Simplify text completion generator boundary

ts/packages/flow/src/__tests__/text-completion-common.test.ts

@@ -0,0 +1,24 @@
+import { describe, expect, it } from "@effect/vitest";
+import type { LlmChunk } from "@trustgraph/base";
+import { providerRuntimeError, toAsyncGenerator } from "../model/text-completion/common.js";
+
+const emptyChunkIterator = (): AsyncIterable<LlmChunk> => ({
+  [Symbol.asyncIterator]: () => ({
+    next: () => Promise.resolve({ done: true, value: undefined }),
+  }),
+});
+
+describe("text completion common helpers", () => {
+  it("maps fallback generator throw failures into tagged provider errors", async () => {
+    const generator = toAsyncGenerator(
+      emptyChunkIterator(),
+      (error) => providerRuntimeError("test-provider", error),
+    );
+
+    await expect(generator.throw("provider failed")).rejects.toMatchObject({
+      _tag: "TextCompletionProviderError",
+      provider: "test-provider",
+      message: "provider failed",
+    });
+  });
+});

ts/packages/flow/src/model/text-completion/common.ts

@@ -86,16 +86,16 @@ export const toAsyncGenerator = (
   const iterator = iterable[Symbol.asyncIterator]();
   let generator: AsyncGenerator<LlmChunk>;
   generator = {
-    next: (value?: unknown) => iterator.next(value as never),
+    next: (value?: unknown) => iterator.next(value),
     return: (value?: unknown) =>
       iterator.return === undefined
-        ? Promise.resolve({ done: true, value: value as LlmChunk })
-        : iterator.return(value as never) as Promise<IteratorResult<LlmChunk>>,
+        ? Promise.resolve({ done: true, value })
+        : iterator.return(value),
     throw: (error?: unknown) =>
       iterator.throw === undefined
-        ? Effect.runPromise(Effect.fail(mapError(error))) as Promise<IteratorResult<LlmChunk>>
-        : iterator.throw(error) as Promise<IteratorResult<LlmChunk>>,
+        ? Promise.reject(mapError(error))
+        : iterator.throw(error),
     [Symbol.asyncIterator]: () => generator,
-  } as AsyncGenerator<LlmChunk>;
+  };
   return generator;
 };