BoundaryML/baml · error · anyhow::Error

Error occurred while rendering prompt

Error message

Error occurred while rendering prompt: {minijinja_err}

What it means

When minijinja rendering fails inside render_prompt, BAML walks the error's source chain, appends each level as 'caused by: ...', and re-raises everything under this single anyhow error. It is a wrapper: the real diagnosis is in the chained causes (which include errors 810–818 above).

Solutions

1. Read the full message including every 'caused by:' segment — the root cause is at the end of the chain.
2. Fix the underlying template issue identified in the innermost cause (undefined variable, bad filter arg, etc.).
3. Test the template with a minimal render call to isolate which expression fails.
4. If the cause is unclear, render a stripped-down version of the template to bisect the failing expression.

Example fix

// Fix the root cause shown in the chain, e.g.:
// before: {{ user | format }}
// after:  {{ user | format(type='json') }}

match render_prompt(...) {
    Err(e) => {
        // print full chain: {e:#} includes every 'caused by' level
        eprintln!("prompt render failed: {e:#}");
        // branch on the innermost cause to apply the targeted fix
    }
    Ok(p) => p,
}

When it happens

Trigger: Any template rendering failure — missing variables, bad filters, role() misuse, unparseable media markers — surfaces as this wrapper error with the underlying minijinja error and its causes embedded in the message.

Common situations: Prompt templates referencing undefined variables; syntax errors in .baml templates; filter misuse (format/role errors above); debugging CI failures where only the top-level message is logged.

Related errors

• BamlImage has no callable attribute
• CannotUnpack
• Encountered impossible jinja expression during parsing
• Expected a statically defined string, not expression
• Failed to get template

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/30040511744327fd. Report an issue: GitHub.

Appendix: source

Thrown at engine/baml-lib/jinja-runtime/src/lib.rs:685

        default_role,
        allowed_roles,
        remap_role,
        enum_values_by_name,
    });

    match rendered {
        Ok(r) => Ok(r),
        Err(err) => {
            let mut minijinja_err = "".to_string();
            minijinja_err += &format!("{err:#}");

            let mut err = &err as &dyn std::error::Error;
            while let Some(next_err) = err.source() {
                minijinja_err += &format!("\n\ncaused by: {next_err:#}");
                err = next_err;
            }

            anyhow::bail!("Error occurred while rendering prompt: {minijinja_err}");
        }
    }
}

#[cfg(test)]
mod render_tests {

    use std::sync::Once;

    use baml_types::{BamlMap, BamlMediaType};
    use indexmap::IndexMap;

    use super::*;

    static INIT: Once = Once::new();

    pub fn setup_logging() {
        INIT.call_once(|| {

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