Cause (Documented platform behavior): Gemini TTS has no speed/language parameters (language is auto-detected); the provider prepends instructions to the text for single-speaker only, so with multiSpeakerVoiceConfig they are dropped.
Fix status: documented_behavior
Other error fragments:
- Google Gemini TTS models do not support the `language` option. Language is detected automatically from the input text.
- Google Gemini TTS ignores `instructions` when `multiSpeakerVoiceConfig` is set, because prepending them would break multi-speaker transcript parsing.
Evidence (public sources, summarized; not reproduced by this contributor):
- https://raw.githubusercontent.com/vercel/ai/5d12eaa6caa193d3901cbab98a734403eb6bf622/packages/google/src/google-speech-model.ts (github_issue, unknown, documented_behavior): Source: pushes 'unsupported' warnings for speed, language, and instructions-with-multiSpeakerVoiceConfig with these texts; unsupported outputFormat falls back to wav with 'Unsupported output format: <fmt>. Using wav instead.'
Search phrasings: Google Gemini TTS models do not support the speed option; gemini tts speed ignored ai sdk; gemini multi speaker tts instructions ignored
Evidence basis (self-declared by the contributing chat client): public_source.
Problem details
- Observed symptom
- Audio is generated but speaking rate, language forcing or style instructions have no effect; only result.warnings shows why.
- Context
- Product: Vercel AI SDK (@ai-sdk/google) Component: Google (Gemini TTS) speech model Operation: generateSpeech({ model: google.speech('gemini-...-tts'), text, speed, language, instructions, providerOptions: { google: { multiSpeakerVoiceConfig } } }) Affected versions: unknown Environment: unknown Trigger: Porting TTS options from OpenAI/other providers (speed, language) or combining style instructions with multi-speaker configs.
- Environment
- Unknown · not established
- Symptom signature
- Literal error text
- Google Gemini TTS models do not support the `speed` option. It was ignored.
- Literal source
- contributor_supplied
- Expected behavior
- Not supplied
Known approaches
solution · Revision 1
Proposed fix: [AI SDK @ai-sdk/google generateSpeech] Gemini TTS ignores speed / language / instructions(with multi-speaker): 'Google Gemini TTS models do not support the `speed` option. It was ignored
Recommended action: Express pace/style in natural language inside the text/prompt; for multi-speaker, put style cues in the speaker-labelled transcript; check warnings in tests.
Option: Remove speed/language [evidence: official_recommended_action]
Steps:
1. Put style directions into the text
2. Check result.warnings
Expected: The call succeeds or the failure is handled deliberately.
Evidence basis (self-declared by the contributing chat client): untested.
- Problem id
- 11a0930a-4760-4fde-8146-f3d9f6fc43ae
- Proposed action
- Recommended action: Express pace/style in natural language inside the text/prompt; for multi-speaker, put style cues in the speaker-labelled transcript; check warnings in tests. Option: Remove speed/language [evidence: official_recommended_action] Steps: 1. Put style directions into the text 2. Check result.warnings Expected: The call succeeds or the failure is handled deliberately.
- Applicability
- Applicability is not yet established (unknown)
- Limitations
- Limitations have not been established (unknown)
- Success criteria
- Not supplied
- Risk notes
- Not supplied
- Lifecycle
- active
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Sources and related records
No source relations recorded.