# HeyGen Voice vs Smallest.ai Lightning TTS (slim)

> HeyGen Voice scores 73.6 (BB) to Smallest.ai Lightning TTS's 64.4 (B) for text-to-speech. Prices, MCP, x402, uptime and agent notes side by side.

- Full: https://www.anchorterminal.com/compare/heygen-voice-vs-smallest-lightning-tts.md (~5,200 tokens) · this version ~2,980 tokens · JSON https://www.anchorterminal.com/compare/heygen-voice-vs-smallest-lightning-tts.json · canonical https://www.anchorterminal.com/compare/heygen-voice-vs-smallest-lightning-tts
- Index: https://www.anchorterminal.com/llms.txt · API: https://www.anchorterminal.com/api/v1/index.json · MCP: https://www.anchorterminal.com/mcp · Updated: 2026-11-07
- Licence: CC BY 4.0, credit Anchor Terminal and link this page (https://www.anchorterminal.com/terms/#data)

HeyGen Voice scores 73.6 (BB) on agent readiness against Smallest.ai Lightning TTS's 64.4 (B), and leads in 4 of 7 scored categories. Both do text-to-speech.

- HeyGen Voice: BB 73.6, rank #102 of 1486 · https://www.anchorterminal.com/tools/heygen-voice.min.md
- Smallest.ai Lightning TTS: B 64.4, rank #469 of 1486 · https://www.anchorterminal.com/tools/smallest-lightning-tts.min.md
- HeyGen Voice, good for Speech in a voice cloned from one short recording, for agents that already make HeyGen videos or need a cloned narrator with streaming and timestamps. Ahead on Reliability, 75 against 58; Schema & documentation, 97 against 92; Agent ergonomics, 88 against 72; Security & auth, 69 against 52. Also Agent-ready, a grade of BB or better; A hosted endpoint, with nothing to install.
- Smallest.ai Lightning TTS, good for Low-latency speech for voice agents, with English, Hindi code-switching and Indic voices.

| Category | HeyGen Voice | Smallest.ai Lightning TTS |
| --- | --- | --- |
| Reliability (16%) | 75 | 58 |
| Performance (10%) | pending | pending |
| Schema & documentation (13%) | 97 | 92 |
| Agent ergonomics (13%) | 88 | 72 |
| Security & auth (14%) | 69 | 52 |
| Payments & pricing (10%) | 40 | 40 |
| Task success (10%) | pending | pending |
| Maintenance & community (7%) | 65 | 68 |
| Transparency & trust (7%) | 66 | 70 |

| Fact (where they differ) | HeyGen Voice | Smallest.ai Lightning TTS |
| --- | --- | --- |
| Vendor | HeyGen | Smallest AI |
| Hosted endpoint | `https://api.heygen.com` | no (local only) |
| Transports | HTTP | HTTP, websocket |
| Licence | Closed service under HeyGen's Terms of Service (HeyGen Technology, Inc., last updated 23 July 2026). | none |
| Tools exposed | none | 0 |
| Last release | 2026-10-20 | 2026-11-02 |
| Terms last updated | 2026-07-23 | no date given |
| Privacy policy last updated | 2026-08-11 | no date given |
| Customer content may train models | yes | not found in the text |
| Terms restrict automated access | yes | not found in the text |
| Terms restrict benchmarking | yes | not found in the text |
| Agent reviews | 3.5/5 (4) | none |

Why the edge:
- Reliability, HeyGen Voice +17: HeyGen Voice 75 (Status page at status.heygen.com with components for www, app and api.heygen.com and dated incident history (20). It has no voice component. In the 90 days from 3 August, majors hit Video Agent (9 September, about 3.5 hours of 404s on new sessions) and Seedance (16 September, failures between 8:00 and 11:00 PST), and minors or no-impact incidents hit video workflows, video translation, sign-in (10 October) and general service speed (17 and 20 August). None names speech or voices, so partial and minor for this surface (20). The incidents feed and history feed list nothing after 10 October. The speech endpoints publish 30 requests a minute per workspace member (15). A 429 carries Retry-After and X-RateLimit headers, the docs ask for exponential backoff when it doesn't (as for a capacity 429), the OpenAPI file marks 502, 503 and 504 safe to retry, and voice creation and speech to speech take an Idempotency-Key (15). The terms disclaim uninterrupted service and no SLA was found, per the 10 October check (0). Instant voice creation is still in a preview, Turbo Free carries no availability guarantee, and the professional mode carries no label, so half (5). Total 75.); Smallest.ai Lightning TTS 58 (BetterStack status page at status.smallest.ai with components for the Global, India and USA APIs (one health check covers the Agents and Models APIs), docs, the app and Electron, and per-component history (20). In the 90 days from 8 August the health checks on the shared API failed in all three regions on 9, 10, 13, 14 and 15 September (about 8, 21, 33, 17 and 64 minutes of flapping), in the Global and India regions on 11 and 17 September (12 minutes each), and in the USA region alone on 1, 3, 5, 12, 21 and 30 September (6 to 62 minutes) and 6 October (12 minutes). Outages in every region at once are read as wide errors, so several majors (0). Limits are published with numbers but disagree. The limits page says one TTS request at a time per account with up to five sockets, the Lightning v3.1 Pro card says 10 concurrent and the pricing page says 15 streams and 100 requests a minute on pay as you go (8 of 15). 429 is documented with exponential backoff from 500 ms to 30 s and Retry-After when present, and the limits page tells clients to expect 429s at the concurrency ceiling (15). An infrastructure SLA of 99.99 per cent is named for Enterprise only, with no terms, and the terms disclaim any availability commitment (5 of 10). `lightning_v3.1_pro` is stable and only the dated checkpoint is preview (10). 20 + 0 + 8 + 15 + 5 + 10 = 58.)
- Security & auth, HeyGen Voice +17: HeyGen Voice 69 (Model reading of the checklist, with training and retention in place of least-privilege and injection lines. API keys can be restricted to resource read and write scopes, a key without the scope gets 403 insufficient_api_key_scope, `GET /v3/api_keys/self` reports a key's scope mode, and the docs cover rotation (30). The privacy policy (11 August 2026) and the terms (23 July 2026) allow training on user input, with an opt-out by email, and the security page excludes enterprise data by default, per the 10 October check, not re-read on 1 November (10). No retention period for voices or speech was found, deleted data stays in backups for 60 days, and voices can be deleted through the API (5). Each response carries an X-Request-Id used in usage and billing records, and `end_user_id` attributes usage per end user. Audit logs of user and admin actions are for enterprise customers (10 of 15). A valid security.txt (expires 31 December 2026, re-read 1 November), a disclosure policy with safe harbour, SOC 2 Type II stated on the security page with reports on request, and no bug bounty. The report was not read, so the certification counts as a vendor claim (14 of 20). Total 69.); Smallest.ai Lightning TTS 52 (Model reading of the checklist, with training and retention in place of the least-privilege and injection lines. Plain `sk_` keys per organisation, shown once, revoked at once and deletable, with one key per environment advised. The docs say a key grants full account access, and no scope is offered. Short-lived access tokens (`wat_`, 30 to 900 seconds) cover only inference routes and can't mint tokens (22 of 30). The error reference documents the key as a `?token=` query parameter for browser WebSockets, and the token guide passes access tokens as `?api_key=` on the STT socket, so 10 off (12 of 30). The terms (21 October 2026) say Smallest won't use user content or input to train or improve its services. The privacy policy (18 September 2026) lists improving the services with machine learning as a use of usage data and doesn't address uploaded audio or text either way (15 of 20). Zero data retention and the `x-expire-content` header are Enterprise only, or a $1,000 a month HIPAA add-on on pay as you go. Other plans keep content readable in logs and the dashboard, with no retention period published (5 of 15). Usage is tracked per key, request rows sit in a dashboard log viewer, and every response carries `x-request-id`. No audit log of key actions was found (10 of 15). security.txt is valid to 31 December 2026 and the site footer claims SOC 2, GDPR and HIPAA compliance, while the pricing page lists SOC 2 compliance only on Enterprise. No bug bounty, disclosure policy or public advisories found (10 of 20). Total 52.)
- Agent ergonomics, HeyGen Voice +16: HeyGen Voice 88 (API reading of the checklist. The completed response is a URL and a duration, streaming returns WAV parts as they are made, and timestamps come only when asked. Since October `output_format` picks PCM16, MP3, mu-law or A-law at 8 to 48 kHz with an MP3 bitrate, and the stream can return raw bytes (25 of 25, was 20 for WAV only). Voices list newest first with `limit` and `token`, private or public with `type`, and text is capped per request, but there are no output-size controls beyond the text and format (15 of 20). Typed error codes such as voice_not_ready, voice_expired, insufficient_credit and rate_limit_exceeded, each with a documented next step (20). Idempotency-Key on voice creation and speech to speech, safe-retry status codes on speech, a free request never switched to a paid model, and a best-effort `seed` for professional voices (20). Only `voice_id` and `text` are required and `model` defaults to heygen-voice-1, but no official SDK was found, only the HeyGen CLI and agent skills. The HeyGen Voice page's line about SDKs for Python and Node links to none (8 of 15). Total 88.); Smallest.ai Lightning TTS 72 (API reading of the checklist. Eight output formats, a choice of sample rate and word timestamps only when asked (22 of 25). Voices are listed per model and the docs say they filter by language and accent. Paging wasn't confirmed (14 of 20). Errors carry `error` and `message` fields with a shared status-code guide, without TTS-specific codes (14 of 20). Synthesis is stateless and `request_id` is echoed for correlation, but there is no idempotency key and billing of a retried request isn't stated (10 of 20). Two required fields, `text` and `voice_id`, but the default model retires on 30 November. Official Python and Node SDKs, with the Node SDK last published in May 2025 (12 of 15). Total 72.)
- Schema & documentation, HeyGen Voice +5: HeyGen Voice 97 (OpenAPI 3.1 file at developers.heygen.com/openapi/external-api.json (132 paths, 184 operations on 1 November) covering create, list, get and delete voice, completed and streamed speech and speech to speech (25). llms.txt with a Markdown twin of every page (10). Descriptions say what each endpoint is for and when to use another, for example that Turbo refuses professional voices, that Turbo Free is for tests and prototypes with shared capacity, and that catalogue voices go through /v3/voices/speech. Since 13 October the model page and changelog document Turbo, which they didn't on 10 October (20). Request bodies are typed with an enum of three `model` values, numeric ranges for every control, a typed `output_format`, a 5,000-character cap and `additionalProperties: false` (15). curl examples for both endpoints, a copyable prompt for coding agents and a per-endpoint error table with codes and recovery steps (15). Versioned /v3 paths and a public changelog with an RSS feed, dated by month on the page, with day dates only in the feed (12 of 15). Total 97.); Smallest.ai Lightning TTS 92 (OpenAPI 3.1 for the sync and SSE routes and AsyncAPI 2.6 for the WebSocket, with request and event schemas (25). llms.txt and a Markdown copy of every page (10). A transport guide, model cards and the compare page say which model and transport suit which job, but changelog entries correct the language lists and enums the docs and spec carried (15 of 20). Typed fields with enums for model, sample rate, language and output format, an 8,000-character limit and pattern rules on `session_id` and `request_id` (14 of 15). Examples in curl, Python and JavaScript, with error bodies on the reference and a shared error page (13 of 15). Versioned `/v1` paths, a dated Lightning changelog and dated preview checkpoints (15). Total 92.)

| Task | HeyGen Voice | Smallest.ai Lightning TTS |
| --- | --- | --- |
| Narrate a 9,000-character script | $0.13 to $0.27 | $0.16 to $0.18 |