Aider-AI/aider · error · DiffError

Invalid Add File line (missing '+'): {line}

Error message

Invalid Add File line (missing '+'): {line}

What it means

While parsing the body of an '*** Add File:' block, a non-blank line did not start with '+'. Add-file bodies require every content line to be prefixed with '+' (blank lines are tolerated as blank content). Any other leading character is rejected.

Source

Thrown at aider/coders/patch_coder.py:540

            # Stop if we hit another action or end marker
            if norm_line.startswith(
                (
                    "*** End Patch",
                    "*** Update File:",
                    "*** Delete File:",
                    "*** Add File:",
                )
            ):
                break

            # Expect lines to start with '+'
            if not line.startswith("+"):
                # Tolerate blank lines? Or require '+'? Reference implies '+' required.
                if norm_line.strip() == "":
                    # Treat blank line as adding a blank line
                    added_lines.append("")
                else:
                    raise DiffError(f"Invalid Add File line (missing '+'): {line}")
            else:
                added_lines.append(line[1:])  # Strip leading '+'

            index += 1

        action = PatchAction(type=ActionType.ADD, path="", new_content="\n".join(added_lines))
        return action, index

    def apply_edits(self, edits: List[PatchAction]):
        """
        Applies the parsed PatchActions to the corresponding files.
        """
        if not edits:
            return

        # Group edits by original path? Not strictly needed if processed sequentially.

        # Edits are now List[Tuple[str, PatchAction]]

View on GitHub (pinned to 5dc9490bb3)

Solutions

1. Prefix every content line in the Add File block with '+'
2. Regenerate — aider's retry loop typically gets the model to fix prefix mistakes
3. Validate add-block lines with a linter/regex ('^($|\+|\*\*\*)') before applying

Example fix

// before
*** Add File: a.txt
hello

// after
*** Add File: a.txt
+hello

def add_body_prefixed(patch_text: str) -> bool:
    in_add = False
    for line in patch_text.splitlines():
        if line.startswith("*** Add File: "):
            in_add = True
        elif line.startswith("*** "):
            in_add = False
        elif in_add and line and not line.startswith("+"):
            return False
    return True

try:
    edits = coder.get_edits(reply)
except DiffError as e:
    if "Invalid Add File line (missing '+')" in str(e):
        reply = auto_prefix_add_body(reply)
        edits = coder.get_edits(reply)
    else:
        raise

When it happens

Trigger: Model emits raw file content without '+' prefixes inside an Add File block; a '-' or ' ' diff-style line leaks in; a nested '***' header terminates the block and the stray line follows.

Common situations: Model mixes unified-diff habits into the patch format; long generated files where the model forgets prefixes partway; hand-written patches pasted from editors.

Related errors

• Delete File action missing path.
• Add File action missing path.
• Duplicate action for file: {path}
• Unknown or misplaced line while parsing patch: {line}
• Bad/missing filename. The filename must be alone on the line

AI-assisted analysis of Aider-AI/aider@5dc9490bb3 (2026-08-15). Data as JSON: /api/errors/d1369e0414ca4f0e. Report an issue: GitHub.