BoundaryML/baml · error · RuntimeError

{e:?}

Error message

{e:?}

What it means

Ruby FFI generic serialization error: converting the HTTP response's headers into a Ruby hash via serde_magnus failed, with the conversion error carried verbatim in {e:?. It mirrors the request-side headers accessor and fires when the header map contains something serde_magnus cannot map into a Ruby value — effectively a wrapper/runtime version mismatch.

Source

Thrown at engine/language_client_ruby/ext/ruby_ffi/src/types/response.rs:30

impl HTTPResponse {
    pub fn to_s(&self) -> String {
        format!(
            "HTTPResponse(status={}, headers={}, body={})",
            self.inner.status,
            serde_json::to_string_pretty(&self.inner.headers()).unwrap_or_default(),
            serde_json::to_string_pretty(&self.inner.body.as_serde_value()).unwrap_or_default()
        )
    }

    pub fn status(&self) -> u16 {
        self.inner.status
    }

    pub fn headers(ruby: &Ruby, rb_self: &Self) -> Result<magnus::Value> {
        // Convert headers to Ruby hash
        serde_magnus::serialize(&rb_self.inner.headers())
            .map_err(|e| Error::new(ruby.exception_runtime_error(), format!("{e:?}")))
    }

    pub fn body(&self) -> HTTPBody {
        // TODO: Avoid clone.
        HTTPBody::from(self.inner.body.clone())
    }

    pub fn define_in_ruby(module: &RModule) -> Result<()> {
        let cls = module.define_class("HTTPResponse", class::object())?;

        cls.define_method("to_s", method!(HTTPResponse::to_s, 0))?;
        cls.define_method("status", method!(HTTPResponse::status, 0))?;
        cls.define_method("headers", method!(HTTPResponse::headers, 0))?;
        cls.define_method("body", method!(HTTPResponse::body, 0))?;

        Ok(())
    }
}

View on GitHub (pinned to bd85ce9dee)

Solutions

1. Reinstall/rebuild the baml gem so the extension matches the runtime.
2. Update to the latest baml version.
3. Capture the debug serde error from the message to identify the failing header.

Example fix

// before
h = response.headers # RuntimeError: {e:?}
// after
begin
  h = response.headers
rescue RuntimeError => e
  h = {}
  logger.warn("response headers serialization failed: #{e.message}")
end

begin
  headers = response.headers
rescue RuntimeError => e
  headers = {}
end

When it happens

Trigger: Calling headers on an FFI HTTPResponse when the internal header map cannot be serialized to Ruby.

Common situations: Gem/native-extension version mismatch, or header values the serde-to-Ruby bridge cannot represent; often after partial upgrades of the baml gem.

Understand the failure class

Background: json.Marshal / "failed to marshal" errors in Go: why "unsupported type" happens and how to fix it — this error's family across 22 libraries.

Related errors

• {e:?}
• failed coercing BAML value to Ruby value: {e:?}
• failed to convert Ruby object to JSON, errors were: {} Ruby
• encoding method arguments: %w
• unsupported map key type: %s

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/49bf88f27ec2a955. Report an issue: GitHub.