fix(mcp): add model_size parameter to voicebox.speak (#895)

The MCP voicebox.speak tool built its GenerationRequest without a
model_size, so every agent-triggered generation fell back to the schema
default ("1.7B"). There was no way to reach the 0.6B Qwen variant (or
TADA's 1B/3B) through MCP, and callers paid a model reload whenever the
requested size differed from what was already loaded.

Thread an optional model_size through voicebox.speak and the _speak
helper into GenerationRequest, mirroring the REST /generate surface.
Omitting it passes None, which generate_speech normalizes to the engine
default, so existing callers are unaffected.

Add backend/tests/test_mcp_speak.py covering the forwarded value, the
omitted-default path, and rejection of an invalid size.

Fixes #884

backend/tests/test_mcp_speak.py

@@ -0,0 +1,91 @@
+"""Tests for the voicebox.speak MCP tool's ``model_size`` plumbing (issue #884).
+
+The MCP speak path used to build its ``GenerationRequest`` without a
+``model_size``, so every agent-triggered generation silently fell back to the
+schema default ("1.7B") — there was no way to reach 0.6B (or TADA's 1B/3B)
+through MCP. These tests pin the fix: ``_speak`` now forwards ``model_size``
+straight into the request, matching the REST ``/generate`` surface.
+"""
+
+import pytest
+from pydantic import ValidationError
+
+import backend.routes.generations as generations
+from backend.mcp_server import tools
+
+
+class _FakeGeneration:
+    """Minimal stand-in for GenerationResponse consumed by ``_speak_response``."""
+
+    def model_dump(self, mode="json"):
+        return {"id": "gen-test", "status": "generating"}
+
+
+@pytest.fixture
+def captured_request(monkeypatch):
+    """Replace the real (torch-backed) generate_speech with a capturing stub.
+
+    ``_speak`` imports ``generate_speech`` lazily from ``routes.generations``,
+    so patching the attribute on that module intercepts the call and lets us
+    inspect the ``GenerationRequest`` it would have run.
+    """
+    captured = {}
+
+    async def fake_generate_speech(req, db):
+        captured["req"] = req
+        return _FakeGeneration()
+
+    monkeypatch.setattr(generations, "generate_speech", fake_generate_speech)
+    # Isolate the unit from the MCP event bus — _speak_response fires a
+    # speak-start event we don't care about here.
+    monkeypatch.setattr(tools.mcp_events, "publish", lambda *a, **k: None)
+    return captured
+
+
+@pytest.mark.asyncio
+async def test_speak_forwards_explicit_model_size(captured_request):
+    await tools._speak(
+        profile_id="p1",
+        profile_name="Morgan",
+        text="hello",
+        engine="qwen",
+        language="en",
+        personality=False,
+        model_size="0.6B",
+        db=None,
+    )
+    assert captured_request["req"].model_size == "0.6B"
+
+
+@pytest.mark.asyncio
+async def test_speak_omitted_model_size_is_none(captured_request):
+    # Omitted → None; generate_speech normalizes None to the engine default,
+    # so this reproduces the pre-fix behaviour for callers that don't ask.
+    await tools._speak(
+        profile_id="p1",
+        profile_name="Morgan",
+        text="hello",
+        engine="qwen",
+        language="en",
+        personality=False,
+        db=None,
+    )
+    assert captured_request["req"].model_size is None
+
+
+@pytest.mark.asyncio
+async def test_speak_rejects_invalid_model_size(captured_request):
+    # The GenerationRequest schema pattern is the single source of truth for
+    # valid sizes; a bad value is rejected before any generation runs.
+    with pytest.raises(ValidationError):
+        await tools._speak(
+            profile_id="p1",
+            profile_name="Morgan",
+            text="hello",
+            engine="qwen",
+            language="en",
+            personality=False,
+            model_size="9B",
+            db=None,
+        )
+    assert "req" not in captured_request