diff --git a/pi-coding-agent-ui.el b/pi-coding-agent-ui.el
index 1f9e71d..2b84120 100644
--- a/pi-coding-agent-ui.el
+++ b/pi-coding-agent-ui.el
@@ -588,6 +588,45 @@ TYPE is :chat or :input.  Returns the buffer."
             (:input (pi-coding-agent-input-mode))))
         buf))))
 
+;;;; Project Buffer Discovery
+
+(defun pi-coding-agent-project-buffers ()
+  "Return all pi chat buffers for the current project directory.
+Matches buffer names by prefix against the abbreviated project dir.
+Returns a list ordered by `buffer-list' recency (most recent first)."
+  (let ((prefix (format "*pi-coding-agent-chat:%s"
+                        (abbreviate-file-name
+                         (pi-coding-agent--session-directory)))))
+    (cl-remove-if-not
+     (lambda (buf)
+       (string-prefix-p prefix (buffer-name buf)))
+     (buffer-list))))
+
+;;;; Window Hiding
+
+(defun pi-coding-agent--hide-buffers ()
+  "Hide all pi windows (chat and input) for the current session.
+Uses `delete-window' when the frame has other windows, or
+`bury-buffer' for sole-window frames.  A second pass buries any
+pi buffer that `switch-to-prev-buffer' may have landed on."
+  (let* ((chat-buf (pi-coding-agent--get-chat-buffer))
+         (input-buf (pi-coding-agent--get-input-buffer))
+         (pi-bufs (list chat-buf input-buf)))
+    ;; First pass: delete or bury each pi window
+    (dolist (buf pi-bufs)
+      (when (buffer-live-p buf)
+        (dolist (win (get-buffer-window-list buf nil t))
+          (if (> (length (window-list (window-frame win))) 1)
+              (delete-window win)
+            (with-selected-window win
+              (bury-buffer))))))
+    ;; Second pass: if bury-buffer landed on the paired pi buffer, bury again
+    (dolist (buf pi-bufs)
+      (when (buffer-live-p buf)
+        (dolist (win (get-buffer-window-list buf nil t))
+          (with-selected-window win
+            (bury-buffer)))))))
+
 ;;;; Buffer-Local Session Variables
 
 (defvar-local pi-coding-agent--process nil
diff --git a/pi-coding-agent.el b/pi-coding-agent.el
index 725dfa9..86eb918 100644
--- a/pi-coding-agent.el
+++ b/pi-coding-agent.el
@@ -41,7 +41,7 @@
 ;;     Install from: https://github.com/misohena/phscroll
 ;;
 ;; Usage:
-;;   M-x pi           Start a session in current project
+;;   M-x pi           DWIM: toggle, reuse, or create session
 ;;   C-u M-x pi       Start a named session
 ;;
 ;; Key Bindings:
@@ -129,24 +129,54 @@ Returns the chat buffer."
 ;;;###autoload
 (defun pi-coding-agent (&optional session)
   "Start or switch to pi coding agent session in current project.
-With prefix arg, prompt for SESSION name to allow multiple sessions.
-If already in a pi buffer and no SESSION specified, redisplays current session."
+Without prefix arg, DWIM behavior:
+  - From a pi buffer: toggle visibility (hide windows).
+  - Existing session for project: switch to it.
+  - No session: create a new one.
+With prefix arg, prompt for SESSION name to create a new session.
+When called from Lisp with a string SESSION, create or switch to
+that named session (backward-compatible)."
   (interactive
    (list (when current-prefix-arg
            (read-string "Session name: "))))
   (pi-coding-agent--check-dependencies)
-  (let (chat-buf input-buf)
-    (if (and (derived-mode-p 'pi-coding-agent-chat-mode 'pi-coding-agent-input-mode)
-             (not session))
-        ;; Already in pi buffer with no new session requested - use current session
-        (setq chat-buf (pi-coding-agent--get-chat-buffer)
-              input-buf (pi-coding-agent--get-input-buffer))
-      ;; Find or create session for current directory
-      (let ((dir (pi-coding-agent--session-directory)))
-        (setq chat-buf (pi-coding-agent--setup-session dir session))
-        (setq input-buf (buffer-local-value 'pi-coding-agent--input-buffer chat-buf))))
-    ;; Display and focus
-    (pi-coding-agent--display-buffers chat-buf input-buf)))
+  (cond
+   ;; In a pi buffer with no session arg: toggle visibility
+   ((and (derived-mode-p 'pi-coding-agent-chat-mode 'pi-coding-agent-input-mode)
+         (not session))
+    (pi-coding-agent--hide-buffers))
+   ;; Existing session for this project (no session arg): reuse it
+   ((and (not session)
+         (car (pi-coding-agent-project-buffers)))
+    (let* ((chat-buf (car (pi-coding-agent-project-buffers)))
+           (input-buf (buffer-local-value 'pi-coding-agent--input-buffer chat-buf)))
+      (pi-coding-agent--display-buffers chat-buf input-buf)))
+   ;; Otherwise: create or find named session
+   (t
+    (let* ((dir (pi-coding-agent--session-directory))
+           (chat-buf (pi-coding-agent--setup-session dir session))
+           (input-buf (buffer-local-value 'pi-coding-agent--input-buffer chat-buf)))
+      (pi-coding-agent--display-buffers chat-buf input-buf)))))
+
+;;;###autoload
+(defun pi-coding-agent-toggle ()
+  "Toggle pi coding agent window visibility for the current project.
+If pi windows are visible, hide them.  If hidden but a session
+exists, show them.  If no session exists, signal an error."
+  (interactive)
+  (let ((chat-buf (car (pi-coding-agent-project-buffers))))
+    (cond
+     ;; No session at all
+     ((null chat-buf)
+      (user-error "No pi session for this project"))
+     ;; Session visible: hide it
+     ((get-buffer-window-list chat-buf nil t)
+      (with-current-buffer chat-buf
+        (pi-coding-agent--hide-buffers)))
+     ;; Session hidden: show it
+     (t
+      (let ((input-buf (buffer-local-value 'pi-coding-agent--input-buffer chat-buf)))
+        (pi-coding-agent--display-buffers chat-buf input-buf))))))
 
 ;;;; Performance Optimizations
 
diff --git a/test/pi-coding-agent-test-common.el b/test/pi-coding-agent-test-common.el
index cb2160a..e8bf6a1 100644
--- a/test/pi-coding-agent-test-common.el
+++ b/test/pi-coding-agent-test-common.el
@@ -119,8 +119,8 @@ Automatically cleans up chat and input buffers."
                ((symbol-function 'pi-coding-agent--display-buffers) #'ignore))
        (unwind-protect
            (progn (pi-coding-agent) ,@body)
-         (ignore-errors (kill-buffer (pi-coding-agent--chat-buffer-name ,dir nil)))
-         (ignore-errors (kill-buffer (pi-coding-agent--input-buffer-name ,dir nil)))))))
+         (ignore-errors (kill-buffer (pi-coding-agent--buffer-name :chat ,dir nil)))
+         (ignore-errors (kill-buffer (pi-coding-agent--buffer-name :input ,dir nil)))))))
 
 ;;;; Two-Session Fixture
 
diff --git a/test/pi-coding-agent-test.el b/test/pi-coding-agent-test.el
index ff33c70..b1da447 100644
--- a/test/pi-coding-agent-test.el
+++ b/test/pi-coding-agent-test.el
@@ -30,5 +30,62 @@
     (with-current-buffer "*pi-coding-agent-input:/tmp/pi-coding-agent-test-modes/*"
       (should (derived-mode-p 'pi-coding-agent-input-mode)))))
 
+;;; DWIM & Toggle
+
+(ert-deftest pi-coding-agent-test-dwim-reuses-existing-session ()
+  "Calling `pi-coding-agent' from a non-pi buffer reuses the existing session."
+  (pi-coding-agent-test-with-mock-session "/tmp/pi-coding-agent-test-dwim/"
+    ;; Session exists; now call from a non-pi buffer in the same project
+    (with-temp-buffer
+      (setq default-directory "/tmp/pi-coding-agent-test-dwim/")
+      (cl-letf (((symbol-function 'project-current) (lambda (&rest _) nil))
+                ((symbol-function 'pi-coding-agent--display-buffers) #'ignore))
+        (pi-coding-agent))
+      ;; Should not have created a second chat buffer
+      (should (= 1 (length (cl-remove-if-not
+                             (lambda (b)
+                               (string-prefix-p "*pi-coding-agent-chat:/tmp/pi-coding-agent-test-dwim/"
+                                                (buffer-name b)))
+                             (buffer-list))))))))
+
+(ert-deftest pi-coding-agent-test-new-session-with-prefix-arg ()
+  "\\[universal-argument] \\[pi-coding-agent] creates a named session."
+  (pi-coding-agent-test-with-mock-session "/tmp/pi-coding-agent-test-named/"
+    (cl-letf (((symbol-function 'project-current) (lambda (&rest _) nil))
+              ((symbol-function 'pi-coding-agent--start-process) (lambda (_) nil))
+              ((symbol-function 'pi-coding-agent--display-buffers) #'ignore)
+              ((symbol-function 'read-string) (lambda (&rest _) "my-session")))
+      (let ((default-directory "/tmp/pi-coding-agent-test-named/")
+            (current-prefix-arg '(4)))
+        (call-interactively #'pi-coding-agent)))
+    (unwind-protect
+        (should (get-buffer "*pi-coding-agent-chat:/tmp/pi-coding-agent-test-named/<my-session>*"))
+      (ignore-errors
+        (kill-buffer "*pi-coding-agent-chat:/tmp/pi-coding-agent-test-named/<my-session>*"))
+      (ignore-errors
+        (kill-buffer "*pi-coding-agent-input:/tmp/pi-coding-agent-test-named/<my-session>*")))))
+
+(ert-deftest pi-coding-agent-test-project-buffers-finds-session ()
+  "`pi-coding-agent-project-buffers' returns chat buffer for the current project."
+  (pi-coding-agent-test-with-mock-session "/tmp/pi-coding-agent-test-projbuf/"
+    (let ((default-directory "/tmp/pi-coding-agent-test-projbuf/"))
+      (cl-letf (((symbol-function 'project-current) (lambda (&rest _) nil)))
+        (should (= 1 (length (pi-coding-agent-project-buffers))))
+        (should (string-prefix-p "*pi-coding-agent-chat:"
+                                 (buffer-name (car (pi-coding-agent-project-buffers)))))))))
+
+(ert-deftest pi-coding-agent-test-project-buffers-excludes-other-projects ()
+  "`pi-coding-agent-project-buffers' returns nil for a different project."
+  (pi-coding-agent-test-with-mock-session "/tmp/pi-coding-agent-test-projbuf-a/"
+    (let ((default-directory "/tmp/pi-coding-agent-test-projbuf-b/"))
+      (cl-letf (((symbol-function 'project-current) (lambda (&rest _) nil)))
+        (should (null (pi-coding-agent-project-buffers)))))))
+
+(ert-deftest pi-coding-agent-test-toggle-no-session-errors ()
+  "`pi-coding-agent-toggle' signals `user-error' when no session exists."
+  (let ((default-directory "/tmp/pi-coding-agent-test-no-session/"))
+    (cl-letf (((symbol-function 'project-current) (lambda (&rest _) nil)))
+      (should-error (pi-coding-agent-toggle) :type 'user-error))))
+
 (provide 'pi-coding-agent-test)
 ;;; pi-coding-agent-test.el ends here