fix(channels): harden message split boundaries

nanobot/channels/signal/tests/test_signal_markdown.py

@@ -393,6 +393,14 @@ def test_partition_styles_keeps_offset_in_indented_chunk():
     assert _partition_styles(plain, chunks, ["11:4:BOLD"]) == [[], ["4:4:BOLD"]]
 
 
+def test_partition_styles_keeps_offset_after_crlf_boundary():
+    plain = "header\r\n    code"
+    chunks = split_message(plain, 10)
+
+    assert chunks == ["header", "    code"]
+    assert _partition_styles(plain, chunks, ["12:4:BOLD"]) == [[], ["4:4:BOLD"]]
+
+
 def test_partition_styles_long_message_preserves_chunk_one_styles():
     """A bold span deep in the message must follow the message into chunk 1."""
     # Two ~30-char paragraphs separated by a blank line, then **tail**.

nanobot/utils/helpers.py

@@ -633,6 +633,7 @@ def split_message(content: str, max_len: int = 2000) -> list[str]:
         return [content]
     if len(content) <= max_len:
         return [content]
+    original_content = content
     chunks: list[str] = []
     while content:
         if len(content) <= max_len:
@@ -642,8 +643,14 @@ def split_message(content: str, max_len: int = 2000) -> list[str]:
         cut = content[:max_len]
         # Consume only the newline itself so indentation starts the next chunk.
         newline_pos = cut.rfind("\n")
-        if newline_pos > 0:
-            chunks.append(content[:newline_pos])
+        if newline_pos >= 0:
+            # Exclude both bytes of a CRLF boundary from the emitted chunk.
+            line_end = newline_pos
+            if line_end > 0 and content[line_end - 1] == "\r":
+                line_end -= 1
+            chunk = content[:line_end]
+            if chunk.strip():
+                chunks.append(chunk)
             content = content[newline_pos + 1 :]
             continue
 
@@ -652,11 +659,31 @@ def split_message(content: str, max_len: int = 2000) -> list[str]:
         space_pos = cut.rfind(" ")
         if space_pos > 0 and cut[:space_pos].strip():
             chunks.append(content[:space_pos])
-            content = content[space_pos:].lstrip()
+            content = content[space_pos:].lstrip(" \t")
+            # A space boundary may sit immediately before a line break. Drop
+            # that delimiter too, without stripping the next line's indent.
+            if content.startswith("\r\n"):
+                content = content[2:]
+            elif content.startswith("\n"):
+                content = content[1:]
             continue
 
-        chunks.append(content[:max_len])
+        # Do not split between the two code points of a CRLF delimiter.
+        if cut.endswith("\r") and content[max_len : max_len + 1] == "\n":
+            chunk = cut[:-1]
+            if chunk.strip():
+                chunks.append(chunk)
+            content = content[max_len + 1 :]
+            continue
+
+        chunk = content[:max_len]
+        if chunk.strip():
+            chunks.append(chunk)
         content = content[max_len:]
+        if not chunk.strip():
+            # Keep any remaining indentation so the final non-blank chunk can
+            # retain as much of it as the channel limit permits.
+            continue
         # A delimiter can sit immediately after the hard-break boundary. Keep
         # ordinary space trimming, but consume only the newline so indentation
         # on the following line is preserved.
@@ -665,7 +692,10 @@ def split_message(content: str, max_len: int = 2000) -> list[str]:
             content = content[2:]
         elif content.startswith("\n"):
             content = content[1:]
-    return chunks
+    # Preserve the historical non-empty-input contract for callers that take
+    # the first chunk directly. This fallback is only reachable for content
+    # made entirely of whitespace.
+    return chunks or [original_content[:max_len]]
 
 
 def build_assistant_message(

tests/utils/test_helpers.py

@@ -39,6 +39,42 @@ def test_split_message_preserves_indentation_when_newline_is_at_hard_break():
     ]
 
 
+def test_split_message_handles_crlf_before_hard_break():
+    content = "header\r\n    indented code"
+
+    assert split_message(content, max_len=18) == ["header", "    indented code"]
+    assert split_message("abcdefg\r\n    code", max_len=8) == [
+        "abcdefg",
+        "    code",
+    ]
+
+
+def test_split_message_preserves_indent_after_space_then_newline_boundary():
+    content = "abcdef \n    code"
+
+    assert split_message(content, max_len=7) == ["abcdef", "    cod", "e"]
+    assert split_message(content.replace("\n", "\r\n"), max_len=7) == [
+        "abcdef",
+        "    cod",
+        "e",
+    ]
+
+
+def test_split_message_drops_blank_chunks_from_long_indentation():
+    content = "head\n" + " " * 20 + "x"
+
+    chunks = split_message(content, max_len=8)
+
+    assert chunks == ["head", "    x"]
+    assert all(chunk.strip() for chunk in chunks)
+
+
+def test_split_message_drops_whitespace_only_line_at_boundary():
+    content = "    \nhello world"
+
+    assert split_message(content, max_len=8) == ["hello", "world"]
+
+
 def test_split_message_drops_whitespace_only_tail_after_hard_break():
     prefix = "abcdefgh"
 
@@ -46,6 +82,12 @@ def test_split_message_drops_whitespace_only_tail_after_hard_break():
     assert split_message(prefix + " ", max_len=8) == [prefix]
 
 
+def test_split_message_keeps_one_chunk_for_all_whitespace_input():
+    content = " " * 10
+
+    assert split_message(content, max_len=4) == [" " * 4]
+
+
 def test_split_message_nonpositive_maxlen_returns_unsplit():
     content = "alpha beta gamma delta"