One tool call, counted twice: a Google GenAI streaming double-dip in Sentry's JS SDK

This is a submission for DEV's Summer Bug Smash: Clear the Lineup powered by Sentry . The bug When you call @google/genai in streaming mode and the model asks to run a tool, Sentry's JavaScript SDK records that tool call to the span twice. One tool call in, two entries out. The attribute that carries them is gen_ai.response.tool_calls . It should hold one object per call. For a single streamed controlLight call it held two. Worse, the two did not even agree on their shape. Here is a real capture, which I come back to at the end: [ { "id" : "call_2079699" , "args" :{ "colorTemperature" : "warm" , "brightness" : 30 }, "name" : "controlLight" }, { "type" : "function" , "id" : "call_2079699" , "name" : "controlLight" , "arguments" :{ "colorTemperature" : "warm" , "brightness" : 30 }} ] Same id, same call, listed twice. One entry keys the parameters under args , the other under arguments . Anything reading this later sees two tool invocations where the model made one. Following the value The streaming instrumentation lives in packages/server-utils/src/ai/google-genai/streaming.ts . Every chunk of the stream runs through handleCandidateContent . That function wrote tool calls from two places: function handleCandidateContent ( chunk , state , recordOutputs ) { if ( Array . isArray ( chunk . functionCalls )) { state . toolCalls . push (... chunk . functionCalls ); // push #1 } for ( const candidate of chunk . candidates ?? []) { // ...finish reasons... for ( const part of candidate ?. content ?. parts ?? []) { if ( recordOutputs && part . text ) state . responseTexts . push ( part . text ); if ( part . functionCall ) { state . toolCalls . push ({ // push #2 type : ' function ' , id : part . functionCall . id , name : part . functionCall . name , arguments : part . functionCall . args , }); } } } } Push #1 spreads chunk.functionCalls into the accumulator. Push #2 walks candidate.content.parts and pushes every functionCall it finds. They look like two different sources. They