Generate Speech with Gemini TTS

Turn text into natural-sounding speech via Google's Gemini 3.1 Flash TTS preview. Supports single-clip generation, parallel batch mode, prosody tags ([whisper], [short pause], [excited], …), and voice preset files.

Arguments

Parse the user's input for:

• •Target: Raw text (quoted), or a path to a text file (--text-file)
• •--voice NAME: Voice preset name. Resolves in this order:
  1. •voices/<NAME>.txt in this skill dir — first non-comment line is the literal Gemini voiceName (for single-line voice-ID presets)
  2. •Literal voice name (e.g. Kore, Puck, Charon) passed straight through
  3. •Default: read tts-voice.txt (Charon ships as the default)
• •--style-prompt TEXT: Director's-notes prefix prepended to the text (e.g. "Speak slowly, with a warm Newcastle accent."). Gemini honors these notes separately from the voice ID.
• •--style-preset NAME: Load a multi-line style directive from voices/<NAME>.txt (e.g. freud, soprano). Comment lines stripped, body collapsed to one paragraph, prepended as a director's note. Mutually exclusive with --style-prompt / --style-file.
• •--style-file PATH: Like --style-preset but takes an explicit file path (for styles kept outside the skill dir).
• •--output path.wav: Where to save the WAV file
• •--speed FLOAT: Post-process tempo multiplier applied via ffmpeg atempo after the Gemini call. 1.0 (default) = no change, 1.8 pairs well with the freud preset (the preset is deliberately slow; speeding to 1.8× tightens the phrasing without losing the character). Quality is best in [0.5, 2.0]; values outside that band chain atempo filters automatically. Gemini 3.1 Flash TTS has no server-side speed parameter, so post-processing is the deterministic path. Requires ffmpeg on PATH.
• •batch file.json: Parallel batch mode subcommand (see shape below)
• •--api-url URL: Override the Gemini endpoint (rarely needed; useful for testing against a proxy)

--voice (Gemini voice ID) and --style-* (director's notes prepended to the text) are independent and compose — e.g. --voice Charon --style-preset freud pairs the Charon baritone with Freud's Viennese pacing.

Configuration

• •Auth: GOOGLE_API_KEY — auto-loaded from ~/.env by generate-tts.py (same var as gen-image). Passed as an x-goog-api-key header so the key never leaks into URL access logs, shell traces, or proxy logs.
• •Default voice: Read from tts-voice.txt in this skill's directory
• •Single entry point: generate-tts.py — handles the HTTP call, WAV assembly, env loading, voice resolution, and parallel batch execution. Stdlib core with Typer for CLI (no requests / httpx / jq / curl deps); the PEP-723 shebang (#!/usr/bin/env -S uv run --script) means it runs without a local venv. Two subcommands: single (one clip) and batch (parallel from JSON manifest).

Model & Endpoint

• •Model: gemini-3.1-flash-tts-preview
• •Endpoint: POST https://generativelanguage.googleapis.com/v1beta/models/gemini-3.1-flash-tts-preview:generateContent
• •Output: base64-encoded raw PCM (16-bit signed LE, 24 kHz, mono) in candidates[0].content.parts[*].inlineData.data. generate-tts.py wraps it in a canonical WAV header (via struct.pack) so the .wav output is directly playable by any audio tool. Sample rate is parsed from the response MIME type (audio/L16;codec=pcm;rate=24000) with 24000 as the fallback.

Directorial Tags

Embed these inline in the text to steer delivery (Gemini interprets them rather than speaking them verbatim):

• •Dynamics: [whisper], [shouting], [sigh], [gasp], [laughs], [crying]
• •Pacing: [short pause], [long pause]
• •Emotion: [excited], [tired], [sarcastic], [amazed], [angry], [confused], [nervous], [calm], [warm]

The model also accepts narrative "director's notes" prefixed to the text — e.g. "Speak slowly, with a warm Newcastle accent: <text>" — per Simon Willison's 2026-04-15 write-up. Use either approach; they compose.

Voices

30 prebuilt voices ship with the model. Ones we've found useful:

Full catalog in tts-voice.txt. To lock in a named preset separately from the default, drop voices/<name>.txt next to this file with the literal voiceName on its first non-comment line.

voices/<name>.txt — two shapes

Files in voices/ serve two distinct purposes:

1. •Single-line voice-ID alias — first non-comment line is a literal Gemini voice name (Kore, Charon, Puck, etc.). Used via --voice <name>. The rest of the file is commentary on when to pick it.
2. •Multi-line style directive — a character/tone description (Freud's Viennese pacing, Tony Soprano's gravel, etc.). Used via --style-preset <name>. Comment lines are stripped; the body is prepended to the text as a director's note. These do NOT pick a Gemini voice — pair them with --voice Charon (or whichever voice ID suits the character) for best results.

Shipped style presets: freud, soprano.

Usage

The script is executable and uses a uv-powered PEP-723 shebang — invoke it directly, no python3 prefix needed:

GEN_TTS="$(git -C ~/gits/chop-conventions rev-parse --show-toplevel)/skills/gen-tts/generate-tts.py"

Single clip — text arg

"$GEN_TTS" single --text "Hello from Larry. [short pause] This is a voice pipeline test." --output /tmp/larry.wav

Single clip — text file or stdin

"$GEN_TTS" single --text-file /tmp/script.txt --output /tmp/read.wav --voice Charon
echo "piped text" | "$GEN_TTS" single --output /tmp/piped.wav

Single clip — character style preset

# Freud's Viennese analyst voice, baritone
"$GEN_TTS" single --text "Tell me about your mother." --voice Charon \
  --style-preset freud --output /tmp/freud.wav

# Freud at 1.8× — same deliberate phrasing, tighter tempo
"$GEN_TTS" single --text "Tell me about your mother." --voice Charon \
  --style-preset freud --speed 1.8 --output /tmp/freud-fast.wav

# Inline director's notes (no preset)
"$GEN_TTS" single --text "Welcome aboard." --voice Puck \
  --style-prompt "Speak with the warmth of a flight attendant greeting family." \
  --output /tmp/welcome.wav

Batch (parallel)

Write a JSON file:

[
  {
    "text": "Good morning. [short pause] Ready to start?",
    "output": "/tmp/morning.wav",
    "voice": "Kore"
  },
  {
    "text": "[excited] You nailed that workout!",
    "output": "/tmp/celebrate.wav",
    "voice": "Puck"
  },
  { "text": "[whisper] Time to wind down.", "output": "/tmp/bedtime.wav" },
  {
    "text": "Tell me about your father.",
    "output": "/tmp/freud.wav",
    "voice": "Charon",
    "style_preset": "freud"
  }
]

Each job object accepts text, output, voice, plus any one of style_prompt / style_preset / style_file, plus an optional speed float. A --style-* or --speed flag on the CLI provides a default that per-job entries can override.

Then:

"$GEN_TTS" batch /tmp/lines.json --max-workers 4

_duration_s is written back into each job object for debug visibility. The batch file is rewritten atomically (tempfile + os.replace) so a crash mid-write never corrupts the input.

Error Handling

• •Missing API key: auto-loads from ~/.env. If still unset, set GOOGLE_API_KEY in env or ~/.env.
• •API error: top-level error.message (malformed request, quota exhausted, billing not enabled, TTS preview not enabled) is surfaced verbatim with an API Error: prefix.
• •Safety-filter rejection (input): promptFeedback.blockReason surfaces as Gemini TTS refused the prompt (blockReason=...) with the specific reason (e.g. PROHIBITED_CONTENT) plus any safetyRatings for diagnosis.
• •Safety-filter rejection (output): candidates[0].finishReason of SAFETY / PROHIBITED_CONTENT / RECITATION / OTHER (anything other than STOP / MAX_TOKENS) surfaces as refused to generate audio (finishReason=...). Most common trigger: certain prosody tags ([excited], [whisper]) combined with flavorful content trip the filter. Retry without the tag or rephrase.
• •Transient 5xx / network errors: one automatic retry with a 2s delay before giving up. Failed-after-retry requests raise HTTP request failed after retry: ....
• •Empty audio: If the decoded PCM is < 1 KB, the script fails fast rather than emitting a useless WAV.
• •Dependencies: Stdlib-only (urllib.request, base64, struct, json). No jq / curl / requests / httpx required — just a Python 3.11+ interpreter managed by uv (the PEP-723 shebang bootstraps it).

Safety

• •Always confirm before batch runs > ~10 clips (API calls cost money).
• •TTS is a preview API — rate limits and pricing can change with no notice.
• •Do NOT check generated WAV files into this repo; save under /tmp/ or the consumer repo's assets dir.