diff --git a/src/hydra-queue-runner/build-remote.cc b/src/hydra-queue-runner/build-remote.cc
index 9466f009..a0408f55 100644
--- a/src/hydra-queue-runner/build-remote.cc
+++ b/src/hydra-queue-runner/build-remote.cc
@@ -169,7 +169,7 @@ void State::buildRemote(ref<Store> destStore,
         unsigned int remoteVersion;
 
         try {
-            to << SERVE_MAGIC_1 << 0x202;
+            to << SERVE_MAGIC_1 << 0x203;
             to.flush();
 
             unsigned int magic = readInt(from);
@@ -180,6 +180,8 @@ void State::buildRemote(ref<Store> destStore,
                 throw Error(format("unsupported ‘nix-store --serve’ protocol version on ‘%1%’") % machine->sshName);
             if (GET_PROTOCOL_MINOR(remoteVersion) >= 1)
                 sendDerivation = false;
+            if (GET_PROTOCOL_MINOR(remoteVersion) < 3 && step->isDeterministic)
+                throw Error("machine ‘%1%’ does not support deterministic builds; please upgrade it to Nix 1.12", machine->sshName);
 
         } catch (EndOfFile & e) {
             child.pid.wait(true);
@@ -261,6 +263,9 @@ void State::buildRemote(ref<Store> destStore,
         to << maxSilentTime << buildTimeout;
         if (GET_PROTOCOL_MINOR(remoteVersion) >= 2)
             to << 64 * 1024 * 1024; // == maxLogSize
+        if (GET_PROTOCOL_MINOR(remoteVersion) >= 3)
+            // FIXME: make the number of repeats configurable.
+            to << (step->isDeterministic ? 1 : 0);
         to.flush();
 
         result.startTime = time(0);
@@ -325,6 +330,11 @@ void State::buildRemote(ref<Store> destStore,
                 case BuildResult::LogLimitExceeded:
                     result.stepStatus = bsLogLimitExceeded;
                     break;
+                case BuildResult::NotDeterministic:
+                    result.stepStatus = bsNotDeterministic;
+                    result.canRetry = false;
+                    result.canCache = true;
+                    break;
                 default:
                     result.stepStatus = bsAborted;
                     break;
diff --git a/src/hydra-queue-runner/queue-monitor.cc b/src/hydra-queue-runner/queue-monitor.cc
index 644919d7..8504a87f 100644
--- a/src/hydra-queue-runner/queue-monitor.cc
+++ b/src/hydra-queue-runner/queue-monitor.cc
@@ -418,6 +418,7 @@ Step::ptr State::createStep(ref<Store> destStore,
     step->drv = readDerivation(drvPath);
 
     step->preferLocalBuild = step->drv.willBuildLocally();
+    step->isDeterministic = get(step->drv.env, "isDetermistic", "0") == "1";
 
     step->systemType = step->drv.platform;
     {
diff --git a/src/hydra-queue-runner/state.hh b/src/hydra-queue-runner/state.hh
index 347bbbb7..2994b9fb 100644
--- a/src/hydra-queue-runner/state.hh
+++ b/src/hydra-queue-runner/state.hh
@@ -35,6 +35,7 @@ typedef enum {
     bsUnsupported = 9,
     bsLogLimitExceeded = 10,
     bsNarSizeLimitExceeded = 11,
+    bsNotDeterministic = 12,
     bsBusy = 100, // not stored
 } BuildStatus;
 
@@ -139,6 +140,7 @@ struct Step
     nix::Derivation drv;
     std::set<std::string> requiredSystemFeatures;
     bool preferLocalBuild;
+    bool isDeterministic;
     std::string systemType; // concatenation of drv.platform and requiredSystemFeatures
 
     struct State
diff --git a/src/root/build.tt b/src/root/build.tt
index 68548b98..e78fc883 100644
--- a/src/root/build.tt
+++ b/src/root/build.tt
@@ -69,6 +69,8 @@ FOR step IN steps; IF step.busy; busy = 1; END; END;
                 <span class="error">Log limit exceeded</span>
               [% ELSIF step.status == 11 %]
                 <span class="error">Output limit exceeded</span>
+              [% ELSIF step.status == 12 %]
+                <span class="error">Non-deterministic build</span>
               [% ELSIF step.errormsg %]
                 <span class="error">Failed: [% HTML.escape(step.errormsg) %]</span>
               [% ELSE %]
diff --git a/src/root/common.tt b/src/root/common.tt
index 3350a277..076ca437 100644
--- a/src/root/common.tt
+++ b/src/root/common.tt
@@ -209,6 +209,8 @@ BLOCK renderBuildStatusIcon;
       <img src="[% c.uri_for("/static/images/emojione-red-x-274c.svg") %]" height="[% size %]" width="[% size %]" title="Log limit exceeded" alt="Log limit exceeded" class="build-status" />
     [% ELSIF buildstatus == 11 %]
       <img src="[% c.uri_for("/static/images/emojione-red-x-274c.svg") %]" height="[% size %]" width="[% size %]" title="Output size limit exceeded" alt="Output size limit exceeded" class="build-status" />
+    [% ELSIF buildstatus == 12 %]
+      <img src="[% c.uri_for("/static/images/emojione-red-x-274c.svg") %]" height="[% size %]" width="[% size %]" title="Non-deterministic build" alt="Non-deterministic build" class="build-status" />
     [% ELSE %]
       <img src="[% c.uri_for("/static/images/emojione-red-x-274c.svg") %]" height="[% size %]" width="[% size %]" title="Failed" alt="Failed" class="build-status" />
     [% END;
@@ -240,6 +242,8 @@ BLOCK renderStatus;
       <span class="error">Log limit exceeded</span>
     [% ELSIF buildstatus == 11 %]
       <span class="error">Output limit exceeded</span>
+    [% ELSIF buildstatus == 12 %]
+      <span class="error">Non-deterministic build</span>
     [% ELSE %]
       <span class="error">Aborted</span>
       (Hydra failure; see <a href="#nix-error">below</a>)
diff --git a/src/sql/hydra.sql b/src/sql/hydra.sql
index 44a2eb14..fc20e069 100644
--- a/src/sql/hydra.sql
+++ b/src/sql/hydra.sql
@@ -211,6 +211,7 @@ create table Builds (
     --   9 = unsupported system type
     --  10 = log limit exceeded
     --  11 = NAR size limit exceeded
+    --  12 = build or step was not deterministic
     buildStatus   integer,
 
     size          bigint,