diff --git a/packages/coding-agent/src/core/agent-session.ts b/packages/coding-agent/src/core/agent-session.ts
index 21c51c3e..a686018d 100644
--- a/packages/coding-agent/src/core/agent-session.ts
+++ b/packages/coding-agent/src/core/agent-session.ts
@@ -634,6 +634,14 @@ export class AgentSession {
 		// Validate API key
 		const apiKey = await this._modelRegistry.getApiKey(this.model);
 		if (!apiKey) {
+			const isOAuth = this._modelRegistry.isUsingOAuth(this.model);
+			if (isOAuth) {
+				throw new Error(
+					`Authentication failed for "${this.model.provider}". ` +
+						`Credentials may have expired or network is unavailable. ` +
+						`Run '/login ${this.model.provider}' to re-authenticate.`,
+				);
+			}
 			throw new Error(
 				`No API key found for ${this.model.provider}.\n\n` +
 					`Use /login, set an API key environment variable, or create ${getAuthPath()}`,
diff --git a/packages/coding-agent/src/core/sdk.ts b/packages/coding-agent/src/core/sdk.ts
index 9d70e097..a1d2f485 100644
--- a/packages/coding-agent/src/core/sdk.ts
+++ b/packages/coding-agent/src/core/sdk.ts
@@ -641,7 +641,18 @@ export async function createAgentSession(options: CreateAgentSessionOptions = {}
 			}
 			const key = await modelRegistry.getApiKeyForProvider(resolvedProvider);
 			if (!key) {
-				throw new Error(`No API key found for provider "${resolvedProvider}"`);
+				const isOAuth = modelRegistry.isUsingOAuth(currentModel);
+				if (isOAuth) {
+					throw new Error(
+						`Authentication failed for "${currentModel.provider}". ` +
+							`Credentials may have expired or network is unavailable. ` +
+							`Run '/login ${currentModel.provider}' to re-authenticate.`,
+					);
+				}
+				throw new Error(
+					`No API key found for "${currentModel.provider}". ` +
+						`Set an API key environment variable or run '/login ${currentModel.provider}'.`,
+				);
 			}
 			return key;
 		},