feat(runner): support structured fallback models

Bind fallback model chains to the active model configuration so defaults and presets do not inherit or merge fallback behavior implicitly. Require explicit fallback providers while preserving per-fallback generation overrides and context-window safety.

Co-authored-by: Cursor <cursoragent@cursor.com>

docs/configuration.md

@@ -672,6 +672,12 @@ Existing configs do not need to change. If you do not set `modelPresets` or `age
       "maxTokens": 8192,
       "contextWindowTokens": 128000,
       "temperature": 0.1,
+      "fallbackModels": [
+        {
+          "provider": "anthropic",
+          "model": "anthropic/claude-sonnet-4-6"
+        }
+      ],
       "modelPreset": null
     }
   },
@@ -682,7 +688,17 @@ Existing configs do not need to change. If you do not set `modelPresets` or `age
       "maxTokens": 4096,
       "contextWindowTokens": 128000,
       "temperature": 0.2,
-      "reasoningEffort": "low"
+      "reasoningEffort": "low",
+      "fallbackModels": [
+        {
+          "provider": "deepseek",
+          "model": "deepseek/deepseek-chat",
+          "maxTokens": 4096,
+          "contextWindowTokens": 64000,
+          "temperature": 0.1,
+          "reasoningEffort": null
+        }
+      ]
     },
     "deep": {
       "model": "anthropic/claude-opus-4-5",
@@ -705,9 +721,53 @@ Existing configs do not need to change. If you do not set `modelPresets` or `age
 | `contextWindowTokens` | Context window size used by prompt building and consolidation decisions. |
 | `temperature` | Sampling temperature. |
 | `reasoningEffort` | Optional reasoning/thinking setting. Provider support varies. |
+| `fallbackModels` | Optional ordered fallback models for this active configuration only. |
 
 `default` is reserved and always means the implicit preset built from `agents.defaults.*`; do not define `modelPresets.default`. Use `/model default` to switch back to `agents.defaults.*`.
 
+### Model Fallbacks
+
+`fallbackModels` belongs to the currently active model configuration. If the active configuration is `agents.defaults`, only `agents.defaults.fallbackModels` is used. If the active configuration is `modelPresets.fast`, only `modelPresets.fast.fallbackModels` is used. nanobot does not inherit or merge fallbacks between defaults and presets.
+
+Each fallback entry must include at least `provider` and `model`. The other fields are optional; omitted values inherit from the active primary configuration for that request.
+
+```json
+{
+  "modelPresets": {
+    "fast": {
+      "model": "MiniMax-M2.7-highspeed",
+      "provider": "minimaxAnthropic",
+      "maxTokens": 4096,
+      "contextWindowTokens": 262144,
+      "temperature": 0.1,
+      "reasoningEffort": null,
+      "fallbackModels": [
+        {
+          "provider": "deepseek",
+          "model": "deepseek-v4-pro",
+          "maxTokens": 4096,
+          "contextWindowTokens": 262144,
+          "temperature": 0.1,
+          "reasoningEffort": null
+        }
+      ]
+    },
+    "deep": {
+      "model": "deepseek-v4-pro",
+      "provider": "deepseek",
+      "maxTokens": 4096,
+      "contextWindowTokens": 262144,
+      "temperature": 0.1,
+      "reasoningEffort": null
+    }
+  }
+}
+```
+
+In this example, `/model fast` can fail over to DeepSeek, but `/model deep` has no fallback because the `deep` preset does not define `fallbackModels`.
+
+Failover only runs when the primary model returns an error before any answer text has been streamed. Fallback models are tried in order. If a fallback has a smaller `contextWindowTokens`, nanobot uses the smallest window in the active chain when building context so the fallback can receive the same prompt.
+
 Set `agents.defaults.modelPreset` to start with a named preset:
 
 ```json