BoundaryML/baml · error

Could not determine mime type for PDF input. Only applicatio

Error message

Could not determine mime type for PDF input. Only application/pdf is allowed.

What it means

BAML only accepts PDFs whose MIME type is application/pdf for PDF media inputs. When a base64 data URL or file is supplied for a pdf input but the MIME type cannot be inferred at all, process_media bails with this error rather than sending an untyped payload to the LLM provider.

Source

Thrown at engine/baml-runtime/src/internal/llm_client/traits/mod.rs:624

            }

            if mime_type.is_none() {
                if let Some(t) = infer::get(&bytes) {
                    mime_type = Some(t.mime_type().to_string());
                }
            }

            // ENFORCEMENT: For PDF, the mime type must be application/pdf
            if part.media_type == BamlMediaType::Pdf {
                match &mime_type {
                    Some(mt) if mt != "application/pdf" => {
                        anyhow::bail!(
                            "File provided for PDF input is not a PDF. Detected mime type: '{}'. Only application/pdf is allowed.",
                            mt
                        );
                    }
                    None => {
                        anyhow::bail!(
                            "Could not determine mime type for PDF input. Only application/pdf is allowed."
                        );
                    }
                    _ => {}
                }
            }

            Ok(BamlMedia::base64(
                part.media_type,
                if render_settings.as_shell_commands {
                    format!(
                        "$(base64 '{}')",
                        media_path
                            .strip_prefix("file://")
                            .unwrap_or(media_path.as_str())
                    )
                } else {
                    BASE64_STANDARD.encode(&bytes)

View on GitHub (pinned to bd85ce9dee)

Solutions

1. Prefix the media value with a data URL: data:application/pdf;base64,<payload>
2. Verify the file is actually a valid PDF (starts with %PDF-) and not corrupted
3. If using a URL instead of base64, ensure the server serves Content-Type application/pdf so inference succeeds
4. Check you declared the parameter type as pdf in the BAML schema and are not reusing an image value

Example fix

// before
client.messages(media_content=b64_pdf_string)
// after
client.messages(media_content="data:application/pdf;base64," + b64_pdf_string)

function isPdfInput(media) {
  const isPdfDataUrl = /^data:application\/pdf;base64,/.test(media);
  const decoded = Buffer.from(media.replace(/^data:\w+\/\w+;base64,/, ''), 'base64');
  const hasPdfMagic = decoded.subarray(0, 5).toString() === '%PDF-';
  return isPdfDataUrl || hasPdfMagic;
}
if (!isPdfInput(mediaValue)) throw new Error('Input must be a valid application/pdf data URL or PDF bytes');

const isPdfDataUrl = (s) => typeof s === 'string' && /^data:application\/pdf;base64,[A-Za-z0-9+/=]+$/.test(s);

When it happens

Trigger: A media value for a pdf-typed BAML parameter lacks a data: URL prefix with a mime type and no decodable magic bytes, so infer returns None (as_base64 yields no mime type).

Common situations: Passing a raw base64 string without a data:application/pdf;base64, prefix; uploading a scanned/corrupted PDF whose magic bytes were stripped; renaming a non-PDF file to .pdf and sending its base64.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

• File provided for PDF input is not a PDF. Detected mime type
• Requested media of MIME type '{}' but fetched '{}' from URL
• baml.media.image.from_url expects 1 argument, got {} at {:?}
• baml.media.image.from_url expects a string argument at {:?}
• Could not unify Media with {:?}

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/fb36789093c1cc7a. Report an issue: GitHub.