Changelog 2026

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Sonic 3.5

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Why you should try it

• More natural speech, pacing, and emotional expression, especially noticeable on expressive, conversational, and support-style transcripts.
• Cleaner audio quality across all languages and voices.
• Better alphanumeric read-out — confirmation codes, order numbers, phone numbers, IDs, and emails sound meaningfully more natural, in all supported languages.
• Step-change multilingual performance, particularly Hebrew, Japanese, Spanish, Hindi, German, Korean, and French.
• English heteronyms — tricky English heteronyms like “read,” “bass,” and “bow” now pronounce correctly in context.

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How to try it

1. Point your API call or Playground request to the model ID sonic-3-latest.
2. Keep your existing voice IDs, request shape, and prompting — no code changes required for most customers.
3. Send us feedback on any voice or transcript that behaves differently than you expect.

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What to know to be successful

• Spell tags still work the same way. If you already wrap alphanumerics in <spell>...</spell>, you don’t need to change anything — you’ll just get better-sounding output. See here for more details.
• If you use custom delimiters (commas/periods between characters or groups) to control pacing, our recommended format has changed. Use spaces between characters and commas between groups, e.g. A B C, 1 2 3 instead of A, B, C. 1, 2, 3.. See Prompting tips for Sonic 3.5 for more details.
• Speed and volume controls are temporarily disabled on sonic-3-latest. If you rely on speed or volume augmentation (including via SSML), stay on sonic-3 for now. We believe that Sonic 3.5 has more natural pacing and you may find that you don’t need to use speed control as much when using this model.
• Timestamps behave slightly differently. If you use end-of-word timestamps for interruption handling, you should not see a meaningful change. If you depend on beginning-of-word timestamps, please test carefully and reach out if you see regressions for your use case.
• Existing Professional Voice Clones (PVCs) do not carry over to sonic-3-latest. Professional Voice Clones are pinned to the base model they were trained on (e.g. sonic-3) and will function as a standard voice clone for this model. For more information, see Clone Voices (Pro).

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Where to look for help

• Sonic 3.5 model overview
• Prompting tips for Sonic 3.5
• Model aliases and snapshots

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Breaking

• Text-to-Agent (T2A) API — Text-to-Agent workflow for Line is deprecated.

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API

• Error responses — For Cartesia-Version: 2026-03-01, we now return structured JSON. See API Errors.
  • API versions before 2026-03-01 continue to return legacy error formats (for example HTTP Title: Message).
  • Voices — PATCH /voices/{id}: voice owners can now update accent and gender. Voice creation validates language. Invalid voice UUIDs and pronunciation-dictionary IDs return 404 instead of ambiguous errors.
• PVC model routing — PVC voices require a dated model ID (e.g. sonic-3-2026-01-12) instead of sonic-3. See Clone Voices (Pro).
• Voice search — Name and metadata search is diacritics-insensitive.

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Playground

• Pro voice clones
  • Clearer language mismatch messaging
  • Background noise removal is now a simple on/off control
  • Fine-tuning model support:
    • Removed support for older models
    • Now only sonic-3-2026-01-12 is supported
• Multilingual agents — Multilingual agent configuration is now supported in the Playground.
• Agents UI — Search by call ID and agent ID.

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Billing

• Concurrency — Organizations can receive notifications when concurrency nears configured limits.

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Model / voice

• Professional Voice Clones — Backend updates improve stability of the professional voice cloning workflow.
• Accents & filters — Additional accent options (e.g. Irish, New Zealand, South African, Belgian) and locale aliases for accent filtering in APIs and Playground.
• Voice Library — 94 new voices across 17 locales (including Arabic, German, English variants, Spanish, Finnish, French, Hebrew, Hindi, Japanese, Korean, Polish, Portuguese, Swedish, Telugu, Thai, and more).

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Self-hosted

• On-premises — API for managing voices on self-hosted deployments.

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Cartesia SDK

• cartesia-js v3.0.0 (Mar 2) — Major updates:
  • New features: flush_id included in chunk and voice changer binary responses; output_format and infill support; inline WebSocket response types; byte endpoint returns ArrayBuffer; improved WebPlayer and client export.
  • Fixes: memory leak and timing issues with abort signals/listeners, handling of empty Content-Length, and TimeoutError now includes a message.
  See cartesia-js releases for full details.

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Line

• History Management API: You can add or replace the history provided to your agent, for example, to summarize a long conversation.
• Custom User Events: You can send bidirectional custom events between your client and the agent. You could use this, for example, if you have a web application with UI interactions.
• Uninterruptible Messages: You can set messages as uninterruptible. A common use case is a legal disclaimer at the beginning of a call.
• End Tool Call Improvements: The default end call tool call is more conservative to prevent calls from ending prematurely.

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API

• Increased reliability of API connections

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Cartesia SDK

• cartesia-python v3.0.0 (Feb 9). See full details in cartesia-python releases.

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Playground

• Shipped a new TTS page
• Shipped a new Voice Creation page
• Shipped a new Agents page

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Model changes

• Improved pronunciation of real-world text patterns across languages
  • Enhanced support for structured and formatted speech patterns: numbers, dates, times, currency, phone numbers, IDs, percentages, and amounts/measurements.
  • Support for various date formats (YYYY-MM-DD, YYYY/MM/DD, 年月日).
  • Support for measurement units (meters, kg, tablespoon, gigabytes, etc.) with locale awareness.
  • Support for domestic and international phone number formats with locale-specific chunking for French, Italian, German, Portuguese, Korean, and more.
  • Improved alphanumeric ID handling with katakana/hiragana readings and Latin acronym transliteration to katakana for Japanese.
  • Improves all languages except English, Hindi & other Indic languages, Arabic, Hebrew, Chinese, Swedish, Georgian, Bulgarian, and Tagalog (targeted for future updates).
• Support for regional and locale-specific pronunciation within languages
  • Regional voices use region-specific terms in addition to accent (e.g. Belgian and Swiss French “nonante” vs. Canadian and French “quatre-vingt-dix”).
  • Region-specific number terminology, currency symbols, date formats, and measurement units.
  • Locale-aware date and time formatting (e.g. Russian year suffixes, French/Spanish time conventions).
  • Locale-aware currency symbol handling (e.g. $ as “dollars” in en_US and “pesos” in es_MX).
  • Locale pronunciation falls back to the primary country for that language (e.g. US for English, Brazil for Portuguese). We will continue to expand locale-aware support.
  • Improves all languages except English, Hindi & other Indic languages, Arabic, Hebrew, Chinese, Swedish, Georgian, Bulgarian, and Tagalog (targeted for future updates). Existing regional pronunciation for English voices (e.g. British) is unaffected.

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Voice changes

• Voice Library: 39 new voices across 21 locales

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Breaking changes effective June 1, 2026

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API

• Regionalization — Calls routed to US, EU, APAC by origin.
• Parameterized outbound calls — Docs
• Pronunciation dictionaries — Docs

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Model changes

• Sonic-3 model versioning scheme introduced
  • New preview track: sonic-3-latest (continuous updates for early access and feedback).
  • Stable track: sonic-3 always points to the most recent stable release.
  • Immutable dated snapshots: sonic-3-YYYY-MM-DD never change.
  • Details: Continuous updates and model snapshots
• Promotion to stable checkpoint: sonic-3-2026-01-12
  • Included improvements: consistent speed & volume, custom IPA pronunciations with stronger adherence, Hindi prosody improvements, Korean prosody/intonation improvements.

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Voice changes

• Featured Voices launched — Curated set of 30+ best-performing voices (e.g. Cathy, Henry).
• Voice Library — December: 25 new voices across 6 languages.
• Voice Library — January: 9 Spanish voices (Mexican, Colombian, Castilian).

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Playground

• Voice library usability improvements (test with your own scripts, call an agent per voice).
• One-click Report Issue on TTS Playground.
• Mini voice picker (recently used + saved) on TTS page.
• PVC UI + reliability (loading skeletons, error messages, better behavior with large datasets and silence).

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Line

• Line SDK v0.2 — Repo. Improved DX, long-running tool-call handling, committed turns, better turn-taking and transcription.