LLM - Weights & Biases Documentation

Defined in: src/genai/llm.ts:86 An LLM call. Emits a chat span with gen_ai.* attributes. Created by weave.startLLM() (or turn.startLLM()) and terminated with end(). Only one LLM may be active in an async context at a time; nest tool/subagent calls under it via startTool / startSubagent. Populate inputMessages / outputMessages / usage / reasoning directly, or via the helper functions (output, think, attachMedia, record). All recorded data is flushed to the span at end().

Examples

const llm = weave.startLLM({model: 'gpt-4o-mini', providerName: 'openai'});

try {
  llm.inputMessages = [{role: 'user', content: prompt}];
  const resp = await openai.chat.completions.create({...});
  llm.output(resp.choices[0].message.content ?? '');
  llm.record({usage: {inputTokens: resp.usage?.prompt_tokens}});
} finally {
  llm.end();
}

const llm = weave.startLLM({
  model: 'gpt-4o-mini',
  providerName: 'openai',
  systemInstructions: ['You are a helpful weather bot.'],
  startTime: new Date('2026-05-29T10:00:00.000Z'),
});

try {
  // ... call the LLM, populate llm.outputMessages / usage ...
} finally {
  llm.end();
}

Extends

SpanBase

Properties

inputMessages

inputMessages: Message[] = []

Defined in: src/genai/llm.ts:93 Input messages sent to the model. Flushed to gen_ai.input.messages on end().

model

readonly model: string

Defined in: src/genai/llm.ts:117

outputMessages

outputMessages: Message[] = []

Defined in: src/genai/llm.ts:98 Assistant messages returned by the model. Flushed to gen_ai.output.messages on end().

providerName

readonly providerName: string

Defined in: src/genai/llm.ts:118

reasoning?

optional reasoning?: Reasoning

Defined in: src/genai/llm.ts:105 Chain-of-thought content. Folded into the last assistant message as a ReasoningPart at serialization time.

usage

usage: Usage = {}

Defined in: src/genai/llm.ts:100 Token counts and cache stats. Flushed to gen_ai.usage.* on end().

Methods

addEvent()

Deprecated. Record this data via setAttributes instead. OpenTelemetry is phasing out the Span Event API (Span.addEvent). This method still works and existing span-event data stays valid. See https://opentelemetry.io/blog/2026/deprecating-span-events/

addEvent(name, attributes?, startTime?): this

Defined in: src/genai/spanBase.ts:82 Add a named event to the span. Useful for marking non-span moments such as context compaction, tool-loop detection, or guardrail trips. Warns and no-ops after end(). Mirrors OTel Span.addEvent.

Parameters

name

string

attributes?

Attributes

startTime?

TimeInput

Returns

this

Example

span.addEvent('context_compacted', {removedMessages: 12});

Inherited from

SpanBase.addEvent

attachMedia()

attachMedia(opts): this

Defined in: src/genai/llm.ts:183 Stage a media attachment for the LLM call. Pick exactly one of content (inline base64 bytes), uri (URI reference), or fileId (pre-uploaded file id). The attachment is glued onto the last user message in inputMessages on end().

Parameters

opts

AttachMediaOpts

Returns

this

attachMediaUrl()

attachMediaUrl(url, opts): this

Defined in: src/genai/llm.ts:192 Convenience for attachMedia({uri, modality}).

Parameters

url

string

opts

modality

Modality

Returns

this

end()

end(opts?): void

Defined in: src/genai/llm.ts:274 Flush accumulated state and close the span. Idempotent. Pass error to mark failed; pass endTime to backdate the close.

Parameters

opts?

SpanEndOptions

Returns

void

output()

output(content): this

Defined in: src/genai/llm.ts:155 Append an assistant message to the response.

Parameters

content

string

Returns

this

record()

record(opts): this

Defined in: src/genai/llm.ts:203 Bulk-set any subset of the mutable fields. Replaces (does not merge). Useful for assigning everything at once after a provider call returns.

Parameters

opts

finishReasons?

string[]

inputMessages?

Message[]

mediaAttachments?

AttachMediaOpts[]

outputMessages?

Message[]

outputType?

string

reasoning?

Reasoning

responseId?

string

responseModel?

string

usage?

Usage

Returns

this

setAttributes()

setAttributes(attributes): this

Defined in: src/genai/spanBase.ts:63 Set multiple attributes on the span at once. Warns and no-ops after end(). Mirrors OTel Span.setAttributes (and the Python SDK’s set_attributes).

Parameters

attributes

Attributes

Returns

this

Example

span.setAttributes({'weave.tag': 'prod', 'gen_ai.response.id': id});

Inherited from

SpanBase.setAttributes

startSubagent()

startSubagent(opts): SubAgent

Defined in: src/genai/llm.ts:261 Start a child SubAgent span nested under this LLM.

Parameters

opts

SubAgentInit

Returns

SubAgent

startTool()

startTool(opts): Tool

Defined in: src/genai/llm.ts:252 Start a child Tool span nested under this LLM.

Parameters

opts

ToolInit

Returns

Tool

think()

think(content): this

Defined in: src/genai/llm.ts:167 Set or extend the model’s reasoning/chain-of-thought content. Accumulates into this.reasoning.content. Folded into the last assistant message as a ReasoningPart at serialization time, matching the Python SDK’s on-the-wire shape.

Parameters

content

string

Returns

this

​Examples

​Extends

​Properties

​inputMessages

​model

​outputMessages

​providerName

​reasoning?

​usage

​Methods

​addEvent()

​Parameters

name

attributes?

startTime?

​Returns

​Example

​Inherited from

​attachMedia()

​Parameters

opts

​Returns

​attachMediaUrl()

​Parameters

url

opts

modality

​Returns

​end()

​Parameters

opts?

​Returns

​output()

​Parameters

content

​Returns

​record()

​Parameters

opts

finishReasons?

inputMessages?

mediaAttachments?

outputMessages?

outputType?

reasoning?

responseId?

responseModel?

usage?

​Returns

​setAttributes()

​Parameters

attributes

​Returns

​Example

​Inherited from

​startSubagent()

​Parameters

opts

​Returns

​startTool()

​Parameters

opts

​Returns

​think()

​Parameters

content

​Returns

Examples

Extends

Properties

inputMessages

model

outputMessages

providerName

reasoning?

usage

Methods

addEvent()

Parameters

Returns

Example

Inherited from

attachMedia()

Parameters

Returns

attachMediaUrl()

Parameters

Returns

end()

Parameters

Returns

output()

Parameters

Returns

record()

Parameters

Returns

setAttributes()

Parameters

Returns

Example

Inherited from

startSubagent()

Parameters

Returns

startTool()

Parameters

Returns

think()

Parameters

Returns