agent: Make assistant panel input size configurable (#37975)

Release Notes:

- Added the `agent. message_editor_min_lines `setting to allow users to
customize the agent panel message editor default size by using a
different minimum number of lines.

<img width="800" height="1316" alt="Screenshot 2025-09-11 at 5 47 18 pm"
src="https://github.com/user-attachments/assets/20990b90-c4f9-4f5c-af59-76358642a273"
/>

---------

Co-authored-by: Danilo Leal <daniloleal09@gmail.com>

assets/settings/default.json

@@ -914,7 +914,11 @@
     /// Whether to have terminal cards in the agent panel expanded, showing the whole command output.
     ///
     /// Default: true
-    "expand_terminal_card": true
+    "expand_terminal_card": true,
+    // Minimum number of lines to display in the agent message editor.
+    //
+    // Default: 4
+    "message_editor_min_lines": 4
   },
   // The settings for slash commands.
   "slash_commands": {

crates/agent_settings/src/agent_settings.rs

@@ -75,6 +75,7 @@ pub struct AgentSettings {
     pub expand_edit_card: bool,
     pub expand_terminal_card: bool,
     pub use_modifier_to_send: bool,
+    pub message_editor_min_lines: usize,
 }
 
 impl AgentSettings {
@@ -107,6 +108,10 @@ impl AgentSettings {
             model,
         });
     }
+
+    pub fn set_message_editor_max_lines(&self) -> usize {
+        self.message_editor_min_lines * 2
+    }
 }
 
 #[derive(Clone, Debug, Serialize, Deserialize, JsonSchema, PartialEq)]
@@ -320,6 +325,10 @@ pub struct AgentSettingsContent {
     ///
     /// Default: false
     use_modifier_to_send: Option<bool>,
+    /// Minimum number of lines of height the agent message editor should have.
+    ///
+    /// Default: 4
+    message_editor_min_lines: Option<usize>,
 }
 
 #[derive(Clone, Copy, Debug, Serialize, Deserialize, JsonSchema, PartialEq, Default)]
@@ -472,6 +481,10 @@ impl Settings for AgentSettings {
                 &mut settings.use_modifier_to_send,
                 value.use_modifier_to_send,
             );
+            merge(
+                &mut settings.message_editor_min_lines,
+                value.message_editor_min_lines,
+            );
 
             settings
                 .model_parameters

crates/agent_ui/src/acp/thread_view.rs

@@ -71,9 +71,6 @@ use crate::{
     RejectOnce, ToggleBurnMode, ToggleProfileSelector,
 };
 
-pub const MIN_EDITOR_LINES: usize = 4;
-pub const MAX_EDITOR_LINES: usize = 8;
-
 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
 enum ThreadFeedback {
     Positive,
@@ -357,8 +354,8 @@ impl AcpThreadView {
                 agent.name(),
                 &placeholder,
                 editor::EditorMode::AutoHeight {
-                    min_lines: MIN_EDITOR_LINES,
-                    max_lines: Some(MAX_EDITOR_LINES),
+                    min_lines: AgentSettings::get_global(cx).message_editor_min_lines,
+                    max_lines: Some(AgentSettings::get_global(cx).set_message_editor_max_lines()),
                 },
                 window,
                 cx,
@@ -857,10 +854,11 @@ impl AcpThreadView {
                     cx,
                 )
             } else {
+                let agent_settings = AgentSettings::get_global(cx);
                 editor.set_mode(
                     EditorMode::AutoHeight {
-                        min_lines: MIN_EDITOR_LINES,
-                        max_lines: Some(MAX_EDITOR_LINES),
+                        min_lines: agent_settings.message_editor_min_lines,
+                        max_lines: Some(agent_settings.set_message_editor_max_lines()),
                     },
                     cx,
                 )

docs/src/ai/agent-settings.md

@@ -170,6 +170,21 @@ The default value is `false`.
 
 > This setting is available via the Agent Panel's settings UI.
 
+### Message Editor Size
+
+Use the `message_editor_min_lines` setting to control minimum number of lines of height the agent message editor should have.
+It is set to `4` by default, and the max number of lines is always double of the minimum.
+
+```json
+{
+  "agent": {
+    "message_editor_min_lines": 4
+  }
+}
+```
+
+> This setting is currently available only in Preview.
+
 ### Modifier to Send
 
 Make a modifier (`cmd` on macOS, `ctrl` on Linux) required to send messages.