fix(proxy/interceptors): validate truncation window length, not absolute end index

_is_plausible_truncation_range compared end_line directly against the
payload's line count, which only holds when a read starts at line 1. For
an offset read (e.g. "showing lines 100-148 of 300"), the payload only
contains that window's ~49 lines, so the check silently rejected a
genuinely truncated, correctly-formatted banner.

Validate the inclusive window length (end_line - start_line + 1) instead,
which is invariant to where the window starts. Also collapses
test_astgrep_flags_truncated_read onto the _read_result_messages() helper
introduced later in the same file, removing a leftover duplicate.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

headroom/proxy/interceptors/astgrep.py

@@ -111,7 +111,8 @@ def _is_plausible_truncation_range(
     # end_line == total_lines means the whole file was shown, not truncated.
     if start_line < 1 or end_line < start_line or end_line >= total_lines:
         return False
-    return end_line <= source_line_count
+    window_length = end_line - start_line + 1
+    return window_length <= source_line_count
 
 
 def _detect_truncation(source: str) -> tuple[int, int] | None:

tests/test_tool_result_interceptors.py

@@ -221,23 +221,7 @@ def test_astgrep_flags_truncated_read(tokenizer):
         "showing lines 1-42 of 90 total (26031 tokens, cap 25000). "
         "Call Read with offset=43 to see more.]\n"
     )
-    messages = [
-        {
-            "role": "assistant",
-            "content": [
-                {
-                    "type": "tool_use",
-                    "id": "abc",
-                    "name": "Read",
-                    "input": {"file_path": "/repo/payments.py"},
-                }
-            ],
-        },
-        {
-            "role": "user",
-            "content": [{"type": "tool_result", "tool_use_id": "abc", "content": truncated_source}],
-        },
-    ]
+    messages = _read_result_messages(truncated_source)
     result = apply_to_messages(messages, tokenizer)
     assert len(result.spans) == 1
     new_content = result.messages[1]["content"][0]["content"]
@@ -343,6 +327,37 @@ def test_astgrep_accepts_truncation_at_start_equals_end(tokenizer):
     assert "showing through line 42 of 90 total" in new_content
 
 
+def test_astgrep_flags_truncated_read_offset_window(tokenizer):
+    """A legitimate offset read (not starting at line 1) must still be
+    recognized -- window length, not the absolute end index, is what's
+    checked against the visible payload."""
+    truncated_source = (
+        _PY_FIXTURE + "\n\n[Truncated: PARTIAL view — /repo/payments.py: "
+        "showing lines 100-148 of 300 total (26031 tokens, cap 25000). "
+        "Call Read with offset=149 to see more.]\n"
+    )
+    messages = _read_result_messages(truncated_source)
+    result = apply_to_messages(messages, tokenizer)
+    new_content = result.messages[1]["content"][0]["content"]
+    assert "truncated upstream" in new_content
+    assert "showing through line 148 of 300 total" in new_content
+
+
+def test_astgrep_ignores_offset_window_exceeding_visible_payload(tokenizer):
+    """An offset window is still checked for plausibility -- a claimed
+    window far longer than what's actually visible must not trigger."""
+    truncated_source = (
+        _PY_FIXTURE + "\n\n[Truncated: PARTIAL view — /repo/payments.py: "
+        "showing lines 100-5148 of 9999 total (26031 tokens, cap 25000). "
+        "Call Read with offset=5149 to see more.]\n"
+    )
+    messages = _read_result_messages(truncated_source)
+    result = apply_to_messages(messages, tokenizer)
+    new_content = result.messages[1]["content"][0]["content"]
+    assert "outlined by ast-grep" in new_content
+    assert "truncated upstream" not in new_content
+
+
 def test_astgrep_skips_small_files(tokenizer):
     small = "def foo(): return 1\n"
     messages = [