diff --git a/autotest/test_mf6_optional_default_value.py b/autotest/test_mf6_optional_default_value.py
new file mode 100644
index 000000000..b60840f92
--- /dev/null
+++ b/autotest/test_mf6_optional_default_value.py
@@ -0,0 +1,89 @@
+"""
+Test that optional package variables with default values aren't written to
+input files when the value matches the DFN-declared default, unless
+write_defaults is set on the simulation data.
+
+Reproduce https://github.com/modflowpy/flopy/issues/2710:
+ModflowPrtprp.coordinate_check_method was introduced in MF6 6.7.0 and
+defaults to "eager". flopy 3.10.0 wrote COORDINATE_CHECK_METHOD EAGER
+unconditionally, causing fatal errors when used with older MF6 binaries.
+
+The fix: get_file_entry() suppresses writing optional scalars whose current
+value equals the DFN-declared default, unless simulation_data.write_defaults
+is True. MF6 applies its own internal default when the keyword is absent,
+so the simulation outcome is identical either way.
+"""
+
+from pathlib import Path
+
+import pytest
+
+import flopy
+
+pytestmark = pytest.mark.mf6
+
+
+def _build_prt_sim(ws, coordinate_check_method="eager"):
+    sim = flopy.mf6.MFSimulation(sim_name="prt", sim_ws=str(ws))
+    flopy.mf6.ModflowTdis(sim, nper=1, perioddata=[(1.0, 1, 1.0)])
+    ems = flopy.mf6.ModflowEms(sim)
+    prt = flopy.mf6.ModflowPrt(sim, modelname="prt")
+    flopy.mf6.ModflowPrtdis(
+        prt,
+        nlay=1,
+        nrow=1,
+        ncol=3,
+        delr=1.0,
+        delc=1.0,
+        top=1.0,
+        botm=0.0,
+    )
+    flopy.mf6.ModflowPrtmip(prt, porosity=0.1)
+    flopy.mf6.ModflowPrtprp(
+        prt,
+        nreleasepts=1,
+        packagedata=[(0, (0, 0, 0), 0.5, 0.5, 0.5)],
+        perioddata={0: ["FIRST"]},
+        coordinate_check_method=coordinate_check_method,
+    )
+    flopy.mf6.ModflowPrtoc(prt, track_filerecord=[("prt.trk",)])
+    sim.register_solution_package(ems, [prt.name])
+    return sim
+
+
+def _prp_text(ws):
+    prp_files = list(Path(ws).glob("*.prp"))
+    assert len(prp_files) == 1, f"expected one .prp file, found: {prp_files}"
+    return prp_files[0].read_text().upper()
+
+
+def test_coordinate_check_method(function_tmpdir):
+    # Default value "eager": should NOT be written.
+    # MF6 applies its own internal default (eager) when the keyword is absent,
+    # so omitting it produces identical simulation behavior.
+    ws = Path(function_tmpdir) / "eager"
+    ws.mkdir()
+    sim = _build_prt_sim(ws, coordinate_check_method="eager")
+    sim.write_simulation()
+    assert "COORDINATE_CHECK_METHOD" not in _prp_text(ws)
+
+    # Non-default value "none": should be written.
+    ws = Path(function_tmpdir) / "none"
+    ws.mkdir()
+    sim = _build_prt_sim(ws, coordinate_check_method="none")
+    sim.write_simulation()
+    text = _prp_text(ws)
+    assert "COORDINATE_CHECK_METHOD" in text
+    assert "NONE" in text
+
+    # write_defaults=True: default value should be written explicitly.
+    # Useful for producing fully self-contained input files independent
+    # of MF6's internal defaults, e.g. for archival or sharing.
+    ws = Path(function_tmpdir) / "write_defaults"
+    ws.mkdir()
+    sim = _build_prt_sim(ws, coordinate_check_method="eager")
+    sim.simulation_data.write_defaults = True
+    sim.write_simulation()
+    text = _prp_text(ws)
+    assert "COORDINATE_CHECK_METHOD" in text
+    assert "EAGER" in text
diff --git a/flopy/mf6/data/mfdatascalar.py b/flopy/mf6/data/mfdatascalar.py
index 94c4f1e3f..22fdc0cb9 100644
--- a/flopy/mf6/data/mfdatascalar.py
+++ b/flopy/mf6/data/mfdatascalar.py
@@ -361,6 +361,20 @@ def get_file_entry(
                 self._simulation_data.debug,
                 ex,
             )
+        if self.structure.optional:
+            data_item = self.structure.data_item_structures[0]
+            if data_item.default_value is not None:
+                current = storage.get_data()
+                try:
+                    matches_default = float(str(current)) == float(
+                        data_item.default_value
+                    )
+                except (ValueError, TypeError):
+                    matches_default = str(current).lower().strip() == (
+                        data_item.default_value.lower().strip()
+                    )
+                if matches_default and not self._simulation_data.write_defaults:
+                    return ""
         if (
             self.structure.type == DatumType.keyword
             or self.structure.type == DatumType.record
diff --git a/flopy/mf6/data/mfstructure.py b/flopy/mf6/data/mfstructure.py
index 3a44b4f9a..649cfbcb9 100644
--- a/flopy/mf6/data/mfstructure.py
+++ b/flopy/mf6/data/mfstructure.py
@@ -614,7 +614,7 @@ def set_value(self, line, common):
                     self.ucase = bool(arr_line[1])
             elif arr_line[0] == "preserve_case":
                 self.preserve_case = self._get_boolean_val(arr_line)
-            elif arr_line[0] == "default_value":
+            elif arr_line[0] in ("default_value", "default"):
                 self.default_value = " ".join(arr_line[1:])
             elif arr_line[0] == "numeric_index":
                 self.numeric_index = self._get_boolean_val(arr_line)
diff --git a/flopy/mf6/mfsimbase.py b/flopy/mf6/mfsimbase.py
index 273970bb6..def3a1803 100644
--- a/flopy/mf6/mfsimbase.py
+++ b/flopy/mf6/mfsimbase.py
@@ -259,6 +259,7 @@ def __init__(self, path: Union[str, PathLike], mfsim):
         self._verbosity_level = VerbosityLevel.normal
         self._max_columns_set_by = None  # Can be None, 'user', or 'auto'
         self.use_pandas = True
+        self.write_defaults = False
 
         self._update_str_format()