BoundaryML/baml · error

only string keys are supported

Error message

only string keys are supported

What it means

Host map entries crossing the FFI boundary must have string-convertible keys. If the host sends a map entry keyed by int or bool, the decoder rejects it since BAML map keys in this surface are strings (enum keys are accepted via their string value).

Source

Thrown at engine/language_client_cffi/src/ctypes/baml_value_decode.rs:56

            c,
            fields
                .into_iter()
                .map(|(k, v)| from_ffi_value_to_baml_value(v).map(|v| (k, v)))
                .collect::<Result<_, _>>()?,
        )),
        crate::ffi::Value::Enum(e, value, _) => Ok(BamlValue::Enum(e, value)),
    }
}

pub(super) fn from_host_kv_to_baml_kv(
    item: crate::baml::cffi::HostMapEntry,
) -> Result<(String, BamlValue), anyhow::Error> {
    use crate::baml::cffi::host_map_entry::Key;
    let key = match item.key {
        Some(Key::StringKey(key)) => key,
        Some(Key::EnumKey(key)) => key.value,
        Some(Key::IntKey(_)) | Some(Key::BoolKey(_)) => {
            anyhow::bail!("only string keys are supported")
        }
        None => anyhow::bail!("Key is missing"),
    };

    let value = item
        .value
        .ok_or(anyhow::anyhow!("Value is null for key {}", key))?;

    Ok((key, BamlValue::decode(value)?))
}

View on GitHub (pinned to bd85ce9dee)

Solutions

1. Convert numeric/boolean keys to strings before passing the map to the BAML function
2. Ensure all map keys are strings (or BAML enums)
3. Adjust the BAML function signature to accept the keyed data as a class or list of pairs

Example fix

// before
my_func({1: "a", 2: "b"})
// after
my_func({"1": "a", "2": "b"})

function stringifyKeys(obj) {
  return Object.fromEntries(Object.entries(obj).map(([k, v]) => [String(k), v]));
}
myFunc(stringifyKeys({1: 'a', 2: 'b'}));

try {
  await bamlClient.MyFunction(mapArg);
} catch (e) {
  if (String(e).includes('only string keys are supported')) {
    console.error('Convert all map keys to strings before calling BAML functions');
  }
}

When it happens

Trigger: Calling a BAML function with a map/dictionary argument whose key is an int or bool (e.g. {1: 'x'} or {true: 'x'}) through the CFFI layer.

Common situations: Python dicts with int keys passed to BAML functions; serializing maps keyed by enums/ids without converting to strings first.

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

• unsupported raw pointer type
• baml.panics.HostContractViolation
• handle type mismatch
• Key must be a string
• Expected Collector, got {}

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/8b42174f72af52ee. Report an issue: GitHub.