diff --git a/src/tool_execution.py b/src/tool_execution.py
index d23dcd631..5dcfad715 100644
--- a/src/tool_execution.py
+++ b/src/tool_execution.py
@@ -305,6 +305,14 @@ def _client_bridge(client_runtime_context: Optional[Dict]) -> Optional[Dict]:
 
 _ROUTED_BRIDGE_TOOLS = TUI_ROUTED_BRIDGE_TOOL_NAMES
 _BRIDGE_TOOL_TIMEOUT_S = 900.0
+# ``write_file`` transports UTF-8 source/text only.  Keep this guard on the
+# bridge route as well as the local WriteFileTool: native/Harbor runs use the
+# bridge for workspace files, so otherwise a model can overwrite a verified
+# PNG/PDF screenshot with a prose status message.
+_TEXT_WRITE_BINARY_SUFFIXES = frozenset({
+    ".bmp", ".gif", ".ico", ".jpeg", ".jpg", ".mp3", ".mp4", ".ogg",
+    ".pdf", ".png", ".wav", ".webm", ".webp", ".zip",
+})
 
 
 async def _route_tool_via_bridge(tool: str, content: str, session_id: Optional[str], client_runtime_context: Optional[Dict]):
@@ -660,6 +668,15 @@ async def _route_tool_via_bridge(tool: str, content: str, session_id: Optional[s
             "error": "write_file: path is required",
             "exit_code": 1,
         }
+    if os.path.splitext(path)[1].casefold() in _TEXT_WRITE_BINARY_SUFFIXES:
+        return f"write_file: {path[:80]}", {
+            "error": (
+                f"write_file: refusing UTF-8 text for binary artifact path {path}. "
+                "Use Python or a format-specific creation tool, then inspect the result."
+            ),
+            "exit_code": 1,
+            "binary_artifact_preserved": True,
+        }
     return f"write_file: {path[:80]}", await _bridge_post(
         bridge,
         "/write",
diff --git a/tests/test_execution_bridge.py b/tests/test_execution_bridge.py
index 81c270b38..6e2614859 100644
--- a/tests/test_execution_bridge.py
+++ b/tests/test_execution_bridge.py
@@ -153,3 +153,32 @@ def test_scoped_execution_bridge_failure_logs_compact_warning(caplog) -> None:
     assert "Command timed out after 900 seconds" in result["error"]
     assert "Traceback" not in caplog.text
     assert "Scoped execution bridge test-environment failed for tool=host_shell" in caplog.text
+
+
+def test_tui_bridge_write_file_rejects_text_for_binary_artifact(monkeypatch) -> None:
+    """The bridge path must preserve rendered media just like local writes."""
+    called = False
+
+    async def fake_post(*_args, **_kwargs):
+        nonlocal called
+        called = True
+        return {"output": "should not run", "exit_code": 0}
+
+    monkeypatch.setattr(tool_execution, "_client_bridge", lambda _context: {"url": "http://bridge", "token": "x"})
+    monkeypatch.setattr(tool_execution, "_bridge_post", fake_post)
+
+    async def invoke():
+        return await tool_execution._route_tool_via_bridge(
+            "write_file",
+            '{"path":"/tmp_workspace/results/screenshot.png","content":"The screenshot is complete."}',
+            "run-1",
+            {"surface": "odysseus-tui"},
+        )
+
+    description, result = asyncio.run(invoke())
+
+    assert description == "write_file: /tmp_workspace/results/screenshot.png"
+    assert result["exit_code"] == 1
+    assert result["binary_artifact_preserved"] is True
+    assert "binary artifact path" in result["error"]
+    assert called is False