Adds a `TTSProvider(ABC)` + `register_tts_provider()` extension point
to the plugin context API, **alongside** the existing config-driven
`tts.providers.<name>: type: command` registry from PR #17843. This is
additive — the command-provider surface stays as the primary way to
add a TTS backend.
The hook covers cases the shell-template grammar can't reasonably
express:
- Native Python SDKs without a CLI (Cartesia, Fish Audio, etc.)
- Streaming synthesis (chunked Opus → voice-bubble delivery)
- Voice metadata API for the `hermes tools` picker
- OAuth-refreshing auth flows
None of the 10 inline built-in providers (`edge`, `openai`,
`elevenlabs`, `minimax`, `gemini`, `mistral`, `xai`, `piper`,
`kittentts`, `neutts`) are migrated to plugins. They stay inline. The
hook is for *new* engines that aren't built-in.
## Resolution order
The dispatcher's resolution order is the load-bearing invariant:
1. `tts.provider` is a built-in name → built-in dispatch. **Always wins.**
2. `tts.provider` matches `tts.providers.<name>` with `command:` set
→ command-provider dispatch (PR #17843).
3. `tts.provider` matches a plugin-registered `TTSProvider`
→ plugin dispatch (new).
4. No match → falls through to Edge TTS default (legacy behavior).
Built-ins-always-win is enforced at THREE layers:
- Registry: `register_provider()` rejects shadowing names with a warning.
- Dispatcher: `_dispatch_to_plugin_provider()` short-circuits built-in
names defensively before consulting the registry.
- Picker: `_plugin_tts_providers()` filters built-in shadows out of
the `hermes tools` row list defensively.
Command-providers-win-over-plugins is enforced at TWO layers:
- The caller in `text_to_speech_tool` checks
`_resolve_command_provider_config` first.
- `_dispatch_to_plugin_provider` re-checks for a same-name command
config defensively so a refactor of the caller can't silently break
the invariant.
## New files
- `agent/tts_provider.py` — `TTSProvider(ABC)` with `synthesize()` (required),
`list_voices()`, `list_models()`, `get_setup_schema()`, `stream()`,
`voice_compatible` (all optional with sane defaults). Mirrors
`agent/image_gen_provider.py` shape.
- `agent/tts_registry.py` — `register_provider`/`get_provider`/`list_providers`
with `_BUILTIN_NAMES` reject-shadowing invariant. Mirrors
`agent/image_gen_registry.py` shape.
- `plugins/tts/...` directory ready for community plugins (none shipped).
## Modified files
- `hermes_cli/plugins.py` — `register_tts_provider()` method on
`PluginContext`. Matches the gating shape of
`register_image_gen_provider()` / `register_browser_provider()`.
- `tools/tts_tool.py` — `_dispatch_to_plugin_provider()` +
`_plugin_provider_is_voice_compatible()` + walrus-elif wiring into
the main dispatcher. Built-in elif chain untouched.
- `hermes_cli/tools_config.py` — `_plugin_tts_providers()` injects
plugin rows into the Text-to-Speech picker category alongside the
10 hardcoded built-in rows.
## Tests
- `tests/agent/test_tts_registry.py` — 47 tests covering registration,
lookup, ABC contract, helpers, AND a `TestBuiltinSync` regression
test that fails if `agent.tts_registry._BUILTIN_NAMES` drifts from
`tools.tts_tool.BUILTIN_TTS_PROVIDERS` (kept duplicated due to
circular import constraints).
- `tests/tools/test_tts_plugin_dispatch.py` — 35 tests covering
built-in-always-wins, command-wins-over-plugin, plugin dispatch,
exception passthrough, voice_compatible helper.
- `tests/hermes_cli/test_tts_picker.py` — 10 tests covering the
picker surface, builtin shadowing defense, integration with
`_visible_providers`.
- `tests/hermes_cli/test_plugins_tts_registration.py` — 3 end-to-end
tests via `PluginManager.discover_and_load()`.
- `tests/plugins/tts/check_parity_vs_main.py` — 9-scenario subprocess
parity harness vs `origin/main`. The only intentional diff is
`fallback_edge → plugin` for the `plugin-installed` scenario.
## Verification
- 95/95 new tests pass.
- 170/170 pre-existing TTS tests (test_tts_command_providers,
test_tts_max_text_length, test_tts_speed, etc.) pass unchanged.
- Parity harness against `origin/main`: 8 OK + 1 expected DIFF.
- E2E smoke: a registered plugin's `synthesize()` is called via
`text_to_speech_tool` with the standard JSON envelope returned.
- Ruff clean on all touched files.
## Docs
- `website/docs/user-guide/features/tts.md` — new "Python plugin
providers" section with a decision table (command-provider vs
plugin), minimal plugin example, and the optional-hook reference.
- `website/docs/user-guide/features/plugins.md` — TTS row updated to
mention both surfaces (command-provider primary, plugin for
SDK/streaming).
Closes#30398
