fix: support custom/third-party model paths in Vertex AI provider

When users specify a custom or third-party MaaS model ID (e.g.
"gpt-oss-120b-maas" or "publishers/openai/models/gpt-oss-120b-maas"),
the VertexHandler now passes it through as-is instead of silently
falling back to the default model. This fixes 404 NOT_FOUND errors
when using non-Google models on Vertex AI.

Closes #12074

src/api/providers/__tests__/vertex.spec.ts

@@ -163,5 +163,56 @@ describe("VertexHandler", () => {
 			expect(excludedCount).toBe(1)
 			expect(includedCount).toBe(1)
 		})
+
+		it("should pass through custom/unknown model IDs instead of falling back to default", () => {
+			const testHandler = new VertexHandler({
+				apiModelId: "gpt-oss-120b-maas",
+				vertexProjectId: "test-project",
+				vertexRegion: "us-central1",
+			})
+
+			const modelInfo = testHandler.getModel()
+			// The critical fix: custom model ID is preserved, not replaced with the default
+			expect(modelInfo.id).toBe("gpt-oss-120b-maas")
+			expect(modelInfo.info).toBeDefined()
+		})
+
+		it("should pass through publisher-qualified model paths as-is", () => {
+			const testHandler = new VertexHandler({
+				apiModelId: "publishers/openai/models/gpt-oss-120b-maas",
+				vertexProjectId: "test-project",
+				vertexRegion: "us-central1",
+			})
+
+			const modelInfo = testHandler.getModel()
+			// Full publisher path is passed through unchanged
+			expect(modelInfo.id).toBe("publishers/openai/models/gpt-oss-120b-maas")
+			expect(modelInfo.info).toBeDefined()
+		})
+
+		it("should still apply edit/apply_diff tool preferences to custom models", () => {
+			const testHandler = new VertexHandler({
+				apiModelId: "publishers/openai/models/gpt-oss-120b-maas",
+				vertexProjectId: "test-project",
+				vertexRegion: "us-central1",
+			})
+
+			const modelInfo = testHandler.getModel()
+			expect(modelInfo.info.excludedTools).toContain("apply_diff")
+			expect(modelInfo.info.includedTools).toContain("edit")
+		})
+
+		it("should fall back to default model when no model ID is provided", () => {
+			const testHandler = new VertexHandler({
+				vertexProjectId: "test-project",
+				vertexRegion: "us-central1",
+			})
+
+			const modelInfo = testHandler.getModel()
+			// Should use the default vertex model, not crash
+			expect(modelInfo.id).toBeDefined()
+			expect(modelInfo.info).toBeDefined()
+			expect(modelInfo.info.maxTokens).toBeGreaterThan(0)
+		})
 	})
 })

src/api/providers/vertex.ts

@@ -14,8 +14,34 @@ export class VertexHandler extends GeminiHandler implements SingleCompletionHand
 
 	override getModel() {
 		const modelId = this.options.apiModelId
-		let id = modelId && modelId in vertexModels ? (modelId as VertexModelId) : vertexDefaultModelId
-		let info: ModelInfo = vertexModels[id]
+
+		let id: string
+		let info: ModelInfo
+
+		if (modelId && modelId in vertexModels) {
+			// Known Vertex model -- use its curated ModelInfo.
+			id = modelId as VertexModelId
+			info = vertexModels[id as VertexModelId]
+		} else if (modelId) {
+			// Custom / third-party MaaS model.
+			// Users may supply either a bare model name (e.g. "gpt-oss-120b-maas")
+			// or a fully-qualified publisher path
+			// (e.g. "publishers/openai/models/gpt-oss-120b-maas").
+			// Both forms are passed through as-is; the @google/genai SDK and
+			// Vertex AI API accept full resource names directly.
+			id = modelId
+			info = {
+				maxTokens: 8192,
+				contextWindow: 128_000,
+				supportsImages: true,
+				supportsPromptCache: false,
+			}
+		} else {
+			// No model specified -- fall back to the default.
+			id = vertexDefaultModelId
+			info = vertexModels[vertexDefaultModelId]
+		}
+
 		const params = getModelParams({
 			format: "gemini",
 			modelId: id,