diff --git a/Makefile b/Makefile
index 8c89c02c..8fa32085 100644
--- a/Makefile
+++ b/Makefile
@@ -11,7 +11,7 @@ export PATH := $(BIN):$(PATH)
 export GOBIN := $(abspath $(BIN))
 # Set to use a different Python interpreter. For example, `PYTHON=python make test`.
 PYTHON ?= python3
-CONFORMANCE_ARGS ?= --strict --expected_failures=tests/conformance/nonconforming.yaml --timeout 10s
+CONFORMANCE_ARGS ?= --strict --strict_message --expected_failures=tests/conformance/nonconforming.yaml --timeout 10s
 ADD_LICENSE_HEADER := $(BIN)/license-header \
 		--license-type apache \
 		--copyright-holder "Buf Technologies, Inc." \
diff --git a/protovalidate/internal/string_format.py b/protovalidate/internal/string_format.py
index 6604029b..7043084f 100644
--- a/protovalidate/internal/string_format.py
+++ b/protovalidate/internal/string_format.py
@@ -12,6 +12,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from decimal import Decimal
+
 import celpy  # type: ignore
 from celpy import celtypes  # type: ignore
 
@@ -146,9 +148,21 @@ def format_string(self, arg: celtypes.Value) -> celpy.Result:
         if isinstance(arg, celtypes.StringType):
             return arg
         if isinstance(arg, celtypes.BytesType):
-            return celtypes.StringType(arg.hex())
+            return celtypes.StringType(arg)
         if isinstance(arg, celtypes.ListType):
             return self.format_list(arg)
+        if isinstance(arg, celtypes.BoolType):
+            # True -> true
+            return celtypes.StringType(str(arg).lower())
+        if isinstance(arg, celtypes.DoubleType):
+            return celtypes.StringType(f"{arg:.0f}")
+        if isinstance(arg, celtypes.DurationType):
+            return celtypes.StringType(self._format_duration(arg))
+        if isinstance(arg, celtypes.TimestampType):
+            base = arg.isoformat()
+            if arg.getMilliseconds() != 0:
+                base = arg.isoformat(timespec="milliseconds")
+            return celtypes.StringType(base.removesuffix("+00:00") + "Z")
         return celtypes.StringType(arg)
 
     def format_value(self, arg: celtypes.Value) -> celpy.Result:
@@ -156,6 +170,10 @@ def format_value(self, arg: celtypes.Value) -> celpy.Result:
             return celtypes.StringType(quote(arg))
         if isinstance(arg, celtypes.UintType):
             return celtypes.StringType(arg)
+        if isinstance(arg, celtypes.DurationType):
+            return celtypes.StringType(f'duration("{self._format_duration(arg)}")')
+        if isinstance(arg, celtypes.DoubleType):
+            return celtypes.StringType(f"{arg:f}")
         return self.format_string(arg)
 
     def format_list(self, arg: celtypes.ListType) -> celpy.Result:
@@ -167,6 +185,9 @@ def format_list(self, arg: celtypes.ListType) -> celpy.Result:
         result += "]"
         return celtypes.StringType(result)
 
+    def _format_duration(self, arg: celtypes.DurationType) -> celpy.Result:
+        return f"{arg.seconds + Decimal(arg.microseconds) / Decimal(1_000_000):f}s"
+
 
 _default_format = StringFormat("en_US")
 format = _default_format.format  # noqa: A001
diff --git a/tests/conformance/nonconforming.yaml b/tests/conformance/nonconforming.yaml
index 4abf19b8..25744e50 100644
--- a/tests/conformance/nonconforming.yaml
+++ b/tests/conformance/nonconforming.yaml
@@ -1,4 +1,5 @@
 # celpy doesn't support nano seconds
+# ref: https://github.com/cloud-custodian/cel-python/issues/43
 standard_constraints/well_known_types/duration:
  - gte_lte/invalid/above
  - lte/invalid