diff --git a/config/defaults.json b/config/defaults.json
index 8a411ba..adbf006 100644
--- a/config/defaults.json
+++ b/config/defaults.json
@@ -15,7 +15,7 @@
   "dictation": false,
   "profanity_filter": false,
   "diarize": false,
-  "diarize_version": "",
+  "diarize_model": "",
   "detect_entities": false,
   "multichannel": false,
   "utterances": false,
diff --git a/static/app.js b/static/app.js
index 6044708..6246112 100644
--- a/static/app.js
+++ b/static/app.js
@@ -305,7 +305,7 @@ function appData() {
       dictation: false,
       profanity_filter: false,
       diarize: false,
-      diarize_version: '',
+      diarize_model: '',
       detect_entities: false,
       multichannel: false,
       utterances: false,
@@ -1089,7 +1089,7 @@ function appData() {
         dictation: false,
         profanity_filter: false,
         diarize: false,
-        diarize_version: '',
+        diarize_model: '',
         detect_entities: false,
         multichannel: false,
         utterances: false,
diff --git a/stt/options.py b/stt/options.py
index 005a1c7..491edc4 100644
--- a/stt/options.py
+++ b/stt/options.py
@@ -173,6 +173,22 @@ def clean_params(params: dict, mode: Mode) -> dict:
                 continue
             _put(result, wire, v)
 
+    # `diarize_model` is mutually exclusive with the deprecated `diarize` and
+    # `diarize_version`. Deepgram does not pick a winner, it rejects outright:
+    #
+    #   400 diarize_model cannot be used together with diarize or
+    #       diarize_version. (INVALID_QUERY_PARAMETER)
+    #
+    # `diarize_model` wins here because it is the current parameter and the only
+    # way to reach the v2 diarizer: the boolean `diarize` ALWAYS routes to v1,
+    # and `diarize_version` is legacy. The UI already makes the two controls
+    # exclusive; this covers the `extra` escape hatch and any saved config from
+    # before the field was renamed, so an old preset degrades to the modern
+    # parameter instead of a hard 400.
+    if "diarize_model" in result:
+        for legacy in ("diarize", "diarize_version"):
+            result.pop(legacy, None)
+
     return result
 
 
diff --git a/templates/index.html b/templates/index.html
index 625f3cf..5cf6b91 100644
--- a/templates/index.html
+++ b/templates/index.html
@@ -389,6 +389,19 @@
       letter-spacing: 0.06em;
     }
 
+    .field-hint {
+      font-size: 11px;
+      line-height: 1.45;
+      color: #8b93a7;
+      margin-top: 5px;
+    }
+    .field-hint code {
+      background: #1a2234;
+      padding: 1px 4px;
+      border-radius: 3px;
+      color: #a5b4fc;
+    }
+
     .field-input {
       background: #1a1f2e;
       border: 1px solid #2d3748;
@@ -1319,9 +1332,14 @@
           </div>
           <div x-show="sections.features" class="param-section-body">
             <div class="checkbox-grid">
+              <!-- Deprecated boolean, kept because it is what legacy customer
+                   configs send and replicating those is a job this tool has.
+                   Always v1. Clears diarize_model because Deepgram 400s on the
+                   pair. -->
               <div class="checkbox-item">
-                <input type="checkbox" id="diarize" x-model="params.diarize">
-                <label for="diarize">Diarize</label>
+                <input type="checkbox" id="diarize" x-model="params.diarize"
+                       @change="if (params.diarize) params.diarize_model = ''">
+                <label for="diarize" title="Deprecated; always selects the v1 diarizer">Diarize (v1, deprecated)</label>
               </div>
               <div class="checkbox-item">
                 <input type="checkbox" id="detect_entities" x-model="params.detect_entities">
@@ -1340,17 +1358,34 @@
               </div>
             </div>
 
-            <!-- Diarize version -->
-            <template x-if="params.diarize">
-              <div class="field mt-2">
-                <div class="field-label">Diarize Version</div>
-                <select class="field-select" x-model="params.diarize_version">
-                  <option value="">Default</option>
-                  <option value="2021-07-14.0">2021-07-14.0</option>
-                  <option value="latest">latest</option>
-                </select>
+            <!-- Diarizer selection.
+                 DO NOT re-gate this behind params.diarize. It used to be, and
+                 that made v2 unreachable: `diarize` is the deprecated boolean
+                 and ALWAYS routes to v1, while `diarize_model` both enables
+                 diarization and picks the version, so it must be sent WITHOUT
+                 `diarize`. Deepgram rejects the pair outright:
+                   400 diarize_model cannot be used together with diarize or
+                       diarize_version. (INVALID_QUERY_PARAMETER)
+                 The two controls are mutually exclusive in the UI for that
+                 reason; setting one clears the other. -->
+            <div class="field mt-2">
+              <div class="field-label">Diarizer (diarize_model)</div>
+              <select class="field-select" x-model="params.diarize_model"
+                      @change="if (params.diarize_model) params.diarize = false">
+                <option value="">Off / use the deprecated Diarize checkbox</option>
+                <option value="v1">v1</option>
+                <option value="v2">v2 (batch only)</option>
+                <option value="latest">latest</option>
+              </select>
+              <div class="field-hint" x-show="params.diarize_model === 'v2' && mode !== 'batch'">
+                v2 is batch only. Deepgram rejects the WebSocket handshake for
+                <code>diarize_model=v2</code> on a stream; use <code>latest</code> there.
               </div>
-            </template>
+              <div class="field-hint" x-show="params.diarize_model">
+                Sends <code>diarize_model</code> and omits <code>diarize</code>.
+                The deprecated <code>diarize</code> checkbox always selects v1.
+              </div>
+            </div>
 
             <!-- Entity types when detect_entities enabled -->
             <template x-if="params.detect_entities">
diff --git a/tests/test_options.py b/tests/test_options.py
index cde024f..f07abf9 100644
--- a/tests/test_options.py
+++ b/tests/test_options.py
@@ -178,7 +178,11 @@ def test_sdk_accepts_every_serialized_param_name():
     # lost. Regression guard for the keyterms crash class. filler_words is NOT in
     # this list: Deepgram's docs mark it stream-unavailable, so it is stripped in
     # streaming mode by design — see test_stream_unavailable_params_are_stripped.
-    for name in ("no_delay", "word_confidence", "diarize_version", "entity_prompt"):
+    # `diarize_version` is deliberately absent from this list: the UI now models
+    # the current `diarize_model`, which deepgram-sdk 7 names as a real kwarg, so
+    # it rides via_kwargs rather than the passthrough. See
+    # test_diarize_model_is_a_named_sdk_kwarg_not_passthrough.
+    for name in ("no_delay", "word_confidence", "entity_prompt", "alternatives"):
         assert name in via_query, f"{name} is not being forwarded"
 
 
@@ -283,3 +287,46 @@ def test_alias_cannot_smuggle_a_denied_wire_name():
         options.PARAM_ALIASES = original
 
     assert "callback" in DENIED_PARAMS
+
+
+def test_diarize_model_wins_over_the_deprecated_diarize_params():
+    """Deepgram rejects the combination outright rather than picking a winner:
+
+        400 diarize_model cannot be used together with diarize or
+            diarize_version. (INVALID_QUERY_PARAMETER)
+
+    Verified live. diarize_model is kept because it is the current parameter and
+    the only route to the v2 diarizer; the boolean `diarize` always selects v1.
+    """
+    from stt.options import clean_params, Mode
+    out = clean_params(
+        {"model": "nova-3", "diarize": True, "extra": {"diarize_model": "v2"}},
+        Mode.BATCH,
+    )
+    assert out["diarize_model"] == "v2"
+    assert "diarize" not in out
+    assert "diarize_version" not in out
+
+    out = clean_params(
+        {"model": "nova-3", "extra": {"diarize_model": "latest", "diarize_version": "latest"}},
+        Mode.BATCH,
+    )
+    assert out["diarize_model"] == "latest"
+    assert "diarize_version" not in out
+
+
+def test_deprecated_diarize_still_works_alone():
+    """Legacy customer configs send the boolean, and replicating those is a job
+    this tool has. It must keep working when diarize_model is absent."""
+    from stt.options import clean_params, Mode
+    out = clean_params({"model": "nova-3", "diarize": True}, Mode.BATCH)
+    assert out["diarize"] is True
+    assert "diarize_model" not in out
+
+
+def test_diarize_model_is_a_named_sdk_kwarg_not_passthrough():
+    """deepgram-sdk 7 added diarize_model to connect(); it should ride as a real
+    kwarg rather than through additional_query_parameters."""
+    import app
+    built = app._params_to_sdk_kwargs({"model": "nova-3", "extra": {"diarize_model": "latest"}})
+    assert built.get("diarize_model") == "latest"