Fix/vertex token count (RooCodeInc#2893)

* feat: add token usage metadata handling in VertexHandler

* feat: add cost calculation for API usage in VertexHandler

* fix: correct vertexai token count calculation

Author: mzsima

.changeset/pretty-eels-breathe.md

@@ -0,0 +1,5 @@
+---
+"claude-dev": patch
+---
+
+fix vertexai token count

src/api/providers/vertex.ts

@@ -5,6 +5,7 @@ import { ApiHandler } from "../"
 import { ApiHandlerOptions, ModelInfo, vertexDefaultModelId, VertexModelId, vertexModels } from "../../shared/api"
 import { ApiStream } from "../transform/stream"
 import { VertexAI } from "@google-cloud/vertexai"
+import { calculateApiCostOpenAI } from "../../utils/cost"
 
 // https://docs.anthropic.com/en/api/claude-on-vertex-ai
 export class VertexHandler implements ApiHandler {
@@ -266,6 +267,17 @@ export class VertexHandler implements ApiHandler {
 					}
 				}
 			}
+			// Handle token usage metadata
+			const { usageMetadata } = await streamingResult.response
+			if (usageMetadata) {
+				const { promptTokenCount = 0, candidatesTokenCount = 0 } = usageMetadata
+				yield {
+					type: "usage",
+					inputTokens: promptTokenCount,
+					outputTokens: candidatesTokenCount,
+					totalCost: calculateApiCostOpenAI(model.info, promptTokenCount, candidatesTokenCount, 0, 0),
+				}
+			}
 		}
 	}