fix(agents): remove empty strings from user content (#8038)

* fix(agents): drop empty strings from rebuilt user content

* fix(agents): normalize empty content before sanitizer early returns

* docs(agents): clarify input sanitization provider limits

backend/packages/harness/deerflow/agents/middlewares/AGENTS.md

@@ -203,3 +203,19 @@ chain instead of skipping the block and reading the remaining text. Tests:
 `tests/test_llm_error_handling_middleware.py`.
 
 Receipts check IDs/anchors/status; uncited results stay unresolved. Legacy no_citation_claims remains readable.
+
+InputSanitization removes empty bare strings from genuine user content lists before
+text-related early returns; all-empty string lists become an empty string. Request
+copies preserve metadata and leave persisted history untouched.
+
+Scope / known provider compatibility limits (#8038 review):
+- On the upload extraction-disagreement (`rfind` failure) fallback, non-empty
+  bare strings are framed individually but remain untyped list parts. Strict
+  Chat Completions endpoints can still reject them; typing those parts is deferred.
+- Empty typed text blocks (`{"type": "text", "text": ""}`) remain unchanged:
+  this fix targets untyped empty strings, not all provider-specific validation.
+  Anthropic can still reject those empty typed blocks.
+- All-empty bare-string lists become `content=""`; this middleware replaces
+  messages in place rather than dropping turns. This is not a guarantee of
+  Anthropic compatibility: an empty user turn can still be rejected. No upstream
+  Gateway / IM empty-turn handling is assumed by this normalization.

backend/packages/harness/deerflow/agents/middlewares/input_sanitization_middleware.py

@@ -335,12 +335,23 @@ class InputSanitizationMiddleware(AgentMiddleware[AgentState]):
         message is never mutated.
         """
         content = msg.content
+        normalized_message = None
+        if isinstance(content, list):
+            filtered = [block for block in content if not (isinstance(block, str) and not block)]
+            if len(filtered) != len(content):
+                # Normalize before early returns, including image-only and
+                # already-framed messages. Use an empty string rather than
+                # introducing an empty content-parts list for all-empty input.
+                content = filtered or ""
+                normalized_message = msg.model_copy(update={"content": deepcopy(content)})
+                if not filtered:
+                    return normalized_message
         text_content, text_blocks = self._extract_text_from_content(content)
 
         # No text at all (e.g. image-only message) — pass through
         if not text_content and not isinstance(content, str):
             logger.debug("_sanitize_message: no text content in message — passing through")
-            return None
+            return normalized_message
 
         # Sanitize only the user's original input when available (set by
         # UploadsMiddleware before it prepends the <current_uploads> block),
@@ -415,8 +426,8 @@ class InputSanitizationMiddleware(AgentMiddleware[AgentState]):
             processed = _check_user_content(text_content)  # fallback
 
         if processed == text_content:
-            # Already clean / already wrapped — no override needed
-            return None
+            # Already clean / already wrapped — only normalization may be needed
+            return normalized_message
 
         if text_blocks:
             new_content = self._rebuild_content(content, processed, text_blocks)

backend/tests/test_input_sanitization_middleware.py

@@ -756,6 +756,26 @@ class TestWrapModelCallSpecialCases:
         assert "<think>" in merged
         assert processed[1] == image_block
 
+    @pytest.mark.parametrize("empty_index", [0, 1, 2, 3, 4])
+    def test_empty_bare_strings_are_removed_without_mutating_original(self, empty_index):
+        mw = _make_middleware()
+        image = {"type": "image_url", "image_url": {"url": "data:image/png;base64,AA=="}}
+        content = [image, {"type": "text", "text": "hello"}, "world", image]
+        content.insert(empty_index, "")
+        msg = HumanMessage(content=content, id="msg-1")
+        original = deepcopy(msg.content)
+        request = _make_request([msg])
+        captured = []
+
+        mw.wrap_model_call(request, lambda req: captured.append(req) or "ok")
+
+        assert captured[0].messages[0].content == [
+            image,
+            {"type": "text", "text": f"{_USER_INPUT_BEGIN}\nhello\nworld\n{_USER_INPUT_END}"},
+            image,
+        ]
+        assert msg.content == original
+
     def test_already_wrapped_no_override(self):
         mw = _make_middleware()
         already = _check_user_content("Hello")
@@ -1537,3 +1557,42 @@ def test_request_local_frame_reuse_is_idempotent_and_keeps_literal_entities(cont
     assert middleware._sanitize_message(message) is None
     assert str(framed).count(_USER_INPUT_ENCODING_NOTE) == 1
     assert "<role>" in framed
+
+
+@pytest.mark.parametrize("case", ["image", "upload", "framed", "empty"])
+@pytest.mark.parametrize("async_call", [False, True])
+@pytest.mark.asyncio
+async def test_empty_strings_are_removed_even_without_text_rebuilding(case, async_call):
+    image = {"type": "image_url", "image_url": {"url": "data:image/png;base64,AA=="}}
+    server = {"type": "text", "text": "<current_uploads>photo.png</current_uploads>"}
+    framed = _check_user_content("hello")
+    content, expected, kwargs = {
+        "image": (["", image], [image], {}),
+        "upload": ([server, "", image], [server, image], {ORIGINAL_USER_CONTENT_KEY: ""}),
+        "framed": (["", framed], [framed], {}),
+        "empty": (["", ""], "", {}),
+    }[case]
+    message = HumanMessage(content=content, id="empty-parts", additional_kwargs=kwargs)
+    original = deepcopy(message)
+    request = _make_request([message])
+    middleware = _make_middleware()
+    captured = []
+
+    def handler(req):
+        captured.append(req)
+        return "ok"
+
+    if async_call:
+
+        async def async_handler(req):
+            return handler(req)
+
+        await middleware.awrap_model_call(request, async_handler)
+    else:
+        middleware.wrap_model_call(request, handler)
+
+    processed = captured[0].messages[0]
+    assert processed.content == expected
+    assert processed.additional_kwargs == kwargs
+    assert processed.id == message.id
+    assert message == original