diff --git a/packages/ai/src/generate-text/stream-text.ts b/packages/ai/src/generate-text/stream-text.ts
index cbd842822d47..5f741a3a3524 100644
--- a/packages/ai/src/generate-text/stream-text.ts
+++ b/packages/ai/src/generate-text/stream-text.ts
@@ -1028,7 +1028,32 @@ class DefaultStreamTextResult<
             return;
           }
 
-          activeText.text += part.text;
+          // PATCH: O(N²)→O(N) text accumulation. Init _chunks lazily; replace .text
+          // with a lazy getter that materializes from the chunks array on read.
+          // _chunks is non-enumerable so it doesn't leak through
+          // JSON.stringify / structuredClone / spread to consumers.
+          if (!(activeText as any)._chunks) {
+            Object.defineProperty(activeText, '_chunks', {
+              value: [activeText.text || ''],
+              writable: true,
+              enumerable: false,
+              configurable: true,
+            });
+            Object.defineProperty(activeText, 'text', {
+              get(): string {
+                const c = (this as any)._chunks;
+                return c.length === 1
+                  ? c[0]
+                  : ((this as any)._chunks = [c.join('')])[0];
+              },
+              set(v: string) {
+                (this as any)._chunks = [v];
+              },
+              enumerable: true,
+              configurable: true,
+            });
+          }
+          (activeText as any)._chunks.push(part.text);
           activeText.providerMetadata =
             part.providerMetadata ?? activeText.providerMetadata;
         }
@@ -1077,7 +1102,29 @@ class DefaultStreamTextResult<
             return;
           }
 
-          activeReasoning.text += part.text;
+          // PATCH: O(N²)→O(N) reasoning accumulation. Same pattern as activeText above.
+          if (!(activeReasoning as any)._chunks) {
+            Object.defineProperty(activeReasoning, '_chunks', {
+              value: [activeReasoning.text || ''],
+              writable: true,
+              enumerable: false,
+              configurable: true,
+            });
+            Object.defineProperty(activeReasoning, 'text', {
+              get(): string {
+                const c = (this as any)._chunks;
+                return c.length === 1
+                  ? c[0]
+                  : ((this as any)._chunks = [c.join('')])[0];
+              },
+              set(v: string) {
+                (this as any)._chunks = [v];
+              },
+              enumerable: true,
+              configurable: true,
+            });
+          }
+          (activeReasoning as any)._chunks.push(part.text);
           activeReasoning.providerMetadata =
             part.providerMetadata ?? activeReasoning.providerMetadata;
         }
diff --git a/packages/ai/src/ui/process-ui-message-stream.ts b/packages/ai/src/ui/process-ui-message-stream.ts
index e904c1999c05..5b58333cd076 100644
--- a/packages/ai/src/ui/process-ui-message-stream.ts
+++ b/packages/ai/src/ui/process-ui-message-stream.ts
@@ -390,6 +390,29 @@ export function processUIMessageStream<UI_MESSAGE extends UIMessage>({
                 providerMetadata: chunk.providerMetadata,
                 state: 'streaming',
               };
+              // PATCH: O(N²)→O(N) text accumulation via _chunks + lazy getter.
+              // See https://github.com/vercel/ai/issues/15670 for context.
+              // _chunks is non-enumerable so it doesn't leak through
+              // JSON.stringify / structuredClone / spread to consumers.
+              Object.defineProperty(textPart, '_chunks', {
+                value: [textPart.text || ''],
+                writable: true,
+                enumerable: false,
+                configurable: true,
+              });
+              Object.defineProperty(textPart, 'text', {
+                get(): string {
+                  const c = (this as any)._chunks;
+                  return c.length === 1
+                    ? c[0]
+                    : ((this as any)._chunks = [c.join('')])[0];
+                },
+                set(v: string) {
+                  (this as any)._chunks = [v];
+                },
+                enumerable: true,
+                configurable: true,
+              });
               state.activeTextParts[chunk.id] = textPart;
               state.message.parts.push(textPart);
               write();
@@ -407,7 +430,8 @@ export function processUIMessageStream<UI_MESSAGE extends UIMessage>({
                     `Ensure a "text-start" chunk is sent before any "text-delta" chunks.`,
                 });
               }
-              textPart.text += chunk.delta;
+              // PATCH: see text-start above — chunks instead of += to avoid O(N²).
+              (textPart as any)._chunks.push(chunk.delta);
               textPart.providerMetadata =
                 chunk.providerMetadata ?? textPart.providerMetadata;
               write();
@@ -451,6 +475,28 @@ export function processUIMessageStream<UI_MESSAGE extends UIMessage>({
                 providerMetadata: chunk.providerMetadata,
                 state: 'streaming',
               };
+              // PATCH: O(N²)→O(N) reasoning accumulation via _chunks + lazy getter.
+              // _chunks is non-enumerable so it doesn't leak through
+              // JSON.stringify / structuredClone / spread to consumers.
+              Object.defineProperty(reasoningPart, '_chunks', {
+                value: [reasoningPart.text || ''],
+                writable: true,
+                enumerable: false,
+                configurable: true,
+              });
+              Object.defineProperty(reasoningPart, 'text', {
+                get(): string {
+                  const c = (this as any)._chunks;
+                  return c.length === 1
+                    ? c[0]
+                    : ((this as any)._chunks = [c.join('')])[0];
+                },
+                set(v: string) {
+                  (this as any)._chunks = [v];
+                },
+                enumerable: true,
+                configurable: true,
+              });
               state.activeReasoningParts[chunk.id] = reasoningPart;
               state.message.parts.push(reasoningPart);
               write();
@@ -468,7 +514,8 @@ export function processUIMessageStream<UI_MESSAGE extends UIMessage>({
                     `Ensure a "reasoning-start" chunk is sent before any "reasoning-delta" chunks.`,
                 });
               }
-              reasoningPart.text += chunk.delta;
+              // PATCH: see reasoning-start above — chunks instead of += to avoid O(N²).
+              (reasoningPart as any)._chunks.push(chunk.delta);
               reasoningPart.providerMetadata =
                 chunk.providerMetadata ?? reasoningPart.providerMetadata;
               write();