fix: insert cachePoint before non-PDF document blocks in _inject_cache_point()

src/strands/models/bedrock.py

@@ -368,8 +368,24 @@ def _inject_cache_point(self, messages: list[dict[str, Any]]) -> None:
                 last_user_idx = msg_idx
 
         if last_user_idx is not None and messages[last_user_idx].get("content"):
-            messages[last_user_idx]["content"].append({"cachePoint": {"type": "default"}})
-            logger.debug("msg_idx=<%s> | added cache point to last user message", last_user_idx)
+            content = messages[last_user_idx]["content"]
+            # Walk backwards to find insertion point: cachePoint must not immediately follow a
+            # non-PDF document block because Bedrock cannot translate that combination to the
+            # Anthropic API format (non-PDF documents are not natively supported by Anthropic,
+            # so the conversion path fails when a cachePoint trails one).
+            insert_idx = len(content)
+            for i in range(len(content) - 1, -1, -1):
+                doc = content[i].get("document", {})
+                if doc and doc.get("format") != "pdf":
+                    insert_idx = i
+                else:
+                    break
+            content.insert(insert_idx, {"cachePoint": {"type": "default"}})
+            logger.debug(
+                "msg_idx=<%s>, insert_idx=<%s> | added cache point to last user message",
+                last_user_idx,
+                insert_idx,
+            )
 
     def _find_last_user_text_message_index(self, messages: Messages) -> int | None:
         """Find the index of the last user message containing text or image content.

tests/strands/models/test_bedrock.py

@@ -2735,6 +2735,84 @@ def test_inject_cache_point_auto_strategy_resolves_to_anthropic_for_claude(bedro
     assert len(formatted[1]["content"]) == 1
 
 
+def test_inject_cache_point_before_non_pdf_document(bedrock_client):
+    """Test that cachePoint is inserted before trailing non-PDF document blocks."""
+    model = BedrockModel(
+        model_id="us.anthropic.claude-sonnet-4-20250514-v1:0", cache_config=CacheConfig(strategy="auto")
+    )
+
+    cleaned_messages = [
+        {
+            "role": "user",
+            "content": [
+                {"text": "Analyze this file\n\n[Attached files: README.md]"},
+                {"document": {"format": "md", "name": "readme", "source": {"bytes": b"..."}}},
+            ],
+        }
+    ]
+
+    model._inject_cache_point(cleaned_messages)
+
+    content = cleaned_messages[0]["content"]
+    assert len(content) == 3
+    # cachePoint must be inserted before the non-PDF document block
+    cache_idx = next(i for i, b in enumerate(content) if "cachePoint" in b)
+    doc_idx = next(i for i, b in enumerate(content) if "document" in b)
+    assert cache_idx < doc_idx, "cachePoint must come before the non-PDF document block"
+    assert content[cache_idx]["cachePoint"]["type"] == "default"
+
+
+def test_inject_cache_point_after_pdf_document(bedrock_client):
+    """Test that cachePoint is appended after a PDF document block (PDF is supported by Anthropic)."""
+    model = BedrockModel(
+        model_id="us.anthropic.claude-sonnet-4-20250514-v1:0", cache_config=CacheConfig(strategy="auto")
+    )
+
+    cleaned_messages = [
+        {
+            "role": "user",
+            "content": [
+                {"text": "Summarize this PDF"},
+                {"document": {"format": "pdf", "name": "report", "source": {"bytes": b"..."}}},
+            ],
+        }
+    ]
+
+    model._inject_cache_point(cleaned_messages)
+
+    content = cleaned_messages[0]["content"]
+    assert len(content) == 3
+    # PDF documents are safe to precede a cachePoint, so cachePoint goes at the end
+    assert "cachePoint" in content[-1]
+    assert content[-1]["cachePoint"]["type"] == "default"
+
+
+def test_inject_cache_point_before_multiple_non_pdf_documents(bedrock_client):
+    """Test that cachePoint is inserted before multiple trailing non-PDF document blocks."""
+    model = BedrockModel(
+        model_id="us.anthropic.claude-sonnet-4-20250514-v1:0", cache_config=CacheConfig(strategy="auto")
+    )
+
+    cleaned_messages = [
+        {
+            "role": "user",
+            "content": [
+                {"text": "Compare these files"},
+                {"document": {"format": "csv", "name": "data1", "source": {"bytes": b"..."}}},
+                {"document": {"format": "xlsx", "name": "data2", "source": {"bytes": b"..."}}},
+            ],
+        }
+    ]
+
+    model._inject_cache_point(cleaned_messages)
+
+    content = cleaned_messages[0]["content"]
+    assert len(content) == 4
+    cache_idx = next(i for i, b in enumerate(content) if "cachePoint" in b)
+    first_doc_idx = next(i for i, b in enumerate(content) if "document" in b)
+    assert cache_idx < first_doc_idx, "cachePoint must come before all non-PDF document blocks"
+
+
 def test_find_last_user_text_message_index_no_user_messages(bedrock_client):
     """Test _find_last_user_text_message_index returns None when no user text messages exist."""
     model = BedrockModel(model_id="test-model")