Simplify text completion generator boundary

ts/EFFECT_NATIVE_REWRITE_AUDIT.md

@@ -13,11 +13,11 @@ Verified source roots:
 - Installed Effect beta used by this workspace: `ts/node_modules/effect`
 
 Current signal counts from `ts/packages` after the 2026-06-02 Text completion
-stream sentinel slice:
+generator boundary slice:
 
 | Signal | Count |
 | --- | ---: |
-| `Effect.runPromise` | 169 |
+| `Effect.runPromise` | 168 |
 | `Map<` | 88 |
 | `WebSocket` | 72 |
 | `new Map` | 62 |
@@ -72,6 +72,10 @@ Notes:
   `Effect.void as Effect.Effect<undefined>` assertions from provider stream
   unfold branches. Counts are unchanged because this was an Effect diagnostic
   and type-channel cleanup.
+- The text completion generator boundary slice removed the
+  `Effect.runPromise(Effect.fail(...))` fallback and the related
+  `AsyncGenerator`/`IteratorResult` assertions from
+  `model/text-completion/common.ts`.
 - `Record<string, any>` and `throwLibrarianServiceError` are now clean in
   `ts/packages`.
 
@@ -565,6 +569,26 @@ Notes:
   - `cd ts && bun run test`
   - `git diff --check`
 
+### 2026-06-02: Text Completion Generator Boundary Slice
+
+- Status: migrated and root-verified.
+- Completed:
+  - `ts/packages/flow/src/model/text-completion/common.ts` now rejects
+    fallback `AsyncGenerator.throw(...)` calls with the mapped tagged provider
+    error directly instead of running `Effect.fail(...)` through
+    `Effect.runPromise`.
+  - The custom generator object no longer uses `as AsyncGenerator`,
+    `as Promise<IteratorResult<LlmChunk>>`, or `as LlmChunk` assertions.
+  - Added a focused unit test for fallback throw mapping.
+- Verification:
+  - `bun run --cwd ts/packages/flow test -- src/__tests__/text-completion-common.test.ts`
+  - `bun run --cwd ts/packages/flow build`
+  - `cd ts && bun run check`
+  - `bun run --cwd ts/packages/flow test`
+  - `cd ts && bun run build`
+  - `cd ts && bun run test`
+  - `git diff --check`
+
 ## Subagent Findings To Preserve
 
 - MCP/workbench:

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;
 };