What causes "error while calling {function_name}" in BoundaryML/baml?

Read the {:?} detail for the exact underlying cause.

BoundaryML/baml · error · RuntimeError

error while calling

Error message

error while calling {function_name}

What it means

Raised when the BAML runtime itself fails while executing a function invoked via call_function. The specific underlying failure (client config, prompt rendering, provider error, parsing) is attached via anyhow context and printed with {:?} after the message.

Solutions

Read the {:?} detail for the exact underlying cause.
Verify the function name exists in your .baml files (or reload the runtime after edits).
Check required API keys are present in the env_vars passed to the runtime/call.
Test the function in the BAML playground / CLI to reproduce outside Ruby.
Wrap the call in begin/rescue and inspect the message to distinguish config vs provider errors.

Example fix

// before
res = Baml.call_function('ExtractResume', args, ctx, {}) # missing API key env
// after
res = Baml.call_function('ExtractResume', args, ctx, ENV.to_h.merge('OPENAI_API_KEY' => api_key))

Defensive patterns

Strategy: try-catch

Validate before calling

raise ArgumentError, "function '#{fn}' not found" unless File.read(baml_files.join).scan(/function\s+#{fn}\b/).any?

Try / catch

begin
  res = Baml.call_function(fn, args, ctx, ENV.to_h)
rescue RuntimeError => e
  logger.error("BAML call_function failed for #{fn}: #{e.message}")
  raise
end

Prevention

Confirm function names against your .baml sources before calling.
Pass required provider API keys via env_vars (ENV.to_h or explicit merge).
Validate client/model config with the BAML CLI or playground.
Add retry-with-backoff for transient provider errors at a higher level.

When it happens

Trigger: baml_runtime.call_function(...) returns Err for any reason: unknown function name, misconfigured LLM client, missing API key in env_vars, provider HTTP failure, or output-parsing failure.

Common situations: Calling a function name that doesn't exist in the .baml files; missing OPENAI_API_KEY/ANTHROPIC_API_KEY; wrong model name in client config; network failures or provider rate limits.

Understand the failure class

Background: "API request failed": what wrapped HTTP errors from external APIs mean and how to find the real cause — this error's family across 29 libraries.

Related errors

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/6f565acd0db4fafd. Report an issue: GitHub.

Appendix: source

Thrown at engine/language_client_ruby/ext/ruby_ffi/src/lib.rs:148

        let mut collectors = Vec::new();
        for i in collector.into_iter() {
            collectors.push(<&Collector>::try_convert(i)?.inner.clone());
        }

        let retval = match rb_self.t.block_on(rb_self.inner.call_function(
            function_name.clone(),
            &args,
            &ctx.inner,
            type_registry.map(|t| &t.inner),
            client_registry.map(|c| c.inner.borrow_mut()).as_deref(),
            Some(collectors),
            env_vars,
            None,
            TripWire::new(None),
        )) {
            (Ok(res), _) => Ok(FunctionResult::new(res)),
            (Err(e), _) => Err(Error::new(
                ruby.exception_runtime_error(),
                format!(
                    "{:?}",
                    e.context(format!("error while calling {function_name}"))
                ),
            )),
        };

        retval
    }

    fn stream_function(
        ruby: &Ruby,
        rb_self: &BamlRuntimeFfi,
        function_name: String,
        args: RHash,
        ctx: &RuntimeContextManager,
        type_registry: Option<&types::type_builder::TypeBuilder>,

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