I am creating an assistant that speaks Hebrew. Hebrew is one of the languages that uses diacritic signs to vocalise words correctly. A well-known example of diacritic signs can be found in German, such as the umlaut letters. The most common umlauts are ä, ö, and ü, which represent modified vowel sounds compared to their non-umlaut counterparts (a, o, and u). Another notable language using diacritics is Arabic. We have tried instructing the LLM to return its results with diacritic signs in Hebrew so that the TTS produces better results. However, even when instructing the LLM to include full diacritics, I am unable to see them in the call log (neither in the messages tab nor the logs). Please see Call ID 5427f3af-25a6-496a-8f31-4c47ce89239f as an example (you can check this call’s system prompt on your side, which generates answers with diacritics). We also tried providing instructions to the LLM in Hebrew, but observed similar results, except for the system message inside the messages tab, which does show diacritic signs. See Call IDs ff27397f-0af8-4c51-a0cc-6f0f9fadcda7 and 50261828-071c-4a67-bca9-fb865596bfe5 with short and long prompts in Hebrew only. Importantly, when we instruct the LLM (e.g., via the ChatGPT Playground, official website, Azure Foundry Playground, etc.) directly, without Vapi involved, it returns answers with full diacritics. Could you please clarify if this is a limitation on the Vapi side—that is, are diacritic signs from the LLM not supported or passed through as expected? Or is there anything I can do to ensure the LLM passes TTS words with diacritic signs? Thanks in advance for your assistance.

Disable voice formatting in your assistant config or use a custom TTS integration..

I am not sure which voice formatting you mean to disable. I think the problem is that Vapi doesn't attach the diacritic signs to the LLM response as i don't see them inside the call log.

Set

voice.chunkPlan.formatPlan.enabled = false

in your vapi assistant config to preserve Hebrew diacritics in both TTS and logs..

I tried to set voice.chunkPlan.formatPlan.enabled = false (and also voice.chunkPlan.enabled = false, and even tried flase for both), but still the same! another idea?

Set voice.inputPreprocessingEnabled = false and voice.chunkPlan.formatPlan.enabled = false then run a test call

Hi Kyle. I tried with the settings you suggested, still the same results. see Call id 31c01290-2580-487c-9264-ab6beda364a3 as an example.

Hey Amir76, You can make use of pronunciation dictionary to add certain pronunciations for certain key words. - The first link below teaches you how to create pronunciation dictionary using 11Labs - The second one guides you on how to add it to the VAPI Let me know if you require further help with this.

this is not what i am looking for. I am looking for something else - the option to ues discritic signs with Vapi

Hey Amir! We're currently out at an offsite, but I'd love to circle back on this with you first thing Monday morning. Would that work for you?

Yes. Tnx

Hi Amir76, I've identified the issue with your Hebrew diacritics! You're very close - you set 3 out of 4 required flags, but there's one missing. The Issue: Our text formatting system requires all four flags to be disabled, but our support team only provided you with three. You're missing inputReformattingEnabled: false. The Complete Fix:

{
  "voice": {
    "inputPreprocessingEnabled": false,
    "inputReformattingEnabled": false,  // ← This was missing!
    "chunkPlan": {
      "enabled": false,
      "formatPlan": {
        "enabled": false
      }
    }
  }
}

Without all four flags set to false, your Hebrew text still went through our text formatters, which stripped the diacritics. Your system message kept them because it bypasses this formatting pipeline, but assistant responses go through TTS formatting.

Hi, i set all 4 flags to false as you suggested and i changed well-tested assistant system prompt to always response with diacritic signs. i still don't see the signs inside the call log messages tab and transcribe tab. i see few signs inside the logs tab but no together with the word itself but each sign seperated so i can't really use it for anything. see call ID 51f12b55-f856-478f-90ac-1bd06f81b5f0 as an example. Can u confirm if this is a bug on the VAPI dashboard only that i don't see the signs inside the messages and transcripts call log screens, and you see on your side that the LLM returned Hebrew with signs correctly on each bot response? I also noticed that during this call, once i heard those words in English :"What was that" for a reason i can't see and i never had such problems before the changes. see this line from the call log: "21:01:48:444 [LOG] Model output: What was that?". i couldn't find such bot response message inside the messages log nor in the transcripts of this call. I assume this is another problem that related to the flags i set or to the system prompt that i instructed to return all responses from the bot using diacritic signs. first let me know if you see the signs correctly on this call example, and then check the other small issue. Tnx

Hi, First off, we want to sincerely apologize for the delay in getting back to you. We understand how frustrating it is to wait - especially when you're counting on us - and we owe you a clear explanation of what’s been happening and how we’re addressing it. Over the past few weeks, we've seen a significant increase in support requests. While this reflects exciting growth, it has also stretched our small team and exposed some real challenges in scaling our support operations. To improve your experience, we’ve taken a step back to reassess our approach. Here’s what we’re implementing: - Smarter Support Through Automation: We’re investing in our AI support systems to help you resolve issues more efficiently. Soon, our support bot will offer expanded capabilities, making it easier to access accurate, instant help—particularly for common or repetitive queries. - Expanding the Support Team: To meet growing demand, we’re adding 2–3 new team members focused on managing support volume and improving response times. - Prioritized SLAs for High-Usage Accounts: We’re introducing service level improvements for users who are growing with us: - Accounts with usage over 1,000 minutes/month will receive prioritized support. - For all general inquiries, we’re establishing a standardized 48-hour response time. We’re confident these steps will lead to faster, more reliable support and help us better serve you as you grow with us. Also, in case you still need help with this ongoing ticket, do let us know, and we will help you get this resolved as soon as possible. Thank you for your continued patience and for being part of our journey. Warm regards, Vapi Team

Hey Amir, did you end up figuring this whole challenge out at all? did choosing other provider solved the added extra words issue? did you manage finding a way to pass the diacritic signs on so they are shown as you wished? (i didn't fully understand if you did, and if yes what exactly have you done). thanks in advanced