diff --git a/src/libstore/build.cc b/src/libstore/build.cc
index 0b51d90ea..07c5bceb2 100644
--- a/src/libstore/build.cc
+++ b/src/libstore/build.cc
@@ -2950,14 +2950,6 @@ struct RestrictedStore : public LocalFSStore, public virtual RestrictedStoreConf
         goal.addDependency(info.path);
     }
 
-    StorePath addToStoreFromDump(Source & dump, const string & name,
-        FileIngestionMethod method = FileIngestionMethod::Recursive, HashType hashAlgo = htSHA256, RepairFlag repair = NoRepair) override
-    {
-        auto path = next->addToStoreFromDump(dump, name, method, hashAlgo, repair);
-        goal.addDependency(path);
-        return path;
-    }
-
     StorePath addTextToStore(const string & name, const string & s,
         const StorePathSet & references, RepairFlag repair = NoRepair) override
     {
diff --git a/src/libstore/content-address.cc b/src/libstore/content-address.cc
index 0885c3d0e..74215c545 100644
--- a/src/libstore/content-address.cc
+++ b/src/libstore/content-address.cc
@@ -37,48 +37,86 @@ std::string renderContentAddress(ContentAddress ca) {
     }, ca);
 }
 
-ContentAddress parseContentAddress(std::string_view rawCa) {
-    auto rest = rawCa;
+std::string renderContentAddressMethod(ContentAddressMethod cam) {
+    return std::visit(overloaded {
+        [](TextHashMethod &th) {
+            return std::string{"text:"} + printHashType(htSHA256);
+        },
+        [](FixedOutputHashMethod &fshm) {
+            return "fixed:" + makeFileIngestionPrefix(fshm.fileIngestionMethod) + printHashType(fshm.hashType);
+        }
+    }, cam);
+}
+
+/*
+  Parses content address strings up to the hash.
+ */
+static ContentAddressMethod parseContentAddressMethodPrefix(std::string_view & rest) {
+    std::string_view wholeInput { rest };
 
     std::string_view prefix;
     {
         auto optPrefix = splitPrefixTo(rest, ':');
         if (!optPrefix)
-            throw UsageError("not a content address because it is not in the form '<prefix>:<rest>': %s", rawCa);
+            throw UsageError("not a content address because it is not in the form '<prefix>:<rest>': %s", wholeInput);
         prefix = *optPrefix;
     }
 
     auto parseHashType_ = [&](){
         auto hashTypeRaw = splitPrefixTo(rest, ':');
         if (!hashTypeRaw)
-            throw UsageError("content address hash must be in form '<algo>:<hash>', but found: %s", rawCa);
+            throw UsageError("content address hash must be in form '<algo>:<hash>', but found: %s", wholeInput);
         HashType hashType = parseHashType(*hashTypeRaw);
         return std::move(hashType);
     };
 
     // Switch on prefix
     if (prefix == "text") {
-        // No parsing of the method, "text" only support flat.
+        // No parsing of the ingestion method, "text" only support flat.
         HashType hashType = parseHashType_();
         if (hashType != htSHA256)
             throw Error("text content address hash should use %s, but instead uses %s",
                 printHashType(htSHA256), printHashType(hashType));
-        return TextHash {
-            .hash = Hash::parseNonSRIUnprefixed(rest, std::move(hashType)),
-        };
+        return TextHashMethod {};
     } else if (prefix == "fixed") {
         // Parse method
         auto method = FileIngestionMethod::Flat;
         if (splitPrefix(rest, "r:"))
             method = FileIngestionMethod::Recursive;
         HashType hashType = parseHashType_();
-        return FixedOutputHash {
-            .method = method,
-            .hash = Hash::parseNonSRIUnprefixed(rest, std::move(hashType)),
+        return FixedOutputHashMethod {
+            .fileIngestionMethod = method,
+            .hashType = std::move(hashType),
         };
     } else
         throw UsageError("content address prefix '%s' is unrecognized. Recogonized prefixes are 'text' or 'fixed'", prefix);
-};
+}
+
+ContentAddress parseContentAddress(std::string_view rawCa) {
+    auto rest = rawCa;
+
+    ContentAddressMethod caMethod = parseContentAddressMethodPrefix(rest);
+
+    return std::visit(
+        overloaded {
+            [&](TextHashMethod thm) {
+                return ContentAddress(TextHash {
+                    .hash = Hash::parseNonSRIUnprefixed(rest, htSHA256)
+                });
+            },
+            [&](FixedOutputHashMethod fohMethod) {
+                return ContentAddress(FixedOutputHash {
+                    .method = fohMethod.fileIngestionMethod,
+                    .hash = Hash::parseNonSRIUnprefixed(rest, std::move(fohMethod.hashType)),
+                });
+            },
+        }, caMethod);
+}
+
+ContentAddressMethod parseContentAddressMethod(std::string_view caMethod) {
+    std::string_view asPrefix {std::string{caMethod} + ":"};
+    return parseContentAddressMethodPrefix(asPrefix);
+}
 
 std::optional<ContentAddress> parseContentAddressOpt(std::string_view rawCaOpt) {
     return rawCaOpt == "" ? std::optional<ContentAddress> {} : parseContentAddress(rawCaOpt);
diff --git a/src/libstore/content-address.hh b/src/libstore/content-address.hh
index 22a039242..f6a6f5140 100644
--- a/src/libstore/content-address.hh
+++ b/src/libstore/content-address.hh
@@ -55,4 +55,23 @@ std::optional<ContentAddress> parseContentAddressOpt(std::string_view rawCaOpt);
 
 Hash getContentAddressHash(const ContentAddress & ca);
 
+/*
+  We only have one way to hash text with references, so this is single-value
+  type is only useful in std::variant.
+*/
+struct TextHashMethod { };
+struct FixedOutputHashMethod {
+  FileIngestionMethod fileIngestionMethod;
+  HashType hashType;
+};
+
+typedef std::variant<
+    TextHashMethod,
+    FixedOutputHashMethod
+  > ContentAddressMethod;
+
+ContentAddressMethod parseContentAddressMethod(std::string_view rawCaMethod);
+
+std::string renderContentAddressMethod(ContentAddressMethod caMethod);
+
 }
diff --git a/src/libstore/daemon.cc b/src/libstore/daemon.cc
index 8adabf549..8897a73f4 100644
--- a/src/libstore/daemon.cc
+++ b/src/libstore/daemon.cc
@@ -2,7 +2,6 @@
 #include "monitor-fd.hh"
 #include "worker-protocol.hh"
 #include "store-api.hh"
-#include "local-store.hh"
 #include "finally.hh"
 #include "affinity.hh"
 #include "archive.hh"
@@ -240,6 +239,18 @@ struct ClientSettings
     }
 };
 
+static void writeValidPathInfo(ref<Store> store, unsigned int clientVersion, Sink & to, std::shared_ptr<const ValidPathInfo> info) {
+    to << (info->deriver ? store->printStorePath(*info->deriver) : "")
+       << info->narHash.to_string(Base16, false);
+    writeStorePaths(*store, to, info->references);
+    to << info->registrationTime << info->narSize;
+    if (GET_PROTOCOL_MINOR(clientVersion) >= 16) {
+        to << info->ultimate
+           << info->sigs
+           << renderContentAddress(info->ca);
+    }
+}
+
 static void performOp(TunnelLogger * logger, ref<Store> store,
     TrustedFlag trusted, RecursiveFlag recursive, unsigned int clientVersion,
     Source & from, BufferedSink & to, unsigned int op)
@@ -350,47 +361,83 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
     }
 
     case wopAddToStore: {
-        HashType hashAlgo;
-        std::string baseName;
-        FileIngestionMethod method;
-        {
-            bool fixed;
-            uint8_t recursive;
-            std::string hashAlgoRaw;
-            from >> baseName >> fixed /* obsolete */ >> recursive >> hashAlgoRaw;
-            if (recursive > (uint8_t) FileIngestionMethod::Recursive)
-                throw Error("unsupported FileIngestionMethod with value of %i; you may need to upgrade nix-daemon", recursive);
-            method = FileIngestionMethod { recursive };
-            /* Compatibility hack. */
-            if (!fixed) {
-                hashAlgoRaw = "sha256";
-                method = FileIngestionMethod::Recursive;
+        if (GET_PROTOCOL_MINOR(clientVersion) >= 25) {
+            auto name = readString(from);
+            auto camStr = readString(from);
+            auto refs = readStorePaths<StorePathSet>(*store, from);
+            bool repairBool;
+            from >> repairBool;
+            auto repair = RepairFlag{repairBool};
+
+            logger->startWork();
+            auto pathInfo = [&]() {
+                // NB: FramedSource must be out of scope before logger->stopWork();
+                ContentAddressMethod contentAddressMethod = parseContentAddressMethod(camStr);
+                FramedSource source(from);
+                // TODO this is essentially RemoteStore::addCAToStore. Move it up to Store.
+                return std::visit(overloaded {
+                    [&](TextHashMethod &_) {
+                        // We could stream this by changing Store
+                        std::string contents = source.drain();
+                        auto path = store->addTextToStore(name, contents, refs, repair);
+                        return store->queryPathInfo(path);
+                    },
+                    [&](FixedOutputHashMethod &fohm) {
+                        if (!refs.empty())
+                            throw UnimplementedError("cannot yet have refs with flat or nar-hashed data");
+                        auto path = store->addToStoreFromDump(source, name, fohm.fileIngestionMethod, fohm.hashType, repair);
+                        return store->queryPathInfo(path);
+                    },
+                }, contentAddressMethod);
+            }();
+            logger->stopWork();
+
+            to << store->printStorePath(pathInfo->path);
+            writeValidPathInfo(store, clientVersion, to, pathInfo);
+
+        } else {
+            HashType hashAlgo;
+            std::string baseName;
+            FileIngestionMethod method;
+            {
+                bool fixed;
+                uint8_t recursive;
+                std::string hashAlgoRaw;
+                from >> baseName >> fixed /* obsolete */ >> recursive >> hashAlgoRaw;
+                if (recursive > (uint8_t) FileIngestionMethod::Recursive)
+                    throw Error("unsupported FileIngestionMethod with value of %i; you may need to upgrade nix-daemon", recursive);
+                method = FileIngestionMethod { recursive };
+                /* Compatibility hack. */
+                if (!fixed) {
+                    hashAlgoRaw = "sha256";
+                    method = FileIngestionMethod::Recursive;
+                }
+                hashAlgo = parseHashType(hashAlgoRaw);
             }
-            hashAlgo = parseHashType(hashAlgoRaw);
+
+            StringSink saved;
+            TeeSource savedNARSource(from, saved);
+            RetrieveRegularNARSink savedRegular { saved };
+
+            if (method == FileIngestionMethod::Recursive) {
+                /* Get the entire NAR dump from the client and save it to
+                  a string so that we can pass it to
+                  addToStoreFromDump(). */
+                ParseSink sink; /* null sink; just parse the NAR */
+                parseDump(sink, savedNARSource);
+            } else
+                parseDump(savedRegular, from);
+
+            logger->startWork();
+            if (!savedRegular.regular) throw Error("regular file expected");
+
+            // FIXME: try to stream directly from `from`.
+            StringSource dumpSource { *saved.s };
+            auto path = store->addToStoreFromDump(dumpSource, baseName, method, hashAlgo);
+            logger->stopWork();
+
+            to << store->printStorePath(path);
         }
-
-        StringSink saved;
-        TeeSource savedNARSource(from, saved);
-        RetrieveRegularNARSink savedRegular { saved };
-
-        if (method == FileIngestionMethod::Recursive) {
-            /* Get the entire NAR dump from the client and save it to
-               a string so that we can pass it to
-               addToStoreFromDump(). */
-            ParseSink sink; /* null sink; just parse the NAR */
-            parseDump(sink, savedNARSource);
-        } else
-            parseDump(savedRegular, from);
-
-        logger->startWork();
-        if (!savedRegular.regular) throw Error("regular file expected");
-
-        // FIXME: try to stream directly from `from`.
-        StringSource dumpSource { *saved.s };
-        auto path = store->addToStoreFromDump(dumpSource, baseName, method, hashAlgo);
-        logger->stopWork();
-
-        to << store->printStorePath(path);
         break;
     }
 
@@ -675,15 +722,7 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
         if (info) {
             if (GET_PROTOCOL_MINOR(clientVersion) >= 17)
                 to << 1;
-            to << (info->deriver ? store->printStorePath(*info->deriver) : "")
-               << info->narHash.to_string(Base16, false);
-            writeStorePaths(*store, to, info->references);
-            to << info->registrationTime << info->narSize;
-            if (GET_PROTOCOL_MINOR(clientVersion) >= 16) {
-                to << info->ultimate
-                   << info->sigs
-                   << renderContentAddress(info->ca);
-            }
+            writeValidPathInfo(store, clientVersion, to, info);
         } else {
             assert(GET_PROTOCOL_MINOR(clientVersion) >= 17);
             to << 0;
@@ -749,59 +788,12 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
             info.ultimate = false;
 
         if (GET_PROTOCOL_MINOR(clientVersion) >= 23) {
-
-            struct FramedSource : Source
-            {
-                Source & from;
-                bool eof = false;
-                std::vector<unsigned char> pending;
-                size_t pos = 0;
-
-                FramedSource(Source & from) : from(from)
-                { }
-
-                ~FramedSource()
-                {
-                    if (!eof) {
-                        while (true) {
-                            auto n = readInt(from);
-                            if (!n) break;
-                            std::vector<unsigned char> data(n);
-                            from(data.data(), n);
-                        }
-                    }
-                }
-
-                size_t read(unsigned char * data, size_t len) override
-                {
-                    if (eof) throw EndOfFile("reached end of FramedSource");
-
-                    if (pos >= pending.size()) {
-                        size_t len = readInt(from);
-                        if (!len) {
-                            eof = true;
-                            return 0;
-                        }
-                        pending = std::vector<unsigned char>(len);
-                        pos = 0;
-                        from(pending.data(), len);
-                    }
-
-                    auto n = std::min(len, pending.size() - pos);
-                    memcpy(data, pending.data() + pos, n);
-                    pos += n;
-                    return n;
-                }
-            };
-
             logger->startWork();
-
             {
                 FramedSource source(from);
                 store->addToStore(info, source, (RepairFlag) repair,
                     dontCheckSigs ? NoCheckSigs : CheckSigs);
             }
-
             logger->stopWork();
         }
 
diff --git a/src/libstore/remote-store.cc b/src/libstore/remote-store.cc
index 27535f1d0..02a8b72b9 100644
--- a/src/libstore/remote-store.cc
+++ b/src/libstore/remote-store.cc
@@ -307,6 +307,8 @@ struct ConnectionHandle
             std::rethrow_exception(ex);
         }
     }
+
+    void withFramedSink(std::function<void(Sink & sink)> fun);
 };
 
 
@@ -420,11 +422,27 @@ void RemoteStore::querySubstitutablePathInfos(const StorePathCAMap & pathsMap, S
 }
 
 
+ref<const ValidPathInfo> RemoteStore::readValidPathInfo(ConnectionHandle & conn, const StorePath & path) {
+    auto deriver = readString(conn->from);
+    auto narHash = Hash::parseAny(readString(conn->from), htSHA256);
+    auto info = make_ref<ValidPathInfo>(path, narHash);
+    if (deriver != "") info->deriver = parseStorePath(deriver);
+    info->references = readStorePaths<StorePathSet>(*this, conn->from);
+    conn->from >> info->registrationTime >> info->narSize;
+    if (GET_PROTOCOL_MINOR(conn->daemonVersion) >= 16) {
+        conn->from >> info->ultimate;
+        info->sigs = readStrings<StringSet>(conn->from);
+        info->ca = parseContentAddressOpt(readString(conn->from));
+    }
+    return info;
+}
+
+
 void RemoteStore::queryPathInfoUncached(const StorePath & path,
     Callback<std::shared_ptr<const ValidPathInfo>> callback) noexcept
 {
     try {
-        std::shared_ptr<ValidPathInfo> info;
+        std::shared_ptr<const ValidPathInfo> info;
         {
             auto conn(getConnection());
             conn->to << wopQueryPathInfo << printStorePath(path);
@@ -440,17 +458,7 @@ void RemoteStore::queryPathInfoUncached(const StorePath & path,
                 bool valid; conn->from >> valid;
                 if (!valid) throw InvalidPath("path '%s' is not valid", printStorePath(path));
             }
-            auto deriver = readString(conn->from);
-            auto narHash = Hash::parseAny(readString(conn->from), htSHA256);
-            info = std::make_shared<ValidPathInfo>(path, narHash);
-            if (deriver != "") info->deriver = parseStorePath(deriver);
-            info->references = readStorePaths<StorePathSet>(*this, conn->from);
-            conn->from >> info->registrationTime >> info->narSize;
-            if (GET_PROTOCOL_MINOR(conn->daemonVersion) >= 16) {
-                conn->from >> info->ultimate;
-                info->sigs = readStrings<StringSet>(conn->from);
-                info->ca = parseContentAddressOpt(readString(conn->from));
-            }
+            info = readValidPathInfo(conn, path);
         }
         callback(std::move(info));
     } catch (...) { callback.rethrow(); }
@@ -525,6 +533,84 @@ std::optional<StorePath> RemoteStore::queryPathFromHashPart(const std::string &
 }
 
 
+ref<const ValidPathInfo> RemoteStore::addCAToStore(Source & dump, const string & name, ContentAddressMethod caMethod, StorePathSet references, RepairFlag repair)
+{
+    auto conn(getConnection());
+
+    if (GET_PROTOCOL_MINOR(conn->daemonVersion) >= 25) {
+
+        conn->to
+            << wopAddToStore
+            << name
+            << renderContentAddressMethod(caMethod);
+        writeStorePaths(*this, conn->to, references);
+        conn->to << repair;
+
+        conn.withFramedSink([&](Sink & sink) {
+            dump.drainInto(sink);
+        });
+
+        auto path = parseStorePath(readString(conn->from));
+        return readValidPathInfo(conn, path);
+    }
+    else {
+        if (repair) throw Error("repairing is not supported when building through the Nix daemon protocol < 1.25");
+
+        std::visit(overloaded {
+            [&](TextHashMethod thm) -> void {
+                std::string s = dump.drain();
+                conn->to << wopAddTextToStore << name << s;
+                writeStorePaths(*this, conn->to, references);
+                conn.processStderr();
+            },
+            [&](FixedOutputHashMethod fohm) -> void {
+                conn->to
+                    << wopAddToStore
+                    << name
+                    << ((fohm.hashType == htSHA256 && fohm.fileIngestionMethod == FileIngestionMethod::Recursive) ? 0 : 1) /* backwards compatibility hack */
+                    << (fohm.fileIngestionMethod == FileIngestionMethod::Recursive ? 1 : 0)
+                    << printHashType(fohm.hashType);
+
+                try {
+                    conn->to.written = 0;
+                    conn->to.warn = true;
+                    connections->incCapacity();
+                    {
+                        Finally cleanup([&]() { connections->decCapacity(); });
+                        if (fohm.fileIngestionMethod == FileIngestionMethod::Recursive) {
+                            dump.drainInto(conn->to);
+                        } else {
+                            std::string contents = dump.drain();
+                            dumpString(contents, conn->to);
+                        }
+                    }
+                    conn->to.warn = false;
+                    conn.processStderr();
+                } catch (SysError & e) {
+                    /* Daemon closed while we were sending the path. Probably OOM
+                      or I/O error. */
+                    if (e.errNo == EPIPE)
+                        try {
+                            conn.processStderr();
+                        } catch (EndOfFile & e) { }
+                    throw;
+                }
+
+            }
+        }, caMethod);
+        auto path = parseStorePath(readString(conn->from));
+        return queryPathInfo(path);
+    }
+}
+
+StorePath RemoteStore::addToStoreFromDump(Source & dump, const string & name,
+        FileIngestionMethod method, HashType hashType, RepairFlag repair)
+{
+    StorePathSet references;
+    return addCAToStore(dump, name, FixedOutputHashMethod{ .fileIngestionMethod = method, .hashType = hashType }, references, repair)->path;
+}
+
+
 void RemoteStore::addToStore(const ValidPathInfo & info, Source & source,
     RepairFlag repair, CheckSigsFlag checkSigs)
 {
@@ -565,78 +651,9 @@ void RemoteStore::addToStore(const ValidPathInfo & info, Source & source,
                  << repair << !checkSigs;
 
         if (GET_PROTOCOL_MINOR(conn->daemonVersion) >= 23) {
-
-            conn->to.flush();
-
-            std::exception_ptr ex;
-
-            struct FramedSink : BufferedSink
-            {
-                ConnectionHandle & conn;
-                std::exception_ptr & ex;
-
-                FramedSink(ConnectionHandle & conn, std::exception_ptr & ex) : conn(conn), ex(ex)
-                { }
-
-                ~FramedSink()
-                {
-                    try {
-                        conn->to << 0;
-                        conn->to.flush();
-                    } catch (...) {
-                        ignoreException();
-                    }
-                }
-
-                void write(const unsigned char * data, size_t len) override
-                {
-                    /* Don't send more data if the remote has
-                       encountered an error. */
-                    if (ex) {
-                        auto ex2 = ex;
-                        ex = nullptr;
-                        std::rethrow_exception(ex2);
-                    }
-                    conn->to << len;
-                    conn->to(data, len);
-                };
-            };
-
-            /* Handle log messages / exceptions from the remote on a
-               separate thread. */
-            std::thread stderrThread([&]()
-            {
-                try {
-                    conn.processStderr(nullptr, nullptr, false);
-                } catch (...) {
-                    ex = std::current_exception();
-                }
-            });
-
-            Finally joinStderrThread([&]()
-            {
-                if (stderrThread.joinable()) {
-                    stderrThread.join();
-                    if (ex) {
-                        try {
-                            std::rethrow_exception(ex);
-                        } catch (...) {
-                            ignoreException();
-                        }
-                    }
-                }
-            });
-
-            {
-                FramedSink sink(conn, ex);
+            conn.withFramedSink([&](Sink & sink) {
                 copyNAR(source, sink);
-                sink.flush();
-            }
-
-            stderrThread.join();
-            if (ex)
-                std::rethrow_exception(ex);
-
+            });
         } else if (GET_PROTOCOL_MINOR(conn->daemonVersion) >= 21) {
             conn.processStderr(0, &source);
         } else {
@@ -647,57 +664,11 @@ void RemoteStore::addToStore(const ValidPathInfo & info, Source & source,
 }
 
 
-StorePath RemoteStore::addToStore(const string & name, const Path & _srcPath,
-    FileIngestionMethod method, HashType hashAlgo, PathFilter & filter, RepairFlag repair)
-{
-    if (repair) throw Error("repairing is not supported when building through the Nix daemon");
-
-    auto conn(getConnection());
-
-    Path srcPath(absPath(_srcPath));
-
-    conn->to
-        << wopAddToStore
-        << name
-        << ((hashAlgo == htSHA256 && method == FileIngestionMethod::Recursive) ? 0 : 1) /* backwards compatibility hack */
-        << (method == FileIngestionMethod::Recursive ? 1 : 0)
-        << printHashType(hashAlgo);
-
-    try {
-        conn->to.written = 0;
-        conn->to.warn = true;
-        connections->incCapacity();
-        {
-            Finally cleanup([&]() { connections->decCapacity(); });
-            dumpPath(srcPath, conn->to, filter);
-        }
-        conn->to.warn = false;
-        conn.processStderr();
-    } catch (SysError & e) {
-        /* Daemon closed while we were sending the path. Probably OOM
-           or I/O error. */
-        if (e.errNo == EPIPE)
-            try {
-                conn.processStderr();
-            } catch (EndOfFile & e) { }
-        throw;
-    }
-
-    return parseStorePath(readString(conn->from));
-}
-
-
 StorePath RemoteStore::addTextToStore(const string & name, const string & s,
     const StorePathSet & references, RepairFlag repair)
 {
-    if (repair) throw Error("repairing is not supported when building through the Nix daemon");
-
-    auto conn(getConnection());
-    conn->to << wopAddTextToStore << name << s;
-    writeStorePaths(*this, conn->to, references);
-
-    conn.processStderr();
-    return parseStorePath(readString(conn->from));
+    StringSource source(s);
+    return addCAToStore(source, name, TextHashMethod{}, references, repair)->path;
 }
 
 
@@ -993,6 +964,49 @@ std::exception_ptr RemoteStore::Connection::processStderr(Sink * sink, Source *
     return nullptr;
 }
 
+void
+ConnectionHandle::withFramedSink(std::function<void(Sink &sink)> fun) {
+    (*this)->to.flush();
+
+    std::exception_ptr ex;
+
+    /* Handle log messages / exceptions from the remote on a
+        separate thread. */
+    std::thread stderrThread([&]()
+    {
+        try {
+            processStderr(nullptr, nullptr, false);
+        } catch (...) {
+            ex = std::current_exception();
+        }
+    });
+
+    Finally joinStderrThread([&]()
+    {
+        if (stderrThread.joinable()) {
+            stderrThread.join();
+            if (ex) {
+                try {
+                    std::rethrow_exception(ex);
+                } catch (...) {
+                    ignoreException();
+                }
+            }
+        }
+    });
+
+    {
+        FramedSink sink((*this)->to, ex);
+        fun(sink);
+        sink.flush();
+    }
+
+    stderrThread.join();
+    if (ex)
+        std::rethrow_exception(ex);
+
+}
+
 static RegisterStoreImplementation<UDSRemoteStore, UDSRemoteStoreConfig> regStore;
 
 }
diff --git a/src/libstore/remote-store.hh b/src/libstore/remote-store.hh
index 91c748006..735a3c24e 100644
--- a/src/libstore/remote-store.hh
+++ b/src/libstore/remote-store.hh
@@ -63,13 +63,16 @@ public:
     void querySubstitutablePathInfos(const StorePathCAMap & paths,
         SubstitutablePathInfos & infos) override;
 
+    /* Add a content-addressable store path. `dump` will be drained. */
+    ref<const ValidPathInfo> addCAToStore(Source & dump, const string & name, ContentAddressMethod caMethod, StorePathSet references, RepairFlag repair);
+
+    /* Add a content-addressable store path. Does not support references. `dump` will be drained. */
+    StorePath addToStoreFromDump(Source & dump, const string & name,
+        FileIngestionMethod method = FileIngestionMethod::Recursive, HashType hashAlgo = htSHA256, RepairFlag repair = NoRepair) override;
+
     void addToStore(const ValidPathInfo & info, Source & nar,
         RepairFlag repair, CheckSigsFlag checkSigs) override;
 
-    StorePath addToStore(const string & name, const Path & srcPath,
-        FileIngestionMethod method = FileIngestionMethod::Recursive, HashType hashAlgo = htSHA256,
-        PathFilter & filter = defaultPathFilter, RepairFlag repair = NoRepair) override;
-
     StorePath addTextToStore(const string & name, const string & s,
         const StorePathSet & references, RepairFlag repair) override;
 
@@ -139,6 +142,8 @@ protected:
 
     virtual void narFromPath(const StorePath & path, Sink & sink) override;
 
+    ref<const ValidPathInfo> readValidPathInfo(ConnectionHandle & conn, const StorePath & path);
+
 private:
 
     std::atomic_bool failed{false};
diff --git a/src/libstore/store-api.hh b/src/libstore/store-api.hh
index 4d3f07dfc..591140874 100644
--- a/src/libstore/store-api.hh
+++ b/src/libstore/store-api.hh
@@ -449,7 +449,8 @@ public:
     /* Like addToStore(), but the contents of the path are contained
        in `dump', which is either a NAR serialisation (if recursive ==
        true) or simply the contents of a regular file (if recursive ==
-       false). */
+       false).
+       `dump` may be drained */
     // FIXME: remove?
     virtual StorePath addToStoreFromDump(Source & dump, const string & name,
         FileIngestionMethod method = FileIngestionMethod::Recursive, HashType hashAlgo = htSHA256, RepairFlag repair = NoRepair)
diff --git a/src/libstore/worker-protocol.hh b/src/libstore/worker-protocol.hh
index 13cf8d4ab..b100d1550 100644
--- a/src/libstore/worker-protocol.hh
+++ b/src/libstore/worker-protocol.hh
@@ -6,7 +6,7 @@ namespace nix {
 #define WORKER_MAGIC_1 0x6e697863
 #define WORKER_MAGIC_2 0x6478696f
 
-#define PROTOCOL_VERSION 0x118
+#define PROTOCOL_VERSION 0x119
 #define GET_PROTOCOL_MAJOR(x) ((x) & 0xff00)
 #define GET_PROTOCOL_MINOR(x) ((x) & 0x00ff)
 
@@ -18,7 +18,7 @@ typedef enum {
     wopQueryReferences = 5, // obsolete
     wopQueryReferrers = 6,
     wopAddToStore = 7,
-    wopAddTextToStore = 8,
+    wopAddTextToStore = 8, // obsolete since 1.25, Nix 3.0. Use wopAddToStore
     wopBuildPaths = 9,
     wopEnsurePath = 10,
     wopAddTempRoot = 11,
diff --git a/src/libutil/serialise.cc b/src/libutil/serialise.cc
index 00c945113..a469a1e73 100644
--- a/src/libutil/serialise.cc
+++ b/src/libutil/serialise.cc
@@ -93,7 +93,7 @@ void Source::operator () (unsigned char * data, size_t len)
 }
 
 
-std::string Source::drain()
+void Source::drainInto(Sink & sink)
 {
     std::string s;
     std::vector<unsigned char> buf(8192);
@@ -101,12 +101,19 @@ std::string Source::drain()
         size_t n;
         try {
             n = read(buf.data(), buf.size());
-            s.append((char *) buf.data(), n);
+            sink(buf.data(), n);
         } catch (EndOfFile &) {
             break;
         }
     }
-    return s;
+}
+
+
+std::string Source::drain()
+{
+    StringSink s;
+    drainInto(s);
+    return *s.s;
 }
 
 
diff --git a/src/libutil/serialise.hh b/src/libutil/serialise.hh
index 7682a0f19..b41e58f33 100644
--- a/src/libutil/serialise.hh
+++ b/src/libutil/serialise.hh
@@ -69,6 +69,8 @@ struct Source
 
     virtual bool good() { return true; }
 
+    void drainInto(Sink & sink);
+
     std::string drain();
 };
 
@@ -404,4 +406,93 @@ struct StreamToSourceAdapter : Source
 };
 
 
+/* A source that reads a distinct format of concatenated chunks back into its
+   logical form, in order to guarantee a known state to the original stream,
+   even in the event of errors.
+
+   Use with FramedSink, which also allows the logical stream to be terminated
+   in the event of an exception.
+*/
+struct FramedSource : Source
+{
+    Source & from;
+    bool eof = false;
+    std::vector<unsigned char> pending;
+    size_t pos = 0;
+
+    FramedSource(Source & from) : from(from)
+    { }
+
+    ~FramedSource()
+    {
+        if (!eof) {
+            while (true) {
+                auto n = readInt(from);
+                if (!n) break;
+                std::vector<unsigned char> data(n);
+                from(data.data(), n);
+            }
+        }
+    }
+
+    size_t read(unsigned char * data, size_t len) override
+    {
+        if (eof) throw EndOfFile("reached end of FramedSource");
+
+        if (pos >= pending.size()) {
+            size_t len = readInt(from);
+            if (!len) {
+                eof = true;
+                return 0;
+            }
+            pending = std::vector<unsigned char>(len);
+            pos = 0;
+            from(pending.data(), len);
+        }
+
+        auto n = std::min(len, pending.size() - pos);
+        memcpy(data, pending.data() + pos, n);
+        pos += n;
+        return n;
+    }
+};
+
+/* Write as chunks in the format expected by FramedSource.
+
+   The exception_ptr reference can be used to terminate the stream when you
+   detect that an error has occurred on the remote end.
+*/
+struct FramedSink : nix::BufferedSink
+{
+    BufferedSink & to;
+    std::exception_ptr & ex;
+
+    FramedSink(BufferedSink & to, std::exception_ptr & ex) : to(to), ex(ex)
+    { }
+
+    ~FramedSink()
+    {
+        try {
+            to << 0;
+            to.flush();
+        } catch (...) {
+            ignoreException();
+        }
+    }
+
+    void write(const unsigned char * data, size_t len) override
+    {
+        /* Don't send more data if the remote has
+            encountered an error. */
+        if (ex) {
+            auto ex2 = ex;
+            ex = nullptr;
+            std::rethrow_exception(ex2);
+        }
+        to << len;
+        to(data, len);
+    };
+};
+
+
 }
diff --git a/src/libutil/split.hh b/src/libutil/split.hh
index d19d7d8ed..87a23b13e 100644
--- a/src/libutil/split.hh
+++ b/src/libutil/split.hh
@@ -9,7 +9,7 @@ namespace nix {
 
 // If `separator` is found, we return the portion of the string before the
 // separator, and modify the string argument to contain only the part after the
-// separator. Otherwise, wer return `std::nullopt`, and we leave the argument
+// separator. Otherwise, we return `std::nullopt`, and we leave the argument
 // string alone.
 static inline std::optional<std::string_view> splitPrefixTo(std::string_view & string, char separator) {
     auto sepInstance = string.find(separator);