[BOT ISSUE] LangChain4j EmbeddingModel calls are not instrumented

まず BraintrustLangchain.java と LangchainInstrumentationModule.java を読み、既存の chat-model wrapper と builder advice に注目します。embedding モデルと builder をどのようにカバーするべきかを判断する前に、WrappedHttpClient と BraintrustLangchainTest.java を確認します。対応する LangChain4j の embedding 呼び出しが入力/出力のキャプチャと使用量メトリクスを含む Braintrust スパンを生成し、新しいインストゥルメンテーションをテストでカバーできれば完了です。

Summary

The LangChain4j instrumentation wraps OpenAiChatModel, OpenAiStreamingChatModel, and AiServices, but does not instrument any EmbeddingModel implementation. Calls to embeddingModel.embed() or embeddingModel.embedAll() through LangChain4j produce no Braintrust spans, no input/output capture, and no usage metrics.

This is distinct from #60 (which covers missing non-OpenAI chat model providers) and from #71 (which covers Spring AI embedding gaps). LangChain4j's EmbeddingModel is a separate interface from ChatLanguageModel with its own implementations, builders, and HTTP transport.

What is missing

No wrapper methods for EmbeddingModel

BraintrustLangchain.java (lines 22–152) only handles:

• AiServices<T> wrapping (lines 22–85)
• OpenAiChatModel wrapping (lines 87–118)
• OpenAiStreamingChatModel wrapping (lines 120–152)

There is no wrap(OpenTelemetry, EmbeddingModel) method or any reference to embedding models.

No auto-instrumentation for embedding builders

LangchainInstrumentationModule.java (lines 44–145) registers ByteBuddy advice for exactly 3 builder classes:

• OpenAiChatModel$OpenAiChatModelBuilder
• OpenAiStreamingChatModel$OpenAiStreamingChatModelBuilder
• AiServices

No embedding model builder (e.g., OpenAiEmbeddingModel$OpenAiEmbeddingModelBuilder) is targeted.

No fallback to upstream SDK auto-instrumentation

LangChain4j embedding model implementations use LangChain4j's own internal dev.langchain4j.http.client.HttpClient, not the upstream provider SDKs (com.openai:openai-java, etc.). This means the OpenAI auto-instrumentation module (openai_2_8_0) does not intercept embedding calls made through LangChain4j's OpenAiEmbeddingModel.

The WrappedHttpClient class in the LangChain4j module already wraps LangChain4j's internal HTTP client for chat models — the same pattern could be applied to embedding models.

Affected LangChain4j embedding models

LangChain4j class	Provider
OpenAiEmbeddingModel	OpenAI
AzureOpenAiEmbeddingModel	Azure OpenAI
GoogleAiEmbeddingModel	Google AI
VertexAiEmbeddingModel	Google Vertex AI
OllamaEmbeddingModel	Ollama
MistralAiEmbeddingModel	Mistral AI

Braintrust docs status

• LangChain4j is not mentioned on the Braintrust integrations page at https://www.braintrust.dev/docs/integrations/ai-providers: not_found
• No LangChain4j-specific documentation exists for the Java SDK

Upstream sources

• LangChain4j EmbeddingModel interface: https://github.com/langchain4j/langchain4j — EmbeddingModel is a core interface with embed(String), embed(TextSegment), and embedAll(List<TextSegment>) methods
• LangChain4j embedding model implementations: https://docs.langchain4j.dev/integrations/embedding-models — documents 15+ embedding model providers
• LangChain4j OpenAI embedding model: langchain4j-open-ai module provides OpenAiEmbeddingModel as a first-party integration

Local files inspected

• braintrust-sdk/instrumentation/langchain_1_8_0/src/main/java/dev/braintrust/instrumentation/langchain/v1_8_0/BraintrustLangchain.java — lines 22–152 (only chat model and AiServices wrapping; no embedding model handling)
• braintrust-sdk/instrumentation/langchain_1_8_0/src/main/java/dev/braintrust/instrumentation/langchain/v1_8_0/auto/LangchainInstrumentationModule.java — lines 44–145 (only chat model builders and AiServices targeted; no embedding builder)
• braintrust-sdk/instrumentation/langchain_1_8_0/src/main/java/dev/braintrust/instrumentation/langchain/v1_8_0/WrappedHttpClient.java — existing HTTP client wrapper that could be reused for embedding models
• braintrust-sdk/instrumentation/langchain_1_8_0/src/test/java/dev/braintrust/instrumentation/langchain/v1_8_0/BraintrustLangchainTest.java — no embedding model tests exist