BoundaryML/baml · error

File provided for PDF input is not a PDF. Detected mime type

Error message

File provided for PDF input is not a PDF. Detected mime type: '{}'. Only application/pdf is allowed.

What it means

For media parts of type `Pdf`, BAML sniffs the file's MIME type and enforces that it is exactly `application/pdf`. If the detected type differs (or is undetectable), this error explains that only real PDFs are accepted for PDF-typed inputs.

Source

Thrown at engine/baml-runtime/src/internal/llm_client/traits/mod.rs:618

            let mut mime_type = part.mime_type.clone();

            if mime_type.is_none() {
                if let Some(ext) = media_file.extension() {
                    mime_type = Some(format!("{}/{}", part.media_type, ext));
                }
            }

            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 '{}')",

View on GitHub (pinned to bd85ce9dee)

Solutions

1. Verify the file opens in a PDF viewer and starts with the `%PDF-` header; re-export a genuine PDF if not.
2. Check the URL actually returns the PDF (correct status, content-type application/pdf) rather than an HTML error page.
3. Remove the fake extension or change the declared `BamlMediaType` to match the real file type (e.g. Image).
4. Re-download the file if it was truncated/corrupted mid-transfer.

Example fix

// before: renamed Word doc
image_url: baml://report.docx  // typed as Pdf

// after: real PDF export
// $ pandoc report.docx -o report.pdf
image_url: baml://report.pdf

// Sniff the PDF header before passing it as a Pdf media part
const buf = Buffer.from(await fetch(url).then(r => r.arrayBuffer()));
if (!buf.subarray(0, 5).toString('latin1').startsWith('%PDF-')) {
  throw new Error('File is not a real PDF (missing %PDF- header)');
}

function isRealPdf(buf) { return buf && buf.length > 4 && buf.subarray(0, 5).toString('latin1') === '%PDF-'; }

try { await b.F({ pdf: mediaPart }); } catch (e) { if (String(e).includes('not a PDF')) console.error('Re-export the file as a real PDF'); }

When it happens

Trigger: Passing a file (via `baml://` path or media_url) declared as `Pdf` whose bytes sniff as a different type — e.g. a .docx, image, or HTML page renamed to .pdf; also thrown when the MIME type could not be detected at all.

Common situations: Renaming a Word doc or screenshot to `.pdf`, serving a PDF behind a URL that returns an HTML error page (404 page saved as pdf), or corrupted/truncated PDF downloads.

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

• Could not determine mime type for PDF input. Only applicatio
• baml.errors.InvalidArgument
• assertion failed
• baml.media.image.from_url expects 1 argument, got {} at {:?}
• baml.media.image.from_url expects a string argument at {:?}

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/999198674166761a. Report an issue: GitHub.