Implement AbortController in python, typescript, go, and also wasm. Also implement cancel functionality for run test in wasm and add docs for py/ts/go.

First 80% of this work done by @dexhorthy and @hellovai in #2331 :

Built in close collaboration with @hellovai who sat in my apartment and shouted at claude with me all day

Note: plan/Research artifacts in thoughts/ can be removed once they get put in a good location, leaving them in for now

What problem(s) was I solving?

Previously, once a BAML function call was initiated, there was no way to cancel it. This meant:

• Long-running LLM operations couldn't be stopped if no longer needed
• Retry chains would continue even after the user navigated away
• Resources were wasted on unnecessary LLM calls
• Poor user experience when operations needed to be cancelled

What user-facing changes did I ship?

Added abort/cancellation support for all BAML operations using native language patterns:

TypeScript

const controller = new AbortController();
const promise = b.ExtractName("John Doe", { 
  abortController: controller 
});

// Cancel after 100ms
setTimeout(() => controller.abort(), 100);

try {
  await promise;
} catch (e) {
  if (e instanceof BamlAbortError) {
    console.log("Operation cancelled");
  }
}

Python

from baml_py import AbortController

controller = AbortController()
task = b.ExtractName("John Doe", 
  baml_options={"abort_controller": controller})

# Cancel after 100ms
await asyncio.sleep(0.1)
controller.abort()

try:
  await task
except Exception as e:
  if "abort" in str(e).lower():
    print("Operation cancelled")

Go

ctx, cancel := context.WithTimeout(context.Background(), 100*time.Millisecond)
defer cancel()

_, err := b.ExtractName(ctx, "John Doe")
if err != nil && strings.Contains(err.Error(), "context canceled") {
  fmt.Println("Operation cancelled")
}

How I implemented it

• Added cancellation support at the orchestrator level in Rust runtime using stream-cancel crate's Tripwire mechanism
• Created language-specific bindings:
  • TypeScript: Native AbortController support via NAPI bridge
  • Python: Custom AbortController class exposed via PyO3
  • Go: context.Context cancellation with early detection goroutines
• Implemented early cancellation detection that monitors signals immediately when functions are called
• Added new LLMResponse::Cancelled variant to handle cancelled operations
• All changes are backward compatible - existing code works unchanged

How to verify it

Run the tests:

# Python tests
cd integ-tests/python && uv run pytest tests/test_abort_handlers.py -v

# TypeScript tests  
cd integ-tests/typescript && npm test -- tests/abort-handlers.test.ts

# Go tests (requires CFFI rebuild)
cd engine/language_client_cffi && cargo build
cd integ-tests/go && go test -v -run TestAbortHandler

Test status:

• Python abort handler tests (7 tests) - All passing
• TypeScript abort handler tests (11 tests) - All passing
• Go abort handler tests (7 tests) - All passing
• I have ensured make check test passes

Also re-enabled 6 previously disabled test files (PDF, video, OpenAI responses, expression functions) that are now compatible.

Description for the changelog

Added comprehensive abort handler support for TypeScript, Python, and Go, allowing cancellation of in-flight LLM operations through native language cancellation patterns.