fix(bedrock): use non-whitespace placeholder for empty text blocks

Bedrock Converse rejects text content blocks that are empty OR
whitespace-only (ValidationException: "text content blocks must
contain non-whitespace text"). The prior fix attempt substituted a
single space (" ") for missing content -- but a lone space IS
whitespace, so it was rejected by the exact same validation rule it
was meant to satisfy. This caused a deterministic, unrecoverable
retry-loop failure once any blank/whitespace assistant, tool, or user
turn entered history (most commonly via context-compaction rewriting
a turn to a blank string).

Adds _safe_text()/_EMPTY_TEXT_PLACEHOLDER ("(empty)") and applies it
everywhere a blank text block could reach the wire: user/assistant
content conversion, tool results, the assistant-empty-turn fallback,
and the first/last-message user-alternation padding. System-prompt
blocks are the one exception: blank parts are dropped entirely rather
than placeholder-filled, since a system prompt block should never
carry meaningless placeholder text.

Adds tests/agent/test_bedrock_empty_text_blocks.py (11 tests, was
already present uncommitted -- codifies the exact failing history
from issue #9486 and asserts no blank block ever reaches Bedrock).

Verified against the actual failed request dump from this session
(27-message payload) -- replaying it through the fixed converter now
produces zero blank/whitespace-only blocks.

agent/bedrock_adapter.py

@@ -490,6 +490,26 @@ def convert_tools_to_converse(tools: List[Dict]) -> List[Dict]:
     return result
 
 
+# Bedrock's Converse API rejects any text content block whose text is empty
+# OR whitespace-only (ValidationException: "text content blocks must contain
+# non-whitespace text"). A lone space is whitespace and is rejected too — the
+# placeholder MUST itself be non-whitespace. Ref: issue #9486.
+_EMPTY_TEXT_PLACEHOLDER = "(empty)"
+
+
+def _safe_text(text) -> str:
+    """Return ``text`` if it's non-whitespace, else a non-whitespace placeholder.
+
+    Handles None, empty string, and whitespace-only string (spaces, tabs,
+    newlines) — all of which Bedrock's Converse API rejects as text content.
+    """
+    if text is None:
+        return _EMPTY_TEXT_PLACEHOLDER
+    if not isinstance(text, str):
+        text = str(text)
+    return text if text.strip() else _EMPTY_TEXT_PLACEHOLDER
+
+
 def _convert_content_to_converse(content) -> List[Dict]:
     """Convert OpenAI message content (string or list) to Converse content blocks.
 
@@ -497,14 +517,15 @@ def _convert_content_to_converse(content) -> List[Dict]:
       - Plain text strings → [{"text": "..."}]
       - Content arrays with text/image_url parts → mixed text/image blocks
 
-    Filters out empty text blocks — Bedrock's Converse API rejects messages
-    where a text content block has an empty ``text`` field (ValidationException:
-    "text content blocks must be non-empty"). Ref: issue #9486.
+    Replaces empty/whitespace-only text blocks with a non-whitespace
+    placeholder — Bedrock's Converse API rejects messages where a text
+    content block is empty or whitespace-only (ValidationException:
+    "text content blocks must contain non-whitespace text"). Ref: issue #9486.
     """
     if content is None:
-        return [{"text": " "}]
+        return [{"text": _safe_text(content)}]
     if isinstance(content, str):
-        return [{"text": content}] if content.strip() else [{"text": " "}]
+        return [{"text": _safe_text(content)}]
     if isinstance(content, list):
         blocks = []
         for part in content:
@@ -516,7 +537,7 @@ def _convert_content_to_converse(content) -> List[Dict]:
             part_type = part.get("type", "")
             if part_type == "text":
                 text = part.get("text", "")
-                blocks.append({"text": text if text else " "})
+                blocks.append({"text": _safe_text(text)})
             elif part_type == "image_url":
                 image_url = part.get("image_url", {})
                 url = image_url.get("url", "") if isinstance(image_url, dict) else ""
@@ -538,8 +559,8 @@ def _convert_content_to_converse(content) -> List[Dict]:
                     # Remote URL — Converse doesn't support URLs directly,
                     # include as text reference for the model.
                     blocks.append({"text": f"[Image: {url}]"})
-        return blocks if blocks else [{"text": " "}]
-    return [{"text": str(content)}]
+        return blocks if blocks else [{"text": _EMPTY_TEXT_PLACEHOLDER}]
+    return [{"text": _safe_text(content)}]
 
 
 def convert_messages_to_converse(
@@ -569,14 +590,18 @@ def convert_messages_to_converse(
         content = msg.get("content")
 
         if role == "system":
-            # System messages become the system prompt
+            # System messages become the system prompt. Blank/whitespace-only
+            # parts are dropped entirely (not placeholder-filled) since a
+            # system prompt made up of only placeholder text is meaningless.
             if isinstance(content, str) and content.strip():
                 system_blocks.append({"text": content})
             elif isinstance(content, list):
                 for part in content:
                     if isinstance(part, dict) and part.get("type") == "text":
-                        system_blocks.append({"text": part.get("text", "")})
-                    elif isinstance(part, str):
+                        text = part.get("text", "")
+                        if isinstance(text, str) and text.strip():
+                            system_blocks.append({"text": text})
+                    elif isinstance(part, str) and part.strip():
                         system_blocks.append({"text": part})
             continue
 
@@ -587,7 +612,7 @@ def convert_messages_to_converse(
             tool_result_block = {
                 "toolResult": {
                     "toolUseId": tool_call_id,
-                    "content": [{"text": result_content}],
+                    "content": [{"text": _safe_text(result_content)}],
                 }
             }
             # In Converse, tool results go in a "user" role message
@@ -626,7 +651,7 @@ def convert_messages_to_converse(
                 })
 
             if not content_blocks:
-                content_blocks = [{"text": " "}]
+                content_blocks = [{"text": _EMPTY_TEXT_PLACEHOLDER}]
 
             # Merge with previous assistant message if needed (strict alternation)
             if converse_msgs and converse_msgs[-1]["role"] == "assistant":
@@ -652,11 +677,11 @@ def convert_messages_to_converse(
 
     # Converse requires the first message to be from the user
     if converse_msgs and converse_msgs[0]["role"] != "user":
-        converse_msgs.insert(0, {"role": "user", "content": [{"text": " "}]})
+        converse_msgs.insert(0, {"role": "user", "content": [{"text": _EMPTY_TEXT_PLACEHOLDER}]})
 
     # Converse requires the last message to be from the user
     if converse_msgs and converse_msgs[-1]["role"] != "user":
-        converse_msgs.append({"role": "user", "content": [{"text": " "}]})
+        converse_msgs.append({"role": "user", "content": [{"text": _EMPTY_TEXT_PLACEHOLDER}]})
 
     return (system_blocks if system_blocks else None, converse_msgs)
 

tests/agent/test_bedrock_empty_text_blocks.py

@@ -0,0 +1,115 @@
+"""Regression tests for Bedrock Converse empty/whitespace text block rejection.
+
+Bedrock's Converse API raises::
+
+    ValidationException: The model returned the following errors: messages:
+    text content blocks must contain non-whitespace text
+
+for ANY text content block that is empty OR whitespace-only. Anthropic's native
+API tolerates these; Bedrock does not. Such blocks most often appear after
+context compression rewrites an assistant/tool turn into a blank string — once
+in history the block is re-sent on every call and fails deterministically
+(all retries fail identically). Ref: issue #9486.
+
+These tests assert convert_messages_to_converse never emits a blank text block
+(including inside toolResult content) and never uses a whitespace-only
+placeholder (a lone space would be rejected by the same validation).
+"""
+import pytest
+
+from agent.bedrock_adapter import (
+    convert_messages_to_converse,
+    _convert_content_to_converse,
+    _safe_text,
+    _EMPTY_TEXT_PLACEHOLDER,
+)
+
+
+def _iter_text_blocks(msgs):
+    """Yield every text string that will be sent to Bedrock, incl. toolResult."""
+    for m in msgs:
+        for b in m["content"]:
+            if "text" in b:
+                yield b["text"]
+            if "toolResult" in b:
+                for tb in b["toolResult"]["content"]:
+                    if "text" in tb:
+                        yield tb["text"]
+
+
+def test_placeholder_is_non_whitespace():
+    # The core lesson of #9486: a space is whitespace and is itself rejected.
+    assert _EMPTY_TEXT_PLACEHOLDER.strip(), "placeholder must be non-whitespace"
+
+
+@pytest.mark.parametrize("value", ["", "   ", "\n\n", "\t", None])
+def test_safe_text_blank_inputs_become_non_whitespace(value):
+    assert _safe_text(value).strip()
+
+
+def test_safe_text_preserves_real_content():
+    assert _safe_text("hello") == "hello"
+    assert _safe_text("  padded  ") == "  padded  "  # inner content kept verbatim
+
+
+def test_no_blank_blocks_reach_bedrock():
+    """The exact failing history: blank system/assistant/tool/user turns."""
+    messages = [
+        {"role": "system", "content": "You are helpful."},
+        {"role": "system", "content": [{"type": "text", "text": "   "}]},
+        {"role": "user", "content": "search for foo"},
+        {"role": "assistant", "content": "",
+         "tool_calls": [{"id": "tc1",
+                         "function": {"name": "search", "arguments": "{}"}}]},
+        {"role": "tool", "tool_call_id": "tc1", "content": ""},        # empty tool output
+        {"role": "assistant", "content": "   \n\n  "},                 # whitespace-only (compaction)
+        {"role": "user", "content": [{"type": "text", "text": ""}]},
+        {"role": "assistant", "content": None},
+    ]
+    _system, msgs = convert_messages_to_converse(messages)
+    for text in _iter_text_blocks(msgs):
+        assert text.strip(), f"blank text block would be rejected by Bedrock: {text!r}"
+
+
+def test_empty_tool_result_gets_placeholder():
+    """A tool that returns no output must not produce a blank toolResult block."""
+    messages = [
+        {"role": "user", "content": "run it"},
+        {"role": "assistant", "content": "",
+         "tool_calls": [{"id": "t1", "function": {"name": "sh", "arguments": "{}"}}]},
+        {"role": "tool", "tool_call_id": "t1", "content": "   "},
+    ]
+    _system, msgs = convert_messages_to_converse(messages)
+    tool_msg = next(m for m in msgs
+                    if any("toolResult" in b for b in m["content"]))
+    block = next(b for b in tool_msg["content"] if "toolResult" in b)
+    text = block["toolResult"]["content"][0]["text"]
+    assert text.strip()
+
+
+def test_real_content_is_preserved_alongside_blank_siblings():
+    messages = [
+        {"role": "user", "content": [
+            {"type": "text", "text": "  "},
+            {"type": "text", "text": "real question"},
+        ]},
+    ]
+    _system, msgs = convert_messages_to_converse(messages)
+    texts = list(_iter_text_blocks(msgs))
+    assert "real question" in texts
+    assert all(t.strip() for t in texts)
+
+
+def test_blank_system_blocks_dropped_not_blanked():
+    messages = [
+        {"role": "system", "content": [
+            {"type": "text", "text": "keep me"},
+            {"type": "text", "text": "   "},
+        ]},
+        {"role": "user", "content": "hi"},
+    ]
+    system, _msgs = convert_messages_to_converse(messages)
+    assert system is not None
+    for block in system:
+        assert block["text"].strip()
+    assert any(b["text"] == "keep me" for b in system)