diff --git a/src/libstore/content-address.cc b/src/libstore/content-address.cc
index 3d753836f..6cb69d0a9 100644
--- a/src/libstore/content-address.cc
+++ b/src/libstore/content-address.cc
@@ -82,4 +82,16 @@ std::string renderContentAddress(std::optional<ContentAddress> ca) {
     return ca ? renderContentAddress(*ca) : "";
 }
 
+Hash getContentAddressHash(const ContentAddress & ca)
+{
+    return std::visit(overloaded {
+        [](TextHash th) {
+            return th.hash;
+        },
+        [](FixedOutputHash fsh) {
+            return fsh.hash;
+        }
+    }, ca);
+}
+
 }
diff --git a/src/libstore/content-address.hh b/src/libstore/content-address.hh
index ba4797f5b..22a039242 100644
--- a/src/libstore/content-address.hh
+++ b/src/libstore/content-address.hh
@@ -53,4 +53,6 @@ ContentAddress parseContentAddress(std::string_view rawCa);
 
 std::optional<ContentAddress> parseContentAddressOpt(std::string_view rawCaOpt);
 
+Hash getContentAddressHash(const ContentAddress & ca);
+
 }
diff --git a/src/libstore/store-api.cc b/src/libstore/store-api.cc
index c0a8bc9f6..46587a49a 100644
--- a/src/libstore/store-api.cc
+++ b/src/libstore/store-api.cc
@@ -7,6 +7,7 @@
 #include "json.hh"
 #include "derivations.hh"
 #include "url.hh"
+#include "archive.hh"
 
 #include <future>
 
@@ -221,6 +222,37 @@ StorePath Store::computeStorePathForText(const string & name, const string & s,
 }
 
 
+ValidPathInfo Store::addToStoreSlow(std::string_view name, const Path & srcPath,
+    FileIngestionMethod method, HashType hashAlgo,
+    std::optional<Hash> expectedCAHash)
+{
+    /* FIXME: inefficient: we're reading/hashing 'tmpFile' three
+       times. */
+
+    auto hash = method == FileIngestionMethod::Recursive
+        ? hashPath(hashAlgo, srcPath).first
+        : hashFile(hashAlgo, srcPath);
+
+    if (expectedCAHash && expectedCAHash != hash)
+        throw Error("hash mismatch for '%s'", srcPath);
+
+    auto [narHash, narSize] = hashPath(htSHA256, srcPath);
+    ValidPathInfo info(makeFixedOutputPath(method, hash, name));
+    info.narHash = narHash;
+    info.narSize = narSize;
+    info.ca = FixedOutputHash { .method = method, .hash = hash };
+
+    if (!isValidPath(info.path)) {
+        auto source = sinkToSource([&](Sink & sink) {
+            dumpPath(srcPath, sink);
+        });
+        addToStore(info, *source);
+    }
+
+    return info;
+}
+
+
 Store::Store(const Params & params)
     : Config(params)
     , state({(size_t) pathInfoCacheSize})
diff --git a/src/libstore/store-api.hh b/src/libstore/store-api.hh
index b122e05d6..b1dd1f478 100644
--- a/src/libstore/store-api.hh
+++ b/src/libstore/store-api.hh
@@ -450,6 +450,13 @@ public:
         FileIngestionMethod method = FileIngestionMethod::Recursive, HashType hashAlgo = htSHA256,
         PathFilter & filter = defaultPathFilter, RepairFlag repair = NoRepair) = 0;
 
+    /* Copy the contents of a path to the store and register the
+       validity the resulting path, using a constant amount of
+       memory. */
+    ValidPathInfo addToStoreSlow(std::string_view name, const Path & srcPath,
+        FileIngestionMethod method = FileIngestionMethod::Recursive, HashType hashAlgo = htSHA256,
+        std::optional<Hash> expectedCAHash = {});
+
     // FIXME: remove?
     virtual StorePath addToStoreFromDump(const string & dump, const string & name,
         FileIngestionMethod method = FileIngestionMethod::Recursive, HashType hashAlgo = htSHA256, RepairFlag repair = NoRepair)
diff --git a/src/nix-prefetch-url/nix-prefetch-url.cc b/src/nix-prefetch-url/nix-prefetch-url.cc
index 8ca85e894..961e7fb6d 100644
--- a/src/nix-prefetch-url/nix-prefetch-url.cc
+++ b/src/nix-prefetch-url/nix-prefetch-url.cc
@@ -10,7 +10,6 @@
 #include "../nix/legacy.hh"
 #include "progress-bar.hh"
 #include "tarfile.hh"
-#include "archive.hh"
 
 #include <iostream>
 
@@ -154,14 +153,15 @@ static int _main(int argc, char * * argv)
 
         /* If an expected hash is given, the file may already exist in
            the store. */
-        Hash hash, expectedHash(ht);
+        std::optional<Hash> expectedHash;
+        Hash hash;
         std::optional<StorePath> storePath;
         if (args.size() == 2) {
             expectedHash = Hash(args[1], ht);
             const auto recursive = unpack ? FileIngestionMethod::Recursive : FileIngestionMethod::Flat;
-            storePath = store->makeFixedOutputPath(recursive, expectedHash, name);
+            storePath = store->makeFixedOutputPath(recursive, *expectedHash, name);
             if (store->isValidPath(*storePath))
-                hash = expectedHash;
+                hash = *expectedHash;
             else
                 storePath.reset();
         }
@@ -201,28 +201,12 @@ static int _main(int argc, char * * argv)
                     tmpFile = unpacked;
             }
 
-            /* FIXME: inefficient: we're reading/hashing 'tmpFile'
-               three times. */
-            auto [narHash, narSize] = hashPath(htSHA256, tmpFile);
+            const auto method = unpack ? FileIngestionMethod::Recursive : FileIngestionMethod::Flat;
 
-            hash = unpack ? hashPath(ht, tmpFile).first : hashFile(ht, tmpFile);
-
-            if (expectedHash != Hash(ht) && expectedHash != hash)
-                throw Error("hash mismatch for '%1%'", uri);
-
-            const auto recursive = unpack ? FileIngestionMethod::Recursive : FileIngestionMethod::Flat;
-
-            storePath = store->makeFixedOutputPath(recursive, hash, name);
-
-            /* Copy the file to the Nix store. */
-            ValidPathInfo info(*storePath);
-            info.narHash = narHash;
-            info.narSize = narSize;
-            info.ca = FixedOutputHash { .method = recursive, .hash = hash };
-            auto source = sinkToSource([&](Sink & sink) {
-                dumpPath(tmpFile, sink);
-            });
-            store->addToStore(info, *source);
+            auto info = store->addToStoreSlow(name, tmpFile, method, ht, expectedHash);
+            storePath = info.path;
+            assert(info.ca);
+            hash = getContentAddressHash(*info.ca);
         }
 
         stopProgressBar();
diff --git a/src/nix-store/nix-store.cc b/src/nix-store/nix-store.cc
index c0274d4b6..4fa179105 100644
--- a/src/nix-store/nix-store.cc
+++ b/src/nix-store/nix-store.cc
@@ -186,23 +186,8 @@ static void opAddFixed(Strings opFlags, Strings opArgs)
     HashType hashAlgo = parseHashType(opArgs.front());
     opArgs.pop_front();
 
-    for (auto & i : opArgs) {
-        auto hash = method == FileIngestionMethod::Recursive
-            ? hashPath(hashAlgo, i).first
-            : hashFile(hashAlgo, i);
-        auto [narHash, narSize] = hashPath(htSHA256, i);
-        ValidPathInfo info(store->makeFixedOutputPath(method, hash, baseNameOf(i)));
-        info.narHash = narHash;
-        info.narSize = narSize;
-        info.ca = FixedOutputHash { .method = method, .hash = hash };
-
-        auto source = sinkToSource([&](Sink & sink) {
-            dumpPath(i, sink);
-        });
-        store->addToStore(info, *source);
-
-        std::cout << fmt("%s\n", store->printStorePath(info.path));
-    }
+    for (auto & i : opArgs)
+        std::cout << fmt("%s\n", store->printStorePath(store->addToStoreSlow(baseNameOf(i), i, method, hashAlgo).path));
 }