diff --git a/sdk/python/agentfield/harness/_runner.py b/sdk/python/agentfield/harness/_runner.py
index 5e4a8d918..f83ab1d8e 100644
--- a/sdk/python/agentfield/harness/_runner.py
+++ b/sdk/python/agentfield/harness/_runner.py
@@ -294,15 +294,18 @@ async def run(
         # project_dir, or cwd when project_dir is unset. Besides keeping the file
         # inside the agent root, this prevents concurrent runs sharing one cwd
         # from overwriting or deleting each other's fixed output filename.
-        resolved_project_dir = options.get("project_dir")
-        if isinstance(resolved_project_dir, str) and resolved_project_dir:
-            base_dir = resolved_project_dir
-        else:
-            base_dir = resolved_cwd
-        os.makedirs(base_dir, exist_ok=True)
-        temp_output_dir: Optional[str] = tempfile.mkdtemp(
-            prefix=".agentfield-out-", dir=base_dir
-        )
+        # A schema-free run never writes or reads that file, so it must not need
+        # a writable root either: allocating one unconditionally aborted
+        # text-only permission_mode="plan" dispatches before the provider ran.
+        temp_output_dir: Optional[str] = None
+        if schema is not None:
+            resolved_project_dir = options.get("project_dir")
+            if isinstance(resolved_project_dir, str) and resolved_project_dir:
+                base_dir = resolved_project_dir
+            else:
+                base_dir = resolved_cwd
+            os.makedirs(base_dir, exist_ok=True)
+            temp_output_dir = tempfile.mkdtemp(prefix=".agentfield-out-", dir=base_dir)
         output_dir = temp_output_dir
 
         # schema_mode selects how the agent is asked to produce the output:
diff --git a/sdk/python/tests/test_harness_runner.py b/sdk/python/tests/test_harness_runner.py
index adce2b751..27b14e358 100644
--- a/sdk/python/tests/test_harness_runner.py
+++ b/sdk/python/tests/test_harness_runner.py
@@ -240,6 +240,39 @@ async def test_run_without_schema_returns_plain_harness_result(tmp_path):
     assert result.session_id == "sess-1"
 
 
+@pytest.mark.asyncio
+async def test_run_without_schema_does_not_require_writable_project_dir(tmp_path):
+    """A schema-free run must reach the provider without allocating artifacts.
+
+    The per-run ``.agentfield-out-*`` directory only exists to hold the schema
+    output file (#684, #891). Allocating it unconditionally aborted a text-only
+    ``permission_mode="plan"`` run during setup, before the provider was ever
+    dispatched, whenever the project root could not be written to.
+    """
+    blocker = tmp_path / "blocker.txt"
+    blocker.write_text("not a directory", encoding="utf-8")
+    # Cannot be created on any platform: its parent is a regular file.
+    unwritable_root = blocker / "project"
+
+    provider = MockProvider([RawResult(result="plan text")])
+    runner = HarnessRunner()
+
+    with patch("agentfield.harness._runner.build_provider", return_value=provider):
+        result = await runner.run(
+            "hello",
+            provider="codex",
+            permission_mode="plan",
+            cwd=str(tmp_path),
+            project_dir=str(unwritable_root),
+        )
+
+    assert provider.call_count == 1, "schema-free run must dispatch the provider"
+    assert result.is_error is False
+    assert result.result == "plan text"
+    assert result.parsed is None
+    assert list(tmp_path.glob(".agentfield-out-*")) == []
+
+
 @pytest.mark.asyncio
 async def test_run_with_schema_injects_prompt_suffix_and_parses_output(tmp_path):
     provider = FileWritingProvider(json.dumps({"name": "ok", "count": 1}))