std::set<std::string> allowedPathAttrs = {
+    "lastModified",
+    "path",
+    "rev",
+    "revCount",
+};
+
 struct PathInputScheme : InputScheme
 {
+    std::string schemeType() const override { return "path"; }
+
+    const std::set<std::string> & allowedAttrs() const override {
+        return allowedPathAttrs;
+    }
+
     std::optional<Input> inputFromURL(const ParsedURL & url, bool requireTree) const override
     {
         if (url.scheme != "path") return {};
@@ -34,26 +51,11 @@ struct PathInputScheme : InputScheme
         return input;
     }
 
-    std::optional<Input> inputFromAttrs(const Attrs & attrs) const override
+    Attrs preprocessAttrs(const Attrs & attrs) const override
     {
-        if (maybeGetStrAttr(attrs, "type") != "path") return {};
-
         getStrAttr(attrs, "path");
 
-        for (auto & [name, value] : attrs)
-            /* Allow the user to pass in "fake" tree info
-               attributes. This is useful for making a pinned tree
-               work the same as the repository from which is exported
-               (e.g. path:/nix/store/...-source?lastModified=1585388205&rev=b0c285...). */
-            if (name == "type" || name == "rev" || name == "revCount" || name == "lastModified" || name == "narHash" || name == "path")
-                // checked in Input::fromAttrs
-                ;
-            else
-                throw Error("unsupported path input attribute '%s'", name);
-
-        Input input;
-        input.attrs = attrs;
-        return input;
+        return attrs;
     }
 
     bool isLockedByRev() const override { return false; }
diff --git a/lix/libfetchers/tarball.cc b/lix/libfetchers/tarball.cc
index c6fc52161..542c2aba1 100644
--- a/lix/libfetchers/tarball.cc
+++ b/lix/libfetchers/tarball.cc
@@ -230,12 +230,25 @@ try {
     co_return result::current_exception();
 }
 
+// FIXME: some of these only apply to TarballInputScheme.
+static const std::set<std::string> allowedCurlAttrs = {
+    "lastModified",
+    "name",
+    "rev",
+    "revCount",
+    "unpack",
+    "url",
+};
+
 // An input scheme corresponding to a curl-downloadable resource.
 struct CurlInputScheme : InputScheme
 {
-    virtual const std::string inputType() const = 0;
     const std::set<std::string> transportUrlSchemes = {"file", "http", "https"};
 
+    const std::set<std::string> & allowedAttrs() const override {
+        return allowedCurlAttrs;
+    }
+
     bool hasTarballExtension(std::string_view path) const
     {
         return path.ends_with(".zip") || path.ends_with(".tar")
@@ -254,7 +267,7 @@ struct CurlInputScheme : InputScheme
         auto url = _url;
 
         Attrs attrs;
-        attrs.emplace("type", inputType());
+        attrs.emplace("type", schemeType());
 
         url.scheme = parseUrlScheme(url.scheme).transport;
 
@@ -264,24 +277,16 @@ struct CurlInputScheme : InputScheme
         return inputFromAttrs(attrs);
     }
 
-    std::optional<Input> inputFromAttrs(const Attrs & attrs) const override
+    Attrs preprocessAttrs(const Attrs & attrs) const override
     {
-        auto type = maybeGetStrAttr(attrs, "type");
-        if (type != inputType()) return {};
-
-        // FIXME: some of these only apply to TarballInputScheme.
-        std::set<std::string> allowedNames = {"type", "url", "narHash", "name", "unpack", "rev", "revCount", "lastModified"};
         for (auto & [name, value] : attrs)
-            if (!allowedNames.count(name))
-                throw Error("unsupported %s input attribute '%s'. If you wanted to fetch a tarball with a query parameter, please use '{ type = \"tarball\"; url = \"...\"; }'", *type, name);
+            if (name != "type" && name != "narHash" && !allowedAttrs().contains(name))
+                throw UnsupportedAttributeError("unsupported tarball input attribute '%s'. If you wanted to fetch a tarball with a query parameter, please use '{ type = \"tarball\"; url = \"...\"; }'", name);
 
-        Input input;
-        input.attrs = attrs;
-
-        //input.locked = (bool) maybeGetStrAttr(input.attrs, "hash");
-        return input;
+        return attrs;
     }
 
+
     ParsedURL toURL(const Input & input) const override
     {
         auto url = parseURL(getStrAttr(input.attrs, "url"));
@@ -303,14 +308,14 @@ struct CurlInputScheme : InputScheme
 
 struct FileInputScheme : CurlInputScheme
 {
-    const std::string inputType() const override { return "file"; }
+    std::string schemeType() const override { return "file"; }
 
     bool isValidURL(const ParsedURL & url, bool requireTree) const override
     {
         auto parsedUrlScheme = parseUrlScheme(url.scheme);
         return transportUrlSchemes.count(std::string(parsedUrlScheme.transport))
             && (parsedUrlScheme.application
-                ? parsedUrlScheme.application.value() == inputType()
+                ? parsedUrlScheme.application.value() == schemeType()
                 : (!requireTree && !hasTarballExtension(url.path)));
     }
 
@@ -328,7 +333,7 @@ struct FileInputScheme : CurlInputScheme
 
 struct TarballInputScheme : CurlInputScheme
 {
-    const std::string inputType() const override { return "tarball"; }
+    std::string schemeType() const override { return "tarball"; }
 
     bool isValidURL(const ParsedURL & url, bool requireTree) const override
     {
@@ -336,7 +341,7 @@ struct TarballInputScheme : CurlInputScheme
 
         return transportUrlSchemes.count(std::string(parsedUrlScheme.transport))
             && (parsedUrlScheme.application
-                ? parsedUrlScheme.application.value() == inputType()
+                ? parsedUrlScheme.application.value() == schemeType()
                 : (requireTree || hasTarballExtension(url.path)));
     }
 
diff --git a/tests/functional/fetchers.sh b/tests/functional/fetchers.sh
index d1d6d1b63..b9c953862 100644
--- a/tests/functional/fetchers.sh
+++ b/tests/functional/fetchers.sh
@@ -59,7 +59,7 @@ testFetchTreeError \
 # test for unsupported attributes / validation in git fetcher
 testFetchTreeError \
     "\"git+https://github.com/owner/repo?invalid=1\"" \
-    "unsupported Git input attribute 'invalid'"
+    "unsupported input attribute 'invalid' for the 'git' scheme"
 
 testFetchTreeError \
     "\"git+https://github.com/owner/repo?url=foo\"" \
@@ -76,11 +76,11 @@ testFetchTreeError \
 # same for mercurial
 testFetchTreeError \
     "\"hg+https://forge.tld/owner/repo?invalid=1\"" \
-    "unsupported Mercurial input attribute 'invalid'"
+    "unsupported input attribute 'invalid' for the 'hg' scheme"
 
 testFetchTreeError \
     "{ type = \"hg\"; url = \"https://forge.tld/owner/repo\"; invalid = 1; }" \
-    "unsupported Mercurial input attribute 'invalid'"
+    "unsupported input attribute 'invalid' for the 'hg' scheme"
 
 testFetchTreeError \
     "\"hg+https://forge.tld/owner/repo?ref=,\"" \