hashicorp/terraform · error

Failed to generate diff for

Error message

Failed to generate diff for %s: %s

What it means

Thrown by processFile when the -diff flag is set and the internal bytesDiff helper returns an error while computing a unified diff between the original and formatted source. The underlying error is appended with %s.

Solutions

1. Retry without -diff to confirm formatting itself works: `terraform fmt -write <path>`.
2. Run `terraform fmt -check` to identify changed files without producing a diff.
3. Report the issue with the offending file contents if it reproduces, as bytesDiff failures are likely internal bugs.

// Run -diff in a fallback that drops to -list on diff failure.
if d := runFmt("-diff", path); d.HasErrors() {
    _ = runFmt("-list", path) // best-effort fallback
}

When it happens

Trigger: Running `terraform fmt -diff` where bytesDiff fails — typically an internal failure generating the diff textual output. Rare in practice since bytesDiff operates on in-memory byte slices.

Common situations: Edge cases in the diff algorithm with very large files or unusual content; generally indicates an internal bug rather than a user config problem.

Related errors

• Cannot read directory
• cannot serialize updated credentials file
• error converting output values to json
• error reading output values
• Failed to read

AI-assisted analysis of hashicorp/terraform@d32a084675 (2026-08-11). Data as JSON: /api/errors/4805e1a7dfdf3bfe. Report an issue: GitHub.

Appendix: source

Thrown at internal/command/fmt.go:213

	result := c.formatSourceCode(src, path)

	if !bytes.Equal(src, result) {
		// Something was changed
		if c.list {
			fmt.Fprintln(w, path)
		}
		if c.write {
			err := os.WriteFile(path, result, 0644)
			if err != nil {
				diags = diags.Append(fmt.Errorf("Failed to write %s", path))
				return diags
			}
		}
		if c.diff {
			diff, err := bytesDiff(src, result, path)
			if err != nil {
				diags = diags.Append(fmt.Errorf("Failed to generate diff for %s: %s", path, err))
				return diags
			}
			w.Write(diff)
		}
	}

	if !c.list && !c.write && !c.diff {
		_, err = w.Write(result)
		if err != nil {
			diags = diags.Append(fmt.Errorf("Failed to write result"))
		}
	}

	return diags
}

func (c *FmtCommand) processDir(path string, stdout io.Writer) tfdiags.Diagnostics {
	var diags tfdiags.Diagnostics

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