# Retell AI vs Sierra: which do AI models recommend for AI voice agents, October 2026

GTM AI Recommendation Index, October 2026 Edition, AI voice agents. Five of fourteen models named Retell AI first on the direct prompt; zero named Sierra. Page: https://gtm-ai-index.com/customer/ai-voice-agents/retell-ai-vs-sierra/

| | First-choice share | Rank | Negative rate | Labels | Models naming it |
|---|---|---|---|---|---|
| Retell AI | 27% | #1 of 8 | 16% | 58 | 14 of 14 |
| Sierra | 2% | #6 of 8 | 55% | 11 | 7 of 14 |

## The direct prompt, model by model

- Claude Haiku 4.5: retell ai first (first choices: Aloware, Retell AI) (alternatives: Synthflow, Vapi)
- Grok 4.1 Fast: retell ai first (first choices: Retell AI) (alternatives: Bland AI, CloudTalk, Regal, Synthflow)
- Mistral Small: retell ai first (first choices: CloudTalk, Retell AI) (alternatives: ElevenLabs Conversational AI, Leaping AI)
- DeepSeek V4 Flash: retell ai first (first choices: Regal, Retell AI) (alternatives: CloudTalk, Synthflow AI)
- GPT-6 Luna: retell ai first (first choices: Retell AI) (alternatives: PolyAI, Synthflow, Vapi)
- Gemini 3.5 Flash: neither first, one named (first choices: Aloware, Synthflow AI) (alternatives: Bland AI, Retell AI, Vapi)
- Perplexity Sonar: neither first, one named (first choices: CloudTalk) (alternatives: Retell AI, Salesforce Agentforce Voice, Synthflow AI)
- Qwen 3.7 Flash: neither first, one named (first choices: CloudTalk) (alternatives: Bland AI, Retell AI, Rezora IO)
- Kimi K2: neither first, one named (first choices: CloudTalk) (alternatives: Aloware, Retell AI, Synthflow)
- GLM 4.7 FlashX: neither first, one named (first choices: CloudTalk) (alternatives: Bland AI, Retell AI, Synthflow, Vapi.ai)
- MiniMax M2.5: neither first, one named (first choices: CloudTalk, Synthflow) (alternatives: Retell AI)
- Muse Glimmer 30B: neither first, one named (first choices: Aloware, CloudTalk) (alternatives: HubSpot Breeze AI, Retell AI, Synthflow)
- GPT-5.4 mini: neither named (first choices: Twilio) (alternatives: Deepgram Voice Agent API, OpenAI Realtime / Voice agents)
- Llama 4 Maverick: neither named (first choices: Aloware, CloudTalk) (alternatives: GetVoIP, Plivo)

## What the models said about Retell AI

- "some specific products (for example Synthflow, ElevenLabs, Retell AI, Deepgram) have recurring complaints about cost or credit limits" (GPT-5.4 mini, negative prompt, soft negative)
- "Both Vapi.ai and Retell AI can suffer from latency and reliability issues due to reliance on external API providers" (Claude Haiku 4.5, negative prompt, soft negative)
- "While popular among developers, there is significant frustration in the community regarding its operational setup." (Qwen 3.7 Flash, negative prompt, soft negative)
- "Best all-around voice AI platform for production call automation... "best overall — lowest effective cost, highest voice quality, full API and no-code in one platform"" (Muse Glimmer 30B, comparative prompt, first choice)
- "Strong all-around balance of latency, price, and accessibility; often described as a fast, transparent usage-based option with native telephony and quick setup." (Perplexity Sonar, comparative prompt, first choice)
- "*Developer-first / Infrastructure:* Vapi, Retell AI, Vocode, or Bland.ai. (Excellent for high customization, low latency, and BYO keys)." (Gemini 3.5 Flash, scale prompt, first choice)

## What the models said about Sierra

- "Why avoid: Not suitable for mid-market buyers or those with under 100 concurrent calls." (Mistral Small, negative prompt, hard negative)
- "Sierra - if you have under 100 concurrent calls, no dedicated operations team, or if your budget is under $50,000 annually." (Llama 4 Maverick, negative prompt, soft negative)
- "Not recommended for mid-sized B2B companies unless you have a massive support volume and a large budget." (DeepSeek V4 Flash, paraphrase prompt, soft negative)
- "1. Sierra (Best Overall for Mid-Market B2B)" (Claude Haiku 4.5, paraphrase prompt, first choice)
- "Choose Sierra if you are a large enterprise brand looking for a premium, omnichannel solution" (GLM 4.7 FlashX, comparative prompt, alternative)
- "Sierra AI is best for maximum brand governance and tone control" (Claude Haiku 4.5, comparative prompt, alternative)

Share is the count of first choices across the direct, paraphrase, budget and scale prompts over all fourteen models, for a mid-market B2B company; rank is within the category. Comparisons are drawn for the top eight products in each category. Published under CC BY 4.0; the output is the models' output, and nothing here is a recommendation by the index.