vercel/ai · error · UIMessageStreamError

Received text-end for missing text part with ID "${chunk.id}

Error message

Received text-end for missing text part with ID "${chunk.id}". Ensure a "text-start" chunk is sent before any "text-end" chunks.

What it means

The stream processor looks up `state.activeTextParts[chunk.id]` when handling 'text-end'. If the text part was never started (or already ended), no active part exists and a UIMessageStreamError is thrown. Closing a text part requires a matching open text-start.

Source

Thrown at packages/ai/src/ui/process-ui-message-stream.ts:461

                throw new UIMessageStreamError({
                  chunkType: 'text-delta',
                  chunkId: chunk.id,
                  message:
                    `Received text-delta for missing text part with ID "${chunk.id}". ` +
                    `Ensure a "text-start" chunk is sent before any "text-delta" chunks.`,
                });
              }
              textPart.text += chunk.delta;
              textPart.providerMetadata =
                chunk.providerMetadata ?? textPart.providerMetadata;
              write();
              break;
            }

            case 'text-end': {
              const textPart = state.activeTextParts[chunk.id];
              if (textPart == null) {
                throw new UIMessageStreamError({
                  chunkType: 'text-end',
                  chunkId: chunk.id,
                  message:
                    `Received text-end for missing text part with ID "${chunk.id}". ` +
                    `Ensure a "text-start" chunk is sent before any "text-end" chunks.`,
                });
              }
              textPart.state = 'done';
              textPart.providerMetadata =
                chunk.providerMetadata ?? textPart.providerMetadata;
              delete state.activeTextParts[chunk.id];
              write();
              break;
            }

            case 'custom': {
              const customPart: CustomContentUIPart = {
                type: 'custom',

View on GitHub (pinned to 69428b1f8b)

Solutions

Send exactly one 'text-start' with the same id before 'text-end', and emit 'text-end' only once per id.
Verify ids are generated/stable consistently across start/delta/end chunks.
Guard server code against emitting end events after the part was closed (track open part ids server-side).
Catch UIMessageStreamError for text-end chunks and ignore already-closed parts.

Example fix

// before
writer.write({ type: 'text-end', id: 't1' }); // no matching open text-start
// after
writer.write({ type: 'text-start', id: 't1' });
writer.write({ type: 'text-delta', id: 't1', delta: 'Hi' });
writer.write({ type: 'text-end', id: 't1' });

Defensive patterns

Strategy: validation

Validate before calling

const openIds = new Set<string>();
function assertTextOpenBeforeEnd(id: string) {
  if (!openIds.has(id)) throw new Error(`text-end without open text-start for id ${id}`);
  openIds.delete(id); // prevents duplicate ends
}

Try / catch

try {
  await processUIMessageStream(...);
} catch (e) {
  if (e instanceof Error && /text-end for missing text part/.test(e.message)) {
    console.warn('Malformed stream: orphan text-end', e.message);
  } else throw e;
}

Prevention

Emit text-end exactly once per text part and only after text-start.
Generate start/end ids from the same variable to avoid mismatches.
Add server-side lifecycle assertions/unit tests for chunk ordering.
Avoid re-emitting end chunks on retry/resume paths.

When it happens

Trigger: A 'text-end' chunk with an id that never had a 'text-start', or a second 'text-end' for an already-closed text part, in a custom or replayed UI message stream.

Common situations: Duplicate text-end chunks from buggy server code; id mismatches between start and end; resuming streams where the text part was already finalized; hand-rolled SSE emitters out of order.

Related errors

AI-assisted analysis of vercel/ai@69428b1f8b (2026-08-30). Data as JSON: /api/errors/0a094d4f6aa73210. Report an issue: GitHub.