diff --git a/packages/types/src/providers/openai-codex.ts b/packages/types/src/providers/openai-codex.ts
index e9cf5e170c2..051ef4f138e 100644
--- a/packages/types/src/providers/openai-codex.ts
+++ b/packages/types/src/providers/openai-codex.ts
@@ -41,6 +41,23 @@ export const openAiCodexModels = {
 		supportsTemperature: false,
 		description: "GPT-5.1 Codex Max: Maximum capability coding model via ChatGPT subscription",
 	},
+	"gpt-5.1-codex": {
+		maxTokens: 128000,
+		contextWindow: 400000,
+		supportsNativeTools: true,
+		defaultToolProtocol: "native",
+		includedTools: ["apply_patch"],
+		excludedTools: ["apply_diff", "write_to_file"],
+		supportsImages: true,
+		supportsPromptCache: true,
+		supportsReasoningEffort: ["low", "medium", "high"],
+		reasoningEffort: "medium",
+		// Subscription-based: no per-token costs
+		inputPrice: 0,
+		outputPrice: 0,
+		supportsTemperature: false,
+		description: "GPT-5.1 Codex: GPT-5.1 optimized for agentic coding via ChatGPT subscription",
+	},
 	"gpt-5.2-codex": {
 		maxTokens: 128000,
 		contextWindow: 400000,
@@ -57,6 +74,76 @@ export const openAiCodexModels = {
 		supportsTemperature: false,
 		description: "GPT-5.2 Codex: OpenAI's flagship coding model via ChatGPT subscription",
 	},
+	"gpt-5.1": {
+		maxTokens: 128000,
+		contextWindow: 400000,
+		supportsNativeTools: true,
+		defaultToolProtocol: "native",
+		includedTools: ["apply_patch"],
+		excludedTools: ["apply_diff", "write_to_file"],
+		supportsImages: true,
+		supportsPromptCache: true,
+		supportsReasoningEffort: ["none", "low", "medium", "high"],
+		reasoningEffort: "medium",
+		// Subscription-based: no per-token costs
+		inputPrice: 0,
+		outputPrice: 0,
+		supportsVerbosity: true,
+		supportsTemperature: false,
+		description: "GPT-5.1: General GPT-5.1 model via ChatGPT subscription",
+	},
+	"gpt-5": {
+		maxTokens: 128000,
+		contextWindow: 400000,
+		supportsNativeTools: true,
+		defaultToolProtocol: "native",
+		includedTools: ["apply_patch"],
+		excludedTools: ["apply_diff", "write_to_file"],
+		supportsImages: true,
+		supportsPromptCache: true,
+		supportsReasoningEffort: ["minimal", "low", "medium", "high"],
+		reasoningEffort: "medium",
+		// Subscription-based: no per-token costs
+		inputPrice: 0,
+		outputPrice: 0,
+		supportsVerbosity: true,
+		supportsTemperature: false,
+		description: "GPT-5: General GPT-5 model via ChatGPT subscription",
+	},
+	"gpt-5-codex": {
+		maxTokens: 128000,
+		contextWindow: 400000,
+		supportsNativeTools: true,
+		defaultToolProtocol: "native",
+		includedTools: ["apply_patch"],
+		excludedTools: ["apply_diff", "write_to_file"],
+		supportsImages: true,
+		supportsPromptCache: true,
+		supportsReasoningEffort: ["low", "medium", "high"],
+		reasoningEffort: "medium",
+		// Subscription-based: no per-token costs
+		inputPrice: 0,
+		outputPrice: 0,
+		supportsTemperature: false,
+		description: "GPT-5 Codex: GPT-5 optimized for agentic coding via ChatGPT subscription",
+	},
+	"gpt-5-codex-mini": {
+		maxTokens: 128000,
+		contextWindow: 400000,
+		supportsNativeTools: true,
+		defaultToolProtocol: "native",
+		includedTools: ["apply_patch"],
+		excludedTools: ["apply_diff", "write_to_file"],
+		supportsImages: true,
+		supportsPromptCache: true,
+		supportsReasoningEffort: ["low", "medium", "high"],
+		reasoningEffort: "medium",
+		// Subscription-based: no per-token costs
+		inputPrice: 0,
+		outputPrice: 0,
+		supportsTemperature: false,
+		description: "GPT-5 Codex Mini: Faster coding model via ChatGPT subscription",
+	},
 	"gpt-5.1-codex-mini": {
 		maxTokens: 128000,
 		contextWindow: 400000,
diff --git a/src/api/providers/__tests__/openai-codex.spec.ts b/src/api/providers/__tests__/openai-codex.spec.ts
new file mode 100644
index 00000000000..f35d6e61ee7
--- /dev/null
+++ b/src/api/providers/__tests__/openai-codex.spec.ts
@@ -0,0 +1,26 @@
+// npx vitest run api/providers/__tests__/openai-codex.spec.ts
+
+import { OpenAiCodexHandler } from "../openai-codex"
+
+describe("OpenAiCodexHandler.getModel", () => {
+	it.each(["gpt-5.1", "gpt-5", "gpt-5.1-codex", "gpt-5-codex", "gpt-5-codex-mini"])(
+		"should return specified model when a valid model id is provided: %s",
+		(apiModelId) => {
+			const handler = new OpenAiCodexHandler({ apiModelId })
+			const model = handler.getModel()
+
+			expect(model.id).toBe(apiModelId)
+			expect(model.info).toBeDefined()
+			// Default reasoning effort for GPT-5 family
+			expect(model.info.reasoningEffort).toBe("medium")
+		},
+	)
+
+	it("should fall back to default model when an invalid model id is provided", () => {
+		const handler = new OpenAiCodexHandler({ apiModelId: "not-a-real-model" })
+		const model = handler.getModel()
+
+		expect(model.id).toBe("gpt-5.2-codex")
+		expect(model.info).toBeDefined()
+	})
+})
diff --git a/webview-ui/src/components/settings/ApiOptions.tsx b/webview-ui/src/components/settings/ApiOptions.tsx
index 1012b73263f..d10e4cb3dc5 100644
--- a/webview-ui/src/components/settings/ApiOptions.tsx
+++ b/webview-ui/src/components/settings/ApiOptions.tsx
@@ -311,6 +311,7 @@ const ApiOptions = ({
 			// To address that we set the modelId to the default value for th
 			// provider if it's not already set.
 			const validateAndResetModel = (
+				provider: ProviderName,
 				modelId: string | undefined,
 				field: keyof ProviderSettings,
 				defaultValue?: string,
@@ -318,11 +319,32 @@ const ApiOptions = ({
 				// in case we haven't set a default value for a provider
 				if (!defaultValue) return
 
-				// only set default if no model is set, but don't reset invalid models
-				// let users see and decide what to do with invalid model selections
-				const shouldSetDefault = !modelId
+				// 1) If nothing is set, initialize to the provider default.
+				if (!modelId) {
+					setApiConfigurationField(field, defaultValue, false)
+					return
+				}
+
+				// 2) If something *is* set, ensure it's valid for the newly selected provider.
+				//
+				// Without this, switching providers can leave the UI showing a model from the
+				// previously selected provider (including model IDs that don't exist for the
+				// newly selected provider).
+				//
+				// Note: We only validate providers with static model lists.
+				const staticModels = MODELS_BY_PROVIDER[provider]
+				if (!staticModels) {
+					return
+				}
+
+				// Bedrock has a special “custom-arn” pseudo-model that isn't part of MODELS_BY_PROVIDER.
+				if (provider === "bedrock" && modelId === "custom-arn") {
+					return
+				}
 
-				if (shouldSetDefault) {
+				const filteredModels = filterModels(staticModels, provider, organizationAllowList)
+				const isValidModel = !!filteredModels && Object.prototype.hasOwnProperty.call(filteredModels, modelId)
+				if (!isValidModel) {
 					setApiConfigurationField(field, defaultValue, false)
 				}
 			}
@@ -381,13 +403,14 @@ const ApiOptions = ({
 			const config = PROVIDER_MODEL_CONFIG[value]
 			if (config) {
 				validateAndResetModel(
+					value,
 					apiConfiguration[config.field] as string | undefined,
 					config.field,
 					config.default,
 				)
 			}
 		},
-		[setApiConfigurationField, apiConfiguration],
+		[setApiConfigurationField, apiConfiguration, organizationAllowList],
 	)
 
 	const modelValidationError = useMemo(() => {
diff --git a/webview-ui/src/components/settings/__tests__/ApiOptions.spec.tsx b/webview-ui/src/components/settings/__tests__/ApiOptions.spec.tsx
index c835e3062df..c8bcc72c0af 100644
--- a/webview-ui/src/components/settings/__tests__/ApiOptions.spec.tsx
+++ b/webview-ui/src/components/settings/__tests__/ApiOptions.spec.tsx
@@ -4,6 +4,7 @@ import { render, screen, fireEvent } from "@/utils/test-utils"
 import { QueryClient, QueryClientProvider } from "@tanstack/react-query"
 
 import { type ModelInfo, type ProviderSettings, openAiModelInfoSaneDefaults } from "@roo-code/types"
+import { openAiCodexDefaultModelId } from "@roo-code/types"
 
 import * as ExtensionStateContext from "@src/context/ExtensionStateContext"
 const { ExtensionStateContextProvider } = ExtensionStateContext
@@ -297,6 +298,31 @@ const renderApiOptions = (props: Partial<ApiOptionsProps> = {}) => {
 }
 
 describe("ApiOptions", () => {
+	it("resets model to provider default when switching to openai-codex with an invalid prior apiModelId", () => {
+		const mockSetApiConfigurationField = vi.fn()
+
+		renderApiOptions({
+			apiConfiguration: {
+				apiProvider: "anthropic",
+				// Simulate a previously-selected model ID from another provider.
+				// When switching to OpenAI - ChatGPT Plus/Pro, this is invalid and should be reset.
+				apiModelId: "claude-3-5-sonnet-20241022",
+			},
+			setApiConfigurationField: mockSetApiConfigurationField,
+		})
+
+		const providerSelectContainer = screen.getByTestId("provider-select")
+		const providerSelect = providerSelectContainer.querySelector("select") as HTMLSelectElement
+		expect(providerSelect).toBeInTheDocument()
+
+		fireEvent.change(providerSelect, { target: { value: "openai-codex" } })
+
+		// Provider is updated
+		expect(mockSetApiConfigurationField).toHaveBeenCalledWith("apiProvider", "openai-codex")
+		// Model is reset to the provider default since the previous value is invalid for this provider
+		expect(mockSetApiConfigurationField).toHaveBeenCalledWith("apiModelId", openAiCodexDefaultModelId, false)
+	})
+
 	it("shows diff settings, temperature and rate limit controls by default", () => {
 		renderApiOptions({
 			apiConfiguration: {