feat: add codex fast mode and harden subagent launch

pi/packages/ai/src/api/simple-options.ts

@@ -41,6 +41,7 @@ export function buildBaseOptions(
 		maxRetries: options?.maxRetries,
 		maxRetryDelayMs: options?.maxRetryDelayMs,
 		metadata: options?.metadata,
+		serviceTier: options?.serviceTier,
 		env: options?.env,
 	};
 }

pi/packages/ai/src/types.ts

@@ -113,6 +113,13 @@ export interface ProviderResponse {
 	headers: Record<string, string>;
 }
 
+/**
+ * Request-level service tier for providers that support it.
+ * OpenAI Codex / Responses: "priority" is fast mode (higher cost, lower latency), "flex" is the cheaper tier.
+ * See https://developers.openai.com/codex/speed for Codex fast mode semantics.
+ */
+export type ServiceTier = "auto" | "default" | "flex" | "priority" | "scale";
+
 export interface StreamOptions {
 	temperature?: number;
 	maxTokens?: number;
@@ -189,6 +196,12 @@ export interface StreamOptions {
 	 * For example, Anthropic uses `user_id` for abuse tracking and rate limiting.
 	 */
 	metadata?: Record<string, unknown>;
+	/**
+	 * Request-level service tier. Providers that understand it (OpenAI Codex,
+	 * OpenAI Responses) send it as `service_tier`; other providers ignore it.
+	 * "priority" is Codex fast mode (1.5x speed, ~2x cost).
+	 */
+	serviceTier?: ServiceTier | null;
 	/**
 	 * Provider-scoped environment values. These take precedence over process.env for
 	 * provider configuration such as regional settings, endpoint placeholders, and
@@ -773,6 +786,12 @@ export interface Model<TApi extends Api> {
 	contextWindow: number;
 	maxTokens: number;
 	headers?: Record<string, string>;
+	/**
+	 * Optional default request-level service tier for this model (e.g. "priority"
+	 * for Codex fast mode). Configured via models.json; forwarded to providers
+	 * that support `service_tier`.
+	 */
+	serviceTier?: ServiceTier;
 	/** Compatibility overrides for OpenAI-compatible APIs. If not set, auto-detected from baseUrl. */
 	compat?: TApi extends "openai-completions"
 		? OpenAICompletionsCompat

pi/packages/coding-agent/src/core/model-config.ts

@@ -151,6 +151,13 @@ const ModelCostSchema = Type.Object({
 	tiers: Type.Optional(Type.Array(ModelCostTierSchema)),
 });
 
+const ServiceTierSchema = Type.Union([
+	Type.Literal("auto"),
+	Type.Literal("default"),
+	Type.Literal("priority"),
+	Type.Literal("flex"),
+]);
+
 const ModelDefinitionSchema = Type.Object({
 	id: Type.String({ minLength: 1 }),
 	name: Type.Optional(Type.String({ minLength: 1 })),
@@ -163,6 +170,7 @@ const ModelDefinitionSchema = Type.Object({
 	contextWindow: Type.Optional(Type.Number()),
 	maxTokens: Type.Optional(Type.Number()),
 	headers: Type.Optional(Type.Record(Type.String(), Type.String())),
+	serviceTier: Type.Optional(ServiceTierSchema),
 	compat: Type.Optional(ProviderCompatSchema),
 });
 
@@ -183,6 +191,7 @@ const ModelOverrideSchema = Type.Object({
 	contextWindow: Type.Optional(Type.Number()),
 	maxTokens: Type.Optional(Type.Number()),
 	headers: Type.Optional(Type.Record(Type.String(), Type.String())),
+	serviceTier: Type.Optional(ServiceTierSchema),
 	compat: Type.Optional(ProviderCompatSchema),
 });
 

pi/packages/coding-agent/src/core/provider-composer.ts

@@ -117,6 +117,7 @@ function applyModelOverride(model: Model<Api>, override: ModelsJsonModelOverride
 			: model.cost,
 		contextWindow: override.contextWindow ?? model.contextWindow,
 		maxTokens: override.maxTokens ?? model.maxTokens,
+		serviceTier: override.serviceTier ?? model.serviceTier,
 		compat: mergeCompat(model.compat, override.compat),
 	};
 }
@@ -154,6 +155,7 @@ function modelFromJson(
 		contextWindow: definition.contextWindow ?? 128000,
 		maxTokens: definition.maxTokens ?? 16384,
 		headers: undefined,
+		serviceTier: definition.serviceTier,
 		compat: mergeCompat(providerConfig.compat, definition.compat),
 	};
 }

pi/packages/coding-agent/src/core/sdk.ts

@@ -309,8 +309,13 @@ export async function createAgentSession(options: CreateAgentSessionOptions = {}
 			const websocketConnectTimeoutMs =
 				options?.websocketConnectTimeoutMs ?? settingsManager.getWebSocketConnectTimeoutMs();
 			const headerRunner = extensionRunnerRef.current;
+			// Model-level service tier (models.json `serviceTier`, e.g. "priority" for
+			// Codex fast mode) is forwarded as request `service_tier`. Providers that
+			// don't understand it ignore the option.
+			const serviceTier = model.serviceTier;
 			return modelRuntime.streamSimple(model, context, {
 				...options,
+				...(serviceTier ? { serviceTier } : {}),
 				timeoutMs,
 				websocketConnectTimeoutMs,
 				maxRetries: options?.maxRetries ?? providerRetrySettings.maxRetries,