diff --git a/backend/packages/harness/deerflow/sandbox/tools.py b/backend/packages/harness/deerflow/sandbox/tools.py
index 4748af32b..463656d4b 100644
--- a/backend/packages/harness/deerflow/sandbox/tools.py
+++ b/backend/packages/harness/deerflow/sandbox/tools.py
@@ -1840,7 +1840,7 @@ def _truncate_bash_output(output: str, max_chars: int) -> str:
     return f"{output[:head_len]}{marker}{output[-tail_len:] if tail_len > 0 else ''}" + preserved
 
 
-def _truncate_read_file_output(output: str, max_chars: int) -> str:
+def _truncate_read_file_output(output: str, max_chars: int, line_offset: int = 0) -> str:
     """Head-truncate read_file output, preserving the beginning of the file.
 
     Source code and documents are read top-to-bottom; the head contains the
@@ -1849,19 +1849,34 @@ def _truncate_read_file_output(output: str, max_chars: int) -> str:
     The returned string (including the truncation marker) is guaranteed to be
     no longer than max_chars characters. Pass max_chars=0 to disable truncation
     and return the full output unchanged.
+
+    The marker reports the 1-indexed line the character cut lands in, so the
+    model can resume with ``start_line`` without re-reading or skipping
+    content (#5475). ``line_offset`` is the 0-based absolute line number of
+    ``output``'s first line — ranged reads pass the requested ``start_line -
+    1`` so the reported lines are absolute file lines, keeping the resume
+    point from looping back onto the slice's own beginning.
     """
     if max_chars == 0:
         return output
     if len(output) <= max_chars:
         return output
     total = len(output)
-    # Compute the exact worst-case marker length: both numeric fields are at
-    # their maximum (total chars), so this is a tight upper bound.
-    marker_max_len = len(f"\n... [truncated: showing first {total} of {total} chars. Use start_line/end_line to read a specific range] ...")
+    total_lines = output.count("\n") + (0 if output.endswith("\n") else 1)
+    # Compute the exact worst-case marker length: every numeric field is at
+    # its maximum (total + offset), so this is a tight upper bound.
+    bound = total + max(line_offset, 0)
+    marker_max_len = len(f"\n... [truncated: showing first {total} of {total} chars (cut lands in line {bound} of {bound}). Use start_line={bound} — optionally with end_line — to continue without a gap] ...")
     kept = max(0, max_chars - marker_max_len)
     if kept == 0:
         return output[:max_chars]
-    marker = f"\n... [truncated: showing first {kept} of {total} chars. Use start_line/end_line to read a specific range] ..."
+    # 1-indexed line holding the first hidden character: a cut mid-line lands
+    # in the partially shown line, a cut exactly after a newline lands in the
+    # next line — either way resuming at this line leaves no gap. Expressed in
+    # absolute file lines via line_offset.
+    cut_line = line_offset + output[:kept].count("\n") + 1
+    last_line = line_offset + total_lines
+    marker = f"\n... [truncated: showing first {kept} of {total} chars (cut lands in line {cut_line} of {last_line}). Use start_line={cut_line} — optionally with end_line — to continue without a gap] ..."
     return f"{output[:kept]}{marker}"
 
 
@@ -2445,7 +2460,10 @@ def read_file_tool(
             max_chars = sandbox_cfg.read_file_output_max_chars if sandbox_cfg else 50000
         except Exception:
             max_chars = 50000
-        return _truncate_read_file_output(content, max_chars)
+        # Ranged reads hand a slice to the truncator; line_offset converts the
+        # slice-relative cut line back to the absolute file lines the tool's
+        # start_line/end_line arguments speak in (#5475).
+        return _truncate_read_file_output(content, max_chars, line_offset=effective_start - 1)
     except SandboxError as e:
         return f"Error: {e}"
     except FileNotFoundError:
diff --git a/backend/tests/test_tool_output_truncation.py b/backend/tests/test_tool_output_truncation.py
index 4b2bf4652..1d4094ac4 100644
--- a/backend/tests/test_tool_output_truncation.py
+++ b/backend/tests/test_tool_output_truncation.py
@@ -6,8 +6,35 @@ These functions truncate long tool outputs to prevent context window overflow.
 - _truncate_ls_output: head-truncation, for ls tool
 """
 
+import re
+
 from deerflow.sandbox.tools import _truncate_bash_output, _truncate_ls_output, _truncate_read_file_output
 
+
+def _head_and_marker(result: str) -> tuple[str, str]:
+    """Split a truncated read_file result into shown head and trailing marker."""
+    idx = result.rfind("\n... [truncated:")
+    assert idx != -1, "truncation marker missing"
+    return result[:idx], result[idx:]
+
+
+def _line_containing(output: str, char_index: int) -> int:
+    """Return the 0-based line index holding ``output[char_index]``.
+
+    Computed from line spans independently of the truncation marker, so the
+    tests verify the marker's reported line rather than restating its formula.
+    """
+    assert 0 <= char_index < len(output)
+    starts = [0]
+    for line in output.splitlines(keepends=True):
+        starts.append(starts[-1] + len(line))
+    candidate = 0
+    for i, start in enumerate(starts):
+        if start <= char_index:
+            candidate = i
+    return candidate
+
+
 # ---------------------------------------------------------------------------
 # _truncate_bash_output
 # ---------------------------------------------------------------------------
@@ -186,6 +213,63 @@ class TestTruncateReadFileOutput:
         assert "start_line" in result
         assert "end_line" in result
 
+    def test_marker_reports_the_line_the_cut_lands_in(self):
+        # Shape from #5475: a multi-line file cut mid-line, where the old
+        # marker gave only a character count so the model could not compute
+        # which line to resume from.
+        output = "".join(f"def fn_{i}():\n    return {i}\n" for i in range(3000))
+        result = _truncate_read_file_output(output, 50000)
+        head, marker = _head_and_marker(result)
+        cut_line = int(re.search(r"cut lands in line (\d+) of", marker).group(1))
+        total_lines = output.count("\n")
+        assert f"cut lands in line {cut_line} of {total_lines}" in marker
+        assert f"start_line={cut_line}" in marker
+        # The no-gap contract: the reported line is the one holding the first
+        # hidden character (computed independently from line spans).
+        assert _line_containing(output, len(head)) + 1 == cut_line
+
+    def test_first_hidden_character_always_belongs_to_reported_line(self):
+        # Whatever size the cut lands at — mid-line or exactly after a
+        # newline — the reported line is the one holding the first hidden
+        # character, so resuming at start_line=<reported> leaves no gap.
+        lines = [f"line-{i}-with-padding\n" for i in range(1, 3001)]
+        output = "".join(lines)
+        for max_chars in [1000, 5000, 20000, 50000]:
+            result = _truncate_read_file_output(output, max_chars)
+            head, marker = _head_and_marker(result)
+            cut_line = int(re.search(r"cut lands in line (\d+) of", marker).group(1))
+            assert 1 <= cut_line <= len(lines)
+            assert _line_containing(output, len(head)) + 1 == cut_line
+            assert f"start_line={cut_line}" in marker
+            assert len(result) <= max_chars
+
+    def test_marker_reports_total_lines(self):
+        lines = [f"line-{i}-with-padding\n" for i in range(1, 3001)]
+        output = "".join(lines)
+        result = _truncate_read_file_output(output, 50000)
+        _, marker = _head_and_marker(result)
+        assert f"of {output.count(chr(10))}" in marker
+
+    def test_ranged_read_reports_absolute_lines(self):
+        # Ranged reads hand a slice to the truncator; reported lines must be
+        # absolute file lines or the resume hint loops back onto the slice's
+        # own start (the repro from the #5478 review: start_line=831 kept
+        # suggesting start_line=831 forever).
+        slice_lines = [f"row-{i}-content\n" for i in range(1, 5171)]
+        slice_output = "".join(slice_lines)
+        offset = 830  # slice starts at absolute line 831
+        result = _truncate_read_file_output(slice_output, 50000, line_offset=offset)
+        head, marker = _head_and_marker(result)
+        absolute_cut = offset + head.count("\n") + 1
+        absolute_last = offset + len(slice_lines)
+        assert f"cut lands in line {absolute_cut} of {absolute_last}" in marker
+        assert f"start_line={absolute_cut}" in marker
+        assert absolute_cut > offset + 1  # resume strictly advances
+
+    def test_ranged_read_zero_offset_keeps_full_read_semantics(self):
+        output = "".join(f"line-{i}-with-padding\n" for i in range(3000))
+        assert _truncate_read_file_output(output, 50000) == _truncate_read_file_output(output, 50000, line_offset=0)
+
     def test_max_chars_zero_disables_truncation(self):
         output = "X" * 100000
         assert _truncate_read_file_output(output, 0) == output