diff --git a/.github/workflows/build-without-conan.yml b/.github/workflows/build-without-conan.yml
index 74b7583095..f6a6ac5492 100644
--- a/.github/workflows/build-without-conan.yml
+++ b/.github/workflows/build-without-conan.yml
@@ -40,10 +40,9 @@ jobs:
     - name: Install symengine
       run: |
         cd ${TMP_DIR}
-        wget https://github.com/symengine/symengine/archive/refs/tags/v0.14.0.tar.gz
-        tar xzvf v0.14.0.tar.gz
-        cd symengine-0.14.0/
-        sed -i -e 's/cmake_minimum_required(VERSION 2.8.12)/cmake_minimum_required(VERSION 3.5)/g' cmake/SymEngineConfig.cmake.in
+        wget https://github.com/symengine/symengine/archive/refs/tags/v0.15.0.tar.gz
+        tar xzvf v0.15.0.tar.gz
+        cd symengine-0.15.0/
         mkdir build
         cd build
         cmake -GNinja -DCMAKE_INSTALL_PREFIX=${INSTALL_DIR} -DBUILD_TESTS=OFF -DBUILD_BENCHMARKS=OFF -DWITH_SYMENGINE_THREAD_SAFE=ON ..
diff --git a/TKET_VERSION b/TKET_VERSION
index 613f02ec35..3611216b5f 100644
--- a/TKET_VERSION
+++ b/TKET_VERSION
@@ -1 +1 @@
-2.1.96
+2.1.97
diff --git a/build-without-conan.md b/build-without-conan.md
index d6bdc962a7..12517b9938 100644
--- a/build-without-conan.md
+++ b/build-without-conan.md
@@ -48,10 +48,9 @@ make install
 
 ```
 cd ${TMP_DIR}
-wget https://github.com/symengine/symengine/archive/refs/tags/v0.14.0.tar.gz
-tar xzvf v0.14.0.tar.gz
-cd symengine-0.14.0/
-sed -i -e 's/cmake_minimum_required(VERSION 2.8.12)/cmake_minimum_required(VERSION 3.5)/g' cmake/SymEngineConfig.cmake.in
+wget https://github.com/symengine/symengine/archive/refs/tags/v0.15.0.tar.gz
+tar xzvf v0.15.0.tar.gz
+cd symengine-0.15.0/
 mkdir build
 cd build
 cmake -DCMAKE_INSTALL_PREFIX=${INSTALL_DIR} -DBUILD_TESTS=OFF -DBUILD_BENCHMARKS=OFF -DWITH_SYMENGINE_THREAD_SAFE=ON ..
diff --git a/pytket/conanfile.py b/pytket/conanfile.py
index 72f6ef47f7..6e70aa539d 100644
--- a/pytket/conanfile.py
+++ b/pytket/conanfile.py
@@ -53,7 +53,7 @@ def requirements(self):
         self.requires("gmp/tci-6.3.0@tket/stable")
         self.requires("nlohmann_json/3.12.0")
         self.requires("nanobind/tci-3.0.0@tket/stable")
-        self.requires("symengine/tci-0.14.0.3@tket/stable")
+        self.requires("symengine/tci-0.15.0@tket/stable")
         self.requires("tkassert/0.3.6@tket/stable")
         self.requires(f"tket/{tket_version}@tket/stable")
         self.requires("tklog/0.3.5@tket/stable")
diff --git a/tket/conanfile.py b/tket/conanfile.py
index e6a2923435..7247d804e3 100644
--- a/tket/conanfile.py
+++ b/tket/conanfile.py
@@ -143,7 +143,7 @@ def requirements(self):
         self.requires("boost/tci-1.91.0@tket/stable", transitive_headers=True)
         self.requires("eigen/5.0.1", transitive_headers=True)
         self.requires("nlohmann_json/3.12.0", transitive_headers=True)
-        self.requires("symengine/tci-0.14.0.3@tket/stable", transitive_headers=True)
+        self.requires("symengine/tci-0.15.0@tket/stable", transitive_headers=True)
         self.requires("tkassert/0.3.6@tket/stable", transitive_headers=True)
         self.requires("tklog/0.3.5@tket/stable")
         self.requires("tkrng/0.3.5@tket/stable")
diff --git a/tket/src/Gate/Rotation.cpp b/tket/src/Gate/Rotation.cpp
index 461418ea5e..62e24edb6a 100644
--- a/tket/src/Gate/Rotation.cpp
+++ b/tket/src/Gate/Rotation.cpp
@@ -142,6 +142,11 @@ static std::tuple<Expr, Expr, Expr> xyx_angles_from_coeffs(
   // 2 * atan2(B, A).
   // Finally, note that u must be well-defined because we have already dealt
   // with all cases where s = 0.
+  //
+  // NOTE: Although the above symengine issue is fixed as of version 0.15.0, we
+  // retain the "workaround" since it gives better simplifications: symengine is
+  // not good at evaluating expressions combining sign and trig functions (e.g.
+  // "sign(cos(pi/4))".
   TKET_ASSERT(!s_zero);
   if (approx_0(SymEngine::expand(i * j + s * k))) {
     Expr u = expr_div(i, s);