Fixes #1159.

Problem

updateAgentModel() in electron/utils/agent-config.ts replaced the agent's model block wholesale with { primary: modelRef } on every model switch (in-chat picker or Agents dialog). This has two destructive effects:

1. Any hand-configured fallbacks array in that block is silently discarded.
2. The resulting { primary }-only block disables the agent's entire fallback chain: the OpenClaw runtime's resolveSelectedModelFallbacksOverride() treats a per-agent model block that has primary but no fallbacks key as an explicit empty fallback override, which suppresses agents.defaults.model.fallbacks too. The gateway then reports fallbackConfigured: false, and a 429/rate-limit on the primary fails the turn with no fallback attempted.

Nothing in the UI indicates failover was turned off. Full analysis, logs, and reproduction in #1159.

Fix

• Merge instead of replace: spread the existing model block and update primary, preserving fallbacks (and any other fields).
• Inherit defaults into bare blocks: when the resulting block would still lack a fallbacks key (fresh block, or a pre-existing bare-primary block), copy agents.defaults.model.fallbacks into it, so a per-agent primary never silently disables failover.

The modelRef: null path (reset to default) is unchanged — deleting the block correctly falls back to the defaults chain.

Testing

• tsc --noEmit -p tsconfig.node.json clean for this file (one pre-existing unrelated error about the generated _ext-bridge.generated module in a fresh checkout).
• Direct invocation of updateAgentModel() against a real config, all three paths:
  • fresh block → { primary, fallbacks: [defaults chain] } inherited;
  • subsequent switch → primary updated, fallbacks preserved;
  • null → block deleted (unchanged behavior).
• Verified from a real GUI click in a dev build: the in-chat picker switch wrote { "primary": "openrouter/qwen/qwen3.6-flash", "fallbacks": [ ...full defaults chain... ] } and gateway failover kept working.

An alternative design would be to keep per-agent blocks primary-only and change the empty-override semantics in the openclaw runtime instead — happy to rework if maintainers prefer that direction.

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