Backport: fix(ai): skip stringifying text when streaming partial text (#14200)

vercel-ai-sdk[bot] · aayush-kapoor · web-flow · commit 1003609012f1 · 2026-04-07T18:43:34.000Z
This is an automated backport of #14123 to the release-v6.0 branch. FYI @aayush-kapoor Co-authored-by: Aayush Kapoor <83492835+aayush-kapoor@users.noreply.github.com>
diff --git a/.changeset/empty-cups-love.md b/.changeset/empty-cups-love.md
@@ -0,0 +1,5 @@
+---
+"ai": patch
+---
+
+fix(ai): skip stringifying text when streaming partial text
diff --git a/packages/ai/src/generate-text/stream-text.test.ts b/packages/ai/src/generate-text/stream-text.test.ts
@@ -15190,6 +15190,50 @@ describe('streamText', () => {
         `);
       });
 
+      it('should not call JSON.stringify for string partial outputs', async () => {
+        const originalStringify = JSON.stringify;
+        const stringifySpy = vi.spyOn(JSON, 'stringify').mockImplementation(((
+          ...args: Parameters<typeof JSON.stringify>
+        ) => {
+          if (typeof args[0] === 'string') {
+            throw new Error(
+              'JSON.stringify should not be called for string partial outputs',
+            );
+          }
+          return originalStringify.call(JSON, ...args);
+        }) as typeof JSON.stringify);
+
+        try {
+          const result = streamText({
+            model: createTestModel({
+              stream: convertArrayToReadableStream([
+                { type: 'text-start', id: '1' },
+                { type: 'text-delta', id: '1', delta: 'Hello, ' },
+                { type: 'text-delta', id: '1', delta: 'world!' },
+                { type: 'text-end', id: '1' },
+                {
+                  type: 'finish',
+                  finishReason: { unified: 'stop', raw: 'stop' },
+                  usage: testUsage,
+                },
+              ]),
+            }),
+            prompt: 'prompt',
+            output: Output.text(),
+          });
+
+          expect(await convertAsyncIterableToArray(result.partialOutputStream))
+            .toMatchInlineSnapshot(`
+            [
+              "Hello, ",
+              "Hello, world!",
+            ]
+          `);
+        } finally {
+          stringifySpy.mockRestore();
+        }
+      });
+
       it('should resolve output promise with the correct content', async () => {
         const result = streamText({
           model: createTestModel({
diff --git a/packages/ai/src/generate-text/stream-text.ts b/packages/ai/src/generate-text/stream-text.ts
@@ -600,7 +600,7 @@ function createOutputTransformStream<
   let text = '';
   let textChunk = '';
   let textProviderMetadata: ProviderMetadata | undefined = undefined;
-  let lastPublishedJson = '';
+  let lastPublishedValue = '';
 
   function publishTextChunk({
     controller,
@@ -673,11 +673,15 @@ function createOutputTransformStream<
 
       // null should be allowed (valid JSON value) but undefined should not:
       if (result !== undefined) {
-        // only send new json if it has changed:
-        const currentJson = JSON.stringify(result.partial);
-        if (currentJson !== lastPublishedJson) {
+        // only send new value if it has changed:
+        // For string partials (text output), compare directly to avoid unnecessary JSON.stringify overhead
+        const currentValue =
+          typeof result.partial === 'string'
+            ? result.partial
+            : JSON.stringify(result.partial);
+        if (currentValue !== lastPublishedValue) {
           publishTextChunk({ controller, partialOutput: result.partial });
-          lastPublishedJson = currentJson;
+          lastPublishedValue = currentValue;
         }
       }
     },

-Original file line number
+Diff line change
@@ @@ -0,0 +1,5 @@ @@
 +---
 +"ai": patch
 +---
++
 +fix(ai): skip stringifying text when streaming partial text