Aider-AI/aider · error · DiffError

Invalid line prefix in update section: {line}

Error message

Invalid line prefix in update section: {line}

What it means

Raised by PatchCoder's update-section parser when a body line has none of the accepted prefixes: '+' (add), '-' (delete), ' ' (keep/context), or completely blank. The parser is strict — every line in an update section must carry a diff prefix.

Source

Thrown at aider/coders/patch_coder.py:148

        last_mode = mode

        # Determine line type and strip prefix
        if line.startswith("+"):
            mode = "add"
            line_content = line[1:]
        elif line.startswith("-"):
            mode = "delete"
            line_content = line[1:]
        elif line.startswith(" "):
            mode = "keep"
            line_content = line[1:]
        elif line.strip() == "":  # Treat blank lines in patch as context ' '
            mode = "keep"
            line_content = ""  # Keep it as a blank line
        else:
            # Assume lines without prefix are context if format is loose,
            # but strict format requires ' '. Raise error for strictness.
            raise DiffError(f"Invalid line prefix in update section: {line}")

        # If mode changes from add/delete back to keep, finalize the previous chunk
        if mode == "keep" and last_mode != "keep":
            if del_lines or ins_lines:
                chunks.append(
                    Chunk(
                        # orig_index is relative to the start of the *context* block found
                        orig_index=len(context_lines) - len(del_lines),
                        del_lines=del_lines,
                        ins_lines=ins_lines,
                    )
                )
            del_lines, ins_lines = [], []

        # Collect lines based on mode
        if mode == "delete":
            del_lines.append(line_content)
            context_lines.append(line_content)  # Deleted lines are part of the original context

View on GitHub (pinned to 5dc9490bb3)

Solutions

1. Prefix every line in the update section: '+' for added, '-' for removed, ' ' for unchanged context.
2. Blank context lines inside a hunk should be a single space or truly empty only at section boundaries — re-check the failing line named in the error.
3. Return the error to the LLM (it arrives as ValueError 'Error parsing patch content: Invalid line prefix...') so it re-emits a well-formed patch.
4. Prefer the fence-free patch template in prompts/system reminders so the model doesn't wrap hunks in markdown fences.

Example fix

# before
*** Update File: foo.py
@@
 def f():
    return 1   (no prefix — raises)
*** End Patch

# after
*** Update File: foo.py
@@
 def f():
-    return 1
+    return 2
*** End Patch

def unprefixed_body_lines(patch_text: str) -> list[int]:
    bad, in_update = [], False
    for n, l in enumerate(patch_text.splitlines(), 1):
        s = l.strip()
        if s.startswith("*** Update File:"):
            in_update = True
        elif s == "*** End Patch" or s.startswith(("*** Add File:", "*** Delete File:")):
            in_update = False
        elif in_update and l and not l[0] in "+- " and not s.startswith(("@@", "***")):
            bad.append(n)
    return bad  # non-empty line numbers → prefix them before parsing

try:
    coder.get_edits(patch_text)
except ValueError as e:
    if "Invalid line prefix in update section" in str(e):
        patch_text = reemit_with_prefixes(str(e))  # model retry with +/-/space markers
        coder.get_edits(patch_text)
    else:
        raise

When it happens

Trigger: A patch in apply_patch grammar where the model wrote raw unprefixed code lines inside the update body — e.g. pasted source without the leading +/-/space markers, or a markdown fence line slipping into the patch.

Common situations: Models blending 'patch' edit format with plain code output; users hand-authoring patches and forgetting that context lines need a leading space (including for blank context lines inside hunks, which must be ' ' not '').

Related errors

• Invalid patch line found in update section: {line}
• Empty patch section found.
• Error parsing patch content: {e}
• Unexpected error parsing patch: {e}
• Update File action missing path.

AI-assisted analysis of Aider-AI/aider@5dc9490bb3 (2026-08-15). Data as JSON: /api/errors/5b5a8e8060e17d45. Report an issue: GitHub.