LangGraph Anthropic Streaming#

Overview#

appendLangChainChunk in @assistant-ui/react-langgraph accumulates streamed AIMessageChunk messages into a running LangChainMessage. Anthropic emits several non-standard content-block delta types that omit fields other providers always populate, causing recurring bugs in this layer.

Entry point: packages/react-langgraph/src/appendLangChainChunk.ts

How appendLangChainChunk Works#

The function handles three cases 1:

1. Non-chunk messages (type !== "AIMessageChunk") — returned as-is, or merged via mergeStreamedToolCallArgs if both prev and curr are "ai" type.
2. First chunk (no prior "ai" message) — bootstrapped directly, converting tool_call_chunks to tool_calls via chunkToToolCall.
3. Continuation chunks — content blocks and tool call chunks are merged into the accumulated message.

For content blocks, the merge loop originally handled only text and image_url blocks. All other types (e.g., thinking, reasoning, computer_call) were silently dropped.

For tool calls, a two-tier lookup first matches by id, then falls back to index — covering providers like Bedrock that omit id/name on continuation chunks.

Anthropic-Specific Delta Types and Bugs#

Anthropic's streaming protocol emits delta events that do not follow the conventions of OpenAI and other providers. This has caused multiple bugs in appendLangChainChunk.

Bug 1: citations_delta appends "undefined" (Issue #6901)#

@langchain/anthropic emits a citations_delta as a content block typed "text" that carries a citations field but no text field 2. The accumulator's concatenation last.text + item.text stringifies the missing field, producing a literal "undefined" appended to every cited sentence when Anthropic citations are enabled.

The correct fix has two parts 2:

1. Guard text concatenation against a missing text field (i.e., item.text ?? "").
2. Decide whether to carry the citations payload forward onto the accumulated block (upstream @langchain/core keeps it).

Bug 2: signature_delta and thinking blocks dropped (PR #6896)#

Anthropic's extended-thinking feature emits a signature_delta as a thinking-typed block with no thinking field — a metadata-only chunk 3. The original loop discarded all non-text/non-image_url blocks, causing:

• Reasoning content (thinking, reasoning blocks) to disappear after the first chunk.
• Signature metadata to be lost entirely.

Fix (PR #6896): Implemented index-based block accumulation mirroring @langchain/core's _mergeLists. Continuation chunks for an indexed block now update that slot instead of appending a duplicate. thinking blocks now concatenate both thinking content and signature fields across chunks. reasoning blocks accumulate summary array items, reasoning text, and signature fields. Only tool_use and input_json_delta blocks (owned by the tool-call pipeline) are excluded. 3

Bug 3: Mixed content blocks reordered (PR #6882)#

When a chunk contained both image and text blocks, text after an image would be merged into the text before the image because lastIndex was computed once before the loop 4. The fix moves the lastIndex calculation inside the loop so each block finds the correct append position.

Bug 4: computer_call blocks duplicated across chunks#

computer_call blocks split across chunks rendered as duplicate tool-call parts instead of updating a single entry. Fixed as part of PR #6896's index-based accumulation. 3

Type Changes#

PR #6896 updated the type definitions in types.ts to reflect what the wire actually carries 3:

• MessageContentThinking gains optional signature and index fields.
• MessageContentReasoning gains optional signature and index fields.
• MessageContentAudio added to the assistant content union.

Key Files#

Related Issues and PRs#