diff --git a/flake.lock b/flake.lock
index bdb5cfe6d..f045bf7d4 100644
--- a/flake.lock
+++ b/flake.lock
@@ -48,11 +48,11 @@
         "pre-commit-hooks": "pre-commit-hooks"
       },
       "locked": {
-        "lastModified": 1737238254,
-        "narHash": "sha256-GoOwW81pV5T0YhD3yuLZG0+/Qr/Ld4qC71eM0mD3pJA=",
+        "lastModified": 1738053421,
+        "narHash": "sha256-B9HigdAgTs/FxUVcGYIRIxJVPQ+vGp3/G6WRuVKIGds=",
         "ref": "refs/heads/main",
-        "rev": "4cacb5412fedc9f37bc7a502540374f774d923cf",
-        "revCount": 16633,
+        "rev": "f8a592713d14bf0374950c8d3575eb3795e63987",
+        "revCount": 16698,
         "type": "git",
         "url": "https://git.lix.systems/lix-project/lix"
       },
@@ -74,11 +74,11 @@
         "treefmt-nix": "treefmt-nix"
       },
       "locked": {
-        "lastModified": 1737237917,
-        "narHash": "sha256-xd3/Wr4aPc2MY61qyQ6qQ8AGoG3H/DDcJWziP4EhhXY=",
+        "lastModified": 1738008985,
+        "narHash": "sha256-EYmllgk4J7jZPfyf9ps7JWJFiNv9wLncpND23d+7dVo=",
         "ref": "refs/heads/main",
-        "rev": "6482bee40b89ab2c28d2e988848aa9ae0429d78c",
-        "revCount": 621,
+        "rev": "e489d2190a9e7b6a369d3457b49ba80c93f80eae",
+        "revCount": 623,
         "type": "git",
         "url": "https://git.lix.systems/lix-project/nix-eval-jobs"
       },
@@ -126,11 +126,11 @@
     },
     "nixpkgs": {
       "locked": {
-        "lastModified": 1737165118,
-        "narHash": "sha256-s40Kk/OulP3J/1JvC3VT16U4r/Xw6Qdi7SRw3LYkPWs=",
+        "lastModified": 1737672001,
+        "narHash": "sha256-YnHJJ19wqmibLQdUeq9xzE6CjrMA568KN/lFPuSVs4I=",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "6a3ae7a5a12fb8cac2d59d7df7cbd95f9b2f0566",
+        "rev": "035f8c0853c2977b24ffc4d0a42c74f00b182cd8",
         "type": "github"
       },
       "original": {
diff --git a/package.nix b/package.nix
index 8789e058d..60ef1fd0d 100644
--- a/package.nix
+++ b/package.nix
@@ -177,6 +177,7 @@ stdenv.mkDerivation (finalAttrs: {
     boost
     nlohmann_json
     prometheus-cpp
+    nix.passthru.capnproto-lix
   ];
 
   nativeCheckInputs = [
diff --git a/src/hydra-evaluator/hydra-evaluator.cc b/src/hydra-evaluator/hydra-evaluator.cc
index e13ee75a6..b03ba8bfe 100644
--- a/src/hydra-evaluator/hydra-evaluator.cc
+++ b/src/hydra-evaluator/hydra-evaluator.cc
@@ -4,6 +4,7 @@
 #include "lix/libutil/logging.hh"
 #include "lix/libutil/pool.hh"
 #include "lix/libutil/signals.hh"
+#include "lix/libutil/async.hh"
 
 #include <algorithm>
 #include <thread>
@@ -513,7 +514,8 @@ int main(int argc, char * * argv)
 
         std::vector<std::string> args;
 
-        LegacyArgs(argv[0], [&](Strings::iterator & arg, const Strings::iterator & end) {
+        nix::AsyncIoRoot aio;
+        LegacyArgs(aio, argv[0], [&](Strings::iterator & arg, const Strings::iterator & end) {
             if (*arg == "--unlock")
                 unlock = true;
             else if (arg->starts_with("-"))
diff --git a/src/hydra-queue-runner/build-result.cc b/src/hydra-queue-runner/build-result.cc
index fa605319f..2672c4df3 100644
--- a/src/hydra-queue-runner/build-result.cc
+++ b/src/hydra-queue-runner/build-result.cc
@@ -35,7 +35,7 @@ BuildOutput getBuildOutput(
         auto outputS = store->printStorePath(output);
         if (!narMembers.count(outputS)) {
             printInfo("fetching NAR contents of '%s'...", outputS);
-            GeneratorSource source{store->narFromPath(output)};
+            auto source = store->narFromPath(output);
             extractNarData(source, outputS, narMembers);
         }
     }
diff --git a/src/hydra-queue-runner/hydra-queue-runner.cc b/src/hydra-queue-runner/hydra-queue-runner.cc
index f49579f17..c09be0108 100644
--- a/src/hydra-queue-runner/hydra-queue-runner.cc
+++ b/src/hydra-queue-runner/hydra-queue-runner.cc
@@ -21,6 +21,7 @@
 #include "lix/libstore/globals.hh"
 #include "lix/libstore/s3-binary-cache-store.hh"
 #include "lix/libutil/args.hh"
+#include "lix/libutil/async.hh"
 
 using namespace nix;
 using nlohmann::json;
@@ -974,7 +975,8 @@ int main(int argc, char * * argv)
         BuildID buildOne = 0;
         std::optional<std::string> metricsAddrOpt = std::nullopt;
 
-        LegacyArgs(argv[0], [&](Strings::iterator & arg, const Strings::iterator & end) {
+        nix::AsyncIoRoot aio;
+        LegacyArgs(aio, argv[0], [&](Strings::iterator & arg, const Strings::iterator & end) {
             if (*arg == "--unlock")
                 unlock = true;
             else if (*arg == "--status")
diff --git a/src/hydra-queue-runner/nar-extractor.cc b/src/hydra-queue-runner/nar-extractor.cc
index d96ca87c2..0378a9b81 100644
--- a/src/hydra-queue-runner/nar-extractor.cc
+++ b/src/hydra-queue-runner/nar-extractor.cc
@@ -75,11 +75,11 @@ struct Extractor : NARParseVisitor
 
 
 void extractNarData(
-    Source & source,
+    box_ptr<Source> & source,
     const Path & prefix,
     NarMemberDatas & members)
 {
-    auto parser = extractNarDataFilter(source, prefix, members);
+    auto parser = extractNarDataFilter(*source, prefix, members);
     while (parser.next()) {
         // ignore raw data
     }
diff --git a/src/hydra-queue-runner/nar-extractor.hh b/src/hydra-queue-runner/nar-extractor.hh
index dc9d00f46..f05cbe9de 100644
--- a/src/hydra-queue-runner/nar-extractor.hh
+++ b/src/hydra-queue-runner/nar-extractor.hh
@@ -18,7 +18,7 @@ using NarMemberDatas = std::map<nix::Path, NarMemberData>;
 /* Read a NAR from a source and get to some info about every file
    inside the NAR. */
 void extractNarData(
-    nix::Source & source,
+    nix::box_ptr<nix::Source> & source,
     const nix::Path & prefix,
     NarMemberDatas & members);
 
diff --git a/src/hydra-queue-runner/queue-monitor.cc b/src/hydra-queue-runner/queue-monitor.cc
index b66becb0f..6231449bc 100644
--- a/src/hydra-queue-runner/queue-monitor.cc
+++ b/src/hydra-queue-runner/queue-monitor.cc
@@ -1,5 +1,6 @@
 #include "hydra-build-result.hh"
 #include "lix/libstore/globals.hh"
+#include "lix/libutil/async.hh"
 #include "lix/libutil/thread-pool.hh"
 #include "state.hh"
 
@@ -40,13 +41,15 @@ void State::queueMonitorLoop(Connection & conn)
 
     auto destStore = getDestStore();
 
+    AsyncIoRoot aio;
+
     bool quit = false;
     while (!quit) {
         auto t_before_work = std::chrono::steady_clock::now();
 
         localStore->clearPathInfoCache();
 
-        bool done = getQueuedBuilds(conn, destStore);
+        bool done = getQueuedBuilds(aio, conn, destStore);
 
         if (buildOne && buildOneDone) quit = true;
 
@@ -93,7 +96,7 @@ struct PreviousFailure : public std::exception {
 };
 
 
-bool State::getQueuedBuilds(Connection & conn,
+bool State::getQueuedBuilds(AsyncIoRoot & aio, Connection & conn,
     ref<Store> destStore)
 {
     prom.queue_checks_started.Increment();
@@ -176,7 +179,7 @@ bool State::getQueuedBuilds(Connection & conn,
 
         /* Create steps for this derivation and its dependencies. */
         try {
-            step = createStep(destStore, conn, build, build->drvPath,
+            step = createStep(aio, destStore, conn, build, build->drvPath,
                 build, nullptr, finishedDrvs, newSteps, newRunnable);
         } catch (PreviousFailure & ex) {
 
@@ -434,7 +437,7 @@ std::map<DrvOutput, std::optional<StorePath>> State::getMissingRemotePaths(
 }
 
 
-Step::ptr State::createStep(ref<Store> destStore,
+Step::ptr State::createStep(AsyncIoRoot & aio, ref<Store> destStore,
     Connection & conn, Build::ptr build, const StorePath & drvPath,
     Build::ptr referringBuild, Step::ptr referringStep, std::set<StorePath> & finishedDrvs,
     std::set<Step::ptr> & newSteps, std::set<Step::ptr> & newRunnable)
@@ -571,7 +574,7 @@ Step::ptr State::createStep(ref<Store> destStore,
                         printInfo("substituting output ‘%1%’ of ‘%2%’",
                             localStore->printStorePath(path),
                             localStore->printStorePath(drvPath));
-                        localStore->ensurePath(path);
+                        aio.blockOn(localStore->ensurePath(path));
                         // FIXME: should copy directly from substituter to destStore.
                     }
 
@@ -611,7 +614,7 @@ Step::ptr State::createStep(ref<Store> destStore,
 
     /* Create steps for the dependencies. */
     for (auto & i : step->drv->inputDrvs.map) {
-        auto dep = createStep(destStore, conn, build, i.first, nullptr, step, finishedDrvs, newSteps, newRunnable);
+        auto dep = createStep(aio, destStore, conn, build, i.first, nullptr, step, finishedDrvs, newSteps, newRunnable);
         if (dep) {
             auto step_(step->state.lock());
             step_->deps.insert(dep);
diff --git a/src/hydra-queue-runner/state.hh b/src/hydra-queue-runner/state.hh
index 88fe262f4..9e5f21b6f 100644
--- a/src/hydra-queue-runner/state.hh
+++ b/src/hydra-queue-runner/state.hh
@@ -22,6 +22,7 @@
 #include "lix/libstore/pathlocks.hh"
 #include "lix/libstore/serve-protocol.hh"
 #include "lix/libstore/store-api.hh"
+#include "lix/libutil/async.hh"
 #include "lix/libutil/pool.hh"
 #include "lix/libutil/sync.hh"
 #include "nar-extractor.hh"
@@ -541,7 +542,7 @@ private:
     void queueMonitorLoop(Connection & conn);
 
     /* Check the queue for new builds. */
-    bool getQueuedBuilds(Connection & conn, nix::ref<nix::Store> destStore);
+    bool getQueuedBuilds(nix::AsyncIoRoot & aio, Connection & conn, nix::ref<nix::Store> destStore);
 
     /* Handle cancellation, deletion and priority bumps. */
     void processQueueChange(Connection & conn);
@@ -555,7 +556,7 @@ private:
         nix::ref<nix::Store> destStore,
         const std::map<nix::DrvOutput, std::optional<nix::StorePath>> & paths);
 
-    Step::ptr createStep(nix::ref<nix::Store> store,
+    Step::ptr createStep(nix::AsyncIoRoot & aio, nix::ref<nix::Store> store,
         Connection & conn, Build::ptr build, const nix::StorePath & drvPath,
         Build::ptr referringBuild, Step::ptr referringStep, std::set<nix::StorePath> & finishedDrvs,
         std::set<Step::ptr> & newSteps, std::set<Step::ptr> & newRunnable);