BoundaryML/baml · error

Failed to coerce value

Error message

Failed to coerce value: {:?}

What it means

In jsonish::from_str, after coercing the parsed value to the target type, the code checks the coercion flags: if the only evidence was an 'InferedObject' over a plain String (the parser guessed an object from a bare string), it refuses the result with 'Failed to coerce value'. This prevents accepting a string that was silently reinterpreted as the requested object type.

Solutions

1. Make the prompt demand the full object shape with required fields
2. Check the target type — if a scalar is acceptable, use a string/class field type instead
3. Inspect the printed flags in the error to confirm the inferred-object coercion
4. Retry the generation or fall back to manual parsing for this response

Example fix

// schema
// before: class Output { value string }
// model returned: "just a string"
// after: prompt: 'Respond with JSON: {"value": "..."}'

fn is_scalar_only(s: &str) -> bool { let t = s.trim().trim_matches(|c| c == '`'); !(t.starts_with('{') || t.starts_with('[')) }

match jsonish::from_str(&object_type, out) {
    Err(e) if e.to_string().contains("Failed to coerce value") => {
        // schema expects an object but model returned a scalar
        retry_with_schema_example()
    }
    other => other,
}

When it happens

Trigger: from_str(&target_type, s) where the raw text is just a quoted/unquoted string but the target type is a class/object — the coercion produced an inferred object from a string flag.

Common situations: LLM returned a single value (e.g. a name or URL) where the schema expects an object; type changed in the BAML schema but the model output didn't.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

• Arrow types are not supported in CFFI
• baml.json.deserialize failed
• ` ` is declared by the stdlib
• baml.json.serialize failed
• BamlValidationError

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/a7dbfd761ac28027. Report an issue: GitHub.

Appendix: source

Thrown at engine/baml-lib/jsonish/src/lib.rs:267

        of,
        if raw_string_is_done {
            baml_types::StreamingMode::NonStreaming
        } else {
            baml_types::StreamingMode::Streaming
        },
    );

    // Determine the best way to get the desired schema from the parsed schema.

    // Lets try to now coerce the value into the expected schema.
    let parsed_value: BamlValueWithFlags = match target.coerce(&ctx, target, Some(&value)) {
        Ok(v) => {
            if v.conditions()
                .flags()
                .iter()
                .any(|f| matches!(f, Flag::InferedObject(jsonish::Value::String(_, _))))
            {
                anyhow::bail!("Failed to coerce value: {:?}", v.conditions().flags());
            }

            Ok::<BamlValueWithFlags, anyhow::Error>(v)
        }
        Err(e) => anyhow::bail!("Failed to coerce value: {:?}", e),
    }?;

    log::debug!("Parsed JSONish (step 2 of parsing): {parsed_value:#?}");

    // let value: BamlValue = parsed_value.clone().into();
    // println!("from_str value: {value}");
    // println!("-------------------------------------------------");
    // parsed_value.clear_flags();

    Ok(parsed_value)
}

impl ResponseBamlValue {

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