fix: ensure Vertex Claude Sonnet 1M context window is fully supported in streaming and UI (PR #7848)

Author: victoraranda

src/api/providers/anthropic-vertex.ts

@@ -104,6 +104,10 @@ export class AnthropicVertexHandler extends BaseProvider implements SingleComple
 				: systemPrompt,
 			messages: supportsPromptCache ? addCacheBreakpoints(messages) : messages,
 			stream: true,
+			// Enable 1M context window if flag is set and model matches
+			...(id === ANTHROPIC_VERTEX_1M_CONTEXT_MODEL_ID && this.options.vertex1MContext
+				? { additionalModelRequestFields: { anthropic_beta: ["context-1m-2025-08-07"] } }
+				: {}),
 		}
 
 		const stream = await this.client.messages.create(params)
@@ -220,6 +224,9 @@ export class AnthropicVertexHandler extends BaseProvider implements SingleComple
 					},
 				],
 				stream: false,
+				...(id === ANTHROPIC_VERTEX_1M_CONTEXT_MODEL_ID && this.options.vertex1MContext
+					? { additionalModelRequestFields: { anthropic_beta: ["context-1m-2025-08-07"] } }
+					: {}),
 			}
 
 			const response = await this.client.messages.create(params)

webview-ui/src/components/ui/hooks/useSelectedModel.ts

@@ -214,8 +214,16 @@ function getSelectedModel({
 		}
 		case "vertex": {
 			const id = apiConfiguration.apiModelId ?? vertexDefaultModelId
-			const info = vertexModels[id as keyof typeof vertexModels]
-			return { id, info }
+			const baseInfo = vertexModels[id as keyof typeof vertexModels]
+			// Apply 1M context for Claude Sonnet 4 when enabled
+			if (id === "claude-sonnet-4@20250514" && apiConfiguration.vertex1MContext && baseInfo) {
+				const info: ModelInfo = {
+					...baseInfo,
+					contextWindow: 1_000_000,
+				}
+				return { id, info }
+			}
+			return { id, info: baseInfo }
 		}
 		case "gemini": {
 			const id = apiConfiguration.apiModelId ?? geminiDefaultModelId