From 3997d3f2d7d663f5a8f84a9e26c17d4913304acb Mon Sep 17 00:00:00 2001
From: Dax <mail@thdxr.com>
Date: Tue, 13 Jan 2026 15:55:48 -0500
Subject: feat: add plan mode with enter/exit tools (#8281)

---
 packages/opencode/src/agent/agent.ts               |   6 +-
 .../src/cli/cmd/tui/routes/session/index.tsx       |  17 +++
 .../src/cli/cmd/tui/routes/session/question.tsx    |  61 ++++-----
 packages/opencode/src/flag/flag.ts                 |   1 +
 packages/opencode/src/question/index.ts            |   1 +
 packages/opencode/src/session/index.ts             |   9 ++
 packages/opencode/src/session/prompt.ts            | 143 ++++++++++++++++++---
 .../opencode/src/session/prompt/build-switch.txt   |   2 +
 packages/opencode/src/tool/plan-enter.txt          |  14 ++
 packages/opencode/src/tool/plan-exit.txt           |  13 ++
 packages/opencode/src/tool/plan.ts                 | 130 +++++++++++++++++++
 packages/opencode/src/tool/registry.ts             |   2 +
 packages/opencode/test/util/lock.test.ts           |  72 +++++++++++
 packages/sdk/js/src/v2/gen/types.gen.ts            |   5 +
 packages/util/src/slug.ts                          |  74 +++++++++++
 15 files changed, 504 insertions(+), 46 deletions(-)
 create mode 100644 packages/opencode/src/tool/plan-enter.txt
 create mode 100644 packages/opencode/src/tool/plan-exit.txt
 create mode 100644 packages/opencode/src/tool/plan.ts
 create mode 100644 packages/opencode/test/util/lock.test.ts
 create mode 100644 packages/util/src/slug.ts

(limited to 'packages')

diff --git a/packages/opencode/src/agent/agent.ts b/packages/opencode/src/agent/agent.ts
index ea9d3e3ba..6847d29ab 100644
--- a/packages/opencode/src/agent/agent.ts
+++ b/packages/opencode/src/agent/agent.ts
@@ -53,6 +53,8 @@ export namespace Agent {
         [Truncate.GLOB]: "allow",
       },
       question: "deny",
+      plan_enter: "deny",
+      plan_exit: "deny",
       // mirrors github.com/github/gitignore Node.gitignore pattern for .env files
       read: {
         "*": "allow",
@@ -71,6 +73,7 @@ export namespace Agent {
           defaults,
           PermissionNext.fromConfig({
             question: "allow",
+            plan_enter: "allow",
           }),
           user,
         ),
@@ -84,9 +87,10 @@ export namespace Agent {
           defaults,
           PermissionNext.fromConfig({
             question: "allow",
+            plan_exit: "allow",
             edit: {
               "*": "deny",
-              ".opencode/plan/*.md": "allow",
+              ".opencode/plans/*.md": "allow",
             },
           }),
           user,
diff --git a/packages/opencode/src/cli/cmd/tui/routes/session/index.tsx b/packages/opencode/src/cli/cmd/tui/routes/session/index.tsx
index f87b811ae..b6916bc5a 100644
--- a/packages/opencode/src/cli/cmd/tui/routes/session/index.tsx
+++ b/packages/opencode/src/cli/cmd/tui/routes/session/index.tsx
@@ -196,6 +196,23 @@ export function Session() {
     }
   })
 
+  let lastSwitch: string | undefined = undefined
+  sdk.event.on("message.part.updated", (evt) => {
+    const part = evt.properties.part
+    if (part.type !== "tool") return
+    if (part.sessionID !== route.sessionID) return
+    if (part.state.status !== "completed") return
+    if (part.id === lastSwitch) return
+
+    if (part.tool === "plan_exit") {
+      local.agent.set("build")
+      lastSwitch = part.id
+    } else if (part.tool === "plan_enter") {
+      local.agent.set("plan")
+      lastSwitch = part.id
+    }
+  })
+
   let scroll: ScrollBoxRenderable
   let prompt: PromptRef
   const keybind = useKeybind()
diff --git a/packages/opencode/src/cli/cmd/tui/routes/session/question.tsx b/packages/opencode/src/cli/cmd/tui/routes/session/question.tsx
index ccc0e9b12..5e8ce2380 100644
--- a/packages/opencode/src/cli/cmd/tui/routes/session/question.tsx
+++ b/packages/opencode/src/cli/cmd/tui/routes/session/question.tsx
@@ -32,7 +32,8 @@ export function QuestionPrompt(props: { request: QuestionRequest }) {
   const question = createMemo(() => questions()[store.tab])
   const confirm = createMemo(() => !single() && store.tab === questions().length)
   const options = createMemo(() => question()?.options ?? [])
-  const other = createMemo(() => store.selected === options().length)
+  const custom = createMemo(() => question()?.custom !== false)
+  const other = createMemo(() => custom() && store.selected === options().length)
   const input = createMemo(() => store.custom[store.tab] ?? "")
   const multi = createMemo(() => question()?.multiple === true)
   const customPicked = createMemo(() => {
@@ -203,7 +204,7 @@ export function QuestionPrompt(props: { request: QuestionRequest }) {
       }
     } else {
       const opts = options()
-      const total = opts.length + 1 // options + "Other"
+      const total = opts.length + (custom() ? 1 : 0)
 
       if (evt.name === "up" || evt.name === "k") {
         evt.preventDefault()
@@ -298,35 +299,37 @@ export function QuestionPrompt(props: { request: QuestionRequest }) {
                   )
                 }}
               </For>
-              <box onMouseOver={() => moveTo(options().length)} onMouseUp={() => selectOption()}>
-                <box flexDirection="row" gap={1}>
-                  <box backgroundColor={other() ? theme.backgroundElement : undefined}>
-                    <text fg={other() ? theme.secondary : customPicked() ? theme.success : theme.text}>
-                      {options().length + 1}. Type your own answer
-                    </text>
+              <Show when={custom()}>
+                <box onMouseOver={() => moveTo(options().length)} onMouseUp={() => selectOption()}>
+                  <box flexDirection="row" gap={1}>
+                    <box backgroundColor={other() ? theme.backgroundElement : undefined}>
+                      <text fg={other() ? theme.secondary : customPicked() ? theme.success : theme.text}>
+                        {options().length + 1}. Type your own answer
+                      </text>
+                    </box>
+                    <text fg={theme.success}>{customPicked() ? "✓" : ""}</text>
                   </box>
-                  <text fg={theme.success}>{customPicked() ? "✓" : ""}</text>
+                  <Show when={store.editing}>
+                    <box paddingLeft={3}>
+                      <textarea
+                        ref={(val: TextareaRenderable) => (textarea = val)}
+                        focused
+                        initialValue={input()}
+                        placeholder="Type your own answer"
+                        textColor={theme.text}
+                        focusedTextColor={theme.text}
+                        cursorColor={theme.primary}
+                        keyBindings={bindings()}
+                      />
+                    </box>
+                  </Show>
+                  <Show when={!store.editing && input()}>
+                    <box paddingLeft={3}>
+                      <text fg={theme.textMuted}>{input()}</text>
+                    </box>
+                  </Show>
                 </box>
-                <Show when={store.editing}>
-                  <box paddingLeft={3}>
-                    <textarea
-                      ref={(val: TextareaRenderable) => (textarea = val)}
-                      focused
-                      initialValue={input()}
-                      placeholder="Type your own answer"
-                      textColor={theme.text}
-                      focusedTextColor={theme.text}
-                      cursorColor={theme.primary}
-                      keyBindings={bindings()}
-                    />
-                  </box>
-                </Show>
-                <Show when={!store.editing && input()}>
-                  <box paddingLeft={3}>
-                    <text fg={theme.textMuted}>{input()}</text>
-                  </box>
-                </Show>
-              </box>
+              </Show>
             </box>
           </box>
         </Show>
diff --git a/packages/opencode/src/flag/flag.ts b/packages/opencode/src/flag/flag.ts
index ad6052dec..4cdb54909 100644
--- a/packages/opencode/src/flag/flag.ts
+++ b/packages/opencode/src/flag/flag.ts
@@ -38,6 +38,7 @@ export namespace Flag {
   export const OPENCODE_EXPERIMENTAL_OXFMT = OPENCODE_EXPERIMENTAL || truthy("OPENCODE_EXPERIMENTAL_OXFMT")
   export const OPENCODE_EXPERIMENTAL_LSP_TY = truthy("OPENCODE_EXPERIMENTAL_LSP_TY")
   export const OPENCODE_EXPERIMENTAL_LSP_TOOL = OPENCODE_EXPERIMENTAL || truthy("OPENCODE_EXPERIMENTAL_LSP_TOOL")
+  export const OPENCODE_EXPERIMENTAL_PLAN_MODE = OPENCODE_EXPERIMENTAL || truthy("OPENCODE_EXPERIMENTAL_PLAN_MODE")
 
   function truthy(key: string) {
     const value = process.env[key]?.toLowerCase()
diff --git a/packages/opencode/src/question/index.ts b/packages/opencode/src/question/index.ts
index 23656b53a..d18098a9c 100644
--- a/packages/opencode/src/question/index.ts
+++ b/packages/opencode/src/question/index.ts
@@ -24,6 +24,7 @@ export namespace Question {
       header: z.string().max(12).describe("Very short label (max 12 chars)"),
       options: z.array(Option).describe("Available choices"),
       multiple: z.boolean().optional().describe("Allow selecting multiple choices"),
+      custom: z.boolean().optional().describe("Allow typing a custom answer (default: true)"),
     })
     .meta({
       ref: "QuestionInfo",
diff --git a/packages/opencode/src/session/index.ts b/packages/opencode/src/session/index.ts
index a204913f7..2bb9fc5f8 100644
--- a/packages/opencode/src/session/index.ts
+++ b/packages/opencode/src/session/index.ts
@@ -1,3 +1,5 @@
+import { Slug } from "@opencode-ai/util/slug"
+import pat from "path"
 import { BusEvent } from "@/bus/bus-event"
 import { Bus } from "@/bus"
 import { Decimal } from "decimal.js"
@@ -19,6 +21,7 @@ import { Snapshot } from "@/snapshot"
 
 import type { Provider } from "@/provider/provider"
 import { PermissionNext } from "@/permission/next"
+import path from "path"
 
 export namespace Session {
   const log = Log.create({ service: "session" })
@@ -39,6 +42,7 @@ export namespace Session {
   export const Info = z
     .object({
       id: Identifier.schema("session"),
+      slug: z.string(),
       projectID: z.string(),
       directory: z.string(),
       parentID: Identifier.schema("session").optional(),
@@ -194,6 +198,7 @@ export namespace Session {
   }) {
     const result: Info = {
       id: Identifier.descending("session", input.id),
+      slug: Slug.create(),
       version: Installation.VERSION,
       projectID: Instance.project.id,
       directory: input.directory,
@@ -227,6 +232,10 @@ export namespace Session {
     return result
   }
 
+  export function plan(input: { slug: string; time: { created: number } }) {
+    return path.join(Instance.worktree, ".opencode", "plans", [input.time.created, input.slug].join("-") + ".md")
+  }
+
   export const get = fn(Identifier.schema("session"), async (id) => {
     const read = await Storage.read<Info>(["session", Instance.project.id, id])
     return read as Info
diff --git a/packages/opencode/src/session/prompt.ts b/packages/opencode/src/session/prompt.ts
index 34596e629..5036d3a69 100644
--- a/packages/opencode/src/session/prompt.ts
+++ b/packages/opencode/src/session/prompt.ts
@@ -510,9 +510,10 @@ export namespace SessionPrompt {
       const agent = await Agent.get(lastUser.agent)
       const maxSteps = agent.steps ?? Infinity
       const isLastStep = step >= maxSteps
-      msgs = insertReminders({
+      msgs = await insertReminders({
         messages: msgs,
         agent,
+        session,
       })
 
       const processor = SessionProcessor.create({
@@ -1185,30 +1186,140 @@ export namespace SessionPrompt {
     }
   }
 
-  function insertReminders(input: { messages: MessageV2.WithParts[]; agent: Agent.Info }) {
+  async function insertReminders(input: { messages: MessageV2.WithParts[]; agent: Agent.Info; session: Session.Info }) {
     const userMessage = input.messages.findLast((msg) => msg.info.role === "user")
     if (!userMessage) return input.messages
-    if (input.agent.name === "plan") {
-      userMessage.parts.push({
-        id: Identifier.ascending("part"),
-        messageID: userMessage.info.id,
-        sessionID: userMessage.info.sessionID,
-        type: "text",
-        // TODO (for mr dax): update to use the anthropic full fledged one (see plan-reminder-anthropic.txt)
-        text: PROMPT_PLAN,
-        synthetic: true,
-      })
+
+    // Original logic when experimental plan mode is disabled
+    if (!Flag.OPENCODE_EXPERIMENTAL_PLAN_MODE) {
+      if (input.agent.name === "plan") {
+        userMessage.parts.push({
+          id: Identifier.ascending("part"),
+          messageID: userMessage.info.id,
+          sessionID: userMessage.info.sessionID,
+          type: "text",
+          text: PROMPT_PLAN,
+          synthetic: true,
+        })
+      }
+      const wasPlan = input.messages.some((msg) => msg.info.role === "assistant" && msg.info.agent === "plan")
+      if (wasPlan && input.agent.name === "build") {
+        userMessage.parts.push({
+          id: Identifier.ascending("part"),
+          messageID: userMessage.info.id,
+          sessionID: userMessage.info.sessionID,
+          type: "text",
+          text: BUILD_SWITCH,
+          synthetic: true,
+        })
+      }
+      return input.messages
     }
-    const wasPlan = input.messages.some((msg) => msg.info.role === "assistant" && msg.info.agent === "plan")
-    if (wasPlan && input.agent.name === "build") {
-      userMessage.parts.push({
+
+    // New plan mode logic when flag is enabled
+    const assistantMessage = input.messages.findLast((msg) => msg.info.role === "assistant")
+
+    // Switching from plan mode to build mode
+    if (input.agent.name !== "plan" && assistantMessage?.info.agent === "plan") {
+      const plan = Session.plan(input.session)
+      const exists = await Bun.file(plan).exists()
+      if (exists) {
+        const part = await Session.updatePart({
+          id: Identifier.ascending("part"),
+          messageID: userMessage.info.id,
+          sessionID: userMessage.info.sessionID,
+          type: "text",
+          text: BUILD_SWITCH.replace("{{plan}}", plan),
+          synthetic: true,
+        })
+        userMessage.parts.push(part)
+      }
+    }
+
+    // Entering plan mode
+    if (input.agent.name === "plan" && assistantMessage?.info.agent !== "plan") {
+      const plan = Session.plan(input.session)
+      const exists = await Bun.file(plan).exists()
+      if (!exists) await fs.mkdir(path.dirname(plan), { recursive: true })
+      const part = await Session.updatePart({
         id: Identifier.ascending("part"),
         messageID: userMessage.info.id,
         sessionID: userMessage.info.sessionID,
         type: "text",
-        text: BUILD_SWITCH,
+        text: `<system-reminder>
+Plan mode is active. The user indicated that they do not want you to execute yet -- you MUST NOT make any edits (with the exception of the plan file mentioned below), run any non-readonly tools (including changing configs or making commits), or otherwise make any changes to the system. This supercedes any other instructions you have received.
+
+## Plan File Info:
+${exists ? `A plan file already exists at ${plan}. You can read it and make incremental edits using the edit tool.` : `No plan file exists yet. You should create your plan at ${plan} using the write tool.`}
+You should build your plan incrementally by writing to or editing this file. NOTE that this is the only file you are allowed to edit - other than this you are only allowed to take READ-ONLY actions.
+
+## Plan Workflow
+
+### Phase 1: Initial Understanding
+Goal: Gain a comprehensive understanding of the user's request by reading through code and asking them questions. Critical: In this phase you should only use the explore subagent type.
+
+1. Focus on understanding the user's request and the code associated with their request
+
+2. **Launch up to 3 explore agents IN PARALLEL** (single message, multiple tool calls) to efficiently explore the codebase.
+   - Use 1 agent when the task is isolated to known files, the user provided specific file paths, or you're making a small targeted change.
+   - Use multiple agents when: the scope is uncertain, multiple areas of the codebase are involved, or you need to understand existing patterns before planning.
+   - Quality over quantity - 3 agents maximum, but you should try to use the minimum number of agents necessary (usually just 1)
+   - If using multiple agents: Provide each agent with a specific search focus or area to explore. Example: One agent searches for existing implementations, another explores related components, a third investigates testing patterns
+
+3. After exploring the code, use the question tool to clarify ambiguities in the user request up front.
+
+### Phase 2: Design
+Goal: Design an implementation approach.
+
+Launch general agent(s) to design the implementation based on the user's intent and your exploration results from Phase 1.
+
+You can launch up to 1 agent(s) in parallel.
+
+**Guidelines:**
+- **Default**: Launch at least 1 Plan agent for most tasks - it helps validate your understanding and consider alternatives
+- **Skip agents**: Only for truly trivial tasks (typo fixes, single-line changes, simple renames)
+
+Examples of when to use multiple agents:
+- The task touches multiple parts of the codebase
+- It's a large refactor or architectural change
+- There are many edge cases to consider
+- You'd benefit from exploring different approaches
+
+Example perspectives by task type:
+- New feature: simplicity vs performance vs maintainability
+- Bug fix: root cause vs workaround vs prevention
+- Refactoring: minimal change vs clean architecture
+
+In the agent prompt:
+- Provide comprehensive background context from Phase 1 exploration including filenames and code path traces
+- Describe requirements and constraints
+- Request a detailed implementation plan
+
+### Phase 3: Review
+Goal: Review the plan(s) from Phase 2 and ensure alignment with the user's intentions.
+1. Read the critical files identified by agents to deepen your understanding
+2. Ensure that the plans align with the user's original request
+3. Use question tool to clarify any remaining questions with the user
+
+### Phase 4: Final Plan
+Goal: Write your final plan to the plan file (the only file you can edit).
+- Include only your recommended approach, not all alternatives
+- Ensure that the plan file is concise enough to scan quickly, but detailed enough to execute effectively
+- Include the paths of critical files to be modified
+- Include a verification section describing how to test the changes end-to-end (run the code, use MCP tools, run tests)
+
+### Phase 5: Call plan_exit tool
+At the very end of your turn, once you have asked the user questions and are happy with your final plan file - you should always call plan_exit to indicate to the user that you are done planning.
+This is critical - your turn should only end with either asking the user a question or calling plan_exit. Do not stop unless it's for these 2 reasons.
+
+**Important:** Use question tool to clarify requirements/approach, use plan_exit to request plan approval. Do NOT use question tool to ask "Is this plan okay?" - that's what plan_exit does.
+
+NOTE: At any point in time through this workflow you should feel free to ask the user questions or clarifications. Don't make large assumptions about user intent. The goal is to present a well researched plan to the user, and tie any loose ends before implementation begins.
+</system-reminder>`,
         synthetic: true,
       })
+      userMessage.parts.push(part)
+      return input.messages
     }
     return input.messages
   }
diff --git a/packages/opencode/src/session/prompt/build-switch.txt b/packages/opencode/src/session/prompt/build-switch.txt
index 3737b74d8..40b39b95b 100644
--- a/packages/opencode/src/session/prompt/build-switch.txt
+++ b/packages/opencode/src/session/prompt/build-switch.txt
@@ -2,4 +2,6 @@
 Your operational mode has changed from plan to build.
 You are no longer in read-only mode.
 You are permitted to make file changes, run shell commands, and utilize your arsenal of tools as needed.
+
+A plan file exists at {{plan}}. You should read this file and execute on the plan defined within it.
 </system-reminder>
diff --git a/packages/opencode/src/tool/plan-enter.txt b/packages/opencode/src/tool/plan-enter.txt
new file mode 100644
index 000000000..2e6a69f1f
--- /dev/null
+++ b/packages/opencode/src/tool/plan-enter.txt
@@ -0,0 +1,14 @@
+Use this tool to suggest switching to plan agent when the user's request would benefit from planning before implementation.
+
+If they explicitly mention wanting to create a plan ALWAYS call this tool first.
+
+This tool will ask the user if they want to switch to plan agent.
+
+Call this tool when:
+- The user's request is complex and would benefit from planning first
+- You want to research and design before making changes
+- The task involves multiple files or significant architectural decisions
+
+Do NOT call this tool:
+- For simple, straightforward tasks
+- When the user explicitly wants immediate implementation
diff --git a/packages/opencode/src/tool/plan-exit.txt b/packages/opencode/src/tool/plan-exit.txt
new file mode 100644
index 000000000..988821de3
--- /dev/null
+++ b/packages/opencode/src/tool/plan-exit.txt
@@ -0,0 +1,13 @@
+Use this tool when you have completed the planning phase and are ready to exit plan agent.
+
+This tool will ask the user if they want to switch to build agent to start implementing the plan.
+
+Call this tool:
+- After you have written a complete plan to the plan file
+- After you have clarified any questions with the user
+- When you are confident the plan is ready for implementation
+
+Do NOT call this tool:
+- Before you have created or finalized the plan
+- If you still have unanswered questions about the implementation
+- If the user has indicated they want to continue planning
diff --git a/packages/opencode/src/tool/plan.ts b/packages/opencode/src/tool/plan.ts
new file mode 100644
index 000000000..6cb7a691c
--- /dev/null
+++ b/packages/opencode/src/tool/plan.ts
@@ -0,0 +1,130 @@
+import z from "zod"
+import path from "path"
+import { Tool } from "./tool"
+import { Question } from "../question"
+import { Session } from "../session"
+import { MessageV2 } from "../session/message-v2"
+import { Identifier } from "../id/id"
+import { Provider } from "../provider/provider"
+import { Instance } from "../project/instance"
+import EXIT_DESCRIPTION from "./plan-exit.txt"
+import ENTER_DESCRIPTION from "./plan-enter.txt"
+
+async function getLastModel(sessionID: string) {
+  for await (const item of MessageV2.stream(sessionID)) {
+    if (item.info.role === "user" && item.info.model) return item.info.model
+  }
+  return Provider.defaultModel()
+}
+
+export const PlanExitTool = Tool.define("plan_exit", {
+  description: EXIT_DESCRIPTION,
+  parameters: z.object({}),
+  async execute(_params, ctx) {
+    const session = await Session.get(ctx.sessionID)
+    const plan = path.relative(Instance.worktree, Session.plan(session))
+    const answers = await Question.ask({
+      sessionID: ctx.sessionID,
+      questions: [
+        {
+          question: `Plan at ${plan} is complete. Would you like to switch to the build agent and start implementing?`,
+          header: "Build Agent",
+          custom: false,
+          options: [
+            { label: "Yes", description: "Switch to build agent and start implementing the plan" },
+            { label: "No", description: "Stay with plan agent to continue refining the plan" },
+          ],
+        },
+      ],
+      tool: ctx.callID ? { messageID: ctx.messageID, callID: ctx.callID } : undefined,
+    })
+
+    const answer = answers[0]?.[0]
+    if (answer === "No") throw new Question.RejectedError()
+
+    const model = await getLastModel(ctx.sessionID)
+
+    const userMsg: MessageV2.User = {
+      id: Identifier.ascending("message"),
+      sessionID: ctx.sessionID,
+      role: "user",
+      time: {
+        created: Date.now(),
+      },
+      agent: "build",
+      model,
+    }
+    await Session.updateMessage(userMsg)
+    await Session.updatePart({
+      id: Identifier.ascending("part"),
+      messageID: userMsg.id,
+      sessionID: ctx.sessionID,
+      type: "text",
+      text: `The plan at ${plan} has been approved, you can now edit files. Execute the plan`,
+      synthetic: true,
+    } satisfies MessageV2.TextPart)
+
+    return {
+      title: "Switching to build agent",
+      output: "User approved switching to build agent. Wait for further instructions.",
+      metadata: {},
+    }
+  },
+})
+
+export const PlanEnterTool = Tool.define("plan_enter", {
+  description: ENTER_DESCRIPTION,
+  parameters: z.object({}),
+  async execute(_params, ctx) {
+    const session = await Session.get(ctx.sessionID)
+    const plan = path.relative(Instance.worktree, Session.plan(session))
+
+    const answers = await Question.ask({
+      sessionID: ctx.sessionID,
+      questions: [
+        {
+          question: `Would you like to switch to the plan agent and create a plan saved to ${plan}?`,
+          header: "Plan Mode",
+          custom: false,
+          options: [
+            { label: "Yes", description: "Switch to plan agent for research and planning" },
+            { label: "No", description: "Stay with build agent to continue making changes" },
+          ],
+        },
+      ],
+      tool: ctx.callID ? { messageID: ctx.messageID, callID: ctx.callID } : undefined,
+    })
+
+    const answer = answers[0]?.[0]
+
+    if (answer === "No") throw new Question.RejectedError()
+
+    const model = await getLastModel(ctx.sessionID)
+
+    const userMsg: MessageV2.User = {
+      id: Identifier.ascending("message"),
+      sessionID: ctx.sessionID,
+      role: "user",
+      time: {
+        created: Date.now(),
+      },
+      agent: "plan",
+      model,
+    }
+    await Session.updateMessage(userMsg)
+    await Session.updatePart({
+      id: Identifier.ascending("part"),
+      messageID: userMsg.id,
+      sessionID: ctx.sessionID,
+      type: "text",
+      text: "User has requested to enter plan mode. Switch to plan mode and begin planning.",
+      synthetic: true,
+    } satisfies MessageV2.TextPart)
+
+    return {
+      title: "Switching to plan agent",
+      output: `User confirmed to switch to plan mode. A new message has been created to switch you to plan mode. The plan file will be at ${plan}. Begin planning.`,
+      metadata: {},
+    }
+  },
+})
diff --git a/packages/opencode/src/tool/registry.ts b/packages/opencode/src/tool/registry.ts
index 24faed7f0..35e378f08 100644
--- a/packages/opencode/src/tool/registry.ts
+++ b/packages/opencode/src/tool/registry.ts
@@ -25,6 +25,7 @@ import { Flag } from "@/flag/flag"
 import { Log } from "@/util/log"
 import { LspTool } from "./lsp"
 import { Truncate } from "./truncation"
+import { PlanExitTool, PlanEnterTool } from "./plan"
 
 export namespace ToolRegistry {
   const log = Log.create({ service: "tool.registry" })
@@ -109,6 +110,7 @@ export namespace ToolRegistry {
       SkillTool,
       ...(Flag.OPENCODE_EXPERIMENTAL_LSP_TOOL ? [LspTool] : []),
       ...(config.experimental?.batch_tool === true ? [BatchTool] : []),
+      ...(Flag.OPENCODE_EXPERIMENTAL_PLAN_MODE && Flag.OPENCODE_CLIENT === "cli" ? [PlanExitTool, PlanEnterTool] : []),
       ...custom,
     ]
   }
diff --git a/packages/opencode/test/util/lock.test.ts b/packages/opencode/test/util/lock.test.ts
new file mode 100644
index 000000000..b877311e3
--- /dev/null
+++ b/packages/opencode/test/util/lock.test.ts
@@ -0,0 +1,72 @@
+import { describe, expect, test } from "bun:test"
+import { Lock } from "../../src/util/lock"
+
+function tick() {
+  return new Promise<void>((r) => queueMicrotask(r))
+}
+
+async function flush(n = 5) {
+  for (let i = 0; i < n; i++) await tick()
+}
+
+describe("util.lock", () => {
+  test("writer exclusivity: blocks reads and other writes while held", async () => {
+    const key = "lock:" + Math.random().toString(36).slice(2)
+
+    const state = {
+      writer2: false,
+      reader: false,
+      writers: 0,
+    }
+
+    // Acquire writer1
+    using writer1 = await Lock.write(key)
+    state.writers++
+    expect(state.writers).toBe(1)
+
+    // Start writer2 candidate (should block)
+    const writer2Task = (async () => {
+      const w = await Lock.write(key)
+      state.writers++
+      expect(state.writers).toBe(1)
+      state.writer2 = true
+      // Hold for a tick so reader cannot slip in
+      await tick()
+      return w
+    })()
+
+    // Start reader candidate (should block)
+    const readerTask = (async () => {
+      const r = await Lock.read(key)
+      state.reader = true
+      return r
+    })()
+
+    // Flush microtasks and assert neither acquired
+    await flush()
+    expect(state.writer2).toBe(false)
+    expect(state.reader).toBe(false)
+
+    // Release writer1
+    writer1[Symbol.dispose]()
+    state.writers--
+
+    // writer2 should acquire next
+    const writer2 = await writer2Task
+    expect(state.writer2).toBe(true)
+
+    // Reader still blocked while writer2 held
+    await flush()
+    expect(state.reader).toBe(false)
+
+    // Release writer2
+    writer2[Symbol.dispose]()
+    state.writers--
+
+    // Reader should now acquire
+    const reader = await readerTask
+    expect(state.reader).toBe(true)
+
+    reader[Symbol.dispose]()
+  })
+})
diff --git a/packages/sdk/js/src/v2/gen/types.gen.ts b/packages/sdk/js/src/v2/gen/types.gen.ts
index e423fecea..acc29d9b4 100644
--- a/packages/sdk/js/src/v2/gen/types.gen.ts
+++ b/packages/sdk/js/src/v2/gen/types.gen.ts
@@ -545,6 +545,10 @@ export type QuestionInfo = {
    * Allow selecting multiple choices
    */
   multiple?: boolean
+  /**
+   * Allow typing a custom answer (default: true)
+   */
+  custom?: boolean
 }
 
 export type QuestionRequest = {
@@ -706,6 +710,7 @@ export type PermissionRuleset = Array<PermissionRule>
 
 export type Session = {
   id: string
+  slug: string
   projectID: string
   directory: string
   parentID?: string
diff --git a/packages/util/src/slug.ts b/packages/util/src/slug.ts
new file mode 100644
index 000000000..62cf0e57b
--- /dev/null
+++ b/packages/util/src/slug.ts
@@ -0,0 +1,74 @@
+export namespace Slug {
+  const ADJECTIVES = [
+    "brave",
+    "calm",
+    "clever",
+    "cosmic",
+    "crisp",
+    "curious",
+    "eager",
+    "gentle",
+    "glowing",
+    "happy",
+    "hidden",
+    "jolly",
+    "kind",
+    "lucky",
+    "mighty",
+    "misty",
+    "neon",
+    "nimble",
+    "playful",
+    "proud",
+    "quick",
+    "quiet",
+    "shiny",
+    "silent",
+    "stellar",
+    "sunny",
+    "swift",
+    "tidy",
+    "witty",
+  ] as const
+
+  const NOUNS = [
+    "cabin",
+    "cactus",
+    "canyon",
+    "circuit",
+    "comet",
+    "eagle",
+    "engine",
+    "falcon",
+    "forest",
+    "garden",
+    "harbor",
+    "island",
+    "knight",
+    "lagoon",
+    "meadow",
+    "moon",
+    "mountain",
+    "nebula",
+    "orchid",
+    "otter",
+    "panda",
+    "pixel",
+    "planet",
+    "river",
+    "rocket",
+    "sailor",
+    "squid",
+    "star",
+    "tiger",
+    "wizard",
+    "wolf",
+  ] as const
+
+  export function create() {
+    return [
+      ADJECTIVES[Math.floor(Math.random() * ADJECTIVES.length)],
+      NOUNS[Math.floor(Math.random() * NOUNS.length)],
+    ].join("-")
+  }
+}
-- 
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