add --fork-session support to TUI command

- Add fork option to Args interface
- Add --fork-session flag to TUI command (thread.ts)
- Add validation that --fork-session requires --continue or --session
- Fork session before navigating when --fork-session is set

packages/opencode/src/cli/cmd/tui/app.tsx

@@ -248,10 +248,18 @@ function App() {
         local.model.set({ providerID, modelID }, { recent: true })
       }
       if (args.sessionID) {
-        route.navigate({
-          type: "session",
-          sessionID: args.sessionID,
-        })
+        if (args.fork) {
+          sdk.client.session.fork({ sessionID: args.sessionID }).then((result) => {
+            if (result.data?.id) {
+              route.navigate({ type: "session", sessionID: result.data.id })
+            }
+          })
+        } else {
+          route.navigate({
+            type: "session",
+            sessionID: args.sessionID,
+          })
+        }
       }
     })
   })
@@ -265,7 +273,15 @@ function App() {
       .find((x) => x.parentID === undefined)?.id
     if (match) {
       continued = true
-      route.navigate({ type: "session", sessionID: match })
+      if (args.fork) {
+        sdk.client.session.fork({ sessionID: match }).then((result) => {
+          if (result.data?.id) {
+            route.navigate({ type: "session", sessionID: result.data.id })
+          }
+        })
+      } else {
+        route.navigate({ type: "session", sessionID: match })
+      }
     }
   })
 

packages/opencode/src/cli/cmd/tui/context/args.tsx

@@ -6,6 +6,7 @@ export interface Args {
   prompt?: string
   continue?: boolean
   sessionID?: string
+  fork?: boolean
 }
 
 export const { use: useArgs, provider: ArgsProvider } = createSimpleContext({

packages/opencode/src/cli/cmd/tui/thread.ts

@@ -64,6 +64,11 @@ export const TuiThreadCommand = cmd({
         type: "string",
         describe: "session id to continue",
       })
+      .option("fork-session", {
+        alias: ["fork"],
+        describe: "fork the session before continuing (requires --continue or --session)",
+        type: "boolean",
+      })
       .option("prompt", {
         type: "string",
         describe: "prompt to use",
@@ -73,6 +78,11 @@ export const TuiThreadCommand = cmd({
         describe: "agent to use",
       }),
   handler: async (args) => {
+    if (args.forkSession && !args.continue && !args.session) {
+      UI.error("--fork-session requires --continue or --session")
+      process.exit(1)
+    }
+
     // Resolve relative paths against PWD to preserve behavior when using --cwd flag
     const baseCwd = process.env.PWD ?? process.cwd()
     const cwd = args.project ? path.resolve(baseCwd, args.project) : process.cwd()
@@ -150,6 +160,7 @@ export const TuiThreadCommand = cmd({
         agent: args.agent,
         model: args.model,
         prompt,
+        fork: args.forkSession,
       },
       onExit: async () => {
         await client.call("shutdown", undefined)