How does the fallback mechanism work?

[jmvbambico]

Given my current config:

{
  "preset": "manual",
  "runtime": {
    "fail_fast": true,
    "bypass_watchdog": true,
    "timeout_ms": 15000
  },
  "presets": {
    "manual": {
      "orchestrator": {
        "model": "anthropic/claude-opus-4-6",
        "skills": ["*"],
        "mcps": ["websearch"]
      },
      "oracle": {
        "model": "anthropic/claude-opus-4-6",
        "skills": [],
        "mcps": []
      },
      "designer": {
        "model": "google/antigravity-gemini-3.1-pro",
        "skills": ["agent-browser", "simplify"],
        "mcps": []
      },
      "explorer": {
        "model": "anthropic/claude-haiku-4-5",
        "skills": [],
        "mcps": []
      },
      "librarian": {
        "model": "anthropic/claude-haiku-4-5",
        "skills": [],
        "mcps": ["websearch", "context7", "grep_app"]
      },
      "fixer": {
        "model": "anthropic/claude-haiku-4-5",
        "skills": [],
        "mcps": []
      }
    }
  },
  "fallback": {
    "enabled": true,
    "timeoutMs": 15000,
    "chains": {
      "orchestrator": [
        "anthropic/claude-opus-4-6",
        "google/antigravity-claude-opus-4-6-thinking",
        "google/antigravity-gemini-3.1-pro",
        "opencode/minimax-m2.5-free",
        "opencode/big-pickle"
      ],
      "oracle": [
        "anthropic/claude-opus-4-6",
        "google/antigravity-claude-opus-4-6-thinking",
        "google/antigravity-gemini-3.1-pro",
        "opencode/minimax-m2.5-free",
        "opencode/big-pickle"
      ],
      "designer": [
        "google/antigravity-gemini-3.1-pro",
        "google/antigravity-claude-opus-4-6-thinking",
        "anthropic/claude-sonnet-4-6",
        "opencode/minimax-m2.5-free",
        "opencode/big-pickle"
      ],
      "explorer": [
        "anthropic/claude-haiku-4-5",
        "google/antigravity-claude-haiku-4-5",
        "google/antigravity-gemini-3-flash",
        "opencode/minimax-m2.5-free",
        "opencode/big-pickle"
      ],
      "librarian": [
        "anthropic/claude-haiku-4-5",
        "google/antigravity-claude-haiku-4-5",
        "google/antigravity-gemini-3-flash",
        "opencode/minimax-m2.5-free",
        "opencode/big-pickle"
      ],
      "fixer": [
        "anthropic/claude-haiku-4-5",
        "google/antigravity-claude-haiku-4-5",
        "google/antigravity-gemini-3-flash",
        "opencode/minimax-m2.5-free",
        "opencode/big-pickle"
      ]
    }
  }
}

Whenever I hit rate limits on my Claude sub, it doesn't fallback into the fallback models specified. Or am I missing something? I tried searching the docs about it but I found no reference.

[AlejandroAkbal]

Yeah, I dont think its working at all

[jmvbambico]

Yeah, I dont think its working at all

I did push a PR that was merged for it but I am still testing it. If you updated your plugin lately, do leave feedback if it works now.

[AlejandroAkbal]

On the latest version doesn't work; #180

Yeah, I dont think its working at all

I did push a PR that was merged for it but I am still testing it. If you updated your plugin lately, do leave feedback if it works now.

[vinz-bambico]

tested it last night when I got rate limited... still doesn't work. And to add to the dilemma, it's pretty hard to test given that you need to hit rate limits. And when you do, you can't do anything because, you hit the rate limit hahaha...

Anyway, this library does work: https://github.com/azumag/opencode-rate-limit-fallback but only for the main agent. It doesn't work for agents spawned in the background like omo-slim.

I will look into it more and try to fix but if someone else can do it, I'm all ears.

[AlejandroAkbal]

This is not solved @alvinunreal

let's use the workaround that doesn't fix the real issue reported in #180

[Zaradacht]

We hit the same issue. The working approach in our OpenCode setup is to put the fallback chain directly on each agent’s model field as an ordered array, instead of relying on fallback.chains.

Example:

{
  "fallback": { "enabled": true },
  "agents": {
    "orchestrator": {
      "model": [
        "anthropic/claude-opus-4-6",
        "google/antigravity-claude-opus-4-6-thinking",
        "google/antigravity-gemini-3.1-pro",
        "opencode/minimax-m2.5-free",
        "opencode/big-pickle"
      ]
    }
  }
}

The first model is the primary. On a rate-limit/quota/concurrency error, the runtime fallback hook aborts the failed prompt and re-submits the last user message with the next untried model in that agent’s array.

So the practical fix is: move each fallback.chains.<agent> list into that agent’s model field inside the active preset/agent config, and keep only fallback.enabled.

Prompt to have the orchestrator fix this

Update my oh-my-opencode-slim configuration so runtime model fallback works.

Current problem: I have fallback chains under fallback.chains, but foreground fallback expects each agent model to be an ordered array.

Please modify the active preset/config so that, for every agent with a fallback.chains entry:

1. Move fallback.chains.<agent> into that agent’s model field as an ordered array.
2. Preserve the same model order, with the current primary model first.
3. Keep fallback.enabled set to true.
4. Remove fallback.chains if it is no longer used.
5. Do not change skills, mcps, temperature, options, or other unrelated settings.
6. After editing, show me the final relevant config block.

[mhenke]

Answered by @Zaradacht: fallback.chains is legacy — use the agent model field as an ordered array instead.

[fyq163]

But according to authors-preset, the config value for models is string, does this entry actually supports array?