diff --git a/internal/audio/elevenlabs/capabilities.go b/internal/audio/elevenlabs/capabilities.go
index d981d991..bf515be8 100644
--- a/internal/audio/elevenlabs/capabilities.go
+++ b/internal/audio/elevenlabs/capabilities.go
@@ -11,29 +11,34 @@ var elevenLabsModels = []string{
 }
 
 var (
-	stabilityMin      = 0.0
-	stabilityMax      = 1.0
-	stabilityStep     = 0.01
-	simBoostMin       = 0.0
-	simBoostMax       = 1.0
-	simBoostStep      = 0.01
-	styleMin          = 0.0
-	styleMax          = 1.0
-	styleStep         = 0.01
-	elSpeedMin        = 0.7
-	elSpeedMax        = 1.2
-	elSpeedStep       = 0.01
-	latencyMin        = 0.0
-	latencyMax        = 4.0
-	latencyStep       = 1.0
-	seedMin           = 0.0
-	seedMax           = 4294967295.0 // 2^32-1
-	seedStep          = 1.0
+	stabilityMin  = 0.0
+	stabilityMax  = 1.0
+	stabilityStep = 0.01
+	simBoostMin   = 0.0
+	simBoostMax   = 1.0
+	simBoostStep  = 0.01
+	styleMin      = 0.0
+	styleMax      = 1.0
+	styleStep     = 0.01
+	elSpeedMin    = 0.7
+	elSpeedMax    = 1.2
+	elSpeedStep   = 0.01
+	latencyMin    = 0.0
+	latencyMax    = 4.0
+	latencyStep   = 1.0
+	seedMin       = 0.0
+	seedMax       = 4294967295.0 // 2^32-1
+	seedStep      = 1.0
 )
 
 // elevenLabsParams is the enriched param schema for ElevenLabs TTS.
-// Defaults MUST match the hardcoded values in tts.go:54-59 (characterization fixture):
-//   stability=0.5, similarity_boost=0.75, style=0.0, use_speaker_boost=true, speed=1.0
+// Defaults MUST match the hardcoded values in tts.go (characterization fixture):
+//
+//	stability=0.5, similarity_boost=0.75, style=0.0, use_speaker_boost=true, speed=1.0
+//
+// Group tagging (Finding #7 + §Requirements #7):
+//   - seed, optimize_streaming_latency, language_code, output_format = "advanced"
+//   - all others = "" (basic)
 var elevenLabsParams = []audio.ParamSchema{
 	{
 		Key:         "voice_settings.stability",
@@ -101,6 +106,7 @@ var elevenLabsParams = []audio.ParamSchema{
 		Min:         &seedMin,
 		Max:         &seedMax,
 		Step:        &seedStep,
+		Group:       "advanced",
 	},
 	{
 		Key:         "optimize_streaming_latency",
@@ -111,6 +117,7 @@ var elevenLabsParams = []audio.ParamSchema{
 		Min:         &latencyMin,
 		Max:         &latencyMax,
 		Step:        &latencyStep,
+		Group:       "advanced",
 	},
 	{
 		Key:         "language_code",
@@ -118,6 +125,55 @@ var elevenLabsParams = []audio.ParamSchema{
 		Label:       "Language Code",
 		Description: "ISO 639-1 language hint (e.g. 'en', 'vi'). Advanced.",
 		Default:     "",
+		Group:       "advanced",
+	},
+	// output_format — URL query param controlling codec + bitrate.
+	// 27 values per ElevenLabs API docs. Default mp3_44100_128 matches the
+	// hardcoded fallback in tts.go so the defaults-invariant test stays green.
+	// Tier note: mp3_44100_192 requires Creator+; pcm_*/wav_* require Pro+.
+	// Backend forwards whatever the user sets; API returns 403 on tier violation.
+	{
+		Key:   "output_format",
+		Type:  audio.ParamTypeEnum,
+		Label: "Output Format",
+		Description: "Audio codec and bitrate. Higher quality tiers require Creator+ or Pro+ subscription. " +
+			"Advanced.",
+		Default: "mp3_44100_128",
+		Group:   "advanced",
+		Enum: []audio.EnumOption{
+			// MP3 variants
+			{Value: "mp3_22050_32", Label: "MP3 22 kHz 32 kbps"},
+			{Value: "mp3_44100_32", Label: "MP3 44.1 kHz 32 kbps"},
+			{Value: "mp3_44100_64", Label: "MP3 44.1 kHz 64 kbps"},
+			{Value: "mp3_44100_96", Label: "MP3 44.1 kHz 96 kbps"},
+			{Value: "mp3_44100_128", Label: "MP3 44.1 kHz 128 kbps (default)"},
+			{Value: "mp3_44100_192", Label: "MP3 44.1 kHz 192 kbps (Creator+)"},
+			// Opus variants
+			{Value: "opus_16000_32", Label: "Opus 16 kHz 32 kbps"},
+			{Value: "opus_24000_32", Label: "Opus 24 kHz 32 kbps"},
+			{Value: "opus_24000_64", Label: "Opus 24 kHz 64 kbps"},
+			{Value: "opus_24000_96", Label: "Opus 24 kHz 96 kbps"},
+			{Value: "opus_48000_32", Label: "Opus 48 kHz 32 kbps"},
+			// PCM variants (Pro+)
+			{Value: "pcm_8000", Label: "PCM 8 kHz (Pro+)"},
+			{Value: "pcm_16000", Label: "PCM 16 kHz (Pro+)"},
+			{Value: "pcm_22050", Label: "PCM 22 kHz (Pro+)"},
+			{Value: "pcm_24000", Label: "PCM 24 kHz (Pro+)"},
+			{Value: "pcm_32000", Label: "PCM 32 kHz (Pro+)"},
+			{Value: "pcm_44100", Label: "PCM 44.1 kHz (Pro+)"},
+			{Value: "pcm_48000", Label: "PCM 48 kHz (Pro+)"},
+			// WAV variants (Pro+)
+			{Value: "wav_8000", Label: "WAV 8 kHz (Pro+)"},
+			{Value: "wav_16000", Label: "WAV 16 kHz (Pro+)"},
+			{Value: "wav_22050", Label: "WAV 22 kHz (Pro+)"},
+			{Value: "wav_24000", Label: "WAV 24 kHz (Pro+)"},
+			{Value: "wav_32000", Label: "WAV 32 kHz (Pro+)"},
+			{Value: "wav_44100", Label: "WAV 44.1 kHz (Pro+)"},
+			{Value: "wav_48000", Label: "WAV 48 kHz (Pro+)"},
+			// μ-law and a-law
+			{Value: "ulaw_8000", Label: "μ-law 8 kHz"},
+			{Value: "alaw_8000", Label: "A-law 8 kHz"},
+		},
 	},
 }
 
diff --git a/internal/audio/elevenlabs/format_meta_test.go b/internal/audio/elevenlabs/format_meta_test.go
new file mode 100644
index 00000000..7223a148
--- /dev/null
+++ b/internal/audio/elevenlabs/format_meta_test.go
@@ -0,0 +1,193 @@
+package elevenlabs_test
+
+import (
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/nextlevelbuilder/goclaw/internal/audio"
+	"github.com/nextlevelbuilder/goclaw/internal/audio/elevenlabs"
+)
+
+// TestFormatMeta_AllFormats is a table-driven test covering all 27 ElevenLabs
+// output_format values plus the fallback for unknown input.
+// Finding #7: aligned to researcher-02 — no pcm_mulaw_* variants,
+// single audio/pcm MIME for all pcm_* variants.
+func TestFormatMeta_AllFormats(t *testing.T) {
+	cases := []struct {
+		format   string
+		wantExt  string
+		wantMime string
+	}{
+		// MP3 variants (6 from capabilities enum)
+		{"mp3_22050_32", "mp3", "audio/mpeg"},
+		{"mp3_44100_32", "mp3", "audio/mpeg"},
+		{"mp3_44100_64", "mp3", "audio/mpeg"},
+		{"mp3_44100_96", "mp3", "audio/mpeg"},
+		{"mp3_44100_128", "mp3", "audio/mpeg"},
+		{"mp3_44100_192", "mp3", "audio/mpeg"},
+		// Opus variants (5)
+		{"opus_16000_32", "opus", "audio/opus"},
+		{"opus_24000_32", "opus", "audio/opus"},
+		{"opus_24000_64", "opus", "audio/opus"},
+		{"opus_24000_96", "opus", "audio/opus"},
+		{"opus_48000_32", "opus", "audio/opus"},
+		// PCM variants (7) — all use single audio/pcm MIME (Finding #7)
+		{"pcm_8000", "pcm", "audio/pcm"},
+		{"pcm_16000", "pcm", "audio/pcm"},
+		{"pcm_22050", "pcm", "audio/pcm"},
+		{"pcm_24000", "pcm", "audio/pcm"},
+		{"pcm_32000", "pcm", "audio/pcm"},
+		{"pcm_44100", "pcm", "audio/pcm"},
+		{"pcm_48000", "pcm", "audio/pcm"},
+		// WAV variants (7)
+		{"wav_8000", "wav", "audio/wav"},
+		{"wav_16000", "wav", "audio/wav"},
+		{"wav_22050", "wav", "audio/wav"},
+		{"wav_24000", "wav", "audio/wav"},
+		{"wav_32000", "wav", "audio/wav"},
+		{"wav_44100", "wav", "audio/wav"},
+		{"wav_48000", "wav", "audio/wav"},
+		// μ-law and a-law (2)
+		{"ulaw_8000", "ulaw", "audio/basic"},
+		{"alaw_8000", "alaw", "audio/x-alaw"},
+		// Fallback for unknown/empty
+		{"unknown_format", "mp3", "audio/mpeg"},
+		{"", "mp3", "audio/mpeg"},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.format, func(t *testing.T) {
+			gotExt, gotMime := elevenlabs.FormatMeta(tc.format)
+			if gotExt != tc.wantExt {
+				t.Errorf("FormatMeta(%q) ext = %q, want %q", tc.format, gotExt, tc.wantExt)
+			}
+			if gotMime != tc.wantMime {
+				t.Errorf("FormatMeta(%q) mime = %q, want %q", tc.format, gotMime, tc.wantMime)
+			}
+		})
+	}
+}
+
+// TestSynthesize_OutputFormat_URLRoundTrip verifies output_format flows into the
+// URL query param.
+func TestSynthesize_OutputFormat_URLRoundTrip(t *testing.T) {
+	cfg := elevenlabs.Config{APIKey: "k", VoiceID: "v"}
+	_, path := captureElevenLabsBody(t, cfg, audio.TTSOptions{
+		Params: map[string]any{"output_format": "pcm_44100"},
+	})
+	if !containsStr(path, "output_format=pcm_44100") {
+		t.Errorf("expected output_format=pcm_44100 in URL, got: %s", path)
+	}
+}
+
+// TestSynthesize_OutputFormat_SynthResult verifies SynthResult.Extension and
+// MimeType are derived from output_format for a pcm_44100 request.
+func TestSynthesize_OutputFormat_SynthResult(t *testing.T) {
+	cfg := elevenlabs.Config{APIKey: "k", VoiceID: "v"}
+	result := synthesizeAndCapture(t, cfg, audio.TTSOptions{
+		Params: map[string]any{"output_format": "pcm_44100"},
+	})
+	if result.Extension != "pcm" {
+		t.Errorf("Extension: got %q, want pcm", result.Extension)
+	}
+	if result.MimeType != "audio/pcm" {
+		t.Errorf("MimeType: got %q, want audio/pcm", result.MimeType)
+	}
+}
+
+// TestSynthesize_OpusContract_TelegramPath verifies Finding #11:
+// when opts.Format="opus", MIME is always "audio/ogg; codecs=opus" regardless
+// of any user-set output_format param. Telegram rejects plain "audio/opus" MIME.
+func TestSynthesize_OpusContract_TelegramPath(t *testing.T) {
+	cfg := elevenlabs.Config{APIKey: "k", VoiceID: "v"}
+	result := synthesizeAndCapture(t, cfg, audio.TTSOptions{
+		Format: "opus",
+		Params: map[string]any{"output_format": "opus_24000_96"},
+	})
+	if result.MimeType != "audio/ogg; codecs=opus" {
+		t.Errorf("Telegram opus contract FAILED: MimeType = %q, want audio/ogg; codecs=opus", result.MimeType)
+	}
+	if result.Extension != "ogg" {
+		t.Errorf("Telegram opus contract FAILED: Extension = %q, want ogg", result.Extension)
+	}
+}
+
+// TestSynthesize_OutputFormat_InvalidChars verifies Finding #4:
+// output_format with injection characters is rejected before any HTTP call.
+func TestSynthesize_OutputFormat_InvalidChars(t *testing.T) {
+	cases := []string{
+		"mp3_44100_128&api_key=LEAK",
+		"opus_48000\n",
+		"pcm 44100",
+		"mp3%20injection",
+	}
+	for _, bad := range cases {
+		t.Run(bad, func(t *testing.T) {
+			cfg := elevenlabs.Config{APIKey: "k", VoiceID: "v"}
+			_, err := synthesizeWithError(t, cfg, audio.TTSOptions{
+				Params: map[string]any{"output_format": bad},
+			})
+			if err == nil {
+				t.Errorf("output_format=%q: expected validation error, got nil", bad)
+			}
+		})
+	}
+}
+
+// TestSynthesize_LanguageCode_InvalidChars verifies Finding #4:
+// language_code with injection characters is rejected before any HTTP call.
+func TestSynthesize_LanguageCode_InvalidChars(t *testing.T) {
+	cases := []string{
+		"en&voice_id=victim",
+		"en\r\nX-Injected: yes",
+		"toolong_codex",
+	}
+	for _, bad := range cases {
+		t.Run(bad, func(t *testing.T) {
+			cfg := elevenlabs.Config{APIKey: "k", VoiceID: "v"}
+			_, err := synthesizeWithError(t, cfg, audio.TTSOptions{
+				Params: map[string]any{"language_code": bad},
+			})
+			if err == nil {
+				t.Errorf("language_code=%q: expected validation error, got nil", bad)
+			}
+		})
+	}
+}
+
+// synthesizeAndCapture runs Synthesize against a mock server and returns the
+// *audio.SynthResult. Fails the test on any error.
+func synthesizeAndCapture(t *testing.T, cfg elevenlabs.Config, opts audio.TTSOptions) *audio.SynthResult {
+	t.Helper()
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		io.Copy(io.Discard, r.Body)
+		w.Header().Set("Content-Type", "audio/mpeg")
+		w.Write([]byte("AUDIO"))
+	}))
+	t.Cleanup(srv.Close)
+	cfg.BaseURL = srv.URL
+	p := elevenlabs.NewTTSProvider(cfg)
+	result, err := p.Synthesize(t.Context(), "hello", opts)
+	if err != nil {
+		t.Fatalf("Synthesize failed: %v", err)
+	}
+	return result
+}
+
+// synthesizeWithError runs Synthesize and returns (result, err) without failing
+// the test — used to assert expected errors.
+func synthesizeWithError(t *testing.T, cfg elevenlabs.Config, opts audio.TTSOptions) (*audio.SynthResult, error) {
+	t.Helper()
+	// Use a mock server so validation-path errors (pre-HTTP) are still exercised.
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		io.Copy(io.Discard, r.Body)
+		w.Header().Set("Content-Type", "audio/mpeg")
+		w.Write([]byte("AUDIO"))
+	}))
+	t.Cleanup(srv.Close)
+	cfg.BaseURL = srv.URL
+	p := elevenlabs.NewTTSProvider(cfg)
+	return p.Synthesize(t.Context(), "hello", opts)
+}
diff --git a/internal/audio/elevenlabs/golden_test.go b/internal/audio/elevenlabs/golden_test.go
new file mode 100644
index 00000000..6f618861
--- /dev/null
+++ b/internal/audio/elevenlabs/golden_test.go
@@ -0,0 +1,60 @@
+package elevenlabs_test
+
+import (
+	"encoding/json"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/nextlevelbuilder/goclaw/internal/audio"
+	"github.com/nextlevelbuilder/goclaw/internal/audio/elevenlabs"
+)
+
+// TestGolden_DefaultBody verifies the serialized nil-params request body + URL
+// matches the checked-in golden fixture. Finding #10: external golden file catches
+// simultaneous Default+tts.go drift that a self-referential invariant test misses.
+func TestGolden_DefaultBody(t *testing.T) {
+	cfg := elevenlabs.Config{APIKey: "test-key", VoiceID: "test-voice"}
+	body, path := captureElevenLabsBody(t, cfg, audio.TTSOptions{})
+
+	full := map[string]any{
+		"body": body,
+		"path": path,
+	}
+	gotFull, err := json.MarshalIndent(full, "", "  ")
+	if err != nil {
+		t.Fatalf("marshal body: %v", err)
+	}
+
+	goldenPath := filepath.Join("testdata", "default_body.golden.json")
+	if _, statErr := os.Stat(goldenPath); os.IsNotExist(statErr) {
+		if err := os.MkdirAll("testdata", 0755); err != nil {
+			t.Fatalf("mkdir testdata: %v", err)
+		}
+		if err := os.WriteFile(goldenPath, gotFull, 0644); err != nil {
+			t.Fatalf("write golden: %v", err)
+		}
+		t.Logf("golden file created: %s", goldenPath)
+		return
+	}
+
+	want, err := os.ReadFile(goldenPath)
+	if err != nil {
+		t.Fatalf("read golden: %v", err)
+	}
+
+	var wantMap, gotMap map[string]any
+	if err := json.Unmarshal(want, &wantMap); err != nil {
+		t.Fatalf("parse golden: %v", err)
+	}
+	if err := json.Unmarshal(gotFull, &gotMap); err != nil {
+		t.Fatalf("parse current: %v", err)
+	}
+
+	wantNorm := canonicalJSON(t, wantMap)
+	gotNorm := canonicalJSON(t, gotMap)
+	if gotNorm != wantNorm {
+		t.Errorf("golden fixture mismatch (update testdata/default_body.golden.json if intentional):\n  got:  %s\n  want: %s",
+			gotNorm, wantNorm)
+	}
+}
diff --git a/internal/audio/elevenlabs/testdata/default_body.golden.json b/internal/audio/elevenlabs/testdata/default_body.golden.json
new file mode 100644
index 00000000..0dd0f2b2
--- /dev/null
+++ b/internal/audio/elevenlabs/testdata/default_body.golden.json
@@ -0,0 +1,14 @@
+{
+  "body": {
+    "model_id": "eleven_multilingual_v2",
+    "text": "hello",
+    "voice_settings": {
+      "similarity_boost": 0.75,
+      "speed": 1,
+      "stability": 0.5,
+      "style": 0,
+      "use_speaker_boost": true
+    }
+  },
+  "path": "/v1/text-to-speech/test-voice?output_format=mp3_44100_128"
+}
\ No newline at end of file
diff --git a/internal/audio/elevenlabs/tts.go b/internal/audio/elevenlabs/tts.go
index 0e4d0f85..b8b4fad4 100644
--- a/internal/audio/elevenlabs/tts.go
+++ b/internal/audio/elevenlabs/tts.go
@@ -4,6 +4,9 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
+	"net/url"
+	"regexp"
+	"strings"
 
 	"github.com/nextlevelbuilder/goclaw/internal/audio"
 )
@@ -27,6 +30,40 @@ func NewTTSProvider(cfg Config) *TTSProvider {
 // Name returns the stable provider identifier used by the Manager.
 func (p *TTSProvider) Name() string { return "elevenlabs" }
 
+// reOutputFormat validates output_format query param: lowercase alphanumeric + underscore only.
+// Finding #4: prevents HTTP Parameter Pollution / injection via user-controlled format string.
+var reOutputFormat = regexp.MustCompile(`^[a-z0-9_]+$`)
+
+// reLanguageCode validates language_code: BCP-47 subset (2–3 letter lang + optional region).
+// Finding #4: prevents injection via user-controlled language code.
+var reLanguageCode = regexp.MustCompile(`^[a-z]{2,3}(-[A-Z]{2})?$`)
+
+// FormatMeta maps an ElevenLabs output_format string to the file extension and
+// MIME type to use in SynthResult. All pcm_* variants use audio/pcm (single MIME
+// per Finding #7 — no per-rate variants). Exported for white-box tests.
+//
+// Telegram opus contract (Finding #11): this function is NOT called when
+// opts.Format=="opus"; that path hardcodes ext=ogg / mime=audio/ogg; codecs=opus.
+func FormatMeta(format string) (ext, mime string) {
+	switch {
+	case strings.HasPrefix(format, "mp3_"):
+		return "mp3", "audio/mpeg"
+	case strings.HasPrefix(format, "opus_"):
+		return "opus", "audio/opus"
+	case strings.HasPrefix(format, "pcm_"):
+		return "pcm", "audio/pcm"
+	case strings.HasPrefix(format, "wav_"):
+		return "wav", "audio/wav"
+	case format == "ulaw_8000":
+		return "ulaw", "audio/basic"
+	case format == "alaw_8000":
+		return "alaw", "audio/x-alaw"
+	default:
+		// Unknown format: fall back to mp3 so callers get a usable MIME type.
+		return "mp3", "audio/mpeg"
+	}
+}
+
 // Synthesize converts text to audio. Opts.Voice/Opts.Model override the
 // configured defaults; Opts.Format="opus" switches to Ogg Opus output.
 // opts.Params keys (nested dot-path):
@@ -39,6 +76,7 @@ func (p *TTSProvider) Name() string { return "elevenlabs" }
 //   - "seed"                            int      (default 0 = omitted)
 //   - "optimize_streaming_latency"      int      (query param, default 0 = omitted)
 //   - "language_code"                   string   (query param, default "" = omitted)
+//   - "output_format"                   string   (query param, default "mp3_44100_128")
 //
 // MUST NOT mutate opts.Params — reads only.
 func (p *TTSProvider) Synthesize(ctx context.Context, text string, opts audio.TTSOptions) (*audio.SynthResult, error) {
@@ -54,15 +92,30 @@ func (p *TTSProvider) Synthesize(ctx context.Context, text string, opts audio.TT
 		return nil, err
 	}
 
-	// Output format: MP3 128kbps default, Opus 64kbps for Telegram voice.
-	outputFormat, ext, mime := "mp3_44100_128", "mp3", "audio/mpeg"
+	// Telegram opus contract (Finding #11): when opts.Format=="opus", force
+	// ogg/Opus MIME regardless of any user-set output_format param. This
+	// preserves Telegram voice-bubble compatibility — Telegram rejects audio/opus
+	// MIME; user param affects upstream codec selection only.
+	var ext, mime string
+	var outputFormat string
 	if opts.Format == "opus" {
-		outputFormat, ext, mime = "opus_48000_64", "ogg", "audio/ogg"
-	}
+		outputFormat = "opus_48000_64"
+		ext = "ogg"
+		mime = "audio/ogg; codecs=opus"
+	} else {
+		// Default output format; may be overridden by opts.Params below.
+		outputFormat = "mp3_44100_128"
+		ext = "mp3"
+		mime = "audio/mpeg"
 
-	// Override output format from params if provided (27 codecs supported).
-	if pf := resolveELString(opts.Params, "output_format", ""); pf != "" {
-		outputFormat = pf
+		// Honor user-set output_format (Finding #4: validate before use).
+		if pf := resolveELString(opts.Params, "output_format", ""); pf != "" {
+			if !reOutputFormat.MatchString(pf) {
+				return nil, fmt.Errorf("elevenlabs: invalid output_format %q (must match ^[a-z0-9_]+$)", pf)
+			}
+			outputFormat = pf
+			ext, mime = FormatMeta(outputFormat)
+		}
 	}
 
 	// Build voice_settings from params, falling back to hardcoded defaults.
@@ -94,14 +147,21 @@ func (p *TTSProvider) Synthesize(ctx context.Context, text string, opts audio.TT
 		return nil, fmt.Errorf("marshal elevenlabs tts request: %w", err)
 	}
 
-	// Build path with query params.
-	path := fmt.Sprintf("/v1/text-to-speech/%s?output_format=%s", voiceID, outputFormat)
+	// Build URL path with query params using url.Values to prevent injection
+	// (Finding #4: no more raw fmt.Sprintf concatenation).
+	q := url.Values{}
+	q.Set("output_format", outputFormat)
 	if latency := resolveELInt(opts.Params, "optimize_streaming_latency", -1); latency > 0 {
-		path += fmt.Sprintf("&optimize_streaming_latency=%d", latency)
+		q.Set("optimize_streaming_latency", fmt.Sprintf("%d", latency))
 	}
 	if lang := resolveELString(opts.Params, "language_code", ""); lang != "" {
-		path += "&language_code=" + lang
+		// Finding #4: validate language_code before appending to URL.
+		if !reLanguageCode.MatchString(lang) {
+			return nil, fmt.Errorf("elevenlabs: invalid language_code %q (must match ^[a-z]{2,3}(-[A-Z]{2})?$)", lang)
+		}
+		q.Set("language_code", lang)
 	}
+	path := "/v1/text-to-speech/" + voiceID + "?" + q.Encode()
 
 	audioBytes, err := p.c.postJSON(ctx, path, bodyJSON, 0)
 	if err != nil {
diff --git a/internal/audio/gemini/capabilities.go b/internal/audio/gemini/capabilities.go
index 5885f221..ed2d417e 100644
--- a/internal/audio/gemini/capabilities.go
+++ b/internal/audio/gemini/capabilities.go
@@ -2,6 +2,68 @@ package gemini
 
 import "github.com/nextlevelbuilder/goclaw/internal/audio"
 
+var (
+	geminiTempMin  = 0.0
+	geminiTempMax  = 2.0
+	geminiTempStep = 0.01
+
+	geminiPenaltyMin  = -2.0
+	geminiPenaltyMax  = 2.0
+	geminiPenaltyStep = 0.01
+)
+
+// geminiParams is the enriched param schema for Gemini TTS.
+// All params have nil Default so they are omitted from the request body
+// when not explicitly set, preserving characterization byte-equivalence.
+//
+// Tagging:
+//   - temperature: basic (Group="")
+//   - seed, presencePenalty, frequencyPenalty: advanced (Group="advanced")
+var geminiParams = []audio.ParamSchema{
+	{
+		Key:  "temperature",
+		Type: audio.ParamTypeRange,
+		Label: "Temperature",
+		// Finding #14: note subtle effect on TTS; primary expressiveness via audio tags.
+		Description: "Sampling temperature (0.0–2.0, default 1.0). Effect is subtle on TTS — primary expressiveness comes from audio tags inserted in the text.",
+		Default:     nil, // omit from body when not set; API default is 1.0
+		Min:         &geminiTempMin,
+		Max:         &geminiTempMax,
+		Step:        &geminiTempStep,
+		Group:       "", // basic
+	},
+	{
+		Key:         "seed",
+		Type:        audio.ParamTypeInteger,
+		Label:       "Seed",
+		Description: "Deterministic seed for reproducible output. Advanced.",
+		Default:     nil,
+		Group:       "advanced",
+	},
+	{
+		Key:         "presencePenalty",
+		Type:        audio.ParamTypeRange,
+		Label:       "Presence Penalty",
+		Description: "Presence penalty (-2.0–2.0). Experimental — effect on TTS is not well-characterised. Advanced.",
+		Default:     nil,
+		Min:         &geminiPenaltyMin,
+		Max:         &geminiPenaltyMax,
+		Step:        &geminiPenaltyStep,
+		Group:       "advanced",
+	},
+	{
+		Key:         "frequencyPenalty",
+		Type:        audio.ParamTypeRange,
+		Label:       "Frequency Penalty",
+		Description: "Frequency penalty (-2.0–2.0). Experimental — effect on TTS is not well-characterised. Advanced.",
+		Default:     nil,
+		Min:         &geminiPenaltyMin,
+		Max:         &geminiPenaltyMax,
+		Step:        &geminiPenaltyStep,
+		Group:       "advanced",
+	},
+}
+
 // Capabilities returns the static capability schema for the Gemini TTS provider.
 // CustomFeatures drives frontend custom slot rendering in Phase C.
 func (p *Provider) Capabilities() audio.ProviderCapabilities {
@@ -11,7 +73,7 @@ func (p *Provider) Capabilities() audio.ProviderCapabilities {
 		RequiresAPIKey: true,
 		Models:         geminiModels,
 		Voices:         geminiVoices,
-		// Params is nil — Gemini TTS has no speed/pitch knobs.
+		Params:         geminiParams,
 		CustomFeatures: map[string]any{
 			"multi_speaker": true,
 			"audio_tags":    true,
diff --git a/internal/audio/gemini/capabilities_test.go b/internal/audio/gemini/capabilities_test.go
index 64d6fe4f..46270086 100644
--- a/internal/audio/gemini/capabilities_test.go
+++ b/internal/audio/gemini/capabilities_test.go
@@ -40,10 +40,11 @@ func TestCapabilities_RequiresAPIKey(t *testing.T) {
 	}
 }
 
-func TestCapabilities_ParamsEmpty(t *testing.T) {
+func TestCapabilities_ParamsPopulated(t *testing.T) {
 	p := NewProvider(Config{APIKey: "k"})
 	caps := p.Capabilities()
-	if caps.Params != nil {
-		t.Errorf("want Params nil (no speed/pitch), got %v", caps.Params)
+	// Phase 1: Gemini now exposes 4 params (temperature + seed + presencePenalty + frequencyPenalty).
+	if len(caps.Params) != 4 {
+		t.Errorf("want 4 params, got %d", len(caps.Params))
 	}
 }
diff --git a/internal/audio/gemini/defaults_invariant_test.go b/internal/audio/gemini/defaults_invariant_test.go
new file mode 100644
index 00000000..31d16d64
--- /dev/null
+++ b/internal/audio/gemini/defaults_invariant_test.go
@@ -0,0 +1,141 @@
+package gemini_test
+
+import (
+	"encoding/base64"
+	"encoding/json"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/nextlevelbuilder/goclaw/internal/audio"
+	"github.com/nextlevelbuilder/goclaw/internal/audio/gemini"
+)
+
+// captureGeminiBody sends a Synthesize request to a mock server and returns
+// the decoded JSON request body + the URL path used.
+func captureGeminiBody(t *testing.T, cfg gemini.Config, opts audio.TTSOptions) (map[string]any, string) {
+	t.Helper()
+
+	pcm := make([]byte, 64)
+	b64 := base64.StdEncoding.EncodeToString(pcm)
+	respJSON := []byte(`{"candidates":[{"content":{"parts":[{"inlineData":{"data":"` + b64 + `"}}]}}]}`)
+
+	var captured []byte
+	var capturedPath string
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		capturedPath = r.URL.String()
+		b, err := io.ReadAll(r.Body)
+		if err != nil {
+			t.Errorf("read body: %v", err)
+		}
+		captured = b
+		w.Header().Set("Content-Type", "application/json")
+		w.Write(respJSON)
+	}))
+	t.Cleanup(srv.Close)
+
+	cfg.APIBase = srv.URL
+	p := gemini.NewProvider(cfg)
+	_, err := p.Synthesize(t.Context(), "hello", opts)
+	if err != nil {
+		t.Fatalf("Synthesize failed: %v", err)
+	}
+
+	var m map[string]any
+	if err := json.Unmarshal(captured, &m); err != nil {
+		t.Fatalf("body not valid JSON: %v — body: %s", err, captured)
+	}
+	return m, capturedPath
+}
+
+// TestDefaults_PreserveLegacyBody verifies that populating opts.Params with all
+// Capabilities defaults produces a body byte-equivalent to the nil-Params request.
+// Guards against silent default flip when new params land.
+func TestDefaults_PreserveLegacyBody(t *testing.T) {
+	cfg := gemini.Config{APIKey: "test-key"}
+	p := gemini.NewProvider(cfg)
+	caps := p.Capabilities()
+
+	if len(caps.Params) == 0 {
+		t.Skip("Capabilities.Params not yet populated")
+	}
+
+	// Build params map from all schema defaults (only non-nil defaults).
+	params := make(map[string]any)
+	for _, s := range caps.Params {
+		if s.Default != nil {
+			audio.SetNested(params, s.Key, s.Default)
+		}
+	}
+
+	bodyWithDefaults, _ := captureGeminiBody(t, cfg, audio.TTSOptions{Params: params})
+	bodyNilParams, _ := captureGeminiBody(t, cfg, audio.TTSOptions{})
+
+	wantJSON := canonicalGeminiJSON(t, bodyNilParams)
+	gotJSON := canonicalGeminiJSON(t, bodyWithDefaults)
+
+	if gotJSON != wantJSON {
+		t.Errorf("defaults-invariant body FAILED:\n  with-defaults: %s\n  nil-params:    %s",
+			gotJSON, wantJSON)
+	}
+}
+
+// TestGolden_DefaultBody verifies the serialized nil-params request body matches
+// the checked-in golden fixture. Update the fixture file when intentional changes land.
+// Finding #10: external golden file catches simultaneous Default+tts.go drift.
+func TestGolden_DefaultBody(t *testing.T) {
+	cfg := gemini.Config{APIKey: "test-key", Voice: "Kore"}
+	body, _ := captureGeminiBody(t, cfg, audio.TTSOptions{})
+
+	got, err := json.MarshalIndent(body, "", "  ")
+	if err != nil {
+		t.Fatalf("marshal body: %v", err)
+	}
+
+	goldenPath := filepath.Join("testdata", "default_body.golden.json")
+
+	if _, err := os.Stat(goldenPath); os.IsNotExist(err) {
+		// First run: write the golden file.
+		if err := os.MkdirAll("testdata", 0755); err != nil {
+			t.Fatalf("mkdir testdata: %v", err)
+		}
+		if err := os.WriteFile(goldenPath, got, 0644); err != nil {
+			t.Fatalf("write golden: %v", err)
+		}
+		t.Logf("golden file created: %s", goldenPath)
+		return
+	}
+
+	want, err := os.ReadFile(goldenPath)
+	if err != nil {
+		t.Fatalf("read golden: %v", err)
+	}
+
+	// Normalize both through JSON round-trip for deterministic key ordering.
+	var wantMap, gotMap map[string]any
+	if err := json.Unmarshal(want, &wantMap); err != nil {
+		t.Fatalf("parse golden: %v", err)
+	}
+	if err := json.Unmarshal(got, &gotMap); err != nil {
+		t.Fatalf("parse current body: %v", err)
+	}
+	wantNorm := canonicalGeminiJSON(t, wantMap)
+	gotNorm := canonicalGeminiJSON(t, gotMap)
+
+	if gotNorm != wantNorm {
+		t.Errorf("golden fixture mismatch (update testdata/default_body.golden.json if intentional):\n  got:  %s\n  want: %s",
+			gotNorm, wantNorm)
+	}
+}
+
+func canonicalGeminiJSON(t *testing.T, m map[string]any) string {
+	t.Helper()
+	b, err := json.Marshal(m)
+	if err != nil {
+		t.Fatalf("canonicalGeminiJSON: %v", err)
+	}
+	return string(b)
+}
diff --git a/internal/audio/gemini/params_test.go b/internal/audio/gemini/params_test.go
new file mode 100644
index 00000000..cd752101
--- /dev/null
+++ b/internal/audio/gemini/params_test.go
@@ -0,0 +1,140 @@
+package gemini_test
+
+import (
+	"testing"
+
+	"github.com/nextlevelbuilder/goclaw/internal/audio"
+	"github.com/nextlevelbuilder/goclaw/internal/audio/gemini"
+)
+
+// TestSynthesize_Params_Temperature verifies temperature lands in generationConfig.
+func TestSynthesize_Params_Temperature(t *testing.T) {
+	cfg := gemini.Config{APIKey: "k"}
+	body, _ := captureGeminiBody(t, cfg, audio.TTSOptions{
+		Params: map[string]any{"temperature": 1.5},
+	})
+	gc := assertGeminiGenerationConfig(t, body)
+	if v, _ := gc["temperature"].(float64); v != 1.5 {
+		t.Errorf("temperature: got %v, want 1.5", gc["temperature"])
+	}
+}
+
+// TestSynthesize_Params_Seed verifies seed lands in generationConfig.
+func TestSynthesize_Params_Seed(t *testing.T) {
+	cfg := gemini.Config{APIKey: "k"}
+	body, _ := captureGeminiBody(t, cfg, audio.TTSOptions{
+		Params: map[string]any{"seed": 12345},
+	})
+	gc := assertGeminiGenerationConfig(t, body)
+	// JSON round-trip: int comes back as float64.
+	if v, _ := gc["seed"].(float64); int(v) != 12345 {
+		t.Errorf("seed: got %v, want 12345", gc["seed"])
+	}
+}
+
+// TestSynthesize_Params_PresencePenalty verifies presencePenalty in generationConfig.
+func TestSynthesize_Params_PresencePenalty(t *testing.T) {
+	cfg := gemini.Config{APIKey: "k"}
+	body, _ := captureGeminiBody(t, cfg, audio.TTSOptions{
+		Params: map[string]any{"presencePenalty": 0.3},
+	})
+	gc := assertGeminiGenerationConfig(t, body)
+	if v, _ := gc["presencePenalty"].(float64); v != 0.3 {
+		t.Errorf("presencePenalty: got %v, want 0.3", gc["presencePenalty"])
+	}
+}
+
+// TestSynthesize_Params_FrequencyPenalty verifies frequencyPenalty in generationConfig.
+func TestSynthesize_Params_FrequencyPenalty(t *testing.T) {
+	cfg := gemini.Config{APIKey: "k"}
+	body, _ := captureGeminiBody(t, cfg, audio.TTSOptions{
+		Params: map[string]any{"frequencyPenalty": -0.1},
+	})
+	gc := assertGeminiGenerationConfig(t, body)
+	if v, _ := gc["frequencyPenalty"].(float64); v != -0.1 {
+		t.Errorf("frequencyPenalty: got %v, want -0.1", gc["frequencyPenalty"])
+	}
+}
+
+// TestSynthesize_Params_AllFour verifies all four params land together.
+func TestSynthesize_Params_AllFour(t *testing.T) {
+	cfg := gemini.Config{APIKey: "k"}
+	body, _ := captureGeminiBody(t, cfg, audio.TTSOptions{
+		Params: map[string]any{
+			"temperature":      1.5,
+			"seed":             12345,
+			"presencePenalty":  0.3,
+			"frequencyPenalty": -0.1,
+		},
+	})
+	gc := assertGeminiGenerationConfig(t, body)
+	if v, _ := gc["temperature"].(float64); v != 1.5 {
+		t.Errorf("temperature: got %v, want 1.5", gc["temperature"])
+	}
+	if v, _ := gc["seed"].(float64); int(v) != 12345 {
+		t.Errorf("seed: got %v, want 12345", gc["seed"])
+	}
+	if v, _ := gc["presencePenalty"].(float64); v != 0.3 {
+		t.Errorf("presencePenalty: got %v, want 0.3", gc["presencePenalty"])
+	}
+	if v, _ := gc["frequencyPenalty"].(float64); v != -0.1 {
+		t.Errorf("frequencyPenalty: got %v, want -0.1", gc["frequencyPenalty"])
+	}
+}
+
+// TestSynthesize_Params_NilParams_NoExtraKeys verifies nil params produces no
+// extra keys in generationConfig beyond the mandatory ones.
+func TestSynthesize_Params_NilParams_NoExtraKeys(t *testing.T) {
+	cfg := gemini.Config{APIKey: "k"}
+	body, _ := captureGeminiBody(t, cfg, audio.TTSOptions{})
+	gc := assertGeminiGenerationConfig(t, body)
+
+	for _, extra := range []string{"temperature", "seed", "presencePenalty", "frequencyPenalty"} {
+		if _, ok := gc[extra]; ok {
+			t.Errorf("nil params: generationConfig must not contain %q", extra)
+		}
+	}
+}
+
+// TestCapabilities_Gemini_HasFourParams confirms Capabilities returns exactly
+// the four documented params with correct Group tags.
+func TestCapabilities_Gemini_HasFourParams(t *testing.T) {
+	p := gemini.NewProvider(gemini.Config{APIKey: "k"})
+	caps := p.Capabilities()
+
+	type want struct {
+		group string
+	}
+	expected := map[string]want{
+		"temperature":      {group: ""},
+		"seed":             {group: "advanced"},
+		"presencePenalty":  {group: "advanced"},
+		"frequencyPenalty": {group: "advanced"},
+	}
+
+	found := map[string]bool{}
+	for _, p := range caps.Params {
+		if w, ok := expected[p.Key]; ok {
+			found[p.Key] = true
+			if p.Group != w.group {
+				t.Errorf("param %q: Group got %q, want %q", p.Key, p.Group, w.group)
+			}
+		}
+	}
+	for key := range expected {
+		if !found[key] {
+			t.Errorf("param %q not found in Capabilities", key)
+		}
+	}
+}
+
+// assertGeminiGenerationConfig extracts generationConfig from the body and fails
+// if it is missing or not a map.
+func assertGeminiGenerationConfig(t *testing.T, body map[string]any) map[string]any {
+	t.Helper()
+	gc, ok := body["generationConfig"].(map[string]any)
+	if !ok {
+		t.Fatalf("generationConfig missing or not a map: %#v", body["generationConfig"])
+	}
+	return gc
+}
diff --git a/internal/audio/gemini/testdata/default_body.golden.json b/internal/audio/gemini/testdata/default_body.golden.json
new file mode 100644
index 00000000..cfbe67a1
--- /dev/null
+++ b/internal/audio/gemini/testdata/default_body.golden.json
@@ -0,0 +1,23 @@
+{
+  "contents": [
+    {
+      "parts": [
+        {
+          "text": "hello"
+        }
+      ]
+    }
+  ],
+  "generationConfig": {
+    "responseModalities": [
+      "AUDIO"
+    ],
+    "speechConfig": {
+      "voiceConfig": {
+        "prebuiltVoiceConfig": {
+          "voiceName": "Kore"
+        }
+      }
+    }
+  }
+}
\ No newline at end of file
diff --git a/internal/audio/gemini/tts.go b/internal/audio/gemini/tts.go
index a4bf2776..6a32870d 100644
--- a/internal/audio/gemini/tts.go
+++ b/internal/audio/gemini/tts.go
@@ -105,14 +105,30 @@ func (p *Provider) Synthesize(ctx context.Context, text string, opts audio.TTSOp
 	// Per Gemini generateContent spec, speechConfig is NESTED under
 	// generationConfig — not a top-level field. Sending it at root returns
 	// 400 "Unknown name 'speechConfig': Cannot find field."
+	generationConfig := map[string]any{
+		"responseModalities": []string{"AUDIO"},
+		"speechConfig":       speechConfig,
+	}
+	// Merge optional params into generationConfig — only when explicitly present
+	// in opts.Params (nil default = omit from body, preserving characterization).
+	if temp, ok := resolveGeminiFloatExplicit(opts.Params, "temperature"); ok {
+		generationConfig["temperature"] = temp
+	}
+	if seed, ok := resolveGeminiIntExplicit(opts.Params, "seed"); ok {
+		generationConfig["seed"] = seed
+	}
+	if pp, ok := resolveGeminiFloatExplicit(opts.Params, "presencePenalty"); ok {
+		generationConfig["presencePenalty"] = pp
+	}
+	if fp, ok := resolveGeminiFloatExplicit(opts.Params, "frequencyPenalty"); ok {
+		generationConfig["frequencyPenalty"] = fp
+	}
+
 	reqBody := map[string]any{
 		"contents": []map[string]any{
 			{"parts": []map[string]any{{"text": text}}},
 		},
-		"generationConfig": map[string]any{
-			"responseModalities": []string{"AUDIO"},
-			"speechConfig":       speechConfig,
-		},
+		"generationConfig": generationConfig,
 	}
 
 	bodyBytes, err := json.Marshal(reqBody)
@@ -237,6 +253,49 @@ func (p *Provider) requestAudio(ctx context.Context, model string, bodyBytes []b
 	}, nil
 }
 
+// resolveGeminiFloatExplicit returns (value, true) only when the key is explicitly
+// present in opts.Params, so callers can omit the generationConfig field entirely
+// when not set. Mirrors resolveMiniMaxBoolExplicit in minimax/tts.go.
+func resolveGeminiFloatExplicit(params map[string]any, key string) (float64, bool) {
+	if params == nil {
+		return 0, false
+	}
+	v, ok := audio.GetNested(params, key)
+	if !ok {
+		return 0, false
+	}
+	switch n := v.(type) {
+	case float64:
+		return n, true
+	case float32:
+		return float64(n), true
+	case int:
+		return float64(n), true
+	}
+	return 0, false
+}
+
+// resolveGeminiIntExplicit returns (value, true) only when the key is explicitly
+// present in opts.Params as an integer-compatible type.
+func resolveGeminiIntExplicit(params map[string]any, key string) (int, bool) {
+	if params == nil {
+		return 0, false
+	}
+	v, ok := audio.GetNested(params, key)
+	if !ok {
+		return 0, false
+	}
+	switch n := v.(type) {
+	case int:
+		return n, true
+	case int64:
+		return int(n), true
+	case float64:
+		return int(n), true
+	}
+	return 0, false
+}
+
 // isTransientFinishReason reports whether a Gemini finishReason represents a
 // non-deterministic failure that's worth retrying. OTHER is the catch-all the
 // preview TTS endpoint emits when it just fails to produce audio for no
diff --git a/internal/audio/minimax/capabilities.go b/internal/audio/minimax/capabilities.go
index 322db1f6..211f5921 100644
--- a/internal/audio/minimax/capabilities.go
+++ b/internal/audio/minimax/capabilities.go
@@ -24,9 +24,12 @@ var (
 
 // minimaxParams is the enriched param schema for MiniMax TTS.
 // Defaults MUST match the hardcoded body in tts.go (characterization fixture):
-//   speed=1.0, vol=1.0 (omitted from original = 1.0 default), pitch=0.
-// Note: "vol" key is not in the original body; adding it with default 1.0
-// is safe because server default is also 1.0.
+//
+//	speed=1.0, vol=1.0 (omitted from original = 1.0 default), pitch=0.
+//
+// Group tagging (§Requirements #7):
+//   - audio.bitrate, audio.sample_rate, audio.channel, pronunciation_dict = "advanced"
+//   - all others = "" (basic)
 var minimaxParams = []audio.ParamSchema{
 	{
 		Key:         "speed",
@@ -99,6 +102,7 @@ var minimaxParams = []audio.ParamSchema{
 		Type:    audio.ParamTypeEnum,
 		Label:   "Sample Rate",
 		Default: "",
+		Group:   "advanced",
 		Enum: []audio.EnumOption{
 			{Value: "", Label: "Default"},
 			{Value: "8000", Label: "8 kHz"},
@@ -114,6 +118,7 @@ var minimaxParams = []audio.ParamSchema{
 		Type:    audio.ParamTypeEnum,
 		Label:   "Bitrate (MP3 only)",
 		Default: "",
+		Group:   "advanced",
 		Enum: []audio.EnumOption{
 			{Value: "", Label: "Default"},
 			{Value: "32000", Label: "32 kbps"},
@@ -130,12 +135,63 @@ var minimaxParams = []audio.ParamSchema{
 		Type:    audio.ParamTypeEnum,
 		Label:   "Channels",
 		Default: "",
+		Group:   "advanced",
 		Enum: []audio.EnumOption{
 			{Value: "", Label: "Default"},
 			{Value: "1", Label: "Mono"},
 			{Value: "2", Label: "Stereo"},
 		},
 	},
+	// language_boost: top-level hint for language/accent recognition.
+	{
+		Key:   "language_boost",
+		Type:  audio.ParamTypeEnum,
+		Label: "Language Boost",
+		Description: "Accent/language recognition boost. Helps the model produce more natural " +
+			"pronunciation for the selected language.",
+		Default: "",
+		Enum: []audio.EnumOption{
+			{Value: "", Label: "Auto (default)"},
+			{Value: "Chinese,Yue", Label: "Cantonese"},
+			{Value: "Chinese", Label: "Mandarin"},
+			{Value: "English", Label: "English"},
+			{Value: "Arabic", Label: "Arabic"},
+			{Value: "Russian", Label: "Russian"},
+			{Value: "Spanish", Label: "Spanish"},
+			{Value: "French", Label: "French"},
+			{Value: "Portuguese", Label: "Portuguese"},
+			{Value: "German", Label: "German"},
+			{Value: "Turkish", Label: "Turkish"},
+			{Value: "Dutch", Label: "Dutch"},
+			{Value: "Ukrainian", Label: "Ukrainian"},
+			{Value: "Vietnamese", Label: "Vietnamese"},
+			{Value: "Indonesian", Label: "Indonesian"},
+			{Value: "Italian", Label: "Italian"},
+			{Value: "Korean", Label: "Korean"},
+			{Value: "Japanese", Label: "Japanese"},
+			{Value: "Hindi", Label: "Hindi"},
+		},
+	},
+	// subtitle_enable: whether to return subtitle timing data alongside audio.
+	{
+		Key:         "subtitle_enable",
+		Type:        audio.ParamTypeBoolean,
+		Label:       "Subtitle Enable",
+		Description: "When enabled, the API returns word-level subtitle timing in the response.",
+		Default:     nil, // omit when not explicitly set
+	},
+	// pronunciation_dict: JSON array of "word/pinyin" rule strings for custom pronunciation.
+	// Capped at 8KB pre-accept (Finding #6). On parse failure: log + omit, synth continues.
+	{
+		Key:  "pronunciation_dict",
+		Type: audio.ParamTypeText,
+		Label: "Pronunciation Dictionary",
+		Description: `Custom pronunciation rules as a JSON array of "word/pinyin" strings, ` +
+			`e.g. ["omg/Oh my god", "CEO/Chief Executive Officer"]. ` +
+			`Maximum 8 KB. Advanced.`,
+		Default: nil,
+		Group:   "advanced",
+	},
 }
 
 // Capabilities returns the full capability schema for the MiniMax TTS provider.
diff --git a/internal/audio/minimax/golden_test.go b/internal/audio/minimax/golden_test.go
new file mode 100644
index 00000000..5befb957
--- /dev/null
+++ b/internal/audio/minimax/golden_test.go
@@ -0,0 +1,60 @@
+package minimax_test
+
+import (
+	"encoding/json"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/nextlevelbuilder/goclaw/internal/audio"
+	"github.com/nextlevelbuilder/goclaw/internal/audio/minimax"
+)
+
+// TestGolden_DefaultBody verifies the serialized nil-params request body matches
+// the checked-in golden fixture. Finding #10: external golden file catches
+// simultaneous Default+tts.go drift that the self-referential invariant test misses.
+func TestGolden_DefaultBody(t *testing.T) {
+	cfg := minimax.Config{APIKey: "test-key", GroupID: "grp1"}
+	body, reqURL := captureMiniMaxBody(t, cfg, audio.TTSOptions{})
+
+	full := map[string]any{
+		"body": body,
+		"url":  reqURL,
+	}
+	gotFull, err := json.MarshalIndent(full, "", "  ")
+	if err != nil {
+		t.Fatalf("marshal body: %v", err)
+	}
+
+	goldenPath := filepath.Join("testdata", "default_body.golden.json")
+	if _, statErr := os.Stat(goldenPath); os.IsNotExist(statErr) {
+		if err := os.MkdirAll("testdata", 0755); err != nil {
+			t.Fatalf("mkdir testdata: %v", err)
+		}
+		if err := os.WriteFile(goldenPath, gotFull, 0644); err != nil {
+			t.Fatalf("write golden: %v", err)
+		}
+		t.Logf("golden file created: %s", goldenPath)
+		return
+	}
+
+	want, err := os.ReadFile(goldenPath)
+	if err != nil {
+		t.Fatalf("read golden: %v", err)
+	}
+
+	var wantMap, gotMap map[string]any
+	if err := json.Unmarshal(want, &wantMap); err != nil {
+		t.Fatalf("parse golden: %v", err)
+	}
+	if err := json.Unmarshal(gotFull, &gotMap); err != nil {
+		t.Fatalf("parse current: %v", err)
+	}
+
+	wantNorm := canonicalMiniMaxJSON(t, wantMap)
+	gotNorm := canonicalMiniMaxJSON(t, gotMap)
+	if gotNorm != wantNorm {
+		t.Errorf("golden fixture mismatch (update testdata/default_body.golden.json if intentional):\n  got:  %s\n  want: %s",
+			gotNorm, wantNorm)
+	}
+}
diff --git a/internal/audio/minimax/params_test.go b/internal/audio/minimax/params_test.go
index b28bc926..5f180327 100644
--- a/internal/audio/minimax/params_test.go
+++ b/internal/audio/minimax/params_test.go
@@ -76,6 +76,90 @@ func TestSynthesize_AppliesParams_OmitsEmpty_NilParams(t *testing.T) {
 	if _, ok := vs["emotion"]; ok {
 		t.Error("emotion must not appear for nil params")
 	}
+	// New params must also be absent for nil params.
+	if _, ok := body["language_boost"]; ok {
+		t.Error("language_boost must not appear for nil params")
+	}
+	if _, ok := body["subtitle_enable"]; ok {
+		t.Error("subtitle_enable must not appear for nil params")
+	}
+	if _, ok := body["pronunciation_dict"]; ok {
+		t.Error("pronunciation_dict must not appear for nil params")
+	}
+}
+
+// TestSynthesize_AppliesParams_LanguageBoost verifies language_boost top-level field.
+func TestSynthesize_AppliesParams_LanguageBoost(t *testing.T) {
+	cfg := minimax.Config{APIKey: "k", GroupID: "g"}
+	body, _ := captureMiniMaxBody(t, cfg, audio.TTSOptions{
+		Params: map[string]any{"language_boost": "Chinese"},
+	})
+	if lb, _ := body["language_boost"].(string); lb != "Chinese" {
+		t.Errorf("language_boost: got %q, want Chinese", body["language_boost"])
+	}
+}
+
+// TestSynthesize_AppliesParams_SubtitleEnable verifies subtitle_enable top-level field.
+func TestSynthesize_AppliesParams_SubtitleEnable(t *testing.T) {
+	cfg := minimax.Config{APIKey: "k", GroupID: "g"}
+	body, _ := captureMiniMaxBody(t, cfg, audio.TTSOptions{
+		Params: map[string]any{"subtitle_enable": true},
+	})
+	if se, _ := body["subtitle_enable"].(bool); !se {
+		t.Errorf("subtitle_enable: got %v, want true", body["subtitle_enable"])
+	}
+}
+
+// TestSynthesize_AppliesParams_PronunciationDict verifies that a valid JSON array
+// is wrapped in {"tone":[...]} in the request body.
+func TestSynthesize_AppliesParams_PronunciationDict(t *testing.T) {
+	cfg := minimax.Config{APIKey: "k", GroupID: "g"}
+	body, _ := captureMiniMaxBody(t, cfg, audio.TTSOptions{
+		Params: map[string]any{"pronunciation_dict": `["omg/Oh my god"]`},
+	})
+	pd, ok := body["pronunciation_dict"].(map[string]any)
+	if !ok {
+		t.Fatalf("pronunciation_dict: expected object, got %T (%v)", body["pronunciation_dict"], body["pronunciation_dict"])
+	}
+	tone, ok := pd["tone"].([]any)
+	if !ok {
+		t.Fatalf("pronunciation_dict.tone: expected array, got %T", pd["tone"])
+	}
+	if len(tone) != 1 {
+		t.Fatalf("pronunciation_dict.tone: want 1 entry, got %d", len(tone))
+	}
+	if tone[0] != "omg/Oh my god" {
+		t.Errorf("pronunciation_dict.tone[0]: got %q, want omg/Oh my god", tone[0])
+	}
+}
+
+// TestSynthesize_PronunciationDict_InvalidJSON verifies that malformed JSON is
+// silently omitted (synth continues) and the field is absent from the body.
+func TestSynthesize_PronunciationDict_InvalidJSON(t *testing.T) {
+	cfg := minimax.Config{APIKey: "k", GroupID: "g"}
+	body, _ := captureMiniMaxBody(t, cfg, audio.TTSOptions{
+		Params: map[string]any{"pronunciation_dict": `not valid json`},
+	})
+	if _, ok := body["pronunciation_dict"]; ok {
+		t.Error("pronunciation_dict: must be omitted when JSON is invalid")
+	}
+}
+
+// TestSynthesize_PronunciationDict_ExceedsLimit verifies that a value exceeding
+// 8 KB is silently omitted (Finding #6).
+func TestSynthesize_PronunciationDict_ExceedsLimit(t *testing.T) {
+	cfg := minimax.Config{APIKey: "k", GroupID: "g"}
+	// Build a string > 8192 bytes.
+	large := make([]byte, 9000)
+	for i := range large {
+		large[i] = 'a'
+	}
+	body, _ := captureMiniMaxBody(t, cfg, audio.TTSOptions{
+		Params: map[string]any{"pronunciation_dict": string(large)},
+	})
+	if _, ok := body["pronunciation_dict"]; ok {
+		t.Error("pronunciation_dict: must be omitted when value exceeds 8 KB limit")
+	}
 }
 
 func TestSynthesize_DoesNotMutateCallerParams(t *testing.T) {
diff --git a/internal/audio/minimax/testdata/default_body.golden.json b/internal/audio/minimax/testdata/default_body.golden.json
new file mode 100644
index 00000000..377d9458
--- /dev/null
+++ b/internal/audio/minimax/testdata/default_body.golden.json
@@ -0,0 +1,16 @@
+{
+  "body": {
+    "audio_setting": {
+      "format": "mp3"
+    },
+    "model": "speech-02-hd",
+    "stream": false,
+    "text": "hello",
+    "voice_setting": {
+      "pitch": 0,
+      "speed": 1,
+      "voice_id": "Wise_Woman"
+    }
+  },
+  "url": "/t2a_v2?GroupId=grp1"
+}
\ No newline at end of file
diff --git a/internal/audio/minimax/tts.go b/internal/audio/minimax/tts.go
index 757225e5..8801f0d4 100644
--- a/internal/audio/minimax/tts.go
+++ b/internal/audio/minimax/tts.go
@@ -9,12 +9,17 @@ import (
 	"encoding/json"
 	"fmt"
 	"io"
+	"log/slog"
 	"net/http"
 	"time"
 
 	"github.com/nextlevelbuilder/goclaw/internal/audio"
 )
 
+// pronunciationDictMaxBytes is the maximum accepted byte length for the
+// pronunciation_dict param value. Finding #6: cap at 8 KB to prevent DoS.
+const pronunciationDictMaxBytes = 8 * 1024
+
 // Config configures the MiniMax TTS provider.
 type Config struct {
 	APIKey    string
@@ -67,15 +72,18 @@ func (p *Provider) Name() string { return "minimax" }
 // audio in response.data.audio — we decode before returning.
 //
 // opts.Params keys (nested dot-path):
-//   - "speed"              float64  (0.5–2.0, default 1.0)
-//   - "vol"                float64  (0.01–10.0, default 1.0; omitted from body at default)
-//   - "pitch"              int      (-12..12, default 0)
-//   - "emotion"            string   (optional)
-//   - "text_normalization" bool     (optional)
-//   - "audio.format"       string   (mp3/pcm/flac/wav, default "mp3")
-//   - "audio.sample_rate"  string   (optional)
-//   - "audio.bitrate"      string   (optional, mp3 only)
-//   - "audio.channel"      string   (optional)
+//   - "speed"               float64  (0.5–2.0, default 1.0)
+//   - "vol"                 float64  (0.01–10.0, default 1.0; omitted from body at default)
+//   - "pitch"               int      (-12..12, default 0)
+//   - "emotion"             string   (optional)
+//   - "text_normalization"  bool     (optional)
+//   - "audio.format"        string   (mp3/pcm/flac/wav, default "mp3")
+//   - "audio.sample_rate"   string   (optional)
+//   - "audio.bitrate"       string   (optional, mp3 only)
+//   - "audio.channel"       string   (optional)
+//   - "language_boost"      string   (optional, e.g. "Chinese", "English")
+//   - "subtitle_enable"     bool     (optional)
+//   - "pronunciation_dict"  string   (optional JSON array, max 8 KB; Finding #6)
 //
 // MUST NOT mutate opts.Params — reads only.
 func (p *Provider) Synthesize(ctx context.Context, text string, opts audio.TTSOptions) (*audio.SynthResult, error) {
@@ -154,6 +162,33 @@ func (p *Provider) Synthesize(ctx context.Context, text string, opts audio.TTSOp
 		body["text_normalization"] = tn
 	}
 
+	// language_boost: top-level string hint (omit when empty).
+	if lb := resolveMiniMaxString(opts.Params, "language_boost", ""); lb != "" {
+		body["language_boost"] = lb
+	}
+
+	// subtitle_enable: only when explicitly set.
+	if se, ok := resolveMiniMaxBoolExplicit(opts.Params, "subtitle_enable"); ok {
+		body["subtitle_enable"] = se
+	}
+
+	// pronunciation_dict: parse user-pasted JSON array, wrap in {"tone":[...]}.
+	// Finding #6: cap at 8 KB; never log raw value on parse failure — log length only.
+	if pdRaw := resolveMiniMaxString(opts.Params, "pronunciation_dict", ""); pdRaw != "" {
+		if len(pdRaw) > pronunciationDictMaxBytes {
+			slog.Warn("minimax: pronunciation_dict exceeds 8 KB limit, omitting",
+				"length", len(pdRaw))
+		} else {
+			var rules []string
+			if err := json.Unmarshal([]byte(pdRaw), &rules); err != nil {
+				slog.Warn("minimax: invalid pronunciation_dict JSON, omitting",
+					"length", len(pdRaw))
+			} else if len(rules) > 0 {
+				body["pronunciation_dict"] = map[string]any{"tone": rules}
+			}
+		}
+	}
+
 	bodyJSON, err := json.Marshal(body)
 	if err != nil {
 		return nil, fmt.Errorf("marshal minimax tts request: %w", err)
diff --git a/internal/audio/openai/capabilities.go b/internal/audio/openai/capabilities.go
index 74eac236..93157ae1 100644
--- a/internal/audio/openai/capabilities.go
+++ b/internal/audio/openai/capabilities.go
@@ -65,8 +65,9 @@ var openAIParams = []audio.ParamSchema{
 		Key:         "instructions",
 		Type:        audio.ParamTypeText,
 		Label:       "Instructions",
-		Description: "System-level style prompt (gpt-4o-mini-tts only).",
+		Description: "System-level style prompt (gpt-4o-mini-tts only). Advanced.",
 		Default:     "",
+		Group:       "advanced",
 		DependsOn: []audio.Dependency{
 			{Field: "model", Op: "eq", Value: "gpt-4o-mini-tts"},
 		},
diff --git a/internal/audio/openai/golden_test.go b/internal/audio/openai/golden_test.go
new file mode 100644
index 00000000..9abcf559
--- /dev/null
+++ b/internal/audio/openai/golden_test.go
@@ -0,0 +1,65 @@
+package openai_test
+
+import (
+	"encoding/json"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/nextlevelbuilder/goclaw/internal/audio"
+	"github.com/nextlevelbuilder/goclaw/internal/audio/openai"
+)
+
+// TestGolden_DefaultBody verifies the serialized nil-params request body matches
+// the checked-in golden fixture. Finding #10: external golden file catches
+// simultaneous Default+tts.go drift that the self-referential invariant test misses.
+func TestGolden_DefaultBody(t *testing.T) {
+	cfg := openai.Config{APIKey: "test-key"}
+	raw := captureOpenAIBody(t, cfg, audio.TTSOptions{})
+
+	goldenPath := filepath.Join("testdata", "default_body.golden.json")
+	if _, statErr := os.Stat(goldenPath); os.IsNotExist(statErr) {
+		if err := os.MkdirAll("testdata", 0755); err != nil {
+			t.Fatalf("mkdir testdata: %v", err)
+		}
+		// Pretty-print the raw bytes for readability.
+		var m map[string]any
+		if err := json.Unmarshal(raw, &m); err != nil {
+			t.Fatalf("parse body: %v", err)
+		}
+		pretty, _ := json.MarshalIndent(m, "", "  ")
+		if err := os.WriteFile(goldenPath, pretty, 0644); err != nil {
+			t.Fatalf("write golden: %v", err)
+		}
+		t.Logf("golden file created: %s", goldenPath)
+		return
+	}
+
+	want, err := os.ReadFile(goldenPath)
+	if err != nil {
+		t.Fatalf("read golden: %v", err)
+	}
+
+	// Normalize both through JSON round-trip for deterministic key ordering.
+	wantNorm := canonicalOpenAIJSON(t, want)
+	gotNorm := canonicalOpenAIJSON(t, raw)
+	if gotNorm != wantNorm {
+		t.Errorf("golden fixture mismatch (update testdata/default_body.golden.json if intentional):\n  got:  %s\n  want: %s",
+			gotNorm, wantNorm)
+	}
+}
+
+// canonicalOpenAIJSON normalises raw JSON bytes to a deterministic string
+// by round-tripping through map[string]any.
+func canonicalOpenAIJSON(t *testing.T, raw []byte) string {
+	t.Helper()
+	var m map[string]any
+	if err := json.Unmarshal(raw, &m); err != nil {
+		t.Fatalf("canonicalOpenAIJSON unmarshal: %v", err)
+	}
+	b, err := json.Marshal(m)
+	if err != nil {
+		t.Fatalf("canonicalOpenAIJSON marshal: %v", err)
+	}
+	return string(b)
+}
diff --git a/internal/audio/openai/testdata/default_body.golden.json b/internal/audio/openai/testdata/default_body.golden.json
new file mode 100644
index 00000000..f7cd5926
--- /dev/null
+++ b/internal/audio/openai/testdata/default_body.golden.json
@@ -0,0 +1,7 @@
+{
+  "input": "hello",
+  "model": "gpt-4o-mini-tts",
+  "response_format": "mp3",
+  "speed": 1,
+  "voice": "alloy"
+}
\ No newline at end of file
diff --git a/internal/audio/types_capabilities.go b/internal/audio/types_capabilities.go
index d573a503..0cd0ccea 100644
--- a/internal/audio/types_capabilities.go
+++ b/internal/audio/types_capabilities.go
@@ -3,6 +3,7 @@ package audio
 import (
 	"errors"
 	"fmt"
+	"slices"
 	"strings"
 )
 
@@ -58,6 +59,9 @@ type ParamSchema struct {
 	Step        *float64     `json:"step,omitempty"` // for range
 	Enum        []EnumOption `json:"enum,omitempty"`
 	DependsOn   []Dependency `json:"depends_on,omitempty"`
+	// Group categorises the param for collapsible UI sections.
+	// "" (empty) = basic (always visible); "advanced" = collapsed by default.
+	Group string `json:"group,omitempty"`
 }
 
 // ProviderCapabilities is the catalog entry for a single TTS provider.
@@ -109,10 +113,8 @@ func parseKeyPath(key string) ([]string, error) {
 		return nil, errors.New("key path must not be empty")
 	}
 	parts := strings.Split(key, ".")
-	for _, p := range parts {
-		if p == "" {
-			return nil, errors.New("key path must not contain empty segments: " + key)
-		}
+	if slices.Contains(parts, "") {
+		return nil, errors.New("key path must not contain empty segments: " + key)
 	}
 	return parts, nil
 }
diff --git a/internal/audio/validate_params.go b/internal/audio/validate_params.go
new file mode 100644
index 00000000..94489269
--- /dev/null
+++ b/internal/audio/validate_params.go
@@ -0,0 +1,155 @@
+package audio
+
+import (
+	"fmt"
+	"maps"
+)
+
+// ErrTTSParamOutOfRange is returned when a numeric param value is outside the
+// declared Min/Max bounds.
+type ErrTTSParamOutOfRange struct {
+	Key string
+	Val any
+	Min any
+	Max any
+}
+
+func (e ErrTTSParamOutOfRange) Error() string {
+	return fmt.Sprintf("TTS param %q value %v is out of range [%v, %v]", e.Key, e.Val, e.Min, e.Max)
+}
+
+// ErrTTSParamUnknownKey is returned when a user-supplied key is not present in
+// the capability schema for the provider.
+type ErrTTSParamUnknownKey struct {
+	Key string
+}
+
+func (e ErrTTSParamUnknownKey) Error() string {
+	return fmt.Sprintf("TTS param %q is not supported by this provider", e.Key)
+}
+
+// ValidateParams checks all keys in values against the capability schema:
+//   - Unknown keys return ErrTTSParamUnknownKey.
+//   - Numeric values outside declared Min/Max return ErrTTSParamOutOfRange.
+//   - String values not in the Enum list (when non-empty) return an out-of-range error.
+//
+// Empty values map: no-op returns nil.
+// Empty schema: every key is unknown — callers that don't have a schema may pass nil schema to skip.
+// Only the first validation error is returned (fail-fast).
+func ValidateParams(schema []ParamSchema, values map[string]any) error {
+	if len(values) == 0 {
+		return nil
+	}
+
+	// Build a flat key → schema index map for O(1) lookup.
+	// ParamSchema keys use dot-notation (e.g. "voice_settings.stability"), but
+	// values arriving from the HTTP layer use the same dot-notation in a nested map.
+	// We flatten the incoming map to dot-notation keys for matching.
+	flat := flattenMap(values, "")
+
+	// Index schema by key for fast lookup.
+	schemaIdx := make(map[string]ParamSchema, len(schema))
+	for _, s := range schema {
+		schemaIdx[s.Key] = s
+	}
+
+	for flatKey, val := range flat {
+		s, ok := schemaIdx[flatKey]
+		if !ok {
+			return ErrTTSParamUnknownKey{Key: flatKey}
+		}
+
+		// Range / number / integer: enforce Min/Max when declared.
+		if s.Min != nil || s.Max != nil {
+			fval, ok := toFloat64(val)
+			if ok {
+				if s.Min != nil && fval < *s.Min {
+					return ErrTTSParamOutOfRange{Key: flatKey, Val: val, Min: *s.Min, Max: maxOrInf(s.Max)}
+				}
+				if s.Max != nil && fval > *s.Max {
+					return ErrTTSParamOutOfRange{Key: flatKey, Val: val, Min: minOrNegInf(s.Min), Max: *s.Max}
+				}
+			}
+		}
+
+		// Enum: validate against declared options (skip empty value — means "use default").
+		if len(s.Enum) > 0 {
+			if sv, ok := val.(string); ok && sv != "" {
+				if !enumContains(s.Enum, sv) {
+					return ErrTTSParamOutOfRange{Key: flatKey, Val: val, Min: enumFirst(s.Enum), Max: enumLast(s.Enum)}
+				}
+			}
+		}
+	}
+	return nil
+}
+
+// flattenMap converts a nested map[string]any into a flat map with dot-separated keys.
+// prefix is the parent key path (empty string for the root call).
+func flattenMap(m map[string]any, prefix string) map[string]any {
+	out := make(map[string]any)
+	for k, v := range m {
+		fullKey := k
+		if prefix != "" {
+			fullKey = prefix + "." + k
+		}
+		if nested, ok := v.(map[string]any); ok {
+			maps.Copy(out, flattenMap(nested, fullKey))
+		} else {
+			out[fullKey] = v
+		}
+	}
+	return out
+}
+
+// toFloat64 converts common numeric JSON types to float64.
+func toFloat64(v any) (float64, bool) {
+	switch n := v.(type) {
+	case float64:
+		return n, true
+	case float32:
+		return float64(n), true
+	case int:
+		return float64(n), true
+	case int64:
+		return float64(n), true
+	}
+	return 0, false
+}
+
+func enumContains(opts []EnumOption, val string) bool {
+	for _, o := range opts {
+		if o.Value == val {
+			return true
+		}
+	}
+	return false
+}
+
+func enumFirst(opts []EnumOption) any {
+	if len(opts) == 0 {
+		return nil
+	}
+	return opts[0].Value
+}
+
+func enumLast(opts []EnumOption) any {
+	if len(opts) == 0 {
+		return nil
+	}
+	return opts[len(opts)-1].Value
+}
+
+func maxOrInf(p *float64) any {
+	if p == nil {
+		return "+∞"
+	}
+	return *p
+}
+
+func minOrNegInf(p *float64) any {
+	if p == nil {
+		return "-∞"
+	}
+	return *p
+}
diff --git a/internal/http/tts.go b/internal/http/tts.go
index 313230d2..c7296fb7 100644
--- a/internal/http/tts.go
+++ b/internal/http/tts.go
@@ -145,6 +145,30 @@ func (h *TTSHandler) handleSynthesize(w http.ResponseWriter, r *http.Request) {
 		}
 	}
 
+	// Validate tenant params against the provider's capability schema (Finding #3).
+	// Rejects unknown keys and out-of-range values before forwarding to the provider.
+	if len(tenantParams) > 0 {
+		if dp, ok := p.(audio.DescribableProvider); ok {
+			caps := dp.Capabilities()
+			if err := audio.ValidateParams(caps.Params, tenantParams); err != nil {
+				slog.Warn("tts.synthesize.invalid-params", "provider", name, "error", err)
+				var ukErr audio.ErrTTSParamUnknownKey
+				var orErr audio.ErrTTSParamOutOfRange
+				switch {
+				case errors.As(err, &ukErr):
+					msg := i18n.T(locale, i18n.MsgTtsParamUnknownKey, ukErr.Key)
+					http.Error(w, fmt.Sprintf(`{"error":%q}`, msg), http.StatusUnprocessableEntity)
+				case errors.As(err, &orErr):
+					msg := i18n.T(locale, i18n.MsgTtsParamOutOfRange, orErr.Key, orErr.Val, orErr.Min, orErr.Max)
+					http.Error(w, fmt.Sprintf(`{"error":%q}`, msg), http.StatusUnprocessableEntity)
+				default:
+					http.Error(w, fmt.Sprintf(`{"error":%q}`, err.Error()), http.StatusUnprocessableEntity)
+				}
+				return
+			}
+		}
+	}
+
 	// Synthesize with a 15-second deadline.
 	synthCtx, cancel := context.WithTimeout(ctx, synthesizeTimeout)
 	defer cancel()
@@ -284,16 +308,25 @@ func (h *TTSHandler) resolveTenantProvider(ctx context.Context, explicitProvider
 	return provider, providerName, req.Params, nil
 }
 
+// paramsMaxBytes is the maximum byte length accepted for a stored params blob.
+// Finding #6: cap at 16 KB to prevent oversized JSON DoS.
+const paramsMaxBytes = 16 * 1024
+
 // loadParamsBlob reads a JSON blob from system_configs and returns it as map[string]any.
-// Returns nil when the key is absent or the value is not valid JSON.
+// Returns nil when the key is absent, the value exceeds 16 KB, or the value is not valid JSON.
+// Never logs the raw value — logs length only on parse failure (Finding #6).
 func loadParamsBlob(ctx context.Context, sc store.SystemConfigStore, key string) map[string]any {
 	raw, err := sc.Get(ctx, key)
 	if err != nil || raw == "" {
 		return nil
 	}
+	if len(raw) > paramsMaxBytes {
+		slog.Warn("tts: params blob exceeds 16 KB limit, ignoring", "key", key, "length", len(raw))
+		return nil
+	}
 	var m map[string]any
 	if err := json.Unmarshal([]byte(raw), &m); err != nil {
-		slog.Warn("tts: invalid params blob", "key", key, "error", err)
+		slog.Warn("tts: invalid params blob", "key", key, "length", len(raw))
 		return nil
 	}
 	return m
diff --git a/internal/http/tts_config.go b/internal/http/tts_config.go
index dc918c3a..a576cc54 100644
--- a/internal/http/tts_config.go
+++ b/internal/http/tts_config.go
@@ -3,6 +3,7 @@ package http
 import (
 	"context"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"log/slog"
 	"net/http"
@@ -11,6 +12,7 @@ import (
 	"github.com/google/uuid"
 
 	"github.com/nextlevelbuilder/goclaw/internal/audio"
+	"github.com/nextlevelbuilder/goclaw/internal/i18n"
 	"github.com/nextlevelbuilder/goclaw/internal/permissions"
 	"github.com/nextlevelbuilder/goclaw/internal/store"
 )
@@ -236,6 +238,8 @@ func (h *TTSConfigHandler) handleSave(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	locale := store.LocaleFromContext(ctx)
+
 	var req ttsConfigSaveRequest
 	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
 		http.Error(w, fmt.Sprintf(`{"error":"invalid json: %s"}`, err.Error()), http.StatusBadRequest)
@@ -322,29 +326,30 @@ func (h *TTSConfigHandler) handleSave(w http.ResponseWriter, r *http.Request) {
 				return
 			}
 		}
-		// Dual-write: save params blobs alongside legacy flat keys for rollback safety.
+		// Dual-write: validate then save params blobs alongside legacy flat keys.
+		// Finding #3: ValidateParams enforces Min/Max/Enum + rejects unknown keys.
 		if req.OpenAI != nil && req.OpenAI.Params != nil {
-			if !saveParamsBlob(w, ctx, h.systemConfigs.Set, "tts.openai.params", req.OpenAI.Params) {
+			if !validateAndSaveParamsBlob(w, ctx, h.systemConfigs.Set, "tts.openai.params", "openai", req.OpenAI.Params, locale) {
 				return
 			}
 		}
 		if req.ElevenLabs != nil && req.ElevenLabs.Params != nil {
-			if !saveParamsBlob(w, ctx, h.systemConfigs.Set, "tts.elevenlabs.params", req.ElevenLabs.Params) {
+			if !validateAndSaveParamsBlob(w, ctx, h.systemConfigs.Set, "tts.elevenlabs.params", "elevenlabs", req.ElevenLabs.Params, locale) {
 				return
 			}
 		}
 		if req.Edge != nil && req.Edge.Params != nil {
-			if !saveParamsBlob(w, ctx, h.systemConfigs.Set, "tts.edge.params", req.Edge.Params) {
+			if !validateAndSaveParamsBlob(w, ctx, h.systemConfigs.Set, "tts.edge.params", "edge", req.Edge.Params, locale) {
 				return
 			}
 		}
 		if req.MiniMax != nil && req.MiniMax.Params != nil {
-			if !saveParamsBlob(w, ctx, h.systemConfigs.Set, "tts.minimax.params", req.MiniMax.Params) {
+			if !validateAndSaveParamsBlob(w, ctx, h.systemConfigs.Set, "tts.minimax.params", "minimax", req.MiniMax.Params, locale) {
 				return
 			}
 		}
 		if req.Gemini != nil && req.Gemini.Params != nil {
-			if !saveParamsBlob(w, ctx, h.systemConfigs.Set, "tts.gemini.params", req.Gemini.Params) {
+			if !validateAndSaveParamsBlob(w, ctx, h.systemConfigs.Set, "tts.gemini.params", "gemini", req.Gemini.Params, locale) {
 				return
 			}
 		}
@@ -523,3 +528,35 @@ func saveParamsBlob(w http.ResponseWriter, ctx context.Context, setFn func(conte
 	}
 	return saveOrFail(w, ctx, setFn, key, string(blob), key)
 }
+
+// validateAndSaveParamsBlob validates params against the named provider's capability
+// schema (Finding #3) then persists via saveParamsBlob. Returns false and writes
+// 422 on validation failure, 500 on marshal/store error.
+func validateAndSaveParamsBlob(w http.ResponseWriter, ctx context.Context, setFn func(context.Context, string, string) error, key, providerName string, params map[string]any, locale string) bool {
+	// Look up schema from builtin capabilities catalog.
+	var schema []audio.ParamSchema
+	for _, cap := range builtinTTSCapabilities() {
+		if cap.Provider == providerName {
+			schema = cap.Params
+			break
+		}
+	}
+
+	if err := audio.ValidateParams(schema, params); err != nil {
+		slog.Warn("tts.config: invalid params", "provider", providerName, "error", err)
+		var ukErr audio.ErrTTSParamUnknownKey
+		var orErr audio.ErrTTSParamOutOfRange
+		var msg string
+		switch {
+		case errors.As(err, &ukErr):
+			msg = i18n.T(locale, i18n.MsgTtsParamUnknownKey, ukErr.Key)
+		case errors.As(err, &orErr):
+			msg = i18n.T(locale, i18n.MsgTtsParamOutOfRange, orErr.Key, orErr.Val, orErr.Min, orErr.Max)
+		default:
+			msg = err.Error()
+		}
+		http.Error(w, fmt.Sprintf(`{"error":%q}`, msg), http.StatusUnprocessableEntity)
+		return false
+	}
+	return saveParamsBlob(w, ctx, setFn, key, params)
+}
diff --git a/internal/http/tts_config_test.go b/internal/http/tts_config_test.go
index 95d21c8e..f50ebe63 100644
--- a/internal/http/tts_config_test.go
+++ b/internal/http/tts_config_test.go
@@ -4,6 +4,7 @@ import (
 	"context"
 	"encoding/json"
 	"errors"
+	"maps"
 	"net/http"
 	"net/http/httptest"
 	"sync"
@@ -50,9 +51,7 @@ func (s *validationSystemConfigStore) List(ctx context.Context) (map[string]stri
 	s.mu.Lock()
 	defer s.mu.Unlock()
 	out := make(map[string]string, len(s.data))
-	for k, v := range s.data {
-		out[k] = v
-	}
+	maps.Copy(out, s.data)
 	return out, nil
 }
 
@@ -88,9 +87,7 @@ func (s *validationSecretsStore) GetAll(ctx context.Context) (map[string]string,
 	s.mu.Lock()
 	defer s.mu.Unlock()
 	out := make(map[string]string, len(s.data))
-	for k, v := range s.data {
-		out[k] = v
-	}
+	maps.Copy(out, s.data)
 	return out, nil
 }
 
diff --git a/internal/i18n/catalog_en.go b/internal/i18n/catalog_en.go
index f6de025d..fbc2f73b 100644
--- a/internal/i18n/catalog_en.go
+++ b/internal/i18n/catalog_en.go
@@ -206,6 +206,8 @@ func init() {
 		MsgTtsGeminiInvalidModel:  "invalid Gemini TTS model: %s",
 		MsgTtsParamOutOfRange:     "TTS param %q value %v is out of range [%v, %v]",
 		MsgTtsParamDependsOn:      "TTS param %q requires %q to be %q",
+		MsgTtsParamInvalidJSON:    "TTS param %q contains invalid JSON (length %d)",
+		MsgTtsParamUnknownKey:     "TTS param %q is not supported by this provider",
 		MsgTtsMiniMaxVoicesFailed: "failed to fetch MiniMax voices: %s",
 
 		// STT
diff --git a/internal/i18n/catalog_vi.go b/internal/i18n/catalog_vi.go
index a1d18dbc..2dbc3108 100644
--- a/internal/i18n/catalog_vi.go
+++ b/internal/i18n/catalog_vi.go
@@ -206,6 +206,8 @@ func init() {
 		MsgTtsGeminiInvalidModel:  "mô hình Gemini TTS không hợp lệ: %s",
 		MsgTtsParamOutOfRange:     "tham số TTS %q có giá trị %v nằm ngoài phạm vi [%v, %v]",
 		MsgTtsParamDependsOn:      "tham số TTS %q yêu cầu %q phải là %q",
+		MsgTtsParamInvalidJSON:    "tham số TTS %q chứa JSON không hợp lệ (độ dài %d)",
+		MsgTtsParamUnknownKey:     "tham số TTS %q không được nhà cung cấp này hỗ trợ",
 		MsgTtsMiniMaxVoicesFailed: "không tải được danh sách giọng đọc MiniMax: %s",
 
 		// STT
diff --git a/internal/i18n/catalog_zh.go b/internal/i18n/catalog_zh.go
index ab0acad6..1072cccf 100644
--- a/internal/i18n/catalog_zh.go
+++ b/internal/i18n/catalog_zh.go
@@ -206,6 +206,8 @@ func init() {
 		MsgTtsGeminiInvalidModel:  "无效的 Gemini TTS 模型：%s",
 		MsgTtsParamOutOfRange:     "TTS 参数 %q 的值 %v 超出范围 [%v, %v]",
 		MsgTtsParamDependsOn:      "TTS 参数 %q 需要将 %q 设置为 %q",
+		MsgTtsParamInvalidJSON:    "TTS 参数 %q 包含无效的 JSON（长度 %d）",
+		MsgTtsParamUnknownKey:     "TTS 参数 %q 不受此提供商支持",
 		MsgTtsMiniMaxVoicesFailed: "获取 MiniMax 声音列表失败：%s",
 
 		// STT
diff --git a/internal/i18n/keys.go b/internal/i18n/keys.go
index dabf7f81..44836f26 100644
--- a/internal/i18n/keys.go
+++ b/internal/i18n/keys.go
@@ -204,6 +204,8 @@ const (
 	MsgTtsGeminiInvalidModel    = "error.tts_gemini_invalid_model"    // "invalid Gemini TTS model: %s"
 	MsgTtsParamOutOfRange       = "error.tts_param_out_of_range"      // "TTS param %q value %v is out of range [%v, %v]"
 	MsgTtsParamDependsOn        = "error.tts_param_depends_on"        // "TTS param %q requires %q to be %q"
+	MsgTtsParamInvalidJSON      = "error.tts_param_invalid_json"      // "TTS param %q contains invalid JSON (length %d)"
+	MsgTtsParamUnknownKey       = "error.tts_param_unknown_key"       // "TTS param %q is not supported by this provider"
 	MsgTtsMiniMaxVoicesFailed   = "error.tts_minimax_voices_failed"   // "failed to fetch MiniMax voices: %s"
 
 	// --- STT ---