BoundaryML/baml · error

failed to serialize BAML bytecode: {e}

Error message

failed to serialize BAML bytecode: {e}

What it means

After successful compilation, `baml generate` encodes the program into BAML bytecode with `baml_artifact::encode(ArtifactKind::Program, ...)`. If serialization fails, the underlying error is wrapped in this message. Unlike compilation errors this indicates an internal encoding failure rather than a user-source problem.

Source

Thrown at baml_language/crates/baml_cli/src/generate.rs:352

                eprintln!("  [generator.my_client]");
                eprintln!("  output_type = \"python/pydantic\"");
                eprintln!("  output_dir = \"../python\"");
                eprintln!("  naming_convention = \"preserve-case\"");
            }
            return Ok(crate::ExitCode::Other);
        }

        let embedded_baml_toml = build_embedded_baml_toml(&from)?;

        // Build the codegen SymbolPool from the compiler database.
        let pool = baml_ide::build_symbol_pool(&db);

        reporter.spin("Compiling", format!("{} file(s)", source_files.len()));
        let program = db
            .get_bytecode(package)
            .map_err(|e| anyhow!("compilation failed: {e:?}"))?;
        let baml_bytecode = baml_artifact::encode(baml_artifact::ArtifactKind::Program, &program)
            .map_err(|e| anyhow!("failed to serialize BAML bytecode: {e}"))?;
        let source_root = from.join("baml_src");
        let user_baml_files = source_files
            .iter()
            .map(|source_file| {
                let path = source_file.path(&db);
                let relative_path = path
                    .strip_prefix(&source_root)
                    .or_else(|_| path.strip_prefix(&from))
                    .with_context(|| {
                        format!(
                            "BAML source {} is outside project root {}",
                            path.display(),
                            from.display()
                        )
                    })?;
                Ok((
                    relative_path.to_path_buf(),
                    source_file.text(&db).to_string(),

View on GitHub (pinned to bd85ce9dee)

Solutions

1. Retry after updating the BAML CLI to the latest version — encoding bugs are usually fixed upstream.
2. Reduce the failing .baml file to a minimal repro and file a bug with the BAML project including the encoded error message.
3. If building from source, ensure baml_compiler and baml_artifact crates are from the same commit (no mixed versions).
4. As a workaround, bisect your baml_src files to find the construct that breaks encoding.

match run_generate() {
  Err(e) if e.to_string().contains("failed to serialize BAML bytecode") => {
    eprintln!("encoder bug — update CLI or file a minimal repro: {e:#}");
  }
  Err(e) => eprintln!("{e:#}"),
  Ok(_) => {}
}

When it happens

Trigger: `baml_artifact::encode` returning Err for the compiled program — e.g. an artifact-encoding invariant violated by some construct in the program, or a bug in the artifact encoder version.

Common situations: Hitting a compiler/codegen bug with an unusual BAML construct; a version mismatch between the compiler database and the baml_artifact crate in a locally patched build.

Understand the failure class

Background: "JSON serialization failed", "not JSON serializable", "Failed to serialize": why JSON marshaling errors happen and how to fix them — this error's family across 46 libraries.

Related errors

• baml.json.serialize failed: {e:?}
• baml.json.serialize returned non-string value: {other:?}
• unknown method '{}' at {:?}, should have been caught during
• Internal error occurred while resolving repr of field {:?}
• cannot convert value {value} to VM value: class '{name}' not

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/c1a00972521c184b. Report an issue: GitHub.