fix(langchain): do not throw in callback handler if usageDetails are unavailable

[marliessophie]

Disclaimer: Experimental PR review

Greptile Summary

This PR fixes a TypeError in handleLLMEnd where the in operator was applied to llmUsage without first verifying it was a non-null object. When llmUsage is undefined — e.g. when neither extractUsageMetadata nor output.llmOutput?.["tokenUsage"] returns a value — expressions like "promptTokens" in llmUsage would throw instead of gracefully producing undefined.

Key changes:

• Introduces a usageObj guard that casts llmUsage to Record<string, any> only when it is both truthy and an object, safely handling undefined, null, and primitive values.
• Replaces explicit property-existence checks with optional chaining on usageObj, which is semantically equivalent for valid objects and safe for non-objects.
• Behavior is fully preserved when llmUsage is a well-formed UsageMetadata or OpenAI tokenUsage object; only the crash path is eliminated.

Confidence Score: 5/5

Safe to merge — the fix correctly eliminates a real TypeError crash with no behaviour change on the happy path.

The single changed file addresses a well-scoped bug (TypeError from in on undefined). No new logic is introduced, existing behaviour on valid objects is preserved, and all remaining feedback is a P2 style suggestion (redundant ?? undefined trailing expressions) that does not affect correctness.

No files require special attention.

Important Files Changed

Filename	Overview
packages/langchain/src/CallbackHandler.ts	Guards llmUsage against non-object values before property-existence checks, preventing a TypeError thrown by the in operator when llmUsage is undefined; logic and behavior preserved in all non-null paths.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[handleLLMEnd called] --> B[extractUsageMetadata]
    B --> C{UsageMetadata returned?}
    C -- yes --> D[llmUsage = UsageMetadata]
    C -- no --> E[llmUsage = output.llmOutput?.tokenUsage]
    E --> F{llmUsage defined and is object?}
    D --> F
    F -- yes --> G[usageObj = llmUsage as Record]
    F -- no --> H[usageObj = undefined]
    G --> I[Build usageDetails via optional chaining on usageObj]
    H --> I
    I --> J[Augment with input_token_details and output_token_details if present]
    J --> K[handleOtelSpanEnd with usageDetails]

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[github-actions]

@claude review

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Project	Deployment	Actions	Updated (UTC)
langfuse-js	Ready	Preview	Mar 31, 2026 1:51pm

[hassiebp]

Closed in favor of #769