old content does not have a line number prefix for first lin

Error message

old content does not have a line number prefix for first line

What it means

During XML response handling in buildValidate, the model-supplied 'old' content block must begin with a 'pdx-' line-number prefix so it can be matched against the original file. This error is thrown when the first line of the old-content region lacks that prefix, making the edit target impossible to locate. It aborts validation with valid=false.

Source

Thrown at app/server/model/plan/build_validate_and_fix.go:391

		log.Printf("Processing replacement: %d/%d", i+1, len(replacements))

		old := utils.GetXMLContent(replacement, "Old")
		new := utils.GetXMLContent(replacement, "New")

		if old == "" {
			log.Printf("No old content found for replacement")
			return buildValidateResult{valid: false, updated: updated}, fmt.Errorf("no old content found for replacement")
		}

		old = strings.TrimSpace(old)

		// log.Printf("Old content trimmed:\n\n%s", strconv.Quote(old))

		// log.Printf("New content:\n\n%s", strconv.Quote(new))

		if !strings.HasPrefix(old, "pdx-") {
			log.Printf("Old content does not have a line number prefix for first line")
			return buildValidateResult{valid: false, updated: updated}, fmt.Errorf("old content does not have a line number prefix for first line")
		}

		oldLines := strings.Split(old, "\n")

		var lastLine string
		var lastLineNum int
		firstLine := oldLines[0]
		if len(oldLines) > 1 {
			lastLine = oldLines[len(oldLines)-1]
		}

		firstLineNum, err := shared.ExtractLineNumberWithPrefix(firstLine, "pdx-")
		if err != nil {
			log.Printf("Error extracting line number from first line: %v", err)
			return buildValidateResult{valid: false, updated: updated}, fmt.Errorf("error extracting line number from first line: %v", err)
		}

		if lastLine != "" {

View on GitHub (pinned to e2d772072e)

Solutions

Retry the build validation so the model regenerates a correctly prefixed old block
Check prompt/ instructions emphasize that old content must start with 'pdx-N' on the first line
Inspect logged 'Old content' output to see exactly what the model produced and adjust parsing or prompting accordingly

Example fix

// before
old = "func main() {}"
// after
old = "pdx-10 func main() {}"

Defensive patterns

Strategy: validation

Validate before calling

if !strings.HasPrefix(old, "pdx-") {
    return fmt.Errorf("old content must start with pdx- line number prefix")
}

Type guard

func hasLineNumberPrefix(s string) bool {
    idx := strings.Index(s, "pdx-")
    if idx != 0 { return false }
    rest := strings.TrimPrefix(s, "pdx-")
    n, err := strconv.Atoi(strings.Fields(rest)[0])
    return err == nil && n > 0
}

Prevention

Always include line-number prefixes on old-content first lines
Pre-validate model output format before applying edits
Retry generation when prefix validation fails

When it happens

Trigger: The LLM returns a replace/old block whose first line does not start with 'pdx-<num>' (e.g. it omits line-number prefixes or emits raw file content).

Common situations: Models drifting from the prompt format, copying content without the prefixed first line, or emitting an empty/blank old block after trimming.

Related errors

AI-assisted analysis of plandex-ai/plandex@e2d772072e (2026-09-05). Data as JSON: /api/errors/484f46a49689c8e1. Report an issue: GitHub.