diff --git a/packages/server/src/server/agent/agent-manager.ts b/packages/server/src/server/agent/agent-manager.ts
index c6272dc57..9a10ac2a4 100644
--- a/packages/server/src/server/agent/agent-manager.ts
+++ b/packages/server/src/server/agent/agent-manager.ts
@@ -22,6 +22,8 @@ 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",
@@ -162,6 +164,27 @@ 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
@@ -292,6 +315,12 @@ 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(
@@ -313,6 +342,12 @@ 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 }
@@ -374,6 +409,10 @@ 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 815a4acfe..0a1470136 100644
--- a/packages/server/src/server/agent/providers/claude-agent.ts
+++ b/packages/server/src/server/agent/providers/claude-agent.ts
@@ -72,6 +72,10 @@ const DEFAULT_MODES: AgentMode[] = [
   },
 ];
 
+export const VALID_CLAUDE_MODES = new Set(
+  DEFAULT_MODES.map((mode) => mode.id)
+);
+
 type ClaudeAgentConfig = AgentSessionConfig & { provider: "claude" };
 
 export type ClaudeContentChunk = { type: string; [key: string]: any };
diff --git a/packages/server/src/server/agent/providers/codex-agent.ts b/packages/server/src/server/agent/providers/codex-agent.ts
index 43aedd1bc..29a01aafa 100644
--- a/packages/server/src/server/agent/providers/codex-agent.ts
+++ b/packages/server/src/server/agent/providers/codex-agent.ts
@@ -69,6 +69,10 @@ const CODEX_MODES: AgentMode[] = [
   },
 ];
 
+export const VALID_CODEX_MODES = new Set(
+  CODEX_MODES.map((mode) => mode.id)
+);
+
 const DEFAULT_CODEX_MODE_ID = "auto";
 
 const MODE_PRESETS: Record<