Expected message output to have content, but content is…

Error message

Expected message output to have content, but content is empty

What it means

chat_anyhow extracts text from the Converse message's content blocks; if the returned message has zero content blocks, there is nothing to extract and the client bails with this error. Bedrock normally always returns at least one content block in a message, so this indicates a degenerate/unexpected response.

Solutions

Log/dump the raw response (StopReason, usage, output) to see why the model returned no content.
Re-run the request; if reproducible, adjust the prompt or max_tokens so the model can produce output.
Check guardrail/output filter configuration that might be suppressing all content blocks.

Defensive patterns

Strategy: try-catch

Try / catch

try:
    out = baml_client.MyFunction(...)
except Exception as e:
    if 'content is empty' in str(e):
        log.warning('Bedrock returned empty message content; retrying')
        out = retry_with_backoff(baml_client.MyFunction, ...)
    else:
        raise

Prevention

Set adequate max_tokens so the model can produce content
Test prompts against real Bedrock endpoints, not just mocks
Capture StopReason with each call to diagnose empty outputs

When it happens

Trigger: Bedrock Converse returns ConverseOutput::Message whose content array is empty — e.g. the model returned an empty response, output filtering removed all blocks, or a malformed response from a proxy/mocked endpoint.

Common situations: Using mock/proxy Bedrock endpoints in tests that return empty messages; guardrail or output filtering stripping all content; hitting a model that returns empty content on certain prompts.

Understand the failure class

Background: "empty response", "returned no data", "empty embeddings": what HTTP 200-with-empty-body errors mean across libraries — this error's family across 36 libraries.

Related errors

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/b77177b8297e44ba. Report an issue: GitHub.

Appendix: source

Thrown at engine/baml-runtime/src/internal/llm_client/primitive/aws/aws_client.rs:696

            bedrock_config = bedrock_config.endpoint_url(endpoint_url);
        }

        Ok(BedrockRuntimeClient::from_conf(bedrock_config.build()))
    }

    async fn chat_anyhow(&self, response: &ConverseOutput) -> Result<String> {
        let Some(bedrock::types::ConverseOutput::Message(ref message)) = response.output else {
            anyhow::bail!(
                "Expected message output in response, but is type {}",
                "unknown"
            );
        };
        // Try to extract text from all content blocks
        let mut extracted_text = String::new();
        let mut has_text = false;

        if message.content.is_empty() {
            anyhow::bail!("Expected message output to have content, but content is empty");
        }

        for content_block in &message.content {
            if let bedrock::types::ContentBlock::Text(text) = content_block {
                has_text = true;
                extracted_text.push_str(text);
            }
        }

        // If we found at least one text block, return the concatenated text
        if has_text {
            let content = extracted_text;
            return Ok(content);
        }

        // If we didn't find any text blocks, return an error with details about the content
        anyhow::bail!(
            "Expected message output to contain at least one text block, but found none. Content: {:?}",

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