diff --git a/src/libfetchers/fetch-to-store.cc b/src/libfetchers/fetch-to-store.cc
index 5961379ee..b1e8b9d72 100644
--- a/src/libfetchers/fetch-to-store.cc
+++ b/src/libfetchers/fetch-to-store.cc
@@ -1,6 +1,7 @@
 #include "nix/fetchers/fetch-to-store.hh"
 #include "nix/fetchers/fetchers.hh"
 #include "nix/fetchers/fetch-settings.hh"
+#include "nix/util/environment-variables.hh"
 
 namespace nix {
 
@@ -37,6 +38,9 @@ StorePath fetchToStore(
             return res->storePath;
         }
     } else {
+        static auto barf = getEnv("_NIX_TEST_BARF_ON_UNCACHEABLE").value_or("") == "1";
+        if (barf && !filter)
+            throw Error("source path '%s' is uncacheable (filter=%d)", path, (bool) filter);
         // FIXME: could still provide in-memory caching keyed on `SourcePath`.
         debug("source path '%s' is uncacheable", path);
     }
diff --git a/tests/functional/flakes/common.sh b/tests/functional/flakes/common.sh
index 422cab96c..77bc03060 100644
--- a/tests/functional/flakes/common.sh
+++ b/tests/functional/flakes/common.sh
@@ -2,6 +2,8 @@
 
 source ../common.sh
 
+export _NIX_TEST_BARF_ON_UNCACHEABLE=1
+
 # shellcheck disable=SC2034 # this variable is used by tests that source this file
 registry=$TEST_ROOT/registry.json
 
diff --git a/tests/functional/flakes/flake-in-submodule.sh b/tests/functional/flakes/flake-in-submodule.sh
index fe5acf26d..a7d86698d 100755
--- a/tests/functional/flakes/flake-in-submodule.sh
+++ b/tests/functional/flakes/flake-in-submodule.sh
@@ -62,8 +62,8 @@ flakeref=git+file://$rootRepo\?submodules=1\&dir=submodule
 # Check that dirtying a submodule makes the entire thing dirty.
 [[ $(nix flake metadata --json "$flakeref" | jq -r .locked.rev) != null ]]
 echo '"foo"' > "$rootRepo"/submodule/sub.nix
-[[ $(nix eval --json "$flakeref#sub" ) = '"foo"' ]]
-[[ $(nix flake metadata --json "$flakeref" | jq -r .locked.rev) = null ]]
+[[ $(_NIX_TEST_BARF_ON_UNCACHEABLE='' nix eval --json "$flakeref#sub" ) = '"foo"' ]]
+[[ $(_NIX_TEST_BARF_ON_UNCACHEABLE='' nix flake metadata --json "$flakeref" | jq -r .locked.rev) = null ]]
 
 # Test that `nix flake metadata` parses `submodule` correctly.
 cat > "$rootRepo"/flake.nix <<EOF
@@ -75,7 +75,7 @@ EOF
 git -C "$rootRepo" add flake.nix
 git -C "$rootRepo" commit -m "Add flake.nix"
 
-storePath=$(nix flake prefetch --json "$rootRepo?submodules=1" | jq -r .storePath)
+storePath=$(_NIX_TEST_BARF_ON_UNCACHEABLE='' nix flake prefetch --json "$rootRepo?submodules=1" | jq -r .storePath)
 [[ -e "$storePath/submodule" ]]
 
 # Test the use of inputs.self.
diff --git a/tests/functional/flakes/follow-paths.sh b/tests/functional/flakes/follow-paths.sh
index cf27681cb..24f3e213e 100755
--- a/tests/functional/flakes/follow-paths.sh
+++ b/tests/functional/flakes/follow-paths.sh
@@ -131,7 +131,7 @@ EOF
 git -C $flakeFollowsA add flake.nix
 
 expect 1 nix flake lock $flakeFollowsA 2>&1 | grep '/flakeB.*is forbidden in pure evaluation mode'
-expect 1 nix flake lock --impure $flakeFollowsA 2>&1 | grep '/flakeB.*does not exist'
+expect 1 nix flake lock --impure $flakeFollowsA 2>&1 | grep "'flakeB' is too short to be a valid store path"
 
 # Test relative non-flake inputs.
 cat > $flakeFollowsA/flake.nix <<EOF
diff --git a/tests/functional/flakes/mercurial.sh b/tests/functional/flakes/mercurial.sh
index b9045bf6b..b6c14fc26 100755
--- a/tests/functional/flakes/mercurial.sh
+++ b/tests/functional/flakes/mercurial.sh
@@ -27,9 +27,9 @@ nix build -o "$TEST_ROOT/result" "hg+file://$flake2Dir"
 
 (! nix flake metadata --json "hg+file://$flake2Dir" | jq -e -r .revision)
 
-nix eval "hg+file://$flake2Dir"#expr
+_NIX_TEST_BARF_ON_UNCACHEABLE='' nix eval "hg+file://$flake2Dir"#expr
 
-nix eval "hg+file://$flake2Dir"#expr
+_NIX_TEST_BARF_ON_UNCACHEABLE='' nix eval "hg+file://$flake2Dir"#expr
 
 (! nix eval "hg+file://$flake2Dir"#expr --no-allow-dirty)
 
diff --git a/tests/functional/flakes/non-flake-inputs.sh b/tests/functional/flakes/non-flake-inputs.sh
index 05e656042..6b1c6a941 100644
--- a/tests/functional/flakes/non-flake-inputs.sh
+++ b/tests/functional/flakes/non-flake-inputs.sh
@@ -81,7 +81,7 @@ nix build -o "$TEST_ROOT/result" "$flake3Dir#sth" --commit-lock-file
 
 nix registry add --registry "$registry" flake3 "git+file://$flake3Dir"
 
-nix build -o "$TEST_ROOT/result" flake3#fnord
+_NIX_TEST_BARF_ON_UNCACHEABLE='' nix build -o "$TEST_ROOT/result" flake3#fnord
 [[ $(cat "$TEST_ROOT/result") = FNORD ]]
 
 # Check whether flake input fetching is lazy: flake3#sth does not
@@ -91,16 +91,17 @@ clearStore
 mv "$flake2Dir" "$flake2Dir.tmp"
 mv "$nonFlakeDir" "$nonFlakeDir.tmp"
 nix build -o "$TEST_ROOT/result" flake3#sth
-(! nix build -o "$TEST_ROOT/result" flake3#xyzzy)
-(! nix build -o "$TEST_ROOT/result" flake3#fnord)
+(! _NIX_TEST_BARF_ON_UNCACHEABLE='' nix build -o "$TEST_ROOT/result" flake3#xyzzy)
+(! _NIX_TEST_BARF_ON_UNCACHEABLE='' nix build -o "$TEST_ROOT/result" flake3#fnord)
 mv "$flake2Dir.tmp" "$flake2Dir"
 mv "$nonFlakeDir.tmp" "$nonFlakeDir"
-nix build -o "$TEST_ROOT/result" flake3#xyzzy flake3#fnord
+_NIX_TEST_BARF_ON_UNCACHEABLE='' nix build -o "$TEST_ROOT/result" flake3#xyzzy flake3#fnord
 
 # Check non-flake inputs have a sourceInfo and an outPath
 #
 # This may look redundant, but the other checks below happen in a command
 # substitution subshell, so failures there will not exit this shell
+export _NIX_TEST_BARF_ON_UNCACHEABLE='' # FIXME
 nix eval --raw flake3#inputs.nonFlake.outPath
 nix eval --raw flake3#inputs.nonFlake.sourceInfo.outPath
 nix eval --raw flake3#inputs.nonFlakeFile.outPath
diff --git a/tests/functional/flakes/relative-paths-lockfile.sh b/tests/functional/flakes/relative-paths-lockfile.sh
index d91aedd16..662c9329c 100644
--- a/tests/functional/flakes/relative-paths-lockfile.sh
+++ b/tests/functional/flakes/relative-paths-lockfile.sh
@@ -4,6 +4,8 @@ source ./common.sh
 
 requireGit
 
+unset _NIX_TEST_BARF_ON_UNCACHEABLE
+
 # Test a "vendored" subflake dependency. This is a relative path flake
 # which doesn't reference the root flake and has its own lock file.
 #