emrg: the read tool's past-EOF message names the condition, not a descending range

emrg/tools/read_tool.py

@@ -286,10 +286,23 @@ async def execute(self, arguments: dict) -> ToolResult:
         for i, line in enumerate(selected):
             result_lines.append(f"{start + i + 1:6d}\t{line}")
 
+        # Reached exactly when `start_line` is past the last line: `start >= end` forces
+        # `end == total_lines`, so the `lines {start + 1}-{end}` this used to print was
+        # **always descending** — a range that cannot be a range, offered where the caller
+        # asked why nothing came back. Measured 2026-10-10 (`cyc20261010-204628`) on a
+        # 10-line file: `start_line=50` answered `(empty range: lines 50-10 of 10)` and
+        # `start_line=11` answered `(empty range: lines 11-10 of 10)`. The test that pinned
+        # it (`test_read_start_line_beyond_eof`) asserted only that the substring
+        # `empty range` appeared, so the descending range satisfied it. This branch names
+        # the condition and the two numbers it holds — the request, and the subject's size.
         if not result_lines:
+            plural = "" if total_lines == 1 else "s"
             return ToolResult(
                 name="read",
-                content=f"(empty range: lines {start + 1}-{end} of {total_lines})",
+                content=(
+                    f"(no lines: start_line={start_line} is past the end of the file, "
+                    f"which has {total_lines} line{plural})"
+                ),
             )
 
         if offset_note is not None:

tests/test_read_tool.py

@@ -107,11 +107,40 @@ def test_read_binary_fails(temp_file):
 
 
 def test_read_start_line_beyond_eof(temp_file):
+    """The message names the request and the subject's size — and is not a range.
+
+    The branch is reached exactly when `start_line` is past the last line, so the
+    `lines {start + 1}-{end}` it used to print was always descending (measured 2026-10-10,
+    `cyc20261010-204628`: a 10-line file answered `start_line=50` with `lines 50-10 of
+    10`). This test asserted the substring `empty range`, which that descending range
+    satisfies — a wording pinned by the presence of its words rather than by what they
+    say. Both numbers the message holds are asserted now, and the descending shape is
+    refused, so a message naming the wrong start line or the wrong file size fails here.
+    """
     tool = ReadTool()
     f, _ = temp_file
-    # 6 lines (5 + trailing newline), start_line=100 is way beyond
+    # 5 lines (the fixture writes 5, and the trailing newline is not a line)
     result = _run(tool.execute({"file_path": str(f), "start_line": 100}))
-    assert "(empty range" in result.content or "empty range" in result.content
+    assert "start_line=100" in result.content, result.content
+    assert "5 lines" in result.content, result.content
+    assert "100-5" not in result.content, result.content
+    assert "lines 100" not in result.content, result.content
+
+
+def test_a_one_line_file_past_its_end_is_counted_in_the_singular(tmp_path):
+    """The singular half of the message above, which no test reached.
+
+    `plural = "" if total_lines == 1 else "s"` is a branch, and a branch needs a reading:
+    measured 2026-10-10, replacing it with the bare `"s"` left every test in this file
+    green, so a one-line file read past its end could have been made to say `1 lines`
+    with no red anywhere. It is reachable — `start_line=2` on a one-line file goes
+    through this branch — and the plural side above does not reach it.
+    """
+    f = tmp_path / "one.txt"
+    f.write_text("only\n")
+    result = _run(ReadTool().execute({"file_path": str(f), "start_line": 2}))
+    assert "which has 1 line" in result.content, result.content
+    assert "1 lines" not in result.content, result.content
 
 
 def test_read_truncation_message(temp_file):