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perf: fix O(N^2) accumulation in MessageStream and BetaMessageStream #997

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perf: fix O(N^2) accumulation in MessageStream and BetaMessageStream #997

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eedf0cb
perf: fix O(N^2) accumulation in MessageStream and BetaMessageStream
claygeo Apr 12, 2026
b73b736
fix: add defensive copy in BetaMessageStream content_block_start
claygeo Apr 12, 2026
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53 changes: 21 additions & 32 deletions src/lib/BetaMessageStream.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -603,83 +603,72 @@ export class BetaMessageStream<ParsedT = null> implements AsyncIterable<BetaMess

return snapshot;
case 'content_block_start':
snapshot.content.push(event.content_block);
snapshot.content.push({ ...event.content_block });
return snapshot;
case 'content_block_delta': {
const snapshotContent = snapshot.content.at(event.index);

switch (event.delta.type) {
case 'text_delta': {
if (snapshotContent?.type === 'text') {
snapshot.content[event.index] = {
...snapshotContent,
text: (snapshotContent.text || '') + event.delta.text,
};
snapshotContent.text = (snapshotContent.text || '') + event.delta.text;
}
break;
}
case 'citations_delta': {
if (snapshotContent?.type === 'text') {
snapshot.content[event.index] = {
...snapshotContent,
citations: [...(snapshotContent.citations ?? []), event.delta.citation],
};
if (!snapshotContent.citations) {
snapshotContent.citations = [];
}
snapshotContent.citations.push(event.delta.citation);
}
break;
}
case 'input_json_delta': {
if (snapshotContent && tracksToolInput(snapshotContent)) {
// we need to keep track of the raw JSON string as well so that we can
// re-parse it for each delta, for now we just store it as an untyped
// non-enumerable property on the snapshot
// Track the raw JSON string as a non-enumerable property so it's
// hidden from JSON.stringify/serialization but available for re-parsing
let jsonBuf = (snapshotContent as any)[JSON_BUF_PROPERTY] || '';
jsonBuf += event.delta.partial_json;

const newContent = { ...snapshotContent };
Object.defineProperty(newContent, JSON_BUF_PROPERTY, {
value: jsonBuf,
enumerable: false,
writable: true,
});
if (!(JSON_BUF_PROPERTY in snapshotContent)) {
Object.defineProperty(snapshotContent, JSON_BUF_PROPERTY, {
value: jsonBuf,
enumerable: false,
writable: true,
});
} else {
(snapshotContent as any)[JSON_BUF_PROPERTY] = jsonBuf;
}

if (jsonBuf) {
try {
newContent.input = partialParse(jsonBuf);
(snapshotContent as any).input = partialParse(jsonBuf);
} catch (err) {
const error = new AnthropicError(
`Unable to parse tool parameter JSON from model. Please retry your request or adjust your prompt. Error: ${err}. JSON: ${jsonBuf}`,
);
this.#handleError(error);
}
}
snapshot.content[event.index] = newContent;
}
break;
}
case 'thinking_delta': {
if (snapshotContent?.type === 'thinking') {
snapshot.content[event.index] = {
...snapshotContent,
thinking: snapshotContent.thinking + event.delta.thinking,
};
snapshotContent.thinking = snapshotContent.thinking + event.delta.thinking;
}
break;
}
case 'signature_delta': {
if (snapshotContent?.type === 'thinking') {
snapshot.content[event.index] = {
...snapshotContent,
signature: event.delta.signature,
};
snapshotContent.signature = event.delta.signature;
}
break;
}
case 'compaction_delta': {
if (snapshotContent?.type === 'compaction') {
snapshot.content[event.index] = {
...snapshotContent,
content: (snapshotContent.content || '') + event.delta.content,
};
snapshotContent.content = (snapshotContent.content || '') + event.delta.content;
}
break;
}
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46 changes: 19 additions & 27 deletions src/lib/MessageStream.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -605,59 +605,51 @@ export class MessageStream<ParsedT = null> implements AsyncIterable<MessageStrea
switch (event.delta.type) {
case 'text_delta': {
if (snapshotContent?.type === 'text') {
snapshot.content[event.index] = {
...snapshotContent,
text: (snapshotContent.text || '') + event.delta.text,
};
snapshotContent.text = (snapshotContent.text || '') + event.delta.text;
}
break;
}
case 'citations_delta': {
if (snapshotContent?.type === 'text') {
snapshot.content[event.index] = {
...snapshotContent,
citations: [...(snapshotContent.citations ?? []), event.delta.citation],
};
if (!snapshotContent.citations) {
snapshotContent.citations = [];
}
snapshotContent.citations.push(event.delta.citation);
}
break;
}
case 'input_json_delta': {
if (snapshotContent && tracksToolInput(snapshotContent)) {
// we need to keep track of the raw JSON string as well so that we can
// re-parse it for each delta, for now we just store it as an untyped
// non-enumerable property on the snapshot
// Track the raw JSON string as a non-enumerable property so it's
// hidden from JSON.stringify/serialization but available for re-parsing
let jsonBuf = (snapshotContent as any)[JSON_BUF_PROPERTY] || '';
jsonBuf += event.delta.partial_json;

const newContent = { ...snapshotContent };
Object.defineProperty(newContent, JSON_BUF_PROPERTY, {
value: jsonBuf,
enumerable: false,
writable: true,
});
if (!(JSON_BUF_PROPERTY in snapshotContent)) {
Object.defineProperty(snapshotContent, JSON_BUF_PROPERTY, {
value: jsonBuf,
enumerable: false,
writable: true,
});
} else {
(snapshotContent as any)[JSON_BUF_PROPERTY] = jsonBuf;
}

if (jsonBuf) {
newContent.input = partialParse(jsonBuf);
(snapshotContent as any).input = partialParse(jsonBuf);
}
snapshot.content[event.index] = newContent;
}
break;
}
case 'thinking_delta': {
if (snapshotContent?.type === 'thinking') {
snapshot.content[event.index] = {
...snapshotContent,
thinking: snapshotContent.thinking + event.delta.thinking,
};
snapshotContent.thinking = snapshotContent.thinking + event.delta.thinking;
}
break;
}
case 'signature_delta': {
if (snapshotContent?.type === 'thinking') {
snapshot.content[event.index] = {
...snapshotContent,
signature: event.delta.signature,
};
snapshotContent.signature = event.delta.signature;
}
break;
}
Expand Down
63 changes: 63 additions & 0 deletions tests/api-resources/MessageStream.test.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -226,6 +226,69 @@ describe('MessageStream class', () => {
expect(finalText).toBe("I'll check the current weather in Paris for you.");
});

it('accumulates large text responses in linear time', async () => {
const { fetch, handleStreamEvents } = mockFetch();

const anthropic = new Anthropic({ apiKey: '...', fetch });

const NUM_DELTAS = 5000;
const CHUNK = 'abcdefghij'; // 10 chars per delta

// Build a synthetic stream with many text deltas
const events: any[] = [
{
type: 'message_start',
message: {
id: 'msg_perf_test',
type: 'message',
role: 'assistant',
content: [],
model: 'claude-opus-4-20250514',
stop_reason: null,
stop_sequence: null,
usage: { input_tokens: 10, output_tokens: 1 },
},
},
{
type: 'content_block_start',
index: 0,
content_block: { type: 'text', text: '' },
},
];

for (let i = 0; i < NUM_DELTAS; i++) {
events.push({
type: 'content_block_delta',
index: 0,
delta: { type: 'text_delta', text: CHUNK },
});
}

events.push(
{ type: 'content_block_stop', index: 0 },
{ type: 'message_delta', delta: { stop_reason: 'end_turn', stop_sequence: null }, usage: { output_tokens: NUM_DELTAS } },
{ type: 'message_stop' },
);

handleStreamEvents(events);

const stream = anthropic.messages.stream({
max_tokens: 1024,
model: 'claude-opus-4-20250514',
messages: [{ role: 'user', content: 'test' }],
});

const finalMessage = await stream.finalMessage();
const finalText = await stream.finalText();

// Verify correctness
expect(finalText).toBe(CHUNK.repeat(NUM_DELTAS));
expect(finalMessage.content[0]!.type).toBe('text');
if (finalMessage.content[0]!.type === 'text') {
expect(finalMessage.content[0]!.text).toBe(CHUNK.repeat(NUM_DELTAS));
}
});

it('does not throw unhandled rejection with withResponse()', async () => {
const { fetch, handleRequest } = mockFetch();
const anthropic = new Anthropic({
Expand Down