[Feature Request] Chat (chat.z.ai): Support fallback model configuration & mid-session model switching for subagents / agents #922

Problem Statement

Reported by community user Lu Doan on Discord for the web platform (chat.z.ai):

"Is it possible to set a fallback model for subagents? Changing the subagent model currently requires starting a new session, which disrupts my workflow. It would be great if a fallback model feature were available for subagents!"

Currently on chat.z.ai:

1. No Fallback Model for Subagents / Agents: When using agents or subagents in the web chat, tasks rely on a single statically configured model. If that model hits rate limits (429), quota limits, or temporary upstream errors, execution halts instead of gracefully falling back to a secondary backup model.
2. Session Disruption: When users need to adjust the agent or subagent model, the current web chat UI requires starting an entirely new chat session. This destroys active conversational context, uploaded file references, and ongoing project momentum.

Proposed Solution

1. Configurable Fallback Model for Agents & Subagents

Provide an optional fallback model selection in the chat settings or agent setup panel on chat.z.ai:

• Allow selecting a secondary fallback model (e.g., falling back to a secondary available model tier) when the primary model encounters rate limits, capacity constraints, or timeouts.
• When an error occurs during an agent run, automatically retry using the fallback model rather than terminating the run with an error state.

2. Dynamic Mid-Session Model Switching in Chat

Allow users to switch the model of an existing chat session or active agent without forcing a brand new session:

• A model selector dropdown accessible directly in the current chat header or agent configuration drawer.
• Preserves the full conversation thread and context when changing models.

User Impact & Benefits

• Prevents workflow interruptions when quotas or rate limits are reached during web chat tasks.
• Eliminates the need to abandon active chat sessions just to change models.
• Provides a smoother experience for users on chat.z.ai.