feat(stream): add method for filtering reasoning chunks from the ai sdk stream

Author: adarshba

src/lib/core/baseProvider.ts

@@ -61,7 +61,7 @@ export abstract class BaseProvider implements AIProvider {
 
   // Composition modules - Single Responsibility Principle
   private readonly messageBuilder: MessageBuilder;
-  private readonly streamHandler: StreamHandler;
+  protected readonly streamHandler: StreamHandler;
   private readonly generationHandler: GenerationHandler;
   private readonly telemetryHandler: TelemetryHandler;
   private readonly utilities: Utilities;

src/lib/core/modules/StreamHandler.ts

@@ -87,6 +87,51 @@ export class StreamHandler {
     })();
   }
 
+  /**
+   * Create filtered stream from AI SDK fullStream
+   *
+   * Filters out reasoning/thinking chunks from the stream.
+   * Used as safety net to prevent chain-of-thought tokens from leaking to users.
+   *
+   * @param result - AI SDK stream result
+   */
+  createFilteredFullStream(result: {
+    fullStream?: AsyncIterable<unknown>;
+    textStream?: AsyncIterable<string>;
+  }): AsyncGenerator<{ content: string }> {
+    type TextDeltaChunk = { type: "text-delta"; textDelta: string };
+    type ErrorChunk = { type: "error"; error: Record<string, unknown> };
+
+    return (async function* () {
+      if (result.fullStream) {
+        for await (const chunk of result.fullStream) {
+          if (chunk && typeof chunk === "object" && "type" in chunk) {
+            const chunkType = (chunk as { type: string }).type;
+
+            if (chunkType === "error") {
+              const errorChunk = chunk as ErrorChunk;
+              throw new Error(
+                `Streaming error: ${
+                  (errorChunk.error as Record<string, unknown>)?.message ||
+                  "Unknown error"
+                }`,
+              );
+            } else if (chunkType === "text-delta") {
+              yield { content: (chunk as TextDeltaChunk).textDelta };
+            }
+            // Skip: reasoning, reasoning-signature, redacted-reasoning, source, tool-call, tool-result, etc.
+          } else if (typeof chunk === "string") {
+            yield { content: chunk };
+          }
+        }
+      } else if (result.textStream) {
+        for await (const chunk of result.textStream) {
+          yield { content: chunk };
+        }
+      }
+    })();
+  }
+
   /**
    * Create standardized stream result - consolidates result structure
    */

src/lib/providers/googleVertex.ts

@@ -983,8 +983,8 @@ export class GoogleVertexProvider extends BaseProvider {
 
       timeoutController?.cleanup();
 
-      // Transform string stream to content object stream using BaseProvider method
-      const transformedStream = this.createTextStream(result);
+      const transformedStream =
+        this.streamHandler.createFilteredFullStream(result);
 
       // Track tool calls and results for streaming
       const toolCalls: Array<{