diff --git a/.changelog/5239.fixed b/.changelog/5239.fixed
new file mode 100644
index 00000000000..a4a50718193
--- /dev/null
+++ b/.changelog/5239.fixed
@@ -0,0 +1 @@
+`opentelemetry-exporter-otlp-proto-common`, `opentelemetry-exporter-otlp-json-common`: support non-standard attribute value types (e.g. `pathlib.Path`) by coercing to string
diff --git a/exporter/opentelemetry-exporter-otlp-json-common/src/opentelemetry/exporter/otlp/json/common/_internal/__init__.py b/exporter/opentelemetry-exporter-otlp-json-common/src/opentelemetry/exporter/otlp/json/common/_internal/__init__.py
index 7670c23304b..ff3f842dd0f 100644
--- a/exporter/opentelemetry-exporter-otlp-json-common/src/opentelemetry/exporter/otlp/json/common/_internal/__init__.py
+++ b/exporter/opentelemetry-exporter-otlp-json-common/src/opentelemetry/exporter/otlp/json/common/_internal/__init__.py
@@ -83,6 +83,20 @@ def _encode_value(value: Any, allow_null: bool = False) -> JSONAnyValue | None:
                 ]
             )
         )
+    # Third-party instrumentation can inject arbitrary types that cannot be exhaustively
+    # whitelisted; stringify as a best-effort fallback so telemetry is not silently lost.
+    # None is excluded — it must be handled via allow_null=True at the call site.
+    if value is not None:
+        _logger.error(
+            "Invalid type %s for OTLP value; encoding as string. "
+            "This is a bug in the instrumentation.",
+            type(value),
+        )
+        # pylint: disable=broad-exception-caught
+        try:
+            return JSONAnyValue(string_value=str(value))
+        except Exception:
+            pass
     raise TypeError(f"Invalid type {type(value)} of value {value}")
 
 
diff --git a/exporter/opentelemetry-exporter-otlp-json-common/tests/test_common_encoder.py b/exporter/opentelemetry-exporter-otlp-json-common/tests/test_common_encoder.py
index 70a9e746fce..22d58088178 100644
--- a/exporter/opentelemetry-exporter-otlp-json-common/tests/test_common_encoder.py
+++ b/exporter/opentelemetry-exporter-otlp-json-common/tests/test_common_encoder.py
@@ -6,6 +6,7 @@
 import base64
 import unittest
 from logging import ERROR
+from pathlib import Path
 
 from opentelemetry.exporter.otlp.json.common._internal import (
     _encode_array,
@@ -35,7 +36,7 @@
 from opentelemetry.sdk.util.instrumentation import InstrumentationScope
 
 
-class TestCommonEncoder(unittest.TestCase):
+class TestCommonEncoder(unittest.TestCase):  # pylint: disable=too-many-public-methods
     def test_encode_value(self):
         cases = [
             (
@@ -326,3 +327,38 @@ def test_encode_instrumentation_scope_none(self):
         result = _encode_instrumentation_scope(None)
         self.assertEqual(result, JSONInstrumentationScope())
         self.assertEqual(result.to_dict(), {})
+
+    def test_encode_value_pathlib_path(self):
+        path = Path("/models/my-model")
+        with self.assertLogs(
+            "opentelemetry.exporter.otlp.json.common._internal", level=ERROR
+        ) as log_cm:
+            result = _encode_value(path)
+        self.assertEqual(result, JSONAnyValue(string_value=str(path)))
+        self.assertEqual(result.to_dict(), {"stringValue": str(path)})
+        self.assertTrue(any("Invalid type" in msg for msg in log_cm.output))
+
+    def test_encode_attributes_pathlib_path(self):
+        path = Path("/models/my-model")
+        result = _encode_attributes({"model_path": path})
+        self.assertEqual(
+            result,
+            [
+                JSONKeyValue(
+                    key="model_path",
+                    value=JSONAnyValue(string_value=str(path)),
+                )
+            ],
+        )
+
+    def test_encode_value_unstringable_type_raises(self):
+        class _Unstringable:
+            def __str__(self):
+                raise RuntimeError("cannot convert")
+
+        with self.assertLogs(
+            "opentelemetry.exporter.otlp.json.common._internal", level=ERROR
+        ) as log_cm:
+            with self.assertRaises((TypeError, Exception)):
+                _encode_value(_Unstringable())
+        self.assertTrue(any("Invalid type" in msg for msg in log_cm.output))
diff --git a/exporter/opentelemetry-exporter-otlp-proto-common/src/opentelemetry/exporter/otlp/proto/common/_internal/__init__.py b/exporter/opentelemetry-exporter-otlp-proto-common/src/opentelemetry/exporter/otlp/proto/common/_internal/__init__.py
index 8dbbc212f74..137be3b5510 100644
--- a/exporter/opentelemetry-exporter-otlp-proto-common/src/opentelemetry/exporter/otlp/proto/common/_internal/__init__.py
+++ b/exporter/opentelemetry-exporter-otlp-proto-common/src/opentelemetry/exporter/otlp/proto/common/_internal/__init__.py
@@ -79,6 +79,20 @@ def _encode_value(value: Any, allow_null: bool = False) -> PB2AnyValue | None:
                 ]
             )
         )
+    # Third-party instrumentation can inject arbitrary types that cannot be exhaustively
+    # whitelisted; stringify as a best-effort fallback so telemetry is not silently lost.
+    # None is excluded — it must be handled via allow_null=True at the call site.
+    if value is not None:
+        _logger.error(
+            "Invalid type %s for OTLP value; encoding as string. "
+            "This is a bug in the instrumentation.",
+            type(value),
+        )
+        # pylint: disable=broad-exception-caught
+        try:
+            return PB2AnyValue(string_value=str(value))
+        except Exception:
+            pass
     raise Exception(f"Invalid type {type(value)} of value {value}")
 
 
diff --git a/exporter/opentelemetry-exporter-otlp-proto-common/tests/test_attribute_encoder.py b/exporter/opentelemetry-exporter-otlp-proto-common/tests/test_attribute_encoder.py
index 58ce2b337e4..dda76beaabb 100644
--- a/exporter/opentelemetry-exporter-otlp-proto-common/tests/test_attribute_encoder.py
+++ b/exporter/opentelemetry-exporter-otlp-proto-common/tests/test_attribute_encoder.py
@@ -3,9 +3,11 @@
 
 import unittest
 from logging import ERROR
+from pathlib import Path
 
 from opentelemetry.exporter.otlp.proto.common._internal import (
     _encode_attributes,
+    _encode_value,
 )
 from opentelemetry.proto.common.v1.common_pb2 import AnyValue as PB2AnyValue
 from opentelemetry.proto.common.v1.common_pb2 import (
@@ -110,3 +112,37 @@ def test_encode_attributes_error_logs_key(self):
                 PB2KeyValue(key="b", value=PB2AnyValue(int_value=2)),
             ],
         )
+
+    def test_encode_value_pathlib_path(self):
+        path = Path("/models/my-model")
+        with self.assertLogs(
+            "opentelemetry.exporter.otlp.proto.common._internal", level=ERROR
+        ) as log_cm:
+            result = _encode_value(path)
+        self.assertEqual(result, PB2AnyValue(string_value=str(path)))
+        self.assertTrue(any("Invalid type" in msg for msg in log_cm.output))
+
+    def test_encode_attributes_pathlib_path(self):
+        path = Path("/models/my-model")
+        result = _encode_attributes({"model_path": path})
+        self.assertEqual(
+            result,
+            [
+                PB2KeyValue(
+                    key="model_path",
+                    value=PB2AnyValue(string_value=str(path)),
+                )
+            ],
+        )
+
+    def test_encode_value_unstringable_type_raises(self):
+        class _Unstringable:
+            def __str__(self):
+                raise RuntimeError("cannot convert")
+
+        with self.assertLogs(
+            "opentelemetry.exporter.otlp.proto.common._internal", level=ERROR
+        ) as log_cm:
+            with self.assertRaises(Exception):
+                _encode_value(_Unstringable())
+        self.assertTrue(any("Invalid type" in msg for msg in log_cm.output))