BoundaryML/baml · error · VmPanic

baml.panics.AssertionFailed

baml.panics.AssertionFailed

Error message

assertion failed

What it means

Raised when a VM assertion instruction fails, i.e. a runtime check inserted into the BAML program evaluated to false. It carries no payload; the surrounding program is expected to provide context. It surfaces user-authored or compiler-inserted invariant checks.

Solutions

1. Inspect the failing condition and validate inputs before reaching the assertion.
2. Replace the bare assert with an explicit check that returns a descriptive error to the caller.
3. Catch `baml.panics.AssertionFailed` at the boundary only if you cannot fix the condition upstream.

Example fix

// before
assert(items.length > 0);
// after
if (items.length == 0) { return err("items must be non-empty"); }

// BAML: validate the precondition instead of asserting it
if (items.length == 0) { return err("items must be non-empty"); }

// BAML
try {
  run_checked(input);
} catch (e: baml.panics.AssertionFailed) {
  return err("assertion violated for input " + describe(input));
}

When it happens

Trigger: Executing an `assert` (or compiler-generated assertion) whose condition is false at runtime, e.g. `assert(x > 0)` where x is 0 or negative.

Common situations: Assertions validating preconditions on user input, invariants after transformations (e.g. sortedness, non-emptiness), regressions after refactoring where an assumption no longer holds.

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

• baml.panics.Cancelled
• baml.panics.Exit
• baml.panics.IndexOutOfBounds
• baml.panics.IntegerOverflow
• baml.panics.InvalidFieldAccess

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/a5ff0c6ef686af5e. Report an issue: GitHub.

Appendix: source

Thrown at baml_language/crates/bex_vm_types/src/errors.rs:51

    // Raised by array and byte-array subscripting, so the message stays generic
    // ("index", not "array index").
    #[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

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