diff --git a/entry b/entry
index 2072553..4c4efee 100755
--- a/entry
+++ b/entry
@@ -137869,18 +137869,14 @@ function resolveAgent(params) {
   log.debug(
     `\xBB determineAgent: agentOverride=${agentOverride}, payload.agent=${params.payload.agent}, repoSettings.defaultAgent=${params.repoSettings.defaultAgent}`
   );
-  const configuredAgentName = agentOverride || params.payload.agent || params.repoSettings.defaultAgent || null;
+  const configuredAgentName = agentOverride || params.payload.agent || params.repoSettings.defaultAgent || void 0;
   if (configuredAgentName) {
     const agent3 = agents[configuredAgentName];
     if (!agent3) {
       throw new Error(`invalid agent name: ${configuredAgentName}`);
     }
-    const isExplicitOverride = agentOverride !== void 0 || params.payload.agent !== null;
-    if (isExplicitOverride) {
-      log.info(`\xBB selected configured agent: ${agent3.name}`);
-      return agent3;
-    }
-    if (agentHasApiKeys(agent3)) {
+    const isExplicitOverride = agentOverride !== void 0 || params.payload.agent !== void 0;
+    if (isExplicitOverride || agentHasApiKeys(agent3)) {
       log.info(`\xBB selected configured agent: ${agent3.name}`);
       return agent3;
     }
@@ -138183,11 +138179,11 @@ var ToolPermissionInput = type.enumerated("disabled", "enabled");
 var BashPermissionInput = type.enumerated("disabled", "restricted", "enabled");
 var JsonPayload = type({
   "~pullfrog": "true",
-  "agent?": AgentName.or("null"),
+  "agent?": AgentName.or("undefined"),
   "prompt?": "string",
   "repoInstructions?": "string",
   "event?": "object",
-  "effort?": Effort
+  "effort?": Effort.or("undefined")
 });
 var COLLABORATOR_PERMISSIONS = ["admin", "maintain", "write"];
 function isCollaborator(event) {
@@ -138196,13 +138192,13 @@ function isCollaborator(event) {
 }
 var Inputs = type({
   prompt: "string",
-  "effort?": Effort,
-  "agent?": AgentName.or("null"),
+  "effort?": Effort.or("undefined"),
+  "agent?": AgentName.or("undefined"),
   "web?": ToolPermissionInput.or("undefined"),
   "search?": ToolPermissionInput.or("undefined"),
   "write?": ToolPermissionInput.or("undefined"),
   "bash?": BashPermissionInput.or("undefined"),
-  "cwd?": "string|null"
+  "cwd?": type.string.or("undefined")
 });
 function isAgentName(value2) {
   return typeof value2 === "string" && AgentName(value2) instanceof type.errors === false;
@@ -138212,7 +138208,7 @@ function isPayloadEvent(value2) {
 }
 function resolveCwd(cwd2) {
   const workspace = process.env.GITHUB_WORKSPACE;
-  if (!cwd2) return workspace ?? null;
+  if (!cwd2) return workspace;
   if (isAbsolute(cwd2)) return cwd2;
   return workspace ? resolve(workspace, cwd2) : cwd2;
 }
@@ -138220,14 +138216,14 @@ function resolvePayload(repoSettings) {
   const inputs = Inputs.assert({
     prompt: core4.getInput("prompt", { required: true }),
     effort: core4.getInput("effort") || void 0,
-    agent: core4.getInput("agent") || null,
-    cwd: core4.getInput("cwd") || null,
+    agent: core4.getInput("agent") || void 0,
+    cwd: core4.getInput("cwd") || void 0,
     web: core4.getInput("web") || void 0,
     search: core4.getInput("search") || void 0,
     write: core4.getInput("write") || void 0,
     bash: core4.getInput("bash") || void 0
   });
-  const agent2 = inputs.agent !== void 0 && inputs.agent !== "null" && isAgentName(inputs.agent) ? inputs.agent : null;
+  const agent2 = inputs.agent !== void 0 && isAgentName(inputs.agent) ? inputs.agent : void 0;
   let jsonPayload = null;
   try {
     const parsed2 = JSON.parse(inputs.prompt);
@@ -138242,7 +138238,7 @@ function resolvePayload(repoSettings) {
   const rawEvent = jsonPayload?.event;
   const event = isPayloadEvent(rawEvent) ? rawEvent : { trigger: "unknown" };
   const jsonAgent = jsonPayload?.agent;
-  const resolvedAgent = agent2 ?? (jsonAgent !== void 0 && jsonAgent !== "null" && isAgentName(jsonAgent) ? jsonAgent : null);
+  const resolvedAgent = agent2 ?? (jsonAgent !== void 0 && isAgentName(jsonAgent) ? jsonAgent : void 0);
   const shouldRestrict = !isCollaborator(event);
   return {
     "~pullfrog": true,
diff --git a/external.ts b/external.ts
index b4a3d60..daedf78 100644
--- a/external.ts
+++ b/external.ts
@@ -245,7 +245,7 @@ export type PayloadEvent =
 export interface WriteablePayload {
   "~pullfrog": true;
   /** agent slug identifier (e.g., "claude", "codex", "gemini") */
-  agent: AgentName | null;
+  agent?: AgentName | undefined;
   /** the prompt/instructions for the agent to execute (body if @pullfrog tagged + per-trigger instructions) */
   prompt: string;
   /** repo-level instructions (macro-expanded server-side) */
@@ -255,7 +255,7 @@ export interface WriteablePayload {
   /** effort level for model selection (mini, auto, max) - defaults to "auto" */
   effort?: Effort | undefined;
   /** working directory for the agent */
-  cwd?: string | null | undefined;
+  cwd?: string | undefined;
 }
 
 // immutable payload type for agent execution
diff --git a/test/fixtures/bash-test.ts b/test/fixtures/bash-test.ts
index 4cdd8d4..764c0c6 100644
--- a/test/fixtures/bash-test.ts
+++ b/test/fixtures/bash-test.ts
@@ -14,7 +14,6 @@ import type { Payload } from "../../external.ts";
  */
 export default {
   "~pullfrog": true,
-  agent: null,
   prompt: `Create a file called test-runner.js with the following content:
 
 \`\`\`javascript
diff --git a/test/fixtures/basic.ts b/test/fixtures/basic.ts
index b464d1d..b252353 100644
--- a/test/fixtures/basic.ts
+++ b/test/fixtures/basic.ts
@@ -2,7 +2,6 @@ import type { Payload } from "../../external.ts";
 
 export default {
   "~pullfrog": true,
-  agent: null,
   prompt:
     "List all files in the current directory, then create a file called dynamic-test.txt with the content 'This was loaded from a TypeScript file!', then delete it.",
   event: {
diff --git a/utils/agent.ts b/utils/agent.ts
index 1804230..09c19ee 100644
--- a/utils/agent.ts
+++ b/utils/agent.ts
@@ -28,7 +28,7 @@ export function resolveAgent(params: {
     `» determineAgent: agentOverride=${agentOverride}, payload.agent=${params.payload.agent}, repoSettings.defaultAgent=${params.repoSettings.defaultAgent}`
   );
   const configuredAgentName =
-    agentOverride || params.payload.agent || params.repoSettings.defaultAgent || null;
+    agentOverride || params.payload.agent || params.repoSettings.defaultAgent || undefined;
 
   if (configuredAgentName) {
     const agent = agents[configuredAgentName];
@@ -38,14 +38,9 @@ export function resolveAgent(params: {
 
     // if explicitly configured (via override or payload), respect it even without matching keys
     // this allows users to force an agent selection (will fail later with clear error if no keys)
-    const isExplicitOverride = agentOverride !== undefined || params.payload.agent !== null;
-    if (isExplicitOverride) {
-      log.info(`» selected configured agent: ${agent.name}`);
-      return agent;
-    }
-
-    // for repo-level defaults, check if agent has matching keys before selecting
-    if (agentHasApiKeys(agent)) {
+    // for repo-level defaults, only select if agent has matching API keys
+    const isExplicitOverride = agentOverride !== undefined || params.payload.agent !== undefined;
+    if (isExplicitOverride || agentHasApiKeys(agent)) {
       log.info(`» selected configured agent: ${agent.name}`);
       return agent;
     }
diff --git a/utils/payload.ts b/utils/payload.ts
index 881be48..c590ed6 100644
--- a/utils/payload.ts
+++ b/utils/payload.ts
@@ -1,13 +1,7 @@
 import { isAbsolute, resolve } from "node:path";
 import * as core from "@actions/core";
 import { type } from "arktype";
-import {
-  AgentName,
-  type AgentName as AgentNameType,
-  type AuthorPermission,
-  Effort,
-  type PayloadEvent,
-} from "../external.ts";
+import { AgentName, type AuthorPermission, Effort, type PayloadEvent } from "../external.ts";
 import type { RepoSettings } from "./repoSettings.ts";
 
 // tool permission enum types for inputs
@@ -19,11 +13,11 @@ const BashPermissionInput = type.enumerated("disabled", "restricted", "enabled")
 // permissions are derived from event.authorPermission instead
 const JsonPayload = type({
   "~pullfrog": "true",
-  "agent?": AgentName.or("null"),
+  "agent?": AgentName.or("undefined"),
   "prompt?": "string",
   "repoInstructions?": "string",
   "event?": "object",
-  "effort?": Effort,
+  "effort?": Effort.or("undefined"),
 });
 
 // permission levels that indicate collaborator status (have push access)
@@ -42,18 +36,18 @@ function isCollaborator(event: PayloadEvent): boolean {
 // if included, must match the type - so we need to explicitly allow undefined.
 export const Inputs = type({
   prompt: "string",
-  "effort?": Effort,
-  "agent?": AgentName.or("null"),
+  "effort?": Effort.or("undefined"),
+  "agent?": AgentName.or("undefined"),
   "web?": ToolPermissionInput.or("undefined"),
   "search?": ToolPermissionInput.or("undefined"),
   "write?": ToolPermissionInput.or("undefined"),
   "bash?": BashPermissionInput.or("undefined"),
-  "cwd?": "string|null",
+  "cwd?": type.string.or("undefined"),
 });
 
 export type Inputs = typeof Inputs.infer;
 
-function isAgentName(value: unknown): value is AgentNameType {
+function isAgentName(value: unknown): value is AgentName {
   return typeof value === "string" && AgentName(value) instanceof type.errors === false;
 }
 
@@ -61,9 +55,9 @@ function isPayloadEvent(value: unknown): value is PayloadEvent {
   return typeof value === "object" && value !== null && "trigger" in value;
 }
 
-function resolveCwd(cwd: string | null | undefined): string | null {
+function resolveCwd(cwd: string | undefined): string | undefined {
   const workspace = process.env.GITHUB_WORKSPACE;
-  if (!cwd) return workspace ?? null;
+  if (!cwd) return workspace;
   if (isAbsolute(cwd)) return cwd;
   return workspace ? resolve(workspace, cwd) : cwd;
 }
@@ -72,19 +66,17 @@ export function resolvePayload(repoSettings: RepoSettings) {
   const inputs = Inputs.assert({
     prompt: core.getInput("prompt", { required: true }),
     effort: core.getInput("effort") || undefined,
-    agent: core.getInput("agent") || null,
-    cwd: core.getInput("cwd") || null,
+    agent: core.getInput("agent") || undefined,
+    cwd: core.getInput("cwd") || undefined,
     web: core.getInput("web") || undefined,
     search: core.getInput("search") || undefined,
     write: core.getInput("write") || undefined,
     bash: core.getInput("bash") || undefined,
   });
 
-  // convert "null" string to null, validate agent name
-  const agent: AgentNameType | null =
-    inputs.agent !== undefined && inputs.agent !== "null" && isAgentName(inputs.agent)
-      ? inputs.agent
-      : null;
+  // validate agent name
+  const agent: AgentName | undefined =
+    inputs.agent !== undefined && isAgentName(inputs.agent) ? inputs.agent : undefined;
 
   // try to parse prompt as JSON payload (internal invocation)
   let jsonPayload: typeof JsonPayload.infer | null = null;
@@ -108,9 +100,8 @@ export function resolvePayload(repoSettings: RepoSettings) {
 
   // resolve agent from jsonPayload with type guard
   const jsonAgent = jsonPayload?.agent;
-  const resolvedAgent: AgentNameType | null =
-    agent ??
-    (jsonAgent !== undefined && jsonAgent !== "null" && isAgentName(jsonAgent) ? jsonAgent : null);
+  const resolvedAgent: AgentName | undefined =
+    agent ?? (jsonAgent !== undefined && isAgentName(jsonAgent) ? jsonAgent : undefined);
 
   // determine if permissions should be restricted based on event author
   // non-collaborators (read, triage, none, or missing) get restricted bash access