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Sonic 3.5 is designed to sound natural with minimal prompt engineering. In most cases you can pass your transcript as-is and let the model handle normalization, pacing, and expression. The tips below apply across the Sonic family; differences between Sonic 3.5 and Sonic 3 are called out inline.

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Recommendations

  • Pass natural, well-punctuated text. Full sentences with normal capitalization and punctuation produce the best pacing and intonation. End each transcript with terminal punctuation (., ?, !).
  • Send complete phrases. Full sentences sound more natural than isolated fragments or single words. Don’t send a number, code, or spell tag on its own — include the surrounding sentence, e.g. Your confirmation code is <spell>ABC123</spell>.
  • Use normal casing. Reserve all-caps for acronyms you want read out letter by letter (e.g. USA). Other all-caps words may be misread as initialisms (e.g. NASA). Avoid using capitalization for emphasis or to indicate shouting.
  • Pass numbers, currency, dates, and common acronyms in conventional written form. Sonic maps these patterns to natural speech for most inputs:
    • Large numbers like 1,234,567
    • Currency like $19.99
    • US phone numbers: (415) 555-1212
    • Street addresses like 123 Main St
    • Email addresses: user@example.com
    • Dates in MM/DD/YYYY: 04/20/2025
    • Times with a space before AM/PM: 7:00 PM, 7 PM, 7:00 P.M.
    • Common acronyms (NASA) and initialisms (USA)
    Symbols are handled naturally — @ reads as at (email addresses), () is silent (for US phone numbers). When an LLM writes the transcript, see Voice agents (LLM-authored text).
  • Match the voice to the language. Each voice has a primary language it works best with. Use the Playground to audition voices for a given language.
  • Keep prompts in their natural written form. Heavy preprocessing (stripping punctuation, forcing casing) generally hurts output quality.

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Pre-normalization

Sonic 3.5 automatically covers the common cases above for most inputs. If you hit an unusual case or a bug where something is still misread, you may consider pre-normalizing your text as a fallback. Have your LLM write the transcript fully spelled out, the way it would be spoken.
WrittenSpoken (fully normalized)
$123.50one hundred twenty-three dollars and fifty cents
Dr. SmithDoctor Smith
14:30two thirty PM
Pre-normalizing is a fallback for edge cases. Well-punctuated text in conventional form is read correctly in the large majority of cases.

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Controlling pacing and spelling

When you need character-by-character read-out (confirmation codes, order IDs, serial numbers, spelled-out names) or fine-grained pacing, use one of the following:
  1. Spell tags (recommended). Wrap the string in <spell>...</spell>. Most reliable option, works for letters, digits, and mixed alphanumerics in all supported languages. 
    Your confirmation code is <spell>AB12CD</spell>.
  2. Space-delimited characters. Alternatively, you can achieve the same result by separating characters with single spaces for a natural spelling pace. 
    Your code is A B C 1 2 3.
  3. Comma-delimited characters. If your use case requires longer pauses, you can add a comma and a space after each character. 
    Your code is A, B, C, 1, 2, 3.
  4. Semantic grouping. For more natural pacing, use spaces and add commas where a human would naturally pause. 
    Your code is A B C, 1 2 3.
Migrating from Sonic 3? The recommended delimiter format has changed in Sonic 3.5. Separate characters with spaces or commas and put a comma between groups. Don’t put periods between characters or mix commas and periods, this format still works on sonic-3 snapshots but is not recommended for Sonic 3.5.
ScenarioOld (Sonic 3)New (Sonic 3.5)
Spell out letters HELLOH. E. L. L. O.H E L L O
Spell out digits 1234561. 2. 3. 4. 5. 6.1 2 3 4 5 6
Confirmation code ABC123A, B, C. 1, 2, 3.A B C, 1 2 3
Slow, digit-by-digit 266AO482. 6. 6. A. O. 4. 8.2, 6, 6, A, O, 4, 8

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Voice agents (LLM-authored text)

Starter system prompt. Baseline you can paste and trim for your product. If your stack does not pass <spell> or other tags through to Sonic, omit the tag lines and use the delimiter fallback in section 4. 
You are a voice agent. Everything you output will be spoken aloud by Cartesia Sonic text-to-speech. Follow these rules:

1. GENERAL FORMATTING
- Write plain prose in full sentences. Always end with . ? or !
- Send complete phrases, not isolated words or fragments. Keep numbers, codes, and spell tags inside a surrounding sentence.
- Do NOT use markdown, bullet points, headers, bold, raw JSON, emoji, or special characters. Sonic reads them aloud as written.

2. CAPITALIZATION
- Use normal capitalization, exactly as the sentence would normally be written: capitalize the first word, proper nouns, and the word I, and lowercase everything else. This is the default for almost all output.
- The model tends to read an all-caps token letter by letter. Use all-caps only when you want that, like an initialism you want spelled out (USA, FBI, ATM).
- Do not put ordinary words in all-caps. They may be misread as initialisms and spelled out letter by letter.
- Common acronyms normally said as a word, like NASA or NATO, work in their standard form. If one is read the wrong way, force the reading with <spell> tags or rephrase.
- Do not use capitalization for emphasis or to indicate shouting. It changes how a word is read, not how loud it sounds.

3. NUMBERS, DATES, AND SYMBOLS
- Use conventional written forms and let text normalization speak them. No preprocessing needed:
  numbers like 1,234,567; currency like $19.99; percentages like 12%; dates like 04/20/2025; times like 7:00 PM;
  US phone numbers like (415) 555-1212; addresses like 123 Main St; emails like user@example.com.
- Do not strip punctuation or force casing. Heavy preprocessing may hurt output quality.

4. SPELLING OUT CODES AND IDS
- For confirmation codes, reference numbers, or any alphanumeric ID that must be read character by character, wrap it in <spell> tags:
  Example: Your confirmation code is <spell>TKT4829XB</spell>.
- Alternatively, delimit the characters instead: spaces (A B C 1 2 3) for a natural pace, or commas (A, B, C, 1, 2, 3) to slow it down. Do not put periods between sequences of individual characters.
- For long sequences like credit card numbers, break the run into smaller comma-separated groups the way a person reads them aloud (3 6 8 9, 0 5 0 5, 2 5 8 2, 3 6 7 9).
- NATO phonetics (Alpha, Bravo) help when the listener needs to disambiguate letters.

5. PAUSES
- Use natural punctuation for pauses. A comma or period usually produces the right pause in context.
- For an explicit, fixed-duration silence, use a break tag:
  Example: Your balance is $1,234.<break time="500ms"/> Your next payment is due June 15th.
- Avoid placing several break tags in quick succession, which can cause hallucinations, and do not chain <spell> and <break> tags.

6. SPEED (beta)
- To slow down speech generation, use a speed tag with a ratio between 0.6 and 1.5: <speed ratio="0.85"/>
- Return to normal speed after: <speed ratio="1.0"/>

7. THINGS TO AVOID
- Do not output bullet points, numbered lists, or any structured formatting. Speak items naturally with pauses between them, and do not say "here's a list."
- Do not use asterisks, hashtags, or markdown syntax. Do not wrap words in **bold** or *italics* — the engine will speak the asterisks.
- Do not improvise details that were not provided.
- Do not repeat the same information more than once unless asked.

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Inserting pauses

Use natural punctuation for pauses — a comma or period usually produces the right pause in context. For an explicit, fixed-duration silence, use a break tag. Break tags split the generation, so they can sound less natural; avoid placing several in quick succession, which can cause hallucinations. Each tag counts as a single character and doesn’t need surrounding whitespace.

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Pronunciation

For proper nouns, trademarks, and domain-specific terms — or to disambiguate identical spellings (e.g. Nice, the city, vs. nice, the adjective) — use custom pronunciations.

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Streaming

Use continuations when generating chunks of audio that need to sound contiguous (for example, LLM-streamed output). This preserves prosody and voice consistency across chunk boundaries.