diff --git a/.github/workflows/haskell-ci.yml b/.github/workflows/haskell-ci.yml
index 56f55e9..90432fa 100644
--- a/.github/workflows/haskell-ci.yml
+++ b/.github/workflows/haskell-ci.yml
@@ -233,6 +233,7 @@ jobs:
           if [ $((HCNUMVER >= 80200)) -ne 0 ] ; then echo "package io-streams" >> cabal.project ; fi
           if [ $((HCNUMVER >= 80200)) -ne 0 ] ; then echo "    ghc-options: -Werror=missing-methods" >> cabal.project ; fi
           cat >> cabal.project <<EOF
+          allow-newer: bytestring
           EOF
           $HCPKG list --simple-output --names-only | perl -ne 'for (split /\s+/) { print "constraints: $_ installed\n" unless /^(io-streams)$/; }' >> cabal.project.local
           cat cabal.project
@@ -271,6 +272,14 @@ jobs:
         run: |
           rm -f cabal.project.local
           $CABAL v2-build $ARG_COMPILER --disable-tests --disable-benchmarks all
+      - name: prepare for constraint sets
+        run: |
+          rm -f cabal.project.local
+      - name: constraint set bytestring-0.12
+        run: |
+          if [ $((HCNUMVER >= 80000)) -ne 0 ] ; then $CABAL v2-build $ARG_COMPILER --enable-tests --disable-benchmarks --constraint='bytestring >= 0.12' --dependencies-only -j2 all ; fi
+          if [ $((HCNUMVER >= 80000)) -ne 0 ] ; then $CABAL v2-build $ARG_COMPILER --enable-tests --disable-benchmarks --constraint='bytestring >= 0.12' all ; fi
+          if [ $((HCNUMVER >= 80000)) -ne 0 ] ; then $CABAL v2-test $ARG_COMPILER --enable-tests --disable-benchmarks --constraint='bytestring >= 0.12' all ; fi
       - name: save cache
         uses: actions/cache/save@v3
         if: always()
diff --git a/cabal.haskell-ci b/cabal.haskell-ci
index 3ad392c..4409cb0 100644
--- a/cabal.haskell-ci
+++ b/cabal.haskell-ci
@@ -1 +1,25 @@
 branches: master ci*
+
+constraint-set bytestring-0.12
+  -- bytestring-0.12 requires base >=4.9 (GHC 8.0)
+  ghc: >= 8.0
+  constraints: bytestring >= 0.12
+  --
+  -- The following is silently ignored here:
+  --
+  -- raw-project
+  --   allow-newer: bytestring
+  --
+  tests: True
+  run-tests: True
+
+-- The following is meant to be for constraint-set bytestring-0.12 only,
+-- but there is currently no way to enable `allow-newer: bytestring`
+-- just for the constraint set.
+--
+-- Since core library `bytestring` is constrained to `installed`,
+-- it is not harmful to allow newer `bytestring` in the default runs
+-- as well---it will have no effect there.
+--
+raw-project
+  allow-newer: bytestring
diff --git a/io-streams.cabal b/io-streams.cabal
index e08156d..d621b72 100644
--- a/io-streams.cabal
+++ b/io-streams.cabal
@@ -141,7 +141,7 @@ Library
 
   Build-depends:     base               >= 4     && <5,
                      attoparsec         >= 0.10  && <0.15,
-                     bytestring         >= 0.9   && <0.12,
+                     bytestring         >= 0.9   && <0.13,
                      primitive          >= 0.2   && <0.9,
                      process            >= 1.1   && <1.7,
                      text               >=0.10   && <1.3  || >= 2.0 && <2.1,