BoundaryML/baml · error

Typecheck errors

Error message

Typecheck errors: {errors}

What it means

When constructing the async interpreter runtime, the BAML program is typechecked (HIR -> THIR). Any typecheck diagnostics marked as errors abort construction with this aggregate message containing the pretty-printed diagnostics.

Solutions

1. Read the embedded diagnostics in the message and fix each type error in the .baml source.
2. Run the BAML CLI typecheck on your project to see all errors before runtime.
3. Check for type mismatches introduced by recent edits or generated code.

Example fix

// before (baml)
function F(x: int) -> string { ... }
F("hello")
// after (baml)
function F(x: int) -> string { ... }
F(42)

match AsyncInterpreterRuntime::try_from(llm_runtime) {
    Ok(rt) => rt,
    Err(e) => {
        eprintln!("BAML typecheck failed:\n{e:#}");
        std::process::exit(1);
    }
}

When it happens

Trigger: Calling AsyncInterpreterRuntime::try_from (e.g. via the BAML interactive/interpreter entry point) with a .baml source program that fails static typechecking.

Common situations: Passing wrong-typed arguments to functions in BAML code, incompatible return types, or referencing undefined types when programmatically loading BAML sources (playground, tests, interpreter).

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

• array assignment index out of bounds
• array assignment on non-list value at
• array assignment requires a non-negative integer index at
• assign to undeclared variable
• baml.fetch_as expects 1 type argument, got

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/1526627e97010a7a. Report an issue: GitHub.

Appendix: source

Thrown at engine/baml-runtime/src/async_interpreter_runtime.rs:79

}

impl TryFrom<LlmRuntime> for BamlAsyncInterpreterRuntime {
    type Error = anyhow::Error;

    fn try_from(llm_runtime: LlmRuntime) -> Result<Self, Self::Error> {
        #[cfg(not(target_arch = "wasm32"))]
        let async_runtime = Arc::clone(&llm_runtime.async_runtime);

        // Stage 1: AST -> HIR
        let hir_program = hir::Hir::from_ast(&llm_runtime.db.ast);

        // Stage 2: HIR -> THIR (typecheck)
        let mut diagnostics = Diagnostics::new("dummy".into());
        let thir_program = typecheck(&hir_program, &mut diagnostics);

        if diagnostics.has_errors() {
            let errors = diagnostics.to_pretty_string();
            return Err(anyhow::anyhow!("Typecheck errors: {errors}"));
        }

        Ok(Self {
            llm_runtime: Arc::new(llm_runtime),
            thir_program,

            #[cfg(not(target_arch = "wasm32"))]
            async_runtime,
        })
    }
}

impl BamlAsyncInterpreterRuntime {
    pub fn internal(&self) -> &LlmRuntime {
        &self.llm_runtime
    }

    pub fn disassemble(&self, function_name: &str) {

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