google-gemini/gemini-cli · critical · Error

Model policy chain must include an `isLastResort` model.

Error message

Model policy chain must include an `isLastResort` model.

What it means

Thrown by validateModelPolicyChain() when the chain is non-empty but no entry has isLastResort set to true. The last-resort model is the terminal fallback the system uses when all other models in the chain have exhausted their maxAttempts. Without one, the availability loop could fail without a guaranteed recovery path, leaving the user with no model to serve requests.

Source

Thrown at packages/core/src/availability/policyCatalog.ts:158

}

/**
 * Provides a default policy scaffold for models not present in the catalog.
 */
export function createDefaultPolicy(
  model: string,
  options?: { isLastResort?: boolean },
): ModelPolicy {
  return definePolicy({ model, isLastResort: options?.isLastResort });
}

export function validateModelPolicyChain(chain: ModelPolicyChain): void {
  if (chain.length === 0) {
    throw new Error('Model policy chain must include at least one model.');
  }
  const lastResortCount = chain.filter((policy) => policy.isLastResort).length;
  if (lastResortCount === 0) {
    throw new Error('Model policy chain must include an `isLastResort` model.');
  }
  if (lastResortCount > 1) {
    throw new Error('Model policy chain must only have one `isLastResort`.');
  }
}

/**
 * Helper to define a ModelPolicy with default actions and state transitions.
 * Ensures every policy is a fresh instance to avoid shared state.
 */
function definePolicy(config: PolicyConfig): ModelPolicy {
  return {
    model: config.model,
    isLastResort: config.isLastResort,
    maxAttempts: config.maxAttempts,
    actions: { ...DEFAULT_ACTIONS, ...(config.actions ?? {}) },
    stateTransitions: {
      ...DEFAULT_STATE,

View on GitHub (pinned to 5024443c72)

Solutions

1. Mark exactly one entry as isLastResort: true — typically the most reliable, highest-quota model (e.g., the Flash model).
2. Use createSingleModelChain(model) for single-model setups, which automatically sets isLastResort: true.
3. Use the built-in chain builders (getDefaultPolicyChain, getFlashLitePolicyChain) which already include a correct last-resort entry.
4. When customizing a chain, always end with: chain[chain.length - 1].isLastResort = true.

Example fix

// before — no last-resort entry
const chain = [
  definePolicy({ model: 'gemini-pro' }),
  definePolicy({ model: 'gemini-flash' }),
];
validateModelPolicyChain(chain); // throws

// after — mark the fallback
const chain = [
  definePolicy({ model: 'gemini-pro' }),
  definePolicy({ model: 'gemini-flash', isLastResort: true }),
];

// Ensure the chain has exactly one last-resort entry
const hasLastResort = chain.some((p) => p.isLastResort);
if (!hasLastResort && chain.length > 0) {
  chain[chain.length - 1].isLastResort = true;
}
validateModelPolicyChain(chain);

function chainHasLastResort(chain: ModelPolicyChain): boolean {
  return chain.some((p) => p.isLastResort === true);
}

try {
  validateModelPolicyChain(chain);
} catch (e) {
  if (e instanceof Error && e.message.includes('isLastResort')) {
    // Auto-fix: mark the last entry as last-resort
    chain[chain.length - 1].isLastResort = true;
    validateModelPolicyChain(chain);
  } else throw e;
}

When it happens

Trigger: Calling validateModelPolicyChain(chain) where chain has entries but none have isLastResort: true. For example, [{ model: 'gemini-pro', isLastResort: false }, { model: 'gemini-flash', isLastResort: false }].

Common situations: Building a chain programmatically and forgetting to mark the final fallback model as isLastResort; overriding a chain via settings.json that omits the last-resort flag; copying a chain template and stripping the isLastResort field during a transform; all models set isLastResort to false explicitly.

Related errors

• Model policy chain must only have one `isLastResort`.
• Model policy chain must include at least one model.
• ${originalMessage}. The initial COMPUTE_ADC attempt also fai
• Invalid taskId: ${taskId}
• Security violation: Null byte detected in path.

AI-assisted analysis of google-gemini/gemini-cli@5024443c72 (2026-08-12). Data as JSON: /api/errors/cfdf8f81485046e1. Report an issue: GitHub.