diff --git a/packages/server/src/server/agent/agent-manager.ts b/packages/server/src/server/agent/agent-manager.ts
index 9a10ac2a4..c6272dc57 100644
--- a/packages/server/src/server/agent/agent-manager.ts
+++ b/packages/server/src/server/agent/agent-manager.ts
@@ -22,8 +22,6 @@ import type {
 } from "./agent-sdk-types.js";
 import { resolveAgentModel } from "./model-resolver.js";
 import type { AgentRegistry } from "./agent-registry.js";
-import { VALID_CLAUDE_MODES } from "./providers/claude-agent.js";
-import { VALID_CODEX_MODES } from "./providers/codex-agent.js";
 
 export const AGENT_LIFECYCLE_STATUSES = [
   "initializing",
@@ -164,27 +162,6 @@ function isAgentBusy(status: AgentLifecycleStatus): boolean {
   return BUSY_STATUSES.includes(status);
 }
 
-function getValidModesForProvider(provider: AgentProvider): Set<string> {
-  switch (provider) {
-    case "claude":
-      return VALID_CLAUDE_MODES;
-    case "codex":
-      return VALID_CODEX_MODES;
-    default:
-      return new Set();
-  }
-}
-
-function validateMode(provider: AgentProvider, modeId: string): void {
-  const validModes = getValidModesForProvider(provider);
-  if (!validModes.has(modeId)) {
-    const validModesList = Array.from(validModes).join(", ");
-    throw new Error(
-      `Invalid mode '${modeId}' for provider '${provider}'. Valid modes: ${validModesList}`
-    );
-  }
-}
-
 function createAbortError(
   signal: AbortSignal | undefined,
   fallbackMessage: string
@@ -315,12 +292,6 @@ export class AgentManager {
     agentId?: string
   ): Promise<ManagedAgent> {
     const normalizedConfig = await this.normalizeConfig(config);
-
-    // Validate mode if provided
-    if (normalizedConfig.modeId) {
-      validateMode(normalizedConfig.provider, normalizedConfig.modeId);
-    }
-
     const client = this.requireClient(normalizedConfig.provider);
     const session = await client.createSession(normalizedConfig);
     return this.registerSession(
@@ -342,12 +313,6 @@ export class AgentManager {
       provider: handle.provider,
     } as AgentSessionConfig;
     const normalizedConfig = await this.normalizeConfig(mergedConfig);
-
-    // Validate mode if provided
-    if (normalizedConfig.modeId) {
-      validateMode(handle.provider, normalizedConfig.modeId);
-    }
-
     const resumeOverrides =
       normalizedConfig.model !== mergedConfig.model
         ? { ...overrides, model: normalizedConfig.model }
@@ -409,10 +374,6 @@ export class AgentManager {
 
   async setAgentMode(agentId: string, modeId: string): Promise<void> {
     const agent = this.requireAgent(agentId);
-
-    // Validate mode before attempting to set it
-    validateMode(agent.provider, modeId);
-
     await agent.session.setMode(modeId);
     agent.currentModeId = modeId;
     this.emitState(agent);
diff --git a/packages/server/src/server/agent/providers/claude-agent.ts b/packages/server/src/server/agent/providers/claude-agent.ts
index 0a1470136..68253b42d 100644
--- a/packages/server/src/server/agent/providers/claude-agent.ts
+++ b/packages/server/src/server/agent/providers/claude-agent.ts
@@ -72,7 +72,7 @@ const DEFAULT_MODES: AgentMode[] = [
   },
 ];
 
-export const VALID_CLAUDE_MODES = new Set(
+const VALID_CLAUDE_MODES = new Set(
   DEFAULT_MODES.map((mode) => mode.id)
 );
 
@@ -271,6 +271,15 @@ class ClaudeAgentSession implements AgentSession {
     this.claudeSessionId = handle?.sessionId ?? handle?.nativeHandle ?? null;
     this.pendingLocalId = this.claudeSessionId ?? `claude-${randomUUID()}`;
     this.persistence = handle ?? null;
+
+    // Validate mode if provided
+    if (config.modeId && !VALID_CLAUDE_MODES.has(config.modeId)) {
+      const validModesList = Array.from(VALID_CLAUDE_MODES).join(", ");
+      throw new Error(
+        `Invalid mode '${config.modeId}' for Claude provider. Valid modes: ${validModesList}`
+      );
+    }
+
     this.currentMode = (config.modeId as PermissionMode) ?? "default";
     if (this.claudeSessionId) {
       this.loadPersistedHistory(this.claudeSessionId);
@@ -386,6 +395,14 @@ class ClaudeAgentSession implements AgentSession {
   }
 
   async setMode(modeId: string): Promise<void> {
+    // Validate mode
+    if (!VALID_CLAUDE_MODES.has(modeId)) {
+      const validModesList = Array.from(VALID_CLAUDE_MODES).join(", ");
+      throw new Error(
+        `Invalid mode '${modeId}' for Claude provider. Valid modes: ${validModesList}`
+      );
+    }
+
     const normalized = modeId as PermissionMode;
     const query = await this.ensureQuery();
     await query.setPermissionMode(normalized);
diff --git a/packages/server/src/server/agent/providers/codex-agent.ts b/packages/server/src/server/agent/providers/codex-agent.ts
index 29a01aafa..f82de1dae 100644
--- a/packages/server/src/server/agent/providers/codex-agent.ts
+++ b/packages/server/src/server/agent/providers/codex-agent.ts
@@ -69,7 +69,7 @@ const CODEX_MODES: AgentMode[] = [
   },
 ];
 
-export const VALID_CODEX_MODES = new Set(
+const VALID_CODEX_MODES = new Set(
   CODEX_MODES.map((mode) => mode.id)
 );
 
@@ -294,6 +294,15 @@ class CodexAgentSession implements AgentSession {
   constructor(codex: Codex, config: CodexAgentConfig, handle?: AgentPersistenceHandle) {
     this.codex = codex;
     this.config = { ...config };
+
+    // Validate mode if provided
+    if (config.modeId && !VALID_CODEX_MODES.has(config.modeId)) {
+      const validModesList = Array.from(VALID_CODEX_MODES).join(", ");
+      throw new Error(
+        `Invalid mode '${config.modeId}' for Codex provider. Valid modes: ${validModesList}`
+      );
+    }
+
     this.currentMode = this.config.modeId ?? DEFAULT_CODEX_MODE_ID;
     this.threadOptions = this.buildThreadOptions(this.currentMode);
     this.codexSessionDir = this.resolveCodexSessionDir(handle);
@@ -424,8 +433,13 @@ class CodexAgentSession implements AgentSession {
     if (modeId === this.currentMode) {
       return;
     }
-    if (!this.availableModes.some((mode) => mode.id === modeId)) {
-      throw new Error(`Mode '${modeId}' not supported by Codex`);
+
+    // Validate mode
+    if (!VALID_CODEX_MODES.has(modeId)) {
+      const validModesList = Array.from(VALID_CODEX_MODES).join(", ");
+      throw new Error(
+        `Invalid mode '${modeId}' for Codex provider. Valid modes: ${validModesList}`
+      );
     }
 
     this.currentMode = modeId;