Use Match for Effect AI stream parts

ts/EFFECT_NATIVE_REWRITE_AUDIT.md

@@ -433,6 +433,22 @@ Notes:
   - `bun run --cwd ts/packages/flow test -- src/__tests__/gateway-dispatcher.test.ts`
   - `cd ts && bun run check:tsgo`
 
+### 2026-06-04: Effect AI Stream Part Match Slice
+
+- Status: migrated and package-verified.
+- Completed:
+  - `ts/packages/flow/src/model/text-completion/common.ts` now maps
+    `Response.StreamPart` values with `effect/Match` instead of a native
+    `switch`.
+  - The matcher handles `text-delta`, `finish`, and `error` explicitly, while
+    preserving ignored behavior for other valid stream parts with
+    `Match.orElse`.
+  - Text-completion common tests now include an ignored `text-start` stream
+    part before text deltas to prove the fallback path remains silent.
+- Verification:
+  - `bun run --cwd ts/packages/flow test -- src/__tests__/text-completion-common.test.ts`
+  - `cd ts && bun run check:tsgo`
+
 ### 2026-06-02: RAG And Agent Requestor Bridge Slice
 
 - Status: migrated, root-verified, committed, and pushed.
@@ -1862,8 +1878,9 @@ Notes:
     installed client, so do not reopen it as an `acquireRelease` close slice
     without new SDK evidence.
   - Shared text-completion stream iteration and the Mistral content assertion are
-    complete. The remaining provider-layer item is parity-backed Effect AI
-    adapter work, not a direct SDK swap.
+    complete, and the Effect AI stream-part adapter now uses `effect/Match`.
+    The remaining provider-layer item is parity-backed Effect AI adapter work,
+    not a direct SDK swap.
 - Scratch-note follow-ups:
   - `Term` / compact client term serialization is complete for base schema,
     gateway translation, and pure term helper switches. Future work should

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

@@ -1,7 +1,7 @@
 import { describe, expect, it } from "@effect/vitest";
 import type { LlmChunk } from "@trustgraph/base";
 import { Effect, Layer, ManagedRuntime, Stream } from "effect";
-import { AiError, LanguageModel } from "effect/unstable/ai";
+import { AiError, LanguageModel, Response } from "effect/unstable/ai";
 import {
   llmStreamPart,
   makeLanguageModelProvider,
@@ -157,6 +157,7 @@ describe("text completion common helpers", () => {
             ]),
           streamText: () =>
             Stream.fromArray([
+              Response.makePart("text-start", { id: "part-1" }),
               { type: "text-delta", id: "part-1", delta: "hel" },
               { type: "text-delta", id: "part-1", delta: "lo" },
               finishPart(13, 8),

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

@@ -7,7 +7,7 @@ import {
   type LlmResult,
   type LlmProvider,
 } from "@trustgraph/base";
-import { Config, Effect, Layer, ManagedRuntime, Ref, Result, Stream } from "effect";
+import { Config, Effect, Layer, ManagedRuntime, Match, Ref, Result, Stream } from "effect";
 import * as O from "effect/Option";
 import * as Predicate from "effect/Predicate";
 import * as S from "effect/Schema";
@@ -258,29 +258,26 @@ const languageModelStreamChunk = (
   provider: string,
   model: string,
   part: Response.StreamPart<{}>,
-): Effect.Effect<Result.Result<LlmChunk, undefined>, TextCompletionRuntimeError> => {
-  switch (part.type) {
-    case "text-delta":
-      return Effect.succeed(
+): Effect.Effect<Result.Result<LlmChunk, undefined>, TextCompletionRuntimeError> =>
+  Match.value(part).pipe(
+    Match.discriminators("type")({
+      "text-delta": (part) => Effect.succeed(
         part.delta.length > 0
           ? Result.succeed(textChunk(model, part.delta))
           : Result.fail(undefined),
-      );
-    case "finish":
-      return Effect.succeed(
+      ),
+      finish: (part) => Effect.succeed(
         Result.succeed(
           finalChunk(model, {
             inToken: usageInputTokens(part.usage),
             outToken: usageOutputTokens(part.usage),
           }),
         ),
-      );
-    case "error":
-      return Effect.fail(effectAiProviderError(provider, part.error));
-    default:
-      return Effect.succeed(Result.fail(undefined));
-  }
-};
+      ),
+      error: (part) => Effect.fail(effectAiProviderError(provider, part.error)),
+    }),
+    Match.orElse(() => Effect.succeed(Result.fail(undefined))),
+  );
 
 const runLanguageModelStream = <RuntimeRequirements, StreamRequirements extends RuntimeRequirements>(
   runtime: ManagedRuntime.ManagedRuntime<RuntimeRequirements, TextCompletionRuntimeError>,