Stream Decoder Buffer Limits#

Status (as of 2026-10-03): Buffer limits are proposed in PR #8737 (open, not yet merged). The current main branch has no configurable memory bounds on either decoder.

Current behavior (no limits)#

LineDecoderStream accumulates incoming text into a single buffer string until a \n, \r, or \r\n delimiter is found 1. There is no maximum length check; a line that never terminates grows without bound. On stream end, an unterminated buffer throws an error — this is a correctness guard, not a size limit 2.

SSEEventDecoder accumulates unfinished line fragments in lineChunks: string[] and completed data: field values in dataLines: string[] 3. The only constructor option is trailing: "drop" | "dispatch", which controls whether data remaining at stream end is discarded or emitted as a final event 4. Neither array has a size cap.

Risk: A peer that continuously streams without sending a newline or SSE frame boundary can grow process memory until exhaustion. This is the root cause described in PR #8737.

Proposed limits (PR #8737)#

PR #8737 — "fix(streaming): bound decoder buffers without blocking large payloads" — adds configurable, default-bounded limits at multiple layers:

The limits are additive — callers that need to transport payloads larger than 16 MiB can raise them explicitly without changing default behavior 5. Exceeding a limit rejects decoding with a specific line-length or event-length error and discards the rejected frame 5.

The PR modifies these files across the decoder stack:

• packages/assistant-stream/src/core/utils/stream/LineDecoderStream.ts
• packages/assistant-stream/src/core/utils/stream/SSEEventDecoder.ts
• packages/assistant-stream/src/core/serialization/assistant-transport/AssistantTransport.ts
• packages/assistant-stream/src/core/serialization/data-stream/DataStream.ts
• packages/assistant-stream/src/core/serialization/ui-message-stream/UIMessageStream.ts
• packages/core/src/react/runtimes/assistant-transport/useAssistantTransportRuntime.ts
• packages/react-data-stream/src/useDataStreamRuntime.ts

5

Related: incremental SSE fragmentation fix (PR #7262, merged)#

PR #7262 — "fix(assistant-stream): scan fragmented SSE lines incrementally" — is a merged performance fix that replaced the old lineBuffer string accumulation in SSEEventDecoder with the current lineChunks: string[] approach 6. This eliminated quadratic scanning when large events arrive in many small chunks (e.g., a 1 MiB payload in 1 KiB chunks dropped from ~112 ms to ~0.33 ms) 7. This change is not a memory cap — it is an efficiency improvement that happens to change how the buffer is stored.

Where to look#