diff --git a/app/README.md b/app/README.md
index 7190d7e..7341467 100644
--- a/app/README.md
+++ b/app/README.md
@@ -148,6 +148,43 @@ into the field reaches the engine when "Use this" is clicked, the note says so,
 and Close puts the dialog away. The run points `XDG_CONFIG_HOME` at its scratch
 directory, so settings and keys of the real user are never rewritten.
 
+## Custom providers, model parameters
+
+The dialog's **Advanced options** covers the two things the presets cannot:
+a provider of your own, and what the model should do.
+
+- A custom provider is an entry the engine keeps in
+  `<config>/cst-agent/models.json` (layer 1 of the model registry, see
+  `include/cst_models.h`) — the same file a hand-written override would go
+  into, so the GUI and a text editor are interchangeable:
+
+  ```json
+  { "providers": { "myllm": { "api_base": "https://api.example.com/v1",
+                              "key_env": "MYLLM_API_KEY",
+                              "default_model": "m1",
+                              "models": ["m1", "m2"] } } }
+  ```
+
+  `AgentCore::addProvider()` / `removeProvider()` write it
+  (`cst_models_user_set_provider()` / `cst_models_user_drop_provider()`) and
+  reload the registry, so the provider is in the list, in `models[]` on the
+  wire and in `/provider` of the CLI at once. A name a preset uses overrides
+  that preset (a self-hosted endpoint, a proxy); the other content of the file
+  (model rules) is kept, and a file that is not a JSON object is never
+  overwritten. The key is still a key: it is verified against `GET /models` of
+  the new base URL before it is saved, and stored per provider name as before.
+- The generation parameters — **temperature**, **top-p**, **max tokens** — are
+  remembered per provider in `settings.json` (`"params": {...}`,
+  `cst_settings_remember_params()`), and applied to the engine's config by
+  `AgentCore::setGenParams()`, `AgentCore::useProvider()` and
+  `cst_agent_create()` (so a CLI or TUI run sends what the window was set to).
+  An **empty field is not sent at all** — that is not the same as sending 0,
+  which is a real temperature; `cst_llm_build_body()` only adds a field that
+  was set (`tests/test_llm_body.c:test_gen_params`).
+- A long model list (a router lists hundreds) is filtered and capped at 40
+  chips with "… +N more", and the whole dialog body scrolls: the buttons stay
+  put and nothing can spill out of the frame.
+
 ## Attachments
 
 The next prompt can carry files, and they arrive three ways:
diff --git a/app/qml/Main.qml b/app/qml/Main.qml
index 3166e95..1f87de9 100644
--- a/app/qml/Main.qml
+++ b/app/qml/Main.qml
@@ -180,8 +180,11 @@ ApplicationWindow {
         settings.open()
     }
 
-    function applySettings(provider, model, key) {
-        const err = agent.useProvider(provider, model, key)
+    function applySettings(provider, model, key, temperature, topP, maxTokens) {
+        /* The parameters are stored first: useProvider() then picks up what
+         * was remembered for whichever provider it ends on. */
+        const perr = agent.setGenParams(provider, temperature, topP, maxTokens)
+        const err = perr.length ? perr : agent.useProvider(provider, model, key)
         settings.note = err.length ? err
                                    : "using " + agent.provider + " / " + agent.model
         settings.choices = agent.modelChoices(provider)
@@ -470,8 +473,8 @@ ApplicationWindow {
 
     SettingsDialog {
         id: settings
-        onApplyRequested: function (provider, model, key) {
-            win.applySettings(provider, model, key)
+        onApplyRequested: function (provider, model, key, temperature, topP, maxTokens) {
+            win.applySettings(provider, model, key, temperature, topP, maxTokens)
         }
     }
 
diff --git a/app/qml/SettingsDialog.qml b/app/qml/SettingsDialog.qml
index 1bf202e..5310ed8 100644
--- a/app/qml/SettingsDialog.qml
+++ b/app/qml/SettingsDialog.qml
@@ -3,11 +3,18 @@ import QtQuick.Controls
 import QtQuick.Layouts
 
 // Provider, model and key: what the engine needs before it can talk to a model
-// at all. Everything here goes through AgentCore::useProvider(), which moves
-// the engine's own config (its session stays where it is) and remembers the
-// choice. A key is verified against the provider before it is saved and is
-// never read back into this window -- the dialog only ever says where the key
-// it will use comes from.
+// at all. Everything here goes through AgentCore, which moves the engine's own
+// config (its session stays where it is) and remembers the choice. A key is
+// verified against the provider before it is saved and is never read back into
+// this window -- the dialog only ever says where the key it will use comes
+// from.
+//
+// Three things live under "Advanced options", because they are for the odd
+// provider or the odd model: the provider's own entry (name, base URL, key
+// variable, models -- the engine keeps those in <config>/models.json, which is
+// how a provider the presets do not know is added or an existing one is
+// pointed somewhere else) and the generation parameters (temperature, top-p,
+// max tokens), which are remembered per provider.
 Dialog {
     id: dlg
     objectName: "settingsDialog"
@@ -21,15 +28,27 @@ Dialog {
     property var choices: []        // AgentCore::modelChoices(picked)
     property string picked: ""
     property string model: ""
-    property string note: ""        // what the last "Use this" answered
+    property string note: ""        // what the last action answered
     property string keyEnv: ""
     property string keyFrom: ""
     property bool hasKey: false
+    property bool advanced: false
+    property string modelFilter: ""  // narrows a long model list
+    /* The advanced form: the picked provider's entry, editable. */
+    property string advName: ""
+    property string advBase: ""
+    property string advKeyEnv: ""
+    property string advModel: ""
+    property string advModels: ""
+    property string tempText: ""
+    property string topPText: ""
+    property string maxTokText: ""
 
-    signal applyRequested(string provider, string model, string key)
+    signal applyRequested(string provider, string model, string key,
+                          real temperature, real topP, int maxTokens)
 
-    width: Math.min(580, parent ? parent.width - 32 : 580)
-    height: Math.min(560, parent ? parent.height - 60 : 560)
+    width: Math.min(660, parent ? parent.width - 32 : 660)
+    height: Math.min(640, parent ? parent.height - 40 : 640)
 
     /* The Basic style paints a light panel; this window is dark, so the
      * dialog brings its own surface like every other one here. */
@@ -40,6 +59,35 @@ Dialog {
         border.color: Theme.border
     }
 
+    // A long provider list (a router lists hundreds) is a wall of chips: the
+    // filter narrows it and the rest is one line ("+N more").
+    readonly property int chipCap: 40
+    readonly property var chips: {
+        const f = modelFilter.trim().toLowerCase()
+        const out = []
+        for (let i = 0; i < choices.length; i++) {
+            const c = String(choices[i])
+            if (!f.length || c.toLowerCase().indexOf(f) >= 0)
+                out.push(c)
+        }
+        return out
+    }
+    readonly property int chipsHidden: Math.max(0, chips.length - chipCap)
+
+    /* "0.2" -> 0.2; anything empty or unreadable -> 0 (the field is not sent,
+     * which is not the same as sending 0). */
+    function num(text) {
+        const v = parseFloat(String(text).replace(",", "."))
+        return isFinite(v) && v > 0 ? v : 0
+    }
+
+    function pickedEntry() {
+        for (let i = 0; i < providers.length; i++)
+            if (providers[i].name === dlg.picked)
+                return providers[i]
+        return null
+    }
+
     /* Fill it in: the presets, the provider the engine is on now, and the
      * model that provider last used. */
     function load(list, provider, currentModel) {
@@ -51,24 +99,33 @@ Dialog {
             if (list[i].current === true)
                 picked = list[i].name
         choices = picked.length ? agent.modelChoices(picked) : []
+        modelFilter = ""
         syncPick()
     }
 
-    /* Where the picked provider's key would come from. From the entries --
-     * a key itself never reaches the QML side. */
+    /* Where the picked provider's key would come from and what it is set to.
+     * From the entries -- a key itself never reaches the QML side. */
     function syncPick() {
         hasKey = false
         keyFrom = ""
         keyEnv = ""
-        for (let i = 0; i < providers.length; i++) {
-            const p = providers[i]
-            if (p.name !== picked)
-                continue
+        const p = pickedEntry()
+        if (p) {
             hasKey = p.hasKey === true
             keyFrom = p.keyFrom
             keyEnv = p.keyEnv
-            return
+            /* The advanced form describes the picked provider: edit it, or
+             * type another name to add one. */
+            advName = p.name
+            advBase = p.base
+            advKeyEnv = p.keyEnv
+            advModel = p.model
+            advModels = p.custom === true ? agent.modelChoices(p.name).join(", ") : ""
         }
+        const g = picked.length ? agent.genParams(picked) : null
+        tempText = g && g.temperature > 0 ? String(g.temperature) : ""
+        topPText = g && g.top_p > 0 ? String(g.top_p) : ""
+        maxTokText = g && g.max_tokens > 0 ? String(g.max_tokens) : ""
     }
 
     function keyState() {
@@ -90,6 +147,87 @@ Dialog {
         return s
     }
 
+    /* Write the form's entry into the engine's user file and list it again. */
+    function saveProvider() {
+        /* The fields are the truth here, not the properties: a hand-typed
+         * edit is what the user wants saved. */
+        const name = advNameField.text.trim()
+        const err = agent.addProvider(name, advBaseField.text.trim(),
+                                      advKeyEnvField.text.trim(),
+                                      advModelField.text.trim(),
+                                      advModelsField.text)
+        if (err.length) {
+            note = err
+            return
+        }
+        providers = agent.providers()
+        picked = name
+        choices = agent.modelChoices(name)
+        syncPick()
+        note = "saved " + name + " to models.json — it is in the list above now"
+    }
+
+    function removeProvider() {
+        const typed = advNameField.text.trim()
+        const name = typed.length ? typed : picked
+        const rc = agent.removeProvider(name)
+        if (rc < 0) {
+            note = "could not remove " + name
+            return
+        }
+        providers = agent.providers()
+        note = rc === 0
+             ? name + " is not in models.json (a preset of that name stays)"
+             : "removed " + name + " from models.json"
+        if (rc === 1 && providers.length > 0)
+            picked = providers[0].name
+        choices = picked.length ? agent.modelChoices(picked) : []
+        syncPick()
+    }
+
+    /* The form's fields follow the picked provider (and a user edit breaks
+     * the binding, so the push is imperative). */
+    onAdvNameChanged: if (advNameField && advNameField.text !== advName) advNameField.text = advName
+    onAdvBaseChanged: if (advBaseField && advBaseField.text !== advBase) advBaseField.text = advBase
+    onAdvKeyEnvChanged: if (advKeyEnvField && advKeyEnvField.text !== advKeyEnv) advKeyEnvField.text = advKeyEnv
+    onAdvModelChanged: if (advModelField && advModelField.text !== advModel) advModelField.text = advModel
+    onAdvModelsChanged: if (advModelsField && advModelsField.text !== advModels) advModelsField.text = advModels
+    onTempTextChanged: if (tempField && tempField.text !== tempText) tempField.text = tempText
+    onTopPTextChanged: if (topPField && topPField.text !== topPText) topPField.text = topPText
+    onMaxTokTextChanged: if (maxTokField && maxTokField.text !== maxTokText) maxTokField.text = maxTokText
+
+    // One labelled field of the advanced form.
+    component AdvField: ColumnLayout {
+        property string label: ""
+        property alias text: field.text
+        property alias placeholder: field.placeholderText
+        property int hintWidth: 0
+
+        Layout.fillWidth: true
+        spacing: 2
+        Text {
+            text: parent.label
+            color: Theme.textDim
+            font.pixelSize: Theme.smallSize
+        }
+        TextField {
+            id: field
+            Layout.fillWidth: true
+            implicitHeight: Theme.touchTarget
+            selectByMouse: true
+            color: Theme.text
+            placeholderTextColor: Theme.textFaint
+            selectionColor: Theme.accentDim
+            inputMethodHints: Qt.ImhNoPredictiveText
+            background: Rectangle {
+                radius: Theme.radiusSm
+                color: Theme.bg
+                border.width: 1
+                border.color: field.activeFocus ? Theme.accentDim : Theme.border
+            }
+        }
+    }
+
     contentItem: ColumnLayout {
         objectName: "settingsPanel"
         spacing: Theme.gap
@@ -109,201 +247,427 @@ Dialog {
             wrapMode: Text.Wrap
         }
 
+        /* Everything scrolls, so the buttons stay where they are and a long
+         * model list cannot push them out of the dialog. */
         Flickable {
+            id: body
+            objectName: "settingsBody"
             Layout.fillWidth: true
             Layout.fillHeight: true
-            Layout.minimumHeight: 96
+            Layout.minimumHeight: 120
             clip: true
             contentWidth: width
-            contentHeight: plist.implicitHeight
+            contentHeight: bodyCol.implicitHeight
             boundsBehavior: Flickable.StopAtBounds
             ScrollBar.vertical: ScrollBar {}
 
             ColumnLayout {
-                id: plist
+                id: bodyCol
                 width: parent.width
-                spacing: 2
+                spacing: Theme.gap
 
-                Repeater {
-                    model: dlg.providers
-                    delegate: Rectangle {
-                        id: prow
-                        required property var modelData
-                        readonly property bool selected: dlg.picked === modelData.name
+                ColumnLayout {
+                    Layout.fillWidth: true
+                    spacing: 2
 
-                        objectName: "providerRow:" + modelData.name
-                        Layout.fillWidth: true
-                        implicitHeight: prowCol.implicitHeight + 8
-                        radius: Theme.radiusSm
-                        color: prow.selected ? Theme.accentDim
-                                             : prowHover.hovered ? Theme.surfaceAlt
-                                                                 : "transparent"
-                        border.width: 1
-                        border.color: prow.selected ? Theme.accent : "transparent"
+                    Repeater {
+                        model: dlg.providers
+                        delegate: Rectangle {
+                            id: prow
+                            required property var modelData
+                            readonly property bool selected: dlg.picked === modelData.name
 
-                        HoverHandler { id: prowHover }
-                        MouseArea {
-                            anchors.fill: parent
-                            onClicked: {
-                                dlg.picked = prow.modelData.name
-                                modelField.text = prow.modelData.model
-                                dlg.choices = agent.modelChoices(prow.modelData.name)
-                                dlg.syncPick()
+                            objectName: "providerRow:" + modelData.name
+                            Layout.fillWidth: true
+                            implicitHeight: prowCol.implicitHeight + 8
+                            radius: Theme.radiusSm
+                            color: prow.selected ? Theme.accentDim
+                                                 : prowHover.hovered ? Theme.surfaceAlt
+                                                                     : "transparent"
+                            border.width: 1
+                            border.color: prow.selected ? Theme.accent : "transparent"
+
+                            HoverHandler { id: prowHover }
+                            MouseArea {
+                                anchors.fill: parent
+                                onClicked: {
+                                    dlg.picked = prow.modelData.name
+                                    modelField.text = prow.modelData.model
+                                    dlg.choices = agent.modelChoices(prow.modelData.name)
+                                    dlg.modelFilter = ""
+                                    dlg.syncPick()
+                                }
                             }
-                        }
 
-                        ColumnLayout {
-                            id: prowCol
-                            anchors {
-                                left: parent.left
-                                right: parent.right
-                                top: parent.top
-                                margins: 4
-                            }
-                            spacing: 0
+                            ColumnLayout {
+                                id: prowCol
+                                anchors {
+                                    left: parent.left
+                                    right: parent.right
+                                    top: parent.top
+                                    margins: 4
+                                }
+                                spacing: 0
 
-                            RowLayout {
-                                Layout.fillWidth: true
-                                spacing: Theme.gap
-                                Text {
-                                    text: prow.modelData.name
-                                    color: prow.selected ? Theme.accentText : Theme.textDim
-                                    font.pixelSize: Theme.smallSize
-                                    font.bold: prow.selected
+                                RowLayout {
+                                    Layout.fillWidth: true
+                                    spacing: Theme.gap
+                                    Text {
+                                        text: prow.modelData.name
+                                        color: prow.selected ? Theme.accentText : Theme.textDim
+                                        font.pixelSize: Theme.smallSize
+                                        font.bold: prow.selected
+                                    }
+                                    Text {
+                                        visible: prow.modelData.current === true
+                                        text: "in use"
+                                        color: prow.selected ? Theme.okOnAccent : Theme.ok
+                                        font.pixelSize: Theme.smallSize
+                                    }
+                                    Item { Layout.fillWidth: true }
+                                    Text {
+                                        text: prow.modelData.custom === true
+                                              ? "custom" : prow.modelData.source
+                                        color: prow.selected ? Theme.accentTextDim : Theme.textFaint
+                                        font.pixelSize: Theme.smallSize
+                                    }
                                 }
                                 Text {
-                                    visible: prow.modelData.current === true
-                                    text: "in use"
-                                    color: prow.selected ? Theme.okOnAccent : Theme.ok
-                                    font.pixelSize: Theme.smallSize
-                                }
-                                Item { Layout.fillWidth: true }
-                                Text {
-                                    text: prow.modelData.source
+                                    Layout.fillWidth: true
+                                    elide: Text.ElideMiddle
+                                    text: prow.modelData.base + " · " +
+                                          (prow.modelData.hasKey === true
+                                           ? (prow.modelData.keyFrom === "saved"
+                                              ? "key saved"
+                                              : "key from " + prow.modelData.keyFrom)
+                                           : (prow.modelData.keyEnv.length
+                                              ? "no key (" + prow.modelData.keyEnv + ")"
+                                              : "no key"))
                                     color: prow.selected ? Theme.accentTextDim : Theme.textFaint
                                     font.pixelSize: Theme.smallSize
                                 }
                             }
-                            Text {
-                                Layout.fillWidth: true
-                                elide: Text.ElideMiddle
-                                text: prow.modelData.base + " · " +
-                                      (prow.modelData.hasKey === true
-                                       ? (prow.modelData.keyFrom === "saved"
-                                          ? "key saved"
-                                          : "key from " + prow.modelData.keyFrom)
-                                       : (prow.modelData.keyEnv.length
-                                          ? "no key (" + prow.modelData.keyEnv + ")"
-                                          : "no key"))
-                                color: prow.selected ? Theme.accentTextDim : Theme.textFaint
-                                font.pixelSize: Theme.smallSize
-                            }
                         }
                     }
                 }
-            }
-        }
 
-        Text {
-            Layout.fillWidth: true
-            text: "Model"
-            color: Theme.textDim
-            font.pixelSize: Theme.smallSize
-            font.bold: true
-        }
-        RowLayout {
-            Layout.fillWidth: true
-            spacing: Theme.gap
-            TextField {
-                id: modelField
-                objectName: "settingsModel"
-                Layout.fillWidth: true
-                text: dlg.model
-                implicitHeight: Theme.touchTarget
-                selectByMouse: true
-                color: Theme.text
-                placeholderText: "model name"
-                placeholderTextColor: Theme.textFaint
-                selectionColor: Theme.accentDim
-                inputMethodHints: Qt.ImhNoPredictiveText
-                background: Rectangle {
-                    radius: Theme.radiusSm
-                    color: Theme.bg
-                    border.width: 1
-                    border.color: modelField.activeFocus ? Theme.accentDim
-                                                         : Theme.border
+                Text {
+                    Layout.fillWidth: true
+                    text: "Model"
+                    color: Theme.textDim
+                    font.pixelSize: Theme.smallSize
+                    font.bold: true
                 }
-            }
-            Text {
-                Layout.preferredWidth: 90
-                elide: Text.ElideRight
-                text: modelField.text.length ? agent.modelInfo(modelField.text) : ""
-                color: Theme.textFaint
-                font.pixelSize: Theme.smallSize
-            }
-        }
-        Flow {
-            Layout.fillWidth: true
-            spacing: 6
-            visible: dlg.choices.length > 0
-
-            Repeater {
-                model: dlg.choices
-                delegate: Rectangle {
-                    id: chip
-                    required property var modelData
-                    readonly property string name: String(chip.modelData)
-
-                    objectName: "modelChoice:" + chip.name
-                    implicitWidth: chipText.implicitWidth + 14
-                    implicitHeight: 26
-                    radius: Theme.radiusSm
-                    color: chipHover.hovered ? Theme.surfaceHi : "transparent"
-                    border.width: 1
-                    border.color: modelField.text === chip.name ? Theme.accent
-                                                               : Theme.border
-                    HoverHandler { id: chipHover }
+                RowLayout {
+                    Layout.fillWidth: true
+                    spacing: Theme.gap
+                    TextField {
+                        id: modelField
+                        objectName: "settingsModel"
+                        Layout.fillWidth: true
+                        text: dlg.model
+                        implicitHeight: Theme.touchTarget
+                        selectByMouse: true
+                        color: Theme.text
+                        placeholderText: "model name"
+                        placeholderTextColor: Theme.textFaint
+                        selectionColor: Theme.accentDim
+                        inputMethodHints: Qt.ImhNoPredictiveText
+                        background: Rectangle {
+                            radius: Theme.radiusSm
+                            color: Theme.bg
+                            border.width: 1
+                            border.color: modelField.activeFocus ? Theme.accentDim
+                                                                 : Theme.border
+                        }
+                    }
                     Text {
-                        id: chipText
-                        anchors.centerIn: parent
-                        text: chip.name
-                        color: modelField.text === chip.name ? Theme.text
-                                                             : Theme.textDim
+                        Layout.preferredWidth: 90
+                        elide: Text.ElideRight
+                        text: modelField.text.length ? agent.modelInfo(modelField.text) : ""
+                        color: Theme.textFaint
                         font.pixelSize: Theme.smallSize
                     }
-                    TapHandler { onTapped: modelField.text = chip.name }
+                }
+
+                /* A router can list hundreds of models: filter, cap, say how
+                 * many are left instead of filling the dialog. */
+                RowLayout {
+                    Layout.fillWidth: true
+                    visible: dlg.choices.length > 12
+                    spacing: Theme.gap
+                    TextField {
+                        id: filterField
+                        objectName: "settingsFilter"
+                        Layout.fillWidth: true
+                        text: dlg.modelFilter
+                        implicitHeight: Theme.segmentHeight
+                        selectByMouse: true
+                        color: Theme.text
+                        placeholderText: "filter " + dlg.choices.length + " models"
+                        placeholderTextColor: Theme.textFaint
+                        selectionColor: Theme.accentDim
+                        inputMethodHints: Qt.ImhNoPredictiveText
+                        onTextChanged: dlg.modelFilter = text
+                        background: Rectangle {
+                            radius: Theme.radiusSm
+                            color: Theme.bg
+                            border.width: 1
+                            border.color: filterField.activeFocus ? Theme.accentDim
+                                                                  : Theme.border
+                        }
+                    }
+                    Text {
+                        Layout.preferredWidth: 110
+                        elide: Text.ElideRight
+                        text: dlg.chips.length + " shown"
+                              + (dlg.chipsHidden ? " · +" + dlg.chipsHidden + " more" : "")
+                        color: Theme.textFaint
+                        font.pixelSize: Theme.smallSize
+                    }
+                }
+                Flow {
+                    Layout.fillWidth: true
+                    spacing: 6
+                    visible: dlg.choices.length > 0
+
+                    Repeater {
+                        model: dlg.chips.slice(0, dlg.chipCap)
+                        delegate: Rectangle {
+                            id: chip
+                            required property var modelData
+                            readonly property string name: String(chip.modelData)
+
+                            objectName: "modelChoice:" + chip.name
+                            implicitWidth: chipText.implicitWidth + 14
+                            implicitHeight: 26
+                            radius: Theme.radiusSm
+                            color: chipHover.hovered ? Theme.surfaceHi : "transparent"
+                            border.width: 1
+                            border.color: modelField.text === chip.name ? Theme.accent
+                                                                       : Theme.border
+                            HoverHandler { id: chipHover }
+                            Text {
+                                id: chipText
+                                anchors.centerIn: parent
+                                text: chip.name
+                                color: modelField.text === chip.name ? Theme.text
+                                                                     : Theme.textDim
+                                font.pixelSize: Theme.smallSize
+                            }
+                            TapHandler { onTapped: modelField.text = chip.name }
+                        }
+                    }
+                }
+
+                TextField {
+                    id: keyField
+                    objectName: "settingsKey"
+                    Layout.fillWidth: true
+                    implicitHeight: Theme.touchTarget
+                    echoMode: TextInput.Password
+                    selectByMouse: true
+                    color: Theme.text
+                    placeholderText: "API key (leave empty to keep what is there)"
+                    placeholderTextColor: Theme.textFaint
+                    selectionColor: Theme.accentDim
+                    inputMethodHints: Qt.ImhNoPredictiveText | Qt.ImhSensitiveData
+                    onAccepted: dlg.apply()
+                    background: Rectangle {
+                        radius: Theme.radiusSm
+                        color: Theme.bg
+                        border.width: 1
+                        border.color: keyField.activeFocus ? Theme.accentDim : Theme.border
+                    }
+                }
+                Text {
+                    Layout.fillWidth: true
+                    text: dlg.keyState()
+                    color: Theme.textFaint
+                    font.pixelSize: Theme.smallSize
+                    wrapMode: Text.Wrap
+                }
+
+                /* --- advanced ------------------------------------------- */
+                ToolButton {
+                    id: advToggle
+                    objectName: "settingsAdvanced"
+                    Layout.fillWidth: true
+                    implicitHeight: Theme.segmentHeight
+                    onClicked: dlg.advanced = !dlg.advanced
+                    contentItem: RowLayout {
+                        spacing: 6
+                        ThemeIcon {
+                            name: dlg.advanced ? "arrow-down" : "arrow-right"
+                            glyph: dlg.advanced ? "▾" : "▸"
+                            size: 14
+                            color: Theme.textDim
+                        }
+                        Text {
+                            text: "Advanced options"
+                            color: advToggle.hovered ? Theme.text : Theme.textDim
+                            font.pixelSize: Theme.smallSize
+                            font.bold: true
+                        }
+                        Item { Layout.fillWidth: true }
+                        Text {
+                            text: "custom provider · model parameters"
+                            color: Theme.textFaint
+                            font.pixelSize: Theme.smallSize
+                        }
+                    }
+                    background: Rectangle {
+                        radius: Theme.radiusSm
+                        color: advToggle.hovered ? Theme.surfaceAlt : "transparent"
+                    }
+                }
+
+                ColumnLayout {
+                    Layout.fillWidth: true
+                    visible: dlg.advanced
+                    spacing: 6
+
+                    Text {
+                        Layout.fillWidth: true
+                        text: "Provider entry — saved in <config>/cst-agent/models.json. " +
+                              "A name the presets know is overridden, a new one is added."
+                        color: Theme.textFaint
+                        font.pixelSize: Theme.smallSize
+                        wrapMode: Text.Wrap
+                    }
+                    GridLayout {
+                        Layout.fillWidth: true
+                        columns: 2
+                        columnSpacing: Theme.gap
+                        rowSpacing: 4
+
+                        AdvField {
+                            id: advNameField
+                            objectName: "advName"
+                            label: "Name"
+                            placeholder: "myllm"
+                            Layout.preferredWidth: 180
+                        }
+                        AdvField {
+                            id: advBaseField
+                            objectName: "advBase"
+                            label: "Base URL"
+                            placeholder: "https://api.example.com/v1"
+                        }
+                        AdvField {
+                            id: advKeyEnvField
+                            objectName: "advKeyEnv"
+                            label: "Key variable"
+                            placeholder: "MYLLM_API_KEY"
+                        }
+                        AdvField {
+                            id: advModelField
+                            objectName: "advModel"
+                            label: "Default model"
+                            placeholder: "the model to start with"
+                        }
+                        AdvField {
+                            id: advModelsField
+                            objectName: "advModels"
+                            label: "Models it offers (comma separated, optional)"
+                            placeholder: "m1, m2"
+                            Layout.columnSpan: 2
+                        }
+                    }
+                    RowLayout {
+                        Layout.fillWidth: true
+                        spacing: Theme.gap
+                        Button {
+                            id: saveProviderBtn
+                            objectName: "settingsSaveProvider"
+                            text: "Save provider"
+                            implicitHeight: Theme.touchTarget
+                            enabled: advNameField.text.trim().length > 0 &&
+                                     advBaseField.text.trim().length > 0
+                            onClicked: dlg.saveProvider()
+                            background: Rectangle {
+                                radius: Theme.radiusSm
+                                color: saveProviderBtn.hovered ? Theme.surfaceHi : Theme.surfaceAlt
+                                border.width: 1
+                                border.color: Theme.border
+                            }
+                            contentItem: Text {
+                                text: saveProviderBtn.text
+                                color: saveProviderBtn.enabled ? Theme.textDim : Theme.textFaint
+                                font.pixelSize: Theme.bodySize
+                                horizontalAlignment: Text.AlignHCenter
+                                verticalAlignment: Text.AlignVCenter
+                            }
+                        }
+                        Button {
+                            id: removeProviderBtn
+                            objectName: "settingsRemoveProvider"
+                            text: "Remove from models.json"
+                            implicitHeight: Theme.touchTarget
+                            readonly property var entry: dlg.pickedEntry()
+                            enabled: entry !== null && entry.custom === true
+                            ToolTip.visible: hovered
+                            ToolTip.text: "Only the user's own entry goes: a built-in preset of the same name stays"
+                            onClicked: dlg.removeProvider()
+                            background: Rectangle {
+                                radius: Theme.radiusSm
+                                color: removeProviderBtn.hovered && removeProviderBtn.enabled
+                                       ? Theme.surfaceHi : Theme.surfaceAlt
+                                border.width: 1
+                                border.color: Theme.border
+                            }
+                            contentItem: Text {
+                                text: removeProviderBtn.text
+                                color: removeProviderBtn.enabled ? Theme.textDim
+                                                                 : Theme.textFaint
+                                font.pixelSize: Theme.bodySize
+                                horizontalAlignment: Text.AlignHCenter
+                                verticalAlignment: Text.AlignVCenter
+                            }
+                        }
+                        Item { Layout.fillWidth: true }
+                    }
+
+                    Text {
+                        Layout.fillWidth: true
+                        Layout.topMargin: 4
+                        text: "Model parameters — remembered for " +
+                              (dlg.picked.length ? dlg.picked : "this provider") +
+                              ", sent with every request. Empty = the provider's own default."
+                        color: Theme.textFaint
+                        font.pixelSize: Theme.smallSize
+                        wrapMode: Text.Wrap
+                    }
+                    RowLayout {
+                        Layout.fillWidth: true
+                        spacing: Theme.gap
+                        AdvField {
+                            id: tempField
+                            objectName: "advTemperature"
+                            label: "Temperature"
+                            placeholder: "not sent"
+                            Layout.preferredWidth: 130
+                        }
+                        AdvField {
+                            id: topPField
+                            objectName: "advTopP"
+                            label: "Top-p"
+                            placeholder: "not sent"
+                            Layout.preferredWidth: 130
+                        }
+                        AdvField {
+                            id: maxTokField
+                            objectName: "advMaxTokens"
+                            label: "Max tokens"
+                            placeholder: "not sent"
+                            Layout.preferredWidth: 150
+                        }
+                        Item { Layout.fillWidth: true }
+                    }
                 }
             }
         }
 
-        TextField {
-            id: keyField
-            objectName: "settingsKey"
-            Layout.fillWidth: true
-            implicitHeight: Theme.touchTarget
-            echoMode: TextInput.Password
-            selectByMouse: true
-            color: Theme.text
-            placeholderText: "API key (leave empty to keep what is there)"
-            placeholderTextColor: Theme.textFaint
-            selectionColor: Theme.accentDim
-            inputMethodHints: Qt.ImhNoPredictiveText | Qt.ImhSensitiveData
-            onAccepted: dlg.applyRequested(dlg.picked, modelField.text, keyField.text)
-            background: Rectangle {
-                radius: Theme.radiusSm
-                color: Theme.bg
-                border.width: 1
-                border.color: keyField.activeFocus ? Theme.accentDim : Theme.border
-            }
-        }
-        Text {
-            Layout.fillWidth: true
-            text: dlg.keyState()
-            color: Theme.textFaint
-            font.pixelSize: Theme.smallSize
-            wrapMode: Text.Wrap
-        }
-
         RowLayout {
             Layout.fillWidth: true
             spacing: Theme.gap
@@ -342,7 +706,7 @@ Dialog {
                 objectName: "settingsApply"
                 text: "Use this"
                 implicitHeight: Theme.touchTarget
-                onClicked: dlg.applyRequested(dlg.picked, modelField.text, keyField.text)
+                onClicked: dlg.apply()
                 background: Rectangle {
                     radius: Theme.radiusSm
                     color: applyBtn.hovered ? "#5cc0f0" : Theme.accent
@@ -358,4 +722,18 @@ Dialog {
             }
         }
     }
+
+    /* The model parameters belong to the provider that is picked, so they are
+     * applied first: useProvider() then takes over what was remembered for
+     * whichever provider it ends on. */
+    function apply() {
+        dlg.applyRequested(dlg.picked, modelField.text, keyField.text,
+                           dlg.num(tempField.text), dlg.num(topPField.text),
+                           Math.round(dlg.num(maxTokField.text)))
+    }
+
+    onOpened: {
+        keyField.text = ""
+        modelField.forceActiveFocus()
+    }
 }
diff --git a/app/src/core/AgentCore.cpp b/app/src/core/AgentCore.cpp
index 364c6a7..49aaaee 100644
--- a/app/src/core/AgentCore.cpp
+++ b/app/src/core/AgentCore.cpp
@@ -570,6 +570,7 @@ QVariantList AgentCore::providers() const {
     m["keyFrom"] = utf8(from);
     m["keyEnv"] = utf8(p->key_env);
     m["source"] = utf8(cst_provider_source(p));
+    m["custom"] = strcmp(cst_provider_source(p), "user") == 0;
     m["current"] = (m_cfg.provider[0] && strcmp(p->name, m_cfg.provider) == 0);
     out.append(m);
   }
@@ -663,10 +664,122 @@ QString AgentCore::useProvider(const QString &provider, const QString &model,
     snprintf(cfg->model, sizeof(cfg->model), "%s", mdl.constData());
     cst_settings_remember(cfg);
   }
+  /* A base URL edited in the advanced form (a custom provider, or a preset we
+   * override) takes effect without a trip through a different provider. */
+  const cst_provider_t *entry = cst_provider_find(name.constData());
+  if (entry && entry->api_base && strcmp(entry->api_base, cfg->api_base) != 0)
+    snprintf(cfg->api_base, sizeof(cfg->api_base), "%s", entry->api_base);
+  /* And the generation parameters remembered for whichever provider we ended
+   * on (cst_agent_create() does the same for the one it starts with). */
+  cst_settings_t st;
+  if (cst_settings_load(&st) == 0) {
+    const cst_gen_params_t *g = cst_settings_params(&st, cfg->provider);
+    cfg->temperature = g ? g->temperature : 0;
+    cfg->top_p = g ? g->top_p : 0;
+    cfg->max_tokens = g ? g->max_tokens : 0;
+  }
   syncConfig();
   return QString();
 }
 
+QString AgentCore::addProvider(const QString &name, const QString &base,
+                               const QString &keyEnv,
+                               const QString &defaultModel,
+                               const QString &modelsCsv) {
+  const QByteArray n = name.trimmed().toUtf8();
+  const QByteArray b = base.trimmed().toUtf8();
+  const QByteArray ke = keyEnv.trimmed().toUtf8();
+  const QByteArray dm = defaultModel.trimmed().toUtf8();
+  /* "a, b, c" -> the model list the engine will offer for it. */
+  QVector<QByteArray> list;
+  for (const QString &part : modelsCsv.split(QLatin1Char(','), Qt::SkipEmptyParts)) {
+    const QByteArray t = part.trimmed().toUtf8();
+    if (!t.isEmpty() && list.size() < 32)
+      list.append(t);
+  }
+  QVector<const char *> ptrs;
+  for (const QByteArray &p : list)
+    ptrs.append(p.constData()); /* list is complete: no reallocation after */
+  char err[512] = "";
+  if (cst_models_user_set_provider(n.constData(), b.constData(),
+                                   ke.isEmpty() ? nullptr : ke.constData(),
+                                   dm.isEmpty() ? nullptr : dm.constData(),
+                                   ptrs.isEmpty() ? nullptr : ptrs.constData(),
+                                   ptrs.size(), err, sizeof(err)) != 0)
+    return utf8(err[0] ? err : "the provider was refused");
+  /* Editing the live provider's own entry: its new base URL is live at once. */
+  if (m_agent && m_cfg.provider[0] && strcmp(m_cfg.provider, n.constData()) == 0) {
+    const cst_provider_t *p = cst_provider_find(n.constData());
+    if (p && p->api_base && strcmp(p->api_base, m_cfg.api_base) != 0) {
+      cst_agent_config_t *cfg = cst_agent_config_mut(m_agent);
+      snprintf(cfg->api_base, sizeof(cfg->api_base), "%s", p->api_base);
+      syncConfig();
+    }
+  }
+  return QString();
+}
+
+int AgentCore::removeProvider(const QString &name) {
+  const QByteArray n = name.trimmed().toUtf8();
+  if (n.isEmpty())
+    return -1;
+  char err[512] = "";
+  const int rc = cst_models_user_drop_provider(n.constData(), err, sizeof(err));
+  if (rc < 0)
+    emit statusChanged(QStringLiteral("%1: %2")
+                           .arg(name, utf8(err[0] ? err : "not removed")));
+  return rc;
+}
+
+QVariantMap AgentCore::genParams(const QString &provider) const {
+  QVariantMap m;
+  m["temperature"] = 0.0;
+  m["top_p"] = 0.0;
+  m["max_tokens"] = 0;
+  const QByteArray name = provider.trimmed().toUtf8();
+  cst_settings_t st;
+  if (cst_settings_load(&st) == 0) {
+    const cst_gen_params_t *g = cst_settings_params(&st, name.constData());
+    if (g) {
+      m["temperature"] = g->temperature;
+      m["top_p"] = g->top_p;
+      m["max_tokens"] = g->max_tokens;
+    }
+  }
+  /* The live provider's config is what actually goes out: show that, not what
+   * a file says about it. */
+  if (name.size() && m_cfg.provider[0] &&
+      strcmp(m_cfg.provider, name.constData()) == 0) {
+    m["temperature"] = m_cfg.temperature;
+    m["top_p"] = m_cfg.top_p;
+    m["max_tokens"] = m_cfg.max_tokens;
+  }
+  return m;
+}
+
+QString AgentCore::setGenParams(const QString &provider, double temperature,
+                                double topP, int maxTokens) {
+  const QByteArray name = provider.trimmed().toUtf8();
+  if (name.isEmpty())
+    return QStringLiteral("pick a provider first");
+  cst_gen_params_t g;
+  memset(&g, 0, sizeof(g));
+  snprintf(g.provider, sizeof(g.provider), "%s", name.constData());
+  g.temperature = temperature > 0 ? temperature : 0;
+  g.top_p = topP > 0 ? topP : 0;
+  g.max_tokens = maxTokens > 0 ? maxTokens : 0;
+  if (cst_settings_remember_params(name.constData(), &g) != 0)
+    return QStringLiteral("the parameters were not saved");
+  if (m_agent && strcmp(m_cfg.provider, name.constData()) == 0) {
+    cst_agent_config_t *cfg = cst_agent_config_mut(m_agent);
+    cfg->temperature = g.temperature;
+    cfg->top_p = g.top_p;
+    cfg->max_tokens = g.max_tokens;
+    syncConfig();
+  }
+  return QString();
+}
+
 QVariantMap AgentCore::usage() const {
   QVariantMap m;
   if (!m_agent)
diff --git a/app/src/core/AgentCore.h b/app/src/core/AgentCore.h
index f1179fd..b5f7e21 100644
--- a/app/src/core/AgentCore.h
+++ b/app/src/core/AgentCore.h
@@ -75,8 +75,29 @@ class AgentCore : public QObject {
 
   /* The presets: name, base, model (remembered, else the preset's), hasKey
    * and keyFrom ("saved", else the environment variable), keyEnv, source and
-   * current. Never a key: the engine keeps those out of the UI's reach. */
+   * current, custom (the user's own provider, kept in the engine's user
+   * model file). Never a key: the engine keeps those out of the UI's reach. */
   Q_INVOKABLE QVariantList providers() const;
+  /* A provider the engine's presets do not know: it goes into the user model
+   * file (<config>/cst-agent/models.json), which is where the registry keeps
+   * them, and the registry is reloaded, so the next providers() call lists
+   * it. `modelsCsv` is a comma-separated model list (may be empty).
+   * "" when it worked, else the reason. */
+  Q_INVOKABLE QString addProvider(const QString &name, const QString &base,
+                                  const QString &keyEnv,
+                                  const QString &defaultModel,
+                                  const QString &modelsCsv);
+  /* Drop a provider that came from that file (1 = it was there, 0 = it was
+   * not, -1 = with a reason). A preset of the same name stays. */
+  Q_INVOKABLE int removeProvider(const QString &name);
+  /* The generation parameters remembered for a provider (0 = not set, the
+   * field is not sent at all). */
+  Q_INVOKABLE QVariantMap genParams(const QString &provider) const;
+  /* Remember temperature / top-p / max tokens (0 = unset) for a provider and
+   * apply them to the engine when that provider is the live one.
+   * "" when it worked, else the reason. */
+  Q_INVOKABLE QString setGenParams(const QString &provider, double temperature,
+                                   double topP, int maxTokens);
   /* Model candidates for a provider: its own list, what was fetched into the
    * cache, and the remembered pick. No network. */
   Q_INVOKABLE QStringList modelChoices(const QString &provider) const;
diff --git a/app/src/ui/UiTest.cpp b/app/src/ui/UiTest.cpp
index 820016e..240f43f 100644
--- a/app/src/ui/UiTest.cpp
+++ b/app/src/ui/UiTest.cpp
@@ -338,6 +338,112 @@ int runUiTest(AgentCore &core, const QString &size) {
     check(false, QStringLiteral("the dialog has a model field"));
   }
 
+  /* --- advanced: a custom provider, added and removed ------------------- */
+  /* The run has a scratch config dir (main.cpp points XDG_CONFIG_HOME at it),
+   * so the engine's user model file is this run's own. */
+  check(core.genParams(QStringLiteral("deepseek")).value(QStringLiteral("temperature"))
+            .toDouble() == 0.0,
+        QStringLiteral("no generation parameters are saved to begin with"));
+  check(core.setGenParams(QStringLiteral("deepseek"), 0.2, 0.9, 4096).isEmpty(),
+        QStringLiteral("generation parameters are accepted"));
+  {
+    const QVariantMap g = core.genParams(QStringLiteral("deepseek"));
+    check(g.value(QStringLiteral("temperature")).toDouble() > 0.19 &&
+              g.value(QStringLiteral("max_tokens")).toInt() == 4096,
+          QStringLiteral("they are remembered per provider (%1 / %2 / %3)")
+              .arg(g.value(QStringLiteral("temperature")).toDouble())
+              .arg(g.value(QStringLiteral("top_p")).toDouble())
+              .arg(g.value(QStringLiteral("max_tokens")).toInt()));
+  }
+  check(core.addProvider(QStringLiteral("cst-scratch"), QStringLiteral("https://api.example.com/v1"),
+                         QStringLiteral("CST_SCRATCH_KEY"), QStringLiteral("scratch-1"),
+                         QStringLiteral("scratch-1, scratch-2"))
+            .isEmpty(),
+        QStringLiteral("a provider the presets do not know is added"));
+  {
+    bool found = false, custom = false;
+    QString base, models;
+    const QVariantList list = core.providers();
+    for (const QVariant &v : list) {
+      const QVariantMap p = v.toMap();
+      if (p.value(QStringLiteral("name")).toString() != QLatin1String("cst-scratch"))
+        continue;
+      found = true;
+      custom = p.value(QStringLiteral("custom")).toBool();
+      base = p.value(QStringLiteral("base")).toString();
+      models = core.modelChoices(QStringLiteral("cst-scratch")).join(QLatin1Char(','));
+    }
+    check(found && custom && base.contains(QLatin1String("api.example.com")),
+          QStringLiteral("it shows up in the provider list as a custom one"));
+    check(models.contains(QLatin1String("scratch-1")) &&
+              models.contains(QLatin1String("scratch-2")),
+          QStringLiteral("the models it lists are the picker's (%1)").arg(models));
+  }
+  check(!core.addProvider(QStringLiteral("bad name"), QStringLiteral("https://x/v1"),
+                          QString(), QString(), QString())
+             .isEmpty(),
+        QStringLiteral("a name that is not a provider name is refused"));
+  check(core.removeProvider(QStringLiteral("cst-scratch")) == 1,
+        QStringLiteral("the custom provider can be removed again"));
+  {
+    bool gone = true;
+    const QVariantList list = core.providers();
+    for (const QVariant &v : list)
+      if (v.toMap().value(QStringLiteral("name")).toString() ==
+          QLatin1String("cst-scratch"))
+        gone = false;
+    check(gone, QStringLiteral("and it is gone from the list"));
+  }
+  check(core.removeProvider(QStringLiteral("deepseek")) == 0,
+        QStringLiteral("a preset cannot be removed (there is no user entry to drop)"));
+  check(core.setGenParams(QStringLiteral("deepseek"), 0, 0, 0).isEmpty() &&
+            core.genParams(QStringLiteral("deepseek"))
+                    .value(QStringLiteral("temperature"))
+                    .toDouble() == 0.0,
+        QStringLiteral("parameters can be cleared again"));
+
+  /* The dialog drives the same two engine calls: open it, fold the advanced
+   * section out and put a provider in through the form. */
+  clickMouse(win, QStringLiteral("settingsButton"));
+  if (panel && mfield) {
+    clickMouse(win, QStringLiteral("settingsAdvanced"));
+    QQuickItem *nfield = item(win, QStringLiteral("advName"));
+    QQuickItem *bfield = item(win, QStringLiteral("advBase"));
+    check(nfield && nfield->isVisible(), QStringLiteral("Advanced options opens in the dialog"));
+    maybeShot(win, QStringLiteral("-advanced"));
+    if (nfield && bfield) {
+      nfield->setProperty("text", QStringLiteral("cst-form"));
+      bfield->setProperty("text", QStringLiteral("https://form.example.com/v1"));
+      /* The form sits at the end of the dialog's scrolling body: scroll it
+       * into view so the click lands on the button, not on the frame. */
+      QQuickItem *scroller = item(win, QStringLiteral("settingsBody"));
+      if (scroller) {
+        scroller->setProperty("contentY", scroller->property("contentHeight").toReal() -
+                                              scroller->height());
+        QTest::qWait(120); /* the flickable settles */
+      }
+      clickMouse(win, QStringLiteral("settingsSaveProvider"));
+      bool found = false;
+      const QVariantList list = core.providers();
+      for (const QVariant &v : list)
+        if (v.toMap().value(QStringLiteral("name")).toString() ==
+            QLatin1String("cst-form"))
+          found = true;
+      check(found, QStringLiteral("the form in the dialog adds a provider"));
+      QQuickItem *rm = item(win, QStringLiteral("settingsRemoveProvider"));
+      check(rm && rm->property("enabled").toBool(),
+            QStringLiteral("its entry can be removed from the form"));
+      clickMouse(win, QStringLiteral("settingsRemoveProvider"));
+      check(core.removeProvider(QStringLiteral("cst-form")) == 0,
+            QStringLiteral("the form's provider is gone again"));
+    } else {
+      check(false, QStringLiteral("the advanced form has name and base URL fields"));
+    }
+    clickMouse(win, QStringLiteral("settingsClose"));
+  } else {
+    check(false, QStringLiteral("the settings dialog can be reopened"));
+  }
+
   /* --- attachments: a file goes next to the session -------------------- */
   const QString att = core.workspace() + QStringLiteral("/note.txt");
   {
diff --git a/include/cst_agent.h b/include/cst_agent.h
index 66c7feb..baec867 100644
--- a/include/cst_agent.h
+++ b/include/cst_agent.h
@@ -89,6 +89,13 @@ typedef struct {
   cst_protocol_t protocol;
   int max_steps;        /* model calls per user turn */
   size_t tool_out_cap;  /* bytes of one tool output sent to the model */
+  /* What the model is asked to do (Chat Completions fields). 0 = the field
+   * is not sent at all and the provider's own default stands; that is what a
+   * config that was not told otherwise has, so a CLI run is unchanged.
+   * temperature and top_p are doubles, max_tokens the completion cap. */
+  double temperature;
+  double top_p;
+  int max_tokens;
   int headless;
   int confirm_shell;  /* confirm run_shell */
   int confirm_write;  /* confirm file-changing tools */
@@ -271,6 +278,9 @@ typedef struct {
   const char *cache_ttl; /* cache_control ttl (CST_CACHE_ANTHROPIC); NULL = 5m */
   int images_off; /* the model does not take images: send a note instead */
   size_t tool_out_cap;  /* wire cap for role=tool content; 0 = default */
+  double temperature;   /* 0 = not sent */
+  double top_p;         /* 0 = not sent */
+  int max_tokens;       /* 0 = not sent */
   const char *session_path; /* where stored images live; NULL = none */
   volatile int *cancel_flag; /* non-zero → abort (error "cancelled") */
   volatile int *stop_flag;   /* non-zero → stop reading, not an error */
@@ -422,6 +432,16 @@ int cst_creds_prompt_console(const char *provider, char *out, size_t out_sz);
  * <config>/settings.json. */
 #define CST_SETTINGS_MAX_MODELS 16
 #define CST_SETTINGS_MAX_TRUSTED 32
+#define CST_SETTINGS_MAX_PARAMS 16
+/* Generation parameters remembered per provider ("params" in settings.json):
+ * what the user set for that provider's models. 0 = not set, the field is
+ * not sent. */
+typedef struct {
+  char provider[32];
+  double temperature;
+  double top_p;
+  int max_tokens;
+} cst_gen_params_t;
 typedef struct {
   char provider[32];
   char effort[16];
@@ -436,10 +456,18 @@ typedef struct {
     char provider[32];
     char model[128];
   } models[CST_SETTINGS_MAX_MODELS];
+  int nparams;
+  cst_gen_params_t params[CST_SETTINGS_MAX_PARAMS];
 } cst_settings_t;
 int cst_settings_load(cst_settings_t *s);
 int cst_settings_save(const cst_settings_t *s);
 const char *cst_settings_model(const cst_settings_t *s, const char *provider);
+/* The remembered parameters of a provider, or NULL when it has none. */
+const cst_gen_params_t *cst_settings_params(const cst_settings_t *s,
+                                            const char *provider);
+/* Remember parameters for a provider; all-zero (or NULL) forgets them. */
+int cst_settings_remember_params(const char *provider,
+                                 const cst_gen_params_t *p);
 /* Record cfg's provider, model, effort and mode as the last used ones. */
 int cst_settings_remember(const cst_agent_config_t *cfg);
 /* Record an approval mode the user picked (CST_APPROVAL_*). */
diff --git a/include/cst_models.h b/include/cst_models.h
index 1c1d945..9ccb35b 100644
--- a/include/cst_models.h
+++ b/include/cst_models.h
@@ -73,6 +73,22 @@ const char *cst_provider_source(const cst_provider_t *p);
 /* The user file's path (it may not exist). */
 int cst_models_user_path(char *out, size_t out_sz);
 
+/* Add or replace a provider in the user file (layer 1 above) and reload the
+ * registry: this is how a provider the presets do not know becomes one.
+ * name is 1-31 characters of letters, digits, '.', '_' or '-'; api_base is
+ * required; key_env names the variable its key may come from; default_model
+ * and models[] are what the model picker offers (a fetched /models list still
+ * wins). An entry with a preset's name overrides that preset, everything else
+ * in the file (model rules) is kept. 0, or -1 with err filled. */
+int cst_models_user_set_provider(const char *name, const char *api_base,
+                                 const char *key_env,
+                                 const char *default_model,
+                                 const char *const *models, int nmodels,
+                                 char *err, size_t err_sz);
+/* Drop a provider from the user file (a preset of the same name stays).
+ * 1 = removed, 0 = it was not there, -1 with err. */
+int cst_models_user_drop_provider(const char *name, char *err, size_t err_sz);
+
 /* Drop everything and read the built-in data, the user file and the caches
  * again. Provider pointers stay valid but may name another provider. */
 void cst_models_reload(void);
diff --git a/include/cst_version.h b/include/cst_version.h
index 54077fc..807fdc1 100644
--- a/include/cst_version.h
+++ b/include/cst_version.h
@@ -3,7 +3,7 @@
 
 #define CST_VERSION_MAJOR 0
 #define CST_VERSION_MINOR 8
-#define CST_VERSION_PATCH 14
+#define CST_VERSION_PATCH 15
 
 #define CST_VERSION_STR_(x) #x
 #define CST_VERSION_STR(x) CST_VERSION_STR_(x)
diff --git a/src/agent_loop.c b/src/agent_loop.c
index dfaecaa..40eb0d3 100644
--- a/src/agent_loop.c
+++ b/src/agent_loop.c
@@ -1850,6 +1850,9 @@ static void fill_request(cst_agent_t *a, cst_llm_request_t *req,
   req->session_path = cst_session_path(a->session);
   req->cancel_flag = &a->cancel;
   req->tool_out_cap = a->cfg.tool_out_cap;
+  req->temperature = a->cfg.temperature;
+  req->top_p = a->cfg.top_p;
+  req->max_tokens = a->cfg.max_tokens;
   if (cst_protocol_is_native(proto)) {
     req->enable_tools = 1;
     req->tool_cfg = &a->cfg;
@@ -2538,6 +2541,24 @@ cst_agent_t *cst_agent_create(const cst_agent_config_t *cfg) {
     a->cfg.max_steps = CST_DEFAULT_MAX_STEPS;
   if (a->cfg.tool_out_cap == 0)
     a->cfg.tool_out_cap = CST_DEFAULT_TOOL_OUT_CAP;
+  /* The generation parameters the user saved for this provider (Settings ->
+   * Advanced in the app, "params" in settings.json). They are read here so
+   * that the CLI and the TUI send what the GUI was set to; a caller that
+   * named one explicitly wins. */
+  {
+    cst_settings_t st;
+    if (cst_settings_load(&st) == 0) {
+      const cst_gen_params_t *g = cst_settings_params(&st, a->cfg.provider);
+      if (g) {
+        if (a->cfg.temperature <= 0)
+          a->cfg.temperature = g->temperature;
+        if (a->cfg.top_p <= 0)
+          a->cfg.top_p = g->top_p;
+        if (a->cfg.max_tokens <= 0)
+          a->cfg.max_tokens = g->max_tokens;
+      }
+    }
+  }
   a->session = cst_session_open(cfg->session_path[0] ? cfg->session_path
                                                      : "session.jsonl");
   if (!a->session) {
diff --git a/src/llm_openai.c b/src/llm_openai.c
index 52bde6f..d65d6ba 100644
--- a/src/llm_openai.c
+++ b/src/llm_openai.c
@@ -444,6 +444,15 @@ char *cst_llm_build_body(const cst_llm_request_t *req) {
     cJSON *so = cJSON_AddObjectToObject(root, "stream_options");
     cJSON_AddTrueToObject(so, "include_usage");
   }
+  /* The generation parameters the user set (Settings -> Advanced). A field
+   * that was not set is left out: an absent field is the provider's own
+   * default, which "temperature": 0 would not be. */
+  if (req->temperature > 0)
+    cJSON_AddNumberToObject(root, "temperature", req->temperature);
+  if (req->top_p > 0)
+    cJSON_AddNumberToObject(root, "top_p", req->top_p);
+  if (req->max_tokens > 0)
+    cJSON_AddNumberToObject(root, "max_tokens", req->max_tokens);
   cJSON *msgs = cJSON_AddArrayToObject(root, "messages");
   size_t tcap = wire_tool_cap(req);
   cJSON *prev_user = NULL; /* merge consecutive user messages */
diff --git a/src/model_registry.c b/src/model_registry.c
index f562556..e48823f 100644
--- a/src/model_registry.c
+++ b/src/model_registry.c
@@ -1,9 +1,11 @@
 #include "cst_models.h"
+#include "cst_util.h"
 
 #include "cJSON.h"
 #include "os_compat.h"
 
 #include <ctype.h>
+#include <errno.h>
 #include <pthread.h>
 #include <stdio.h>
 #include <stdlib.h>
@@ -326,6 +328,162 @@ int cst_models_user_path(char *out, size_t out_sz) {
   return w > 0 && (size_t)w < out_sz ? 0 : -1;
 }
 
+/* --- the user file: providers the presets do not know -------------------- */
+
+static int valid_provider_name(const char *name) {
+  if (!name || !name[0] || strlen(name) >= 32)
+    return 0;
+  for (const char *q = name; *q; q++)
+    if (!isalnum((unsigned char)*q) && *q != '.' && *q != '_' && *q != '-')
+      return 0;
+  return 1;
+}
+
+/* The user file as a JSON object: an empty one when there is no file, NULL
+ * when there is one that is not an object (never overwrite somebody else's
+ * file, their edit is not worth losing). */
+static cJSON *read_user_doc(char *path, size_t path_sz) {
+  if (cst_models_user_path(path, path_sz) != 0)
+    return NULL;
+  char *text = read_file(path);
+  if (!text)
+    return cJSON_CreateObject();
+  cJSON *doc = cJSON_Parse(text + cst_bom_len(text, strlen(text)));
+  free(text);
+  if (!cJSON_IsObject(doc)) {
+    cJSON_Delete(doc);
+    return NULL;
+  }
+  return doc;
+}
+
+static int write_user_doc(cJSON *doc, const char *path, char *err,
+                          size_t err_sz) {
+  char *text = cJSON_Print(doc);
+  if (!text) {
+    snprintf(err, err_sz, "out of memory");
+    return -1;
+  }
+  char tmp[1200];
+  if (snprintf(tmp, sizeof(tmp), "%s.tmp", path) >= (int)sizeof(tmp)) {
+    cJSON_free(text);
+    snprintf(err, err_sz, "path too long");
+    return -1;
+  }
+  int rc = -1;
+  FILE *f = fopen(tmp, "wb");
+  if (!f) {
+    snprintf(err, err_sz, "%s: cannot write (%s)", tmp, strerror(errno));
+  } else {
+    const int ok = fputs(text, f) >= 0 && fputc('\n', f) != EOF;
+    const int closed = fclose(f);
+    if (ok && closed == 0 &&
+#ifdef _WIN32
+        (remove(path), 1) &&
+#endif
+        rename(tmp, path) == 0)
+      rc = 0;
+    else
+      snprintf(err, err_sz, "%s: cannot replace (%s)", path, strerror(errno));
+    if (rc != 0)
+      remove(tmp);
+  }
+  cJSON_free(text);
+  return rc;
+}
+
+int cst_models_user_set_provider(const char *name, const char *api_base,
+                                 const char *key_env, const char *default_model,
+                                 const char *const *models, int nmodels,
+                                 char *err, size_t err_sz) {
+  char dummy[8];
+  if (!err || !err_sz) {
+    err = dummy;
+    err_sz = sizeof(dummy);
+  }
+  err[0] = 0;
+  if (!valid_provider_name(name)) {
+    snprintf(err, err_sz,
+             "a provider name is 1-31 of letters, digits, '.', '_' or '-'");
+    return -1;
+  }
+  if (!api_base || !api_base[0]) {
+    snprintf(err, err_sz, "a base URL is needed");
+    return -1;
+  }
+  char path[1100];
+  pthread_mutex_lock(&g_mu);
+  cJSON *doc = read_user_doc(path, sizeof(path));
+  if (!doc) {
+    snprintf(err, err_sz, "%s: not a JSON object, left alone", path);
+    pthread_mutex_unlock(&g_mu);
+    return -1;
+  }
+  cJSON *providers = cJSON_GetObjectItemCaseSensitive(doc, "providers");
+  if (!cJSON_IsObject(providers))
+    providers = cJSON_AddObjectToObject(doc, "providers");
+  cJSON *entry = cJSON_CreateObject();
+  cJSON_AddStringToObject(entry, "api_base", api_base);
+  if (key_env && key_env[0])
+    cJSON_AddStringToObject(entry, "key_env", key_env);
+  if (default_model && default_model[0])
+    cJSON_AddStringToObject(entry, "default_model", default_model);
+  if (models && nmodels > 0) {
+    cJSON *arr = cJSON_AddArrayToObject(entry, "models");
+    for (int i = 0; i < nmodels; i++)
+      if (models[i] && models[i][0])
+        cJSON_AddItemToArray(arr, cJSON_CreateString(models[i]));
+  }
+  cJSON_DeleteItemFromObjectCaseSensitive(providers, name);
+  cJSON_AddItemToObject(providers, name, entry);
+  const int rc = write_user_doc(doc, path, err, err_sz);
+  cJSON_Delete(doc);
+  pthread_mutex_unlock(&g_mu);
+  if (rc == 0)
+    cst_models_reload(); /* the provider is there from now on */
+  return rc;
+}
+
+int cst_models_user_drop_provider(const char *name, char *err, size_t err_sz) {
+  char dummy[8];
+  if (!err || !err_sz) {
+    err = dummy;
+    err_sz = sizeof(dummy);
+  }
+  err[0] = 0;
+  if (!valid_provider_name(name)) {
+    snprintf(err, err_sz, "not a provider name");
+    return -1;
+  }
+  char path[1100];
+  if (cst_models_user_path(path, sizeof(path)) != 0) {
+    snprintf(err, err_sz, "no config directory");
+    return -1;
+  }
+  FILE *probe = fopen(path, "rb");
+  if (!probe)
+    return 0; /* no user file: nothing of ours to drop */
+  fclose(probe);
+  pthread_mutex_lock(&g_mu);
+  cJSON *doc = read_user_doc(path, sizeof(path));
+  if (!doc) {
+    snprintf(err, err_sz, "%s: not a JSON object, left alone", path);
+    pthread_mutex_unlock(&g_mu);
+    return -1;
+  }
+  cJSON *providers = cJSON_GetObjectItemCaseSensitive(doc, "providers");
+  const int found = cJSON_IsObject(providers) &&
+                    cJSON_GetObjectItemCaseSensitive(providers, name) != NULL;
+  if (found)
+    cJSON_DeleteItemFromObjectCaseSensitive(providers, name);
+  const int rc = found ? write_user_doc(doc, path, err, err_sz) : 0;
+  cJSON_Delete(doc);
+  pthread_mutex_unlock(&g_mu);
+  if (rc == 0 && found)
+    cst_models_reload();
+  return rc == 0 ? found : -1;
+}
+
 static void load_default_locked(void) {
   reset_locked();
   char e[128];
diff --git a/src/settings.c b/src/settings.c
index f198630..3710fce 100644
--- a/src/settings.c
+++ b/src/settings.c
@@ -72,6 +72,25 @@ int cst_settings_load(cst_settings_t *s) {
              m);
     s->nmodels++;
   }
+  const cJSON *params = cJSON_GetObjectItemCaseSensitive(root, "params");
+  const cJSON *p;
+  cJSON_ArrayForEach(p, params) {
+    if (s->nparams >= CST_SETTINGS_MAX_PARAMS || !p->string || !cJSON_IsObject(p))
+      continue;
+    cst_gen_params_t *g = &s->params[s->nparams];
+    memset(g, 0, sizeof(*g));
+    snprintf(g->provider, sizeof(g->provider), "%s", p->string);
+    const cJSON *v = cJSON_GetObjectItemCaseSensitive(p, "temperature");
+    if (cJSON_IsNumber(v))
+      g->temperature = v->valuedouble;
+    v = cJSON_GetObjectItemCaseSensitive(p, "top_p");
+    if (cJSON_IsNumber(v))
+      g->top_p = v->valuedouble;
+    v = cJSON_GetObjectItemCaseSensitive(p, "max_tokens");
+    if (cJSON_IsNumber(v))
+      g->max_tokens = (int)v->valuedouble;
+    s->nparams++;
+  }
   cJSON_Delete(root);
   return 0;
 }
@@ -102,6 +121,21 @@ int cst_settings_save(const cst_settings_t *s) {
   cJSON *models = cJSON_AddObjectToObject(root, "models");
   for (int i = 0; i < s->nmodels; i++)
     cJSON_AddStringToObject(models, s->models[i].provider, s->models[i].model);
+  if (s->nparams) {
+    cJSON *params = cJSON_AddObjectToObject(root, "params");
+    for (int i = 0; i < s->nparams; i++) {
+      const cst_gen_params_t *g = &s->params[i];
+      if (!g->provider[0])
+        continue;
+      cJSON *o = cJSON_AddObjectToObject(params, g->provider);
+      if (g->temperature > 0)
+        cJSON_AddNumberToObject(o, "temperature", g->temperature);
+      if (g->top_p > 0)
+        cJSON_AddNumberToObject(o, "top_p", g->top_p);
+      if (g->max_tokens > 0)
+        cJSON_AddNumberToObject(o, "max_tokens", g->max_tokens);
+    }
+  }
   char *text = cJSON_Print(root);
   cJSON_Delete(root);
   if (!text)
@@ -180,6 +214,65 @@ int cst_settings_remember(const cst_agent_config_t *cfg) {
   snprintf(s.models[i].provider, sizeof(s.models[i].provider), "%s",
            cfg->provider);
   snprintf(s.models[i].model, sizeof(s.models[i].model), "%s", cfg->model);
+  /* The generation parameters ride along with the provider choice. An
+   * all-zero config (the CLI, the TUI) leaves what the GUI saved alone. */
+  if (cfg->temperature > 0 || cfg->top_p > 0 || cfg->max_tokens > 0) {
+    int pi = 0;
+    while (pi < s.nparams && strcmp(s.params[pi].provider, cfg->provider) != 0)
+      pi++;
+    if (pi == s.nparams) {
+      if (s.nparams >= CST_SETTINGS_MAX_PARAMS)
+        return -1;
+      memset(&s.params[pi], 0, sizeof(s.params[pi]));
+      snprintf(s.params[pi].provider, sizeof(s.params[pi].provider), "%s",
+               cfg->provider);
+      s.nparams++;
+    }
+    s.params[pi].temperature = cfg->temperature;
+    s.params[pi].top_p = cfg->top_p;
+    s.params[pi].max_tokens = cfg->max_tokens;
+  }
+  return cst_settings_save(&s);
+}
+
+const cst_gen_params_t *cst_settings_params(const cst_settings_t *s,
+                                            const char *provider) {
+  for (int i = 0; s && provider && provider[0] && i < s->nparams; i++)
+    if (strcmp(s->params[i].provider, provider) == 0)
+      return &s->params[i];
+  return NULL;
+}
+
+int cst_settings_remember_params(const char *provider,
+                                 const cst_gen_params_t *p) {
+  if (!provider || !provider[0])
+    return -1;
+  cst_settings_t s;
+  cst_settings_load(&s);
+  int i = 0;
+  while (i < s.nparams && strcmp(s.params[i].provider, provider) != 0)
+    i++;
+  const int forget = !p || (p->temperature <= 0 && p->top_p <= 0 &&
+                            p->max_tokens <= 0);
+  if (i == s.nparams) {
+    if (forget)
+      return 0; /* nothing remembered, nothing to forget */
+    if (s.nparams >= CST_SETTINGS_MAX_PARAMS)
+      return -1;
+    memset(&s.params[i], 0, sizeof(s.params[i]));
+    snprintf(s.params[i].provider, sizeof(s.params[i].provider), "%s",
+             provider);
+    s.nparams++;
+  }
+  if (forget) { /* keep the array compact when an entry goes away */
+    memmove(&s.params[i], &s.params[i + 1],
+            sizeof(s.params[i]) * (s.nparams - i - 1));
+    s.nparams--;
+  } else {
+    s.params[i].temperature = p->temperature;
+    s.params[i].top_p = p->top_p;
+    s.params[i].max_tokens = p->max_tokens;
+  }
   return cst_settings_save(&s);
 }
 
diff --git a/tests/test_llm_body.c b/tests/test_llm_body.c
index d8decec..afcb11d 100644
--- a/tests/test_llm_body.c
+++ b/tests/test_llm_body.c
@@ -75,6 +75,40 @@ static void test_escaping(void) {
   cJSON_Delete(j);
 }
 
+/* Settings -> Advanced: the generation parameters go on the wire only when
+ * they were set. An absent field is the provider's own default, which
+ * "temperature": 0 would not be. */
+static void test_gen_params(void) {
+  cst_message_t m[1] = {msg(CST_ROLE_USER, "hi")};
+  cst_llm_request_t r;
+  memset(&r, 0, sizeof(r));
+  r.model = "test-model";
+  r.msgs = m;
+  r.nmsgs = 1;
+  r.temperature = 0.2;
+  r.top_p = 0.9;
+  r.max_tokens = 4096;
+  char *body = cst_llm_build_body(&r);
+  EXPECT(body != NULL, "body with parameters");
+  cJSON *j = body ? cJSON_Parse(body) : NULL;
+  free(body);
+  const cJSON *t = cJSON_GetObjectItemCaseSensitive(j, "temperature");
+  const cJSON *p = cJSON_GetObjectItemCaseSensitive(j, "top_p");
+  const cJSON *mt = cJSON_GetObjectItemCaseSensitive(j, "max_tokens");
+  EXPECT(cJSON_IsNumber(t) && t->valuedouble > 0.19 && t->valuedouble < 0.21,
+         "temperature is sent as a number");
+  EXPECT(cJSON_IsNumber(p) && cJSON_IsNumber(mt) && mt->valueint == 4096,
+         "top_p and max_tokens are sent");
+  cJSON_Delete(j);
+  /* unset: none of the three is in the body at all */
+  cJSON *plain = build(m, 1, 0, 0);
+  EXPECT(!cJSON_GetObjectItemCaseSensitive(plain, "temperature") &&
+             !cJSON_GetObjectItemCaseSensitive(plain, "top_p") &&
+             !cJSON_GetObjectItemCaseSensitive(plain, "max_tokens"),
+         "an unset parameter is not sent at all");
+  cJSON_Delete(plain);
+}
+
 static void test_invalid_utf8(void) {
   const char text[] = "ok \xff\xfe bad \xe4\xb8 trunc";
   cst_message_t m[1] = {msg(CST_ROLE_USER, text)};
@@ -526,6 +560,7 @@ int main(void) {
   test_reasoning_and_prefix();
   test_cache_hints();
   test_tools_snapshot();
+  test_gen_params();
   test_escaping();
   test_invalid_utf8();
   test_tool_calls_and_null_content();