diff --git a/packages/opencode/src/cli/cmd/tui/component/prompt/index.tsx b/packages/opencode/src/cli/cmd/tui/component/prompt/index.tsx
index 941b383e649..9f3a2de4511 100644
--- a/packages/opencode/src/cli/cmd/tui/component/prompt/index.tsx
+++ b/packages/opencode/src/cli/cmd/tui/component/prompt/index.tsx
@@ -203,6 +203,10 @@ export function Prompt(props: PromptProps) {
             setStore("mode", "normal")
             return
           }
+          if (store.mode === "rules") {
+            setStore("mode", "normal")
+            return
+          }
           if (!props.sessionID) return
 
           setStore("interrupt", store.interrupt + 1)
@@ -318,7 +322,7 @@ export function Prompt(props: PromptProps) {
 
   const [store, setStore] = createStore<{
     prompt: PromptInfo
-    mode: "normal" | "shell"
+    mode: "normal" | "shell" | "rules"
     extmarkToPartIndex: Map<number, number>
     interrupt: number
     placeholder: number
@@ -502,6 +506,12 @@ export function Prompt(props: PromptProps) {
         command: inputText,
       })
       setStore("mode", "normal")
+    } else if (store.mode === "rules") {
+      sdk.client.session.rules({
+        sessionID,
+        content: inputText,
+      })
+      setStore("mode", "normal")
     } else if (
       inputText.startsWith("/") &&
       iife(() => {
@@ -640,9 +650,15 @@ export function Prompt(props: PromptProps) {
   const highlight = createMemo(() => {
     if (keybind.leader) return theme.border
     if (store.mode === "shell") return theme.primary
+    if (store.mode === "rules") return theme.success
     return local.agent.color(local.agent.current().name)
   })
 
+  const modeLabel = createMemo(() => {
+    if (store.mode === "shell") return "Shell"
+    if (store.mode === "rules") return "Rules"
+    return Locale.titlecase(local.agent.current().name)
+  })
   const spinnerDef = createMemo(() => {
     const color = local.agent.color(local.agent.current().name)
     return {
@@ -744,6 +760,11 @@ export function Prompt(props: PromptProps) {
                   e.preventDefault()
                   return
                 }
+                if (e.name === "#" && input.visualCursor.offset === 0) {
+                  setStore("mode", "rules")
+                  e.preventDefault()
+                  return
+                }
                 if (store.mode === "shell") {
                   if ((e.name === "backspace" && input.visualCursor.offset === 0) || e.name === "escape") {
                     setStore("mode", "normal")
@@ -751,6 +772,13 @@ export function Prompt(props: PromptProps) {
                     return
                   }
                 }
+                if (store.mode === "rules") {
+                  if ((e.name === "backspace" && input.visualCursor.offset === 0) || e.name === "escape") {
+                    setStore("mode", "normal")
+                    e.preventDefault()
+                    return
+                  }
+                }
                 if (store.mode === "normal") autocomplete.onKeyDown(e)
                 if (!autocomplete.visible) {
                   if (
@@ -849,9 +877,7 @@ export function Prompt(props: PromptProps) {
               syntaxStyle={syntax()}
             />
             <box flexDirection="row" flexShrink={0} paddingTop={1} gap={1}>
-              <text fg={highlight()}>
-                {store.mode === "shell" ? "Shell" : Locale.titlecase(local.agent.current().name)}{" "}
-              </text>
+              <text fg={highlight()}>{modeLabel()} </text>
               <Show when={store.mode === "normal"}>
                 <box flexDirection="row" gap={1}>
                   <text flexShrink={0} fg={keybind.leader ? theme.textMuted : theme.text}>
@@ -983,6 +1009,11 @@ export function Prompt(props: PromptProps) {
                     esc <span style={{ fg: theme.textMuted }}>exit shell mode</span>
                   </text>
                 </Match>
+                <Match when={store.mode === "rules"}>
+                  <text fg={theme.text}>
+                    esc <span style={{ fg: theme.textMuted }}>exit rules mode</span>
+                  </text>
+                </Match>
               </Switch>
             </box>
           </Show>
diff --git a/packages/opencode/src/server/server.ts b/packages/opencode/src/server/server.ts
index f1485ec0150..62d0f8268c8 100644
--- a/packages/opencode/src/server/server.ts
+++ b/packages/opencode/src/server/server.ts
@@ -1310,6 +1310,38 @@ export namespace Server {
           return c.json(msg)
         },
       )
+      .post(
+        "/session/:sessionID/rules",
+        describeRoute({
+          summary: "Save rules",
+          description: "Append content to project AGENTS.md file.",
+          operationId: "session.rules",
+          responses: {
+            200: {
+              description: "Content saved",
+              content: {
+                "application/json": {
+                  schema: resolver(z.boolean()),
+                },
+              },
+            },
+            ...errors(400, 404),
+          },
+        }),
+        validator(
+          "param",
+          z.object({
+            sessionID: z.string().meta({ description: "Session ID" }),
+          }),
+        ),
+        validator("json", SessionPrompt.RulesInput.omit({ sessionID: true })),
+        async (c) => {
+          const sessionID = c.req.valid("param").sessionID
+          const body = c.req.valid("json")
+          await SessionPrompt.rules({ ...body, sessionID })
+          return c.json(true)
+        },
+      )
       .post(
         "/session/:sessionID/revert",
         describeRoute({
diff --git a/packages/opencode/src/session/prompt.ts b/packages/opencode/src/session/prompt.ts
index c9e24f8ca04..fdcf989e150 100644
--- a/packages/opencode/src/session/prompt.ts
+++ b/packages/opencode/src/session/prompt.ts
@@ -25,6 +25,7 @@ import { Bus } from "../bus"
 import { ProviderTransform } from "../provider/transform"
 import { SystemPrompt } from "./system"
 import { Plugin } from "../plugin"
+import { TuiEvent } from "@/cli/cmd/tui/event"
 
 import PROMPT_PLAN from "../session/prompt/plan.txt"
 import BUILD_SWITCH from "../session/prompt/build-switch.txt"
@@ -1389,6 +1390,28 @@ export namespace SessionPrompt {
     return { info: msg, parts: [part] }
   }
 
+  export const RulesInput = z.object({
+    sessionID: Identifier.schema("session"),
+    content: z.string(),
+  })
+  export type RulesInput = z.infer<typeof RulesInput>
+  export async function rules(input: RulesInput) {
+    const agentsPath = path.join(Instance.worktree, "AGENTS.md")
+    const file = Bun.file(agentsPath)
+    const exists = await file.exists()
+
+    const content = exists ? await file.text() : ""
+    const newContent = content === "" ? input.content + "\n" : content.trimEnd() + "\n\n" + input.content + "\n"
+
+    await Bun.write(agentsPath, newContent)
+
+    await Bus.publish(TuiEvent.ToastShow, {
+      message: "Saved to AGENTS.md",
+      variant: "success",
+      duration: 3000,
+    })
+  }
+
   export const CommandInput = z.object({
     messageID: Identifier.schema("message").optional(),
     sessionID: Identifier.schema("session"),
diff --git a/packages/sdk/js/src/v2/gen/sdk.gen.ts b/packages/sdk/js/src/v2/gen/sdk.gen.ts
index 90df76c2234..98bb5c6309a 100644
--- a/packages/sdk/js/src/v2/gen/sdk.gen.ts
+++ b/packages/sdk/js/src/v2/gen/sdk.gen.ts
@@ -99,6 +99,8 @@ import type {
   SessionPromptResponses,
   SessionRevertErrors,
   SessionRevertResponses,
+  SessionRulesErrors,
+  SessionRulesResponses,
   SessionShareErrors,
   SessionShareResponses,
   SessionShellErrors,
@@ -1416,6 +1418,43 @@ export class Session extends HeyApiClient {
     })
   }
 
+  /**
+   * Save rules
+   *
+   * Append content to project AGENTS.md file.
+   */
+  public rules<ThrowOnError extends boolean = false>(
+    parameters: {
+      sessionID: string
+      directory?: string
+      content?: string
+    },
+    options?: Options<never, ThrowOnError>,
+  ) {
+    const params = buildClientParams(
+      [parameters],
+      [
+        {
+          args: [
+            { in: "path", key: "sessionID" },
+            { in: "query", key: "directory" },
+            { in: "body", key: "content" },
+          ],
+        },
+      ],
+    )
+    return (options?.client ?? this.client).post<SessionRulesResponses, SessionRulesErrors, ThrowOnError>({
+      url: "/session/{sessionID}/rules",
+      ...options,
+      ...params,
+      headers: {
+        "Content-Type": "application/json",
+        ...options?.headers,
+        ...params.headers,
+      },
+    })
+  }
+
   /**
    * Revert message
    *
diff --git a/packages/sdk/js/src/v2/gen/types.gen.ts b/packages/sdk/js/src/v2/gen/types.gen.ts
index 9d0bbcc92cd..0f7fc3f0aaa 100644
--- a/packages/sdk/js/src/v2/gen/types.gen.ts
+++ b/packages/sdk/js/src/v2/gen/types.gen.ts
@@ -511,6 +511,48 @@ export type EventSessionCompacted = {
   }
 }
 
+export type EventTuiPromptAppend = {
+  type: "tui.prompt.append"
+  properties: {
+    text: string
+  }
+}
+
+export type EventTuiCommandExecute = {
+  type: "tui.command.execute"
+  properties: {
+    command:
+      | "session.list"
+      | "session.new"
+      | "session.share"
+      | "session.interrupt"
+      | "session.compact"
+      | "session.page.up"
+      | "session.page.down"
+      | "session.half.page.up"
+      | "session.half.page.down"
+      | "session.first"
+      | "session.last"
+      | "prompt.clear"
+      | "prompt.submit"
+      | "agent.cycle"
+      | string
+  }
+}
+
+export type EventTuiToastShow = {
+  type: "tui.toast.show"
+  properties: {
+    title?: string
+    message: string
+    variant: "info" | "success" | "warning" | "error"
+    /**
+     * Duration in milliseconds
+     */
+    duration?: number
+  }
+}
+
 export type EventFileEdited = {
   type: "file.edited"
   properties: {
@@ -637,48 +679,6 @@ export type EventVcsBranchUpdated = {
   }
 }
 
-export type EventTuiPromptAppend = {
-  type: "tui.prompt.append"
-  properties: {
-    text: string
-  }
-}
-
-export type EventTuiCommandExecute = {
-  type: "tui.command.execute"
-  properties: {
-    command:
-      | "session.list"
-      | "session.new"
-      | "session.share"
-      | "session.interrupt"
-      | "session.compact"
-      | "session.page.up"
-      | "session.page.down"
-      | "session.half.page.up"
-      | "session.half.page.down"
-      | "session.first"
-      | "session.last"
-      | "prompt.clear"
-      | "prompt.submit"
-      | "agent.cycle"
-      | string
-  }
-}
-
-export type EventTuiToastShow = {
-  type: "tui.toast.show"
-  properties: {
-    title?: string
-    message: string
-    variant: "info" | "success" | "warning" | "error"
-    /**
-     * Duration in milliseconds
-     */
-    duration?: number
-  }
-}
-
 export type Pty = {
   id: string
   title: string
@@ -748,6 +748,9 @@ export type Event =
   | EventSessionStatus
   | EventSessionIdle
   | EventSessionCompacted
+  | EventTuiPromptAppend
+  | EventTuiCommandExecute
+  | EventTuiToastShow
   | EventFileEdited
   | EventTodoUpdated
   | EventCommandExecuted
@@ -758,9 +761,6 @@ export type Event =
   | EventSessionError
   | EventFileWatcherUpdated
   | EventVcsBranchUpdated
-  | EventTuiPromptAppend
-  | EventTuiCommandExecute
-  | EventTuiToastShow
   | EventPtyCreated
   | EventPtyUpdated
   | EventPtyExited
@@ -3020,6 +3020,44 @@ export type SessionShellResponses = {
 
 export type SessionShellResponse = SessionShellResponses[keyof SessionShellResponses]
 
+export type SessionRulesData = {
+  body?: {
+    content: string
+  }
+  path: {
+    /**
+     * Session ID
+     */
+    sessionID: string
+  }
+  query?: {
+    directory?: string
+  }
+  url: "/session/{sessionID}/rules"
+}
+
+export type SessionRulesErrors = {
+  /**
+   * Bad request
+   */
+  400: BadRequestError
+  /**
+   * Not found
+   */
+  404: NotFoundError
+}
+
+export type SessionRulesError = SessionRulesErrors[keyof SessionRulesErrors]
+
+export type SessionRulesResponses = {
+  /**
+   * Content saved
+   */
+  200: boolean
+}
+
+export type SessionRulesResponse = SessionRulesResponses[keyof SessionRulesResponses]
+
 export type SessionRevertData = {
   body?: {
     messageID: string