Supplying custom_vocabulary to gemini-3.5-transcribe does not seem to change the transcript in any way. The output for audio with unusual names is byte-identical to the same request made without the field.
Environment: gemini-3.5-transcribe (public preview) · google-genai 2.20.0 · Python 3.13.15 · POST /v1beta/interactions · smart mode, language_codes: [“en-GB”]
What I expected: “You can steer the speech model toward uncommon words, technical jargon, brand names, or proper nouns. Supply up to 1,000 terms in the custom_vocabulary array.” Supplied words (spellings) should appear in output.
What happens: With test audio containing 12 invented names, the output misidentifies all of them and is the same whether the custom_vocabulary is provided or not. E.g. “Nykvist” transcribed as “Knighthood”.
Example: 'Ilsenmoor" transcribed to “Illulisat”, “Wrenholt” to “Reinholt”.
Supplied: 'custom_vocabulary': ['Ilsenmoor', 'Ophrendel', 'Nykvist Roan', 'Nykvist', 'Palimyr', 'Sedrilax', 'Wrenholt', 'Ghastrey Vole', 'Ghastrey']}