BoundaryML/baml · critical · VmPanic

baml.panics.Unreachable

baml.panics.Unreachable

Error message

unreachable code executed

What it means

Raised when the VM executes an `unreachable` instruction, meaning a code path the compiler proved (or assumed) impossible was actually reached. This signals a compiler/assumption bug or corrupted/patched bytecode rather than normal user error.

Solutions

  1. Report it as a likely compiler bug with the program that triggered it.
  2. Recompile the program with the current compiler to regenerate bytecode.
  3. Catch `baml.panics.Unreachable` at a top-level boundary to avoid a hard crash while investigating.
Defensive patterns

Strategy: try-catch

Try / catch

// BAML: top-level guard while investigating
try {
  run(program);
} catch (e: baml.panics.Unreachable) {
  report_compiler_bug(e);
}

Prevention

When it happens

Trigger: Executing compiled `unreachable` markers, e.g. after an exhaustive match the compiler considered total but that received an unexpected value, or running bytecode from an incompatible compiler version.

Common situations: Compiler bugs with exhaustiveness checking, hand-edited or stale bytecode, running an old compiled artifact against a newer VM (or vice versa).

Understand the failure class

Background: "This is a bug, please report it": internal invariant violations, unreachable panics, and SNH errors explained — this error's family across 47 libraries.

Related errors

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/464b5cd52a3fd660. Report an issue: GitHub.

Appendix: source

Thrown at baml_language/crates/bex_vm_types/src/errors.rs:54

    #[error("index out of bounds: {index} of {length}")]
    IndexOutOfBounds { index: i64, length: usize },

    #[error("invalid field access: field {field_index} of {field_count}")]
    InvalidFieldAccess {
        field_index: usize,
        field_count: usize,
    },

    #[error("key not found in map")]
    MapKeyNotFound,

    #[error("stack overflow")]
    StackOverflow,

    #[error("assertion failed")]
    AssertionFailed,

    #[error("unreachable code executed")]
    Unreachable,

    #[error("operation cancelled")]
    Cancelled,

    /// A user-caused panic from `baml.sys.panic`, and the stdlib's panic of
    /// record for a user-violated native invariant (e.g. a reflection kind
    /// view's `_ty` field overwritten with a type of a different kind).
    #[error("baml.sys.panic: {message}")]
    UserPanic { message: String },

    /// A clean process-termination request from `baml.sys.exit(code)`.
    ///
    /// Catchable in user code as `baml.panics.Exit` — patterned after
    /// Python's `SystemExit`: code can intercept it for cleanup or
    /// testing, and if nothing catches it the engine surfaces the code
    /// as `EngineError::Exit` and the host terminates with it.
    ///

View on GitHub (pinned to bd85ce9dee)