feat: expose prompt caching TTL setting in UI (#273)

Adds a dropdown to select ENABLE_PROMPT_CACHING_1H or FORCE_PROMPT_CACHING_5M env vars when starting Claude Code sessions. Requires Claude Code v2.1.108+.

src-tauri/src/config.rs

@@ -75,6 +75,11 @@ pub struct ClaudeStartOptions {
     /// Valid values: "low", "medium", "high", "xhigh" (Opus 4.7 only), "max". None uses CLI default.
     #[serde(default)]
     pub effort_level: Option<String>,
+
+    /// Sets `ENABLE_PROMPT_CACHING_1H=1` or `FORCE_PROMPT_CACHING_5M=1` env var (v2.1.108+).
+    /// Values: "1h" → 1-hour TTL, "5m" → force 5-minute TTL. None uses CLI default.
+    #[serde(default)]
+    pub prompt_caching_ttl: Option<String>,
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize)]
@@ -246,6 +251,11 @@ pub struct HikariConfig {
     /// Valid values: "low", "medium", "high", "xhigh" (Opus 4.7 only), "max". None uses CLI default.
     #[serde(default)]
     pub effort_level: Option<String>,
+
+    /// Sets `ENABLE_PROMPT_CACHING_1H=1` or `FORCE_PROMPT_CACHING_5M=1` env var (v2.1.108+).
+    /// Values: "1h" → 1-hour TTL, "5m" → force 5-minute TTL. None uses CLI default.
+    #[serde(default)]
+    pub prompt_caching_ttl: Option<String>,
 }
 
 impl Default for HikariConfig {
@@ -302,6 +312,7 @@ impl Default for HikariConfig {
             show_clear_context_on_plan_accept: true,
             custom_model_option: None,
             effort_level: None,
+            prompt_caching_ttl: None,
         }
     }
 }
@@ -520,6 +531,7 @@ mod tests {
             show_clear_context_on_plan_accept: true,
             custom_model_option: None,
             effort_level: None,
+            prompt_caching_ttl: None,
         };
 
         let json = serde_json::to_string(&config).unwrap();

src-tauri/src/wsl_bridge.rs

@@ -410,6 +410,15 @@ impl WslBridge {
                 }
             }
 
+            // Set prompt caching TTL env var if specified (v2.1.108+)
+            if let Some(ref ttl) = options.prompt_caching_ttl {
+                match ttl.as_str() {
+                    "1h" => { cmd.env("ENABLE_PROMPT_CACHING_1H", "1"); }
+                    "5m" => { cmd.env("FORCE_PROMPT_CACHING_5M", "1"); }
+                    _ => {}
+                }
+            }
+
             cmd
         } else {
             // Running on Windows - use wsl with bash login shell to ensure PATH is loaded
@@ -485,6 +494,15 @@ impl WslBridge {
                 }
             }
 
+            // Set prompt caching TTL env var if specified (v2.1.108+)
+            if let Some(ref ttl) = options.prompt_caching_ttl {
+                match ttl.as_str() {
+                    "1h" => { claude_cmd.push_str("ENABLE_PROMPT_CACHING_1H=1 "); }
+                    "5m" => { claude_cmd.push_str("FORCE_PROMPT_CACHING_5M=1 "); }
+                    _ => {}
+                }
+            }
+
             claude_cmd.push_str(
                 "claude --output-format stream-json --input-format stream-json --verbose",
             );

src/lib/commands/slashCommands.ts

@@ -78,6 +78,7 @@ async function changeDirectory(path: string): Promise<void> {
         show_clear_context_on_plan_accept: config.show_clear_context_on_plan_accept ?? true,
         custom_model_option: config.custom_model_option || null,
         effort_level: config.effort_level || null,
+        prompt_caching_ttl: config.prompt_caching_ttl || null,
       },
     });
 
@@ -166,6 +167,7 @@ async function startNewConversation(): Promise<void> {
         show_clear_context_on_plan_accept: config.show_clear_context_on_plan_accept ?? true,
         custom_model_option: config.custom_model_option || null,
         effort_level: config.effort_level || null,
+        prompt_caching_ttl: config.prompt_caching_ttl || null,
       },
     });
 

src/lib/components/ConfigSidebar.svelte

@@ -79,6 +79,7 @@
     task_loop_commit_prefix: "feat",
     task_loop_include_summary: false,
     effort_level: null,
+    prompt_caching_ttl: null,
   });
 
   let showCustomThemeEditor = $state(false);
@@ -740,6 +741,26 @@
         </p>
       </div>
 
+      <!-- Prompt Caching TTL -->
+      <div class="mb-4">
+        <label class="block text-sm text-[var(--text-primary)] mb-1" for="prompt-caching-ttl"
+          >Prompt caching TTL</label
+        >
+        <select
+          id="prompt-caching-ttl"
+          bind:value={config.prompt_caching_ttl}
+          class="w-full px-3 py-2 rounded border border-[var(--border-color)] bg-[var(--bg-primary)] text-[var(--text-primary)] text-sm focus:outline-none focus:ring-1 focus:ring-[var(--accent-primary)]"
+        >
+          <option value={null}>Default (CLI decides)</option>
+          <option value="1h">1-hour TTL (reduces cost for long sessions)</option>
+          <option value="5m">Force 5-minute TTL</option>
+        </select>
+        <p class="text-xs text-[var(--text-tertiary)] mt-1">
+          Sets <code class="font-mono">ENABLE_PROMPT_CACHING_1H</code> or
+          <code class="font-mono">FORCE_PROMPT_CACHING_5M</code>. Requires Claude Code v2.1.108+.
+        </p>
+      </div>
+
       <!-- Bare Mode -->
       <div class="mb-4">
         <label class="flex items-center gap-3 cursor-pointer">

src/lib/components/ElicitationModal.svelte

@@ -79,6 +79,7 @@
           show_clear_context_on_plan_accept: config.show_clear_context_on_plan_accept ?? true,
           custom_model_option: config.custom_model_option || null,
           effort_level: config.effort_level || null,
+          prompt_caching_ttl: config.prompt_caching_ttl || null,
         },
       });
 

src/lib/components/InputBar.svelte

@@ -413,6 +413,7 @@ User: ${formattedMessage}`;
               show_clear_context_on_plan_accept: config.show_clear_context_on_plan_accept ?? true,
               custom_model_option: config.custom_model_option || null,
               effort_level: config.effort_level || null,
+              prompt_caching_ttl: config.prompt_caching_ttl || null,
             },
           });
 

src/lib/components/PermissionModal.svelte

@@ -100,6 +100,7 @@
           show_clear_context_on_plan_accept: config.show_clear_context_on_plan_accept ?? true,
           custom_model_option: config.custom_model_option || null,
           effort_level: config.effort_level || null,
+          prompt_caching_ttl: config.prompt_caching_ttl || null,
         },
       });
 

src/lib/components/StatusBar.svelte

@@ -98,6 +98,7 @@
     show_clear_context_on_plan_accept: true,
     custom_model_option: null,
     effort_level: null,
+    prompt_caching_ttl: null,
   });
 
   let streamerModeActive = $state(false);
@@ -191,6 +192,7 @@
             currentConfig.show_clear_context_on_plan_accept ?? true,
           custom_model_option: currentConfig.custom_model_option || null,
           effort_level: currentConfig.effort_level || null,
+          prompt_caching_ttl: currentConfig.prompt_caching_ttl || null,
         },
       });
 
@@ -360,6 +362,7 @@
             currentConfig.show_clear_context_on_plan_accept ?? true,
           custom_model_option: currentConfig.custom_model_option || null,
           effort_level: currentConfig.effort_level || null,
+          prompt_caching_ttl: currentConfig.prompt_caching_ttl || null,
         },
       });
 

src/lib/components/TaskLoopPanel.svelte

@@ -229,6 +229,7 @@
           show_clear_context_on_plan_accept: cfg.show_clear_context_on_plan_accept ?? true,
           custom_model_option: cfg.custom_model_option || null,
           effort_level: cfg.effort_level || null,
+          prompt_caching_ttl: cfg.prompt_caching_ttl || null,
         },
       });
     } catch (error) {

src/lib/components/UserQuestionModal.svelte

@@ -119,6 +119,7 @@
           show_clear_context_on_plan_accept: config.show_clear_context_on_plan_accept ?? true,
           custom_model_option: config.custom_model_option || null,
           effort_level: config.effort_level || null,
+          prompt_caching_ttl: config.prompt_caching_ttl || null,
         },
       });
 

src/lib/stores/config.test.ts

@@ -230,6 +230,7 @@ describe("config store", () => {
         show_clear_context_on_plan_accept: true,
         custom_model_option: null,
         effort_level: null,
+        prompt_caching_ttl: null,
       };
 
       expect(config.model).toBe("claude-sonnet-4");
@@ -299,6 +300,7 @@ describe("config store", () => {
         show_clear_context_on_plan_accept: true,
         custom_model_option: null,
         effort_level: null,
+        prompt_caching_ttl: null,
       };
 
       expect(config.model).toBeNull();
@@ -923,6 +925,7 @@ describe("config store", () => {
         show_clear_context_on_plan_accept: true,
         custom_model_option: null,
         effort_level: null,
+        prompt_caching_ttl: null,
       };
 
       const mockInvokeImpl = vi.mocked(invoke);

src/lib/stores/config.ts

@@ -101,6 +101,8 @@ export interface HikariConfig {
   custom_model_option: string | null;
   // Effort level for Claude Code (v2.1.111+) — null means use CLI default
   effort_level: string | null;
+  // Prompt caching TTL override (v2.1.108+) — null means use CLI default
+  prompt_caching_ttl: string | null;
 }
 
 const defaultConfig: HikariConfig = {
@@ -164,6 +166,7 @@ const defaultConfig: HikariConfig = {
   show_clear_context_on_plan_accept: true,
   custom_model_option: null,
   effort_level: null,
+  prompt_caching_ttl: null,
 };
 
 function createConfigStore() {