Safely handles missing reasoning parts

[cdalton713]

Adds a null check to prevent errors when processing a reasoning-end message for a non-existent or already removed reasoning part.

Summary

Getting runtime errors when two reasoning end events come in from OpenAI.

Manual Verification

Added validation that the reasoning event exists.

[vercel]

The reasoning-delta case has the same issue that was just fixed in reasoning-end - it doesn't check if the reasoning part exists before accessing its properties.

View Details

📝 Patch Details

diff --git a/packages/ai/src/ui/process-ui-message-stream.ts b/packages/ai/src/ui/process-ui-message-stream.ts
index e2198e158..6620091dd 100644
--- a/packages/ai/src/ui/process-ui-message-stream.ts
+++ b/packages/ai/src/ui/process-ui-message-stream.ts
@@ -346,9 +346,11 @@ export function processUIMessageStream<UI_MESSAGE extends UIMessage>({
 
             case 'reasoning-delta': {
               const reasoningPart = state.activeReasoningParts[chunk.id];
-              reasoningPart.text += chunk.delta;
-              reasoningPart.providerMetadata =
-                chunk.providerMetadata ?? reasoningPart.providerMetadata;
+              if (reasoningPart) {
+                reasoningPart.text += chunk.delta;
+                reasoningPart.providerMetadata =
+                  chunk.providerMetadata ?? reasoningPart.providerMetadata;
+              }
               write();
               break;
             }

Analysis

Null check missing in reasoning-delta handler can cause TypeError

What fails: processUIMessageStream() in reasoning-delta case accesses reasoningPart.text without null check, causing TypeError when reasoning part doesn't exist

How to reproduce:

// Simulate finish-step clearing activeReasoningParts, then reasoning-delta arriving
const state = { activeReasoningParts: {} };
const chunk = { id: "test-id", delta: "more text" };
const reasoningPart = state.activeReasoningParts[chunk.id]; // undefined
reasoningPart.text += chunk.delta; // TypeError: Cannot read properties of undefined (reading 'text')

Result: TypeError: Cannot read properties of undefined (reading 'text')

Expected: Should gracefully handle missing reasoning parts like reasoning-end case does with if (reasoningPart) null check

The finish-step case resets state.activeReasoningParts = {} which can cause this scenario when delta messages arrive after step completion.

[gr2m]

can you please share a full example where the error occurs, so we can test your fix? If we go ahead, this PR will need a test, and a changeset