zed-industries/zed · error

missing <|fim_middle|> in model output

Error message

missing <|fim_middle|> in model output

What it means

apply_variable_edit splits the model output on the <|fim_middle|> token; its absence means the output is not in the expected prefix/middle/suffix FIM shape, so there is no boundary between context lines and new text.

Source

Thrown at crates/zeta_prompt/src/zeta_prompt.rs:4392

    ///
    /// The model output has the form:
    ///
    /// - prefix context lines
    /// - `<|fim_middle|>`
    /// - new text
    /// - `<|fim_suffix|>`
    /// - suffix context lines
    ///
    /// We locate the prefix/suffix context lines in the original text and replace
    /// everything between them with the new text.
    pub fn apply_variable_edit(
        context: &str,
        model_output: &str,
    ) -> Result<(Range<usize>, String)> {
        let (prefix_context, rest) = model_output
            .split_once("<|fim_middle|>\n")
            .or_else(|| model_output.split_once("<|fim_middle|>"))
            .ok_or_else(|| anyhow::anyhow!("missing <|fim_middle|> in model output"))?;

        let (new_text, suffix_context) = rest
            .split_once("<|fim_suffix|>\n")
            .or_else(|| rest.split_once("<|fim_suffix|>"))
            .unwrap_or((rest, ""));

        let suffix_context = if prefix_context.is_empty() && !suffix_context.is_empty() {
            suffix_context.strip_prefix('\n').unwrap_or(suffix_context)
        } else {
            suffix_context
        };

        let prefix_offset = find_substring_at_line_boundary(context, prefix_context)
            .ok_or_else(|| anyhow!("could not locate prefix lines"))?
            + prefix_context.len();
        let suffix_offset = if suffix_context.is_empty() {
            context.len()
        } else {

View on GitHub (pinned to f4178619ac)

Solutions

1. Reject the output and re-request the edit in proper FIM variable-edit format
2. Check for token-stripping post-processing that removed the fim_middle marker before parsing

When it happens

Trigger: Thrown at crates/zeta_prompt/src/zeta_prompt.rs:4392 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of zed-industries/zed@f4178619ac (2026-08-20). Data as JSON: /api/errors/e561a2ffb6bfde53. Report an issue: GitHub.