diff --git a/ops/model.py b/ops/model.py
index de9b93e06..3a9a1cfe3 100644
--- a/ops/model.py
+++ b/ops/model.py
@@ -3598,11 +3598,13 @@ def secret_set(
             args.extend(['--expire', expire.isoformat()])
         if rotate is not None:
             args += ['--rotate', rotate.value]
-        if content is not None:
-            # The content has already been validated with Secret._validate_content
-            for k, v in content.items():
-                args.append(f'{k}={v}')
-        self._run_for_secret('secret-set', *args)
+        with tempfile.TemporaryDirectory() as tmp:
+            # The content is None or has already been validated with Secret._validate_content
+            for k, v in (content or {}).items():
+                with open(f'{tmp}/{k}', mode='w', encoding='utf-8') as f:
+                    f.write(v)
+                args.append(f'{k}#file={tmp}/{k}')
+            self._run_for_secret('secret-set', *args)
 
     def secret_add(
         self,
@@ -3625,10 +3627,13 @@ def secret_add(
             args += ['--rotate', rotate.value]
         if owner is not None:
             args += ['--owner', owner]
-        # The content has already been validated with Secret._validate_content
-        for k, v in content.items():
-            args.append(f'{k}={v}')
-        result = self._run('secret-add', *args, return_output=True)
+        with tempfile.TemporaryDirectory() as tmp:
+            # The content has already been validated with Secret._validate_content
+            for k, v in content.items():
+                with open(f'{tmp}/{k}', mode='w', encoding='utf-8') as f:
+                    f.write(v)
+                args.append(f'{k}#file={tmp}/{k}')
+            result = self._run('secret-add', *args, return_output=True)
         secret_id = typing.cast(str, result)
         return secret_id.strip()
 
diff --git a/test/test_helpers.py b/test/test_helpers.py
index 393fd7551..84f9b20f6 100644
--- a/test/test_helpers.py
+++ b/test/test_helpers.py
@@ -153,6 +153,15 @@ def write(self, name: str, content: str):
             f.write(
                 """#!/bin/sh
 {{ printf {name}; printf "\\036%s" "$@"; printf "\\034"; }} >> {path}/calls.txt
+
+# Capture key and data from key#file=/some/path arguments
+for word in "$@"; do
+  echo "$word" | grep -q "#file=" || continue
+  key=$(echo "$word" | cut -d'#' -f1)
+  path=$(echo "$word" | cut -d'=' -f2)
+  cp "$path" "{path}/$key.secret"
+done
+
 {content}""".format_map(template_args)
             )
         path.chmod(0o755)
@@ -175,6 +184,9 @@ def calls(self, clear: bool = False) -> typing.List[typing.List[str]]:
                 f.truncate(0)
         return calls
 
+    def secrets(self) -> typing.Dict[str, str]:
+        return {p.stem: p.read_text() for p in self.path.iterdir() if p.suffix == '.secret'}
+
 
 class FakeScriptTest(unittest.TestCase):
     def test_fake_script_works(self):
diff --git a/test/test_model.py b/test/test_model.py
index b0e4f6206..0100570cb 100644
--- a/test/test_model.py
+++ b/test/test_model.py
@@ -24,7 +24,7 @@
 import unittest
 from collections import OrderedDict
 from textwrap import dedent
-from unittest.mock import MagicMock, patch
+from unittest.mock import ANY, MagicMock, patch
 
 import pytest
 
@@ -3349,7 +3349,8 @@ def test_app_add_secret_simple(self, fake_script: FakeScript, model: ops.Model):
         assert secret.id == 'secret:123'
         assert secret.label is None
 
-        assert fake_script.calls(clear=True) == [['secret-add', '--owner', 'application', 'foo=x']]
+        assert fake_script.calls(clear=True) == [['secret-add', '--owner', 'application', ANY]]
+        assert fake_script.secrets() == {'foo': 'x'}
 
     def test_app_add_secret_args(self, fake_script: FakeScript, model: ops.Model):
         fake_script.write('secret-add', 'echo secret:234')
@@ -3379,10 +3380,11 @@ def test_app_add_secret_args(self, fake_script: FakeScript, model: ops.Model):
                 'hourly',
                 '--owner',
                 'application',
-                'foo=x',
-                'bar=y',
+                ANY,
+                ANY,
             ]
         ]
+        assert fake_script.secrets() == {'foo': 'x', 'bar': 'y'}
 
     def test_unit_add_secret_simple(self, fake_script: FakeScript, model: ops.Model):
         fake_script.write('secret-add', 'echo secret:345')
@@ -3392,7 +3394,8 @@ def test_unit_add_secret_simple(self, fake_script: FakeScript, model: ops.Model)
         assert secret.id == 'secret:345'
         assert secret.label is None
 
-        assert fake_script.calls(clear=True) == [['secret-add', '--owner', 'unit', 'foo=x']]
+        assert fake_script.calls(clear=True) == [['secret-add', '--owner', 'unit', ANY]]
+        assert fake_script.secrets() == {'foo': 'x'}
 
     def test_unit_add_secret_args(self, fake_script: FakeScript, model: ops.Model):
         fake_script.write('secret-add', 'echo secret:456')
@@ -3422,10 +3425,11 @@ def test_unit_add_secret_args(self, fake_script: FakeScript, model: ops.Model):
                 'yearly',
                 '--owner',
                 'unit',
-                'foo=w',
-                'bar=z',
+                ANY,
+                ANY,
             ]
         ]
+        assert fake_script.secrets() == {'foo': 'w', 'bar': 'z'}
 
     def test_unit_add_secret_errors(self, model: ops.Model):
         # Additional add_secret tests are done in TestApplication
@@ -3721,10 +3725,11 @@ def test_set_content(self, model: ops.Model, fake_script: FakeScript):
             secret.set_content({'s': 't'})  # ensure it validates content (key too short)
 
         assert fake_script.calls(clear=True) == [
-            ['secret-set', 'secret:x', 'foo=bar'],
+            ['secret-set', 'secret:x', ANY],
             ['secret-info-get', '--label', 'y', '--format=json'],
-            ['secret-set', 'secret:z', 'bar=foo'],
+            ['secret-set', 'secret:z', ANY],
         ]
+        assert fake_script.secrets() == {'foo': 'bar', 'bar': 'foo'}
 
     def test_set_info(self, model: ops.Model, fake_script: FakeScript):
         fake_script.write('secret-set', """exit 0""")