BoundaryML/baml · error

failed to serialize pack envelope: {e}

Error message

failed to serialize pack envelope: {e}

What it means

After building the pack envelope (the packed program artifact), run_with_reporter serializes it via baml_artifact::encode with ArtifactKind::PackedProgram. If encoding fails, the command bails with this message carrying the underlying serialization error.

Source

Thrown at baml_language/crates/baml_cli/src/pack_command.rs:187

        let label = label_for(&targets);
        reporter.spin("Packaging", &label);

        let envelope = PackEnvelope {
            program,
            mode: mode.clone(),
            targets: targets
                .iter()
                .map(|t| baml_exec::TargetEntry {
                    qualified_name: t.qualified_name.clone(),
                    display_name: t.display_name.clone(),
                    subcommand_name: t.subcommand_name.clone(),
                })
                .collect(),
            output_format: self.output_format,
        };
        let serialized =
            baml_artifact::encode(baml_artifact::ArtifactKind::PackedProgram, &envelope)
                .map_err(|e| anyhow!("failed to serialize pack envelope: {e}"))?;

        let target_triple = self.resolved_target_triple()?;
        let host_bytes = read_host_binary(target_triple, reporter)?;
        let basename = self.resolve_output_basename()?;
        let output_path = self
            .output
            .clone()
            .unwrap_or_else(|| default_output_path(&basename, target_triple));

        let mut output_file = std::fs::File::create(&output_path)
            .with_context(|| format!("failed to create {}", output_path.display()))?;
        write_executable(&host_bytes, &serialized, &mut output_file, target_triple)?;

        #[cfg(unix)]
        {
            use std::os::unix::fs::PermissionsExt;
            std::fs::set_permissions(&output_path, std::fs::Permissions::from_mode(0o755))
                .with_context(|| {

View on GitHub (pinned to bd85ce9dee)

Solutions

1. Inspect the embedded `{e}` message for the exact serialization failure.
2. Retry with a different `--output-format` if you passed a non-default one.
3. Update the BAML toolchain (`baml toolchain update`) in case this is a known encoder bug.
4. If it persists, reduce the packed program to a minimal case and file a BAML issue with the error text.

Example fix

// before
baml pack myFn --output-format weird   // hypothetical bad format
// after
baml toolchain update && baml pack myFn

// treat as toolchain bug: capture stderr and retry after update
if (stderr.includes("failed to serialize pack envelope")) {
  await run("baml", ["toolchain", "update"]);
  return run("baml", ["pack", target]);
}

When it happens

Trigger: `baml pack` reaches the envelope-encoding step and baml_artifact::encode returns an error (envelope content not serializable into the packed-program artifact format).

Common situations: Usually an internal issue: an envelope field that violates the artifact schema, or a format/encoding bug after a BAML version change.

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

• failed to initialize engine for resolution: {e:?}
• expression mode (`-e` / `--expression`) is not packageable;
• positional `<TARGET>` is a function name, not a file path. F
• usage: baml self-update unexpected arguments: {}
• usage: baml toolchain install <canary|nightly|version>

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/451a31520dccc5fc. Report an issue: GitHub.