agent: Add expand_code_block setting

When false, fenced code blocks in agent messages start collapsed and can
be expanded individually. Complements expand_edit_card and
expand_terminal_card. Tool output and compaction summaries are
unaffected.

Addresses #58333

Author: cachebag

assets/settings/default.json

@@ -1362,6 +1362,11 @@
     //
     // Default: true
     "expand_terminal_card": true,
+    // Whether to have code blocks in agent messages expanded, showing the whole code.
+    // When disabled, code blocks start collapsed and can be expanded individually.
+    //
+    // Default: true
+    "expand_code_block": true,
     // Command to automatically run when Zed creates a Terminal Thread shell in the agent panel.
     // The command is sent to the shell as if typed, so it is interpreted by your
     // configured shell (including on Windows and remote/WSL projects).

crates/agent/src/tool_permissions.rs

@@ -600,6 +600,7 @@ mod tests {
             enable_feedback: false,
             expand_edit_card: true,
             expand_terminal_card: true,
+            expand_code_block: true,
             terminal_init_command: None,
             cancel_generation_on_terminal_stop: true,
             use_modifier_to_send: true,

crates/agent_settings/src/agent_settings.rs

@@ -240,6 +240,7 @@ pub struct AgentSettings {
     pub enable_feedback: bool,
     pub expand_edit_card: bool,
     pub expand_terminal_card: bool,
+    pub expand_code_block: bool,
     pub terminal_init_command: Option<String>,
     pub thinking_display: ThinkingBlockDisplay,
     pub cancel_generation_on_terminal_stop: bool,
@@ -833,6 +834,7 @@ impl Settings for AgentSettings {
             enable_feedback: agent.enable_feedback.unwrap(),
             expand_edit_card: agent.expand_edit_card.unwrap(),
             expand_terminal_card: agent.expand_terminal_card.unwrap(),
+            expand_code_block: agent.expand_code_block.unwrap(),
             terminal_init_command: agent
                 .terminal_init_command
                 .filter(|command| !command.trim().is_empty()),

crates/agent_ui/src/agent_ui.rs

@@ -1002,6 +1002,7 @@ mod tests {
             enable_feedback: false,
             expand_edit_card: true,
             expand_terminal_card: true,
+            expand_code_block: true,
             terminal_init_command: None,
             cancel_generation_on_terminal_stop: true,
             use_modifier_to_send: true,

crates/agent_ui/src/conversation_view/thread_view.rs

@@ -3955,6 +3955,7 @@ impl ThreadView {
                                                     content.as_view(),
                                                     None,
                                                     true,
+                                                    false,
                                                     window,
                                                     cx,
                                                 )
@@ -7385,14 +7386,22 @@ impl ThreadView {
         window: &Window,
         cx: &Context<Self>,
     ) -> Div {
+        let collapse_code_blocks = !AgentSettings::get_global(cx).expand_code_block;
         v_flex().w_full().gap_3().children(
             content
                 .blocks()
                 .enumerate()
                 .filter(|(_, block)| block.visible_content(cx))
                 .map(|(block_ix, block)| {
                     let content = self.render_output_content_block(
-                        entry_ix, block_ix, block, None, false, window, cx,
+                        entry_ix,
+                        block_ix,
+                        block,
+                        None,
+                        false,
+                        collapse_code_blocks,
+                        window,
+                        cx,
                     );
                     div()
                         .id(("message-content-block", block_ix))
@@ -10189,6 +10198,7 @@ impl ThreadView {
                 content.as_view(),
                 Some(tool_call),
                 card_layout,
+                false,
                 window,
                 cx,
             ),
@@ -10215,6 +10225,7 @@ impl ThreadView {
         content: acp_thread::ContentBlockView<'_>,
         tool_call: Option<&ToolCall>,
         card_layout: bool,
+        collapse_code_blocks: bool,
         window: &Window,
         cx: &Context<Self>,
     ) -> AnyElement {
@@ -10235,6 +10246,7 @@ impl ThreadView {
                     MarkdownStyle::themed(MarkdownFont::Agent, window, cx),
                     cx,
                 )
+                .collapse_code_blocks(collapse_code_blocks)
                 .into_any()
             }
         } else if let Some((resource, _)) = content.embedded_resource() {

crates/settings_content/src/agent.rs

@@ -353,6 +353,11 @@ pub struct AgentSettingsContent {
     ///
     /// Default: true
     pub expand_terminal_card: Option<bool>,
+    /// Whether to have code blocks in agent messages expanded, showing the whole code.
+    /// When disabled, code blocks start collapsed and can be expanded individually.
+    ///
+    /// Default: true
+    pub expand_code_block: Option<bool>,
     /// Command to automatically run when Zed creates a Terminal Thread shell in the agent panel.
     /// The command is sent to the shell as if typed, so it is interpreted by your
     /// configured shell (including on Windows and remote/WSL projects).

crates/settings_ui/src/page_data.rs

@@ -8962,6 +8962,25 @@ fn ai_page(cx: &App) -> SettingsPage {
                 metadata: None,
                 files: USER,
             }),
+            SettingsPageItem::SettingItem(SettingItem {
+                title: "Expand Code Block",
+                description: "Whether to have code blocks in agent messages expanded, showing the whole code. When disabled, code blocks start collapsed and can be expanded individually.",
+                field: Box::new(SettingField {
+                    organization_override: None,
+                    json_path: Some("agent.expand_code_block"),
+                    pick: |settings_content| {
+                        settings_content.agent.as_ref()?.expand_code_block.as_ref()
+                    },
+                    write: |settings_content, value, _| {
+                        settings_content
+                            .agent
+                            .get_or_insert_default()
+                            .expand_code_block = value;
+                    },
+                }),
+                metadata: None,
+                files: USER,
+            }),
             SettingsPageItem::SettingItem(SettingItem {
                 title: "Terminal Thread Init Command",
                 description: "Command to automatically run when Zed creates a Terminal Thread shell in the agent panel. Runs in your configured shell.",