diff --git a/README.md b/README.md
index 9ac1c22..0c65ba7 100644
--- a/README.md
+++ b/README.md
@@ -65,7 +65,7 @@ scripts/               mermaid-validator.mjs (node process used by config/mermai
 
 **Search**: `fff.nvim` handles files and grep (frecency, respects `.gitignore` and `.ignore`). snacks.picker handles everything else. `config/ignore_patterns.lua` holds the shared ignore list used by neo-tree and the snacks pickers.
 
-**Pane navigation** (`config/pane_nav.lua`): `<C-h/j/k/l>` moves between Neovim splits first. At the outermost split, the move goes to herdr, tmux, or kitty, whichever is the nearest ancestor process. Resize is plain `:resize` on `<C-Up/Down/Left/Right>`; smart-splits is disabled.
+**Pane navigation** (`config/pane_nav.lua`): `<C-h/j/k/l>` moves between Neovim splits first. At the outermost split, the move goes to herdr, tmux, or kitty, whichever is the nearest ancestor process. Under tmux the move goes through smart-splits.nvim, loaded at startup, which flags the pane `@pane-is-vim` for `tmux.conf`. There, `<C-Up/Down/Left/Right>` and `<M-h/j/k/l>` are smart-splits resize and cross into tmux panes at the edge. Elsewhere smart-splits stays unloaded and resize is plain `:resize` on `<C-Up/Down/Left/Right>`.
 
 **Mermaid** (`config/mermaid.lua`): tree-sitter highlights ```` ```mermaid ```` fences in markdown/MDX and `.mmd` files. Diagnostics come from mermaid's own parser running in a node process, which exits after 5 idle minutes. Run `:MermaidCheck` to re-validate.
 
@@ -82,7 +82,6 @@ Core or community plugins turned off in `plugins/user.lua` because something els
 | Plugin | Replaced by |
 | --- | --- |
 | `none-ls.nvim`, `mason-null-ls.nvim` | conform (`mason-null-ls` would auto-register every Mason tool as a second formatter/linter client) |
-| `smart-splits.nvim` | `config/pane_nav.lua` + native `:resize` |
 | `toggleterm.nvim` | kitty / snacks terminals |
 | `lsp_signature.nvim` (not imported) | astrolsp's built-in signature help (`features.signature_help = true`) |
 | `refactoring.nvim`, `dev-tools.nvim` (not imported) | LSP code actions (`<C-a>a`) |
diff --git a/lua/config/pane_nav.lua b/lua/config/pane_nav.lua
index 4af482d..3dbae04 100644
--- a/lua/config/pane_nav.lua
+++ b/lua/config/pane_nav.lua
@@ -141,6 +141,11 @@ end
 --- @param dir "left"|"down"|"up"|"right"
 function M.navigate(dir)
   local from = vim.api.nvim_get_current_win()
+  -- Under tmux smart-splits owns the edge handoff (plugins/user.lua).
+  if M.env() == "tmux" and vim.api.nvim_win_get_config(from).relative == "" then
+    require("smart-splits")["move_cursor_" .. dir]()
+    return
+  end
   vim.cmd.wincmd(directions[dir].wincmd)
   if vim.api.nvim_get_current_win() ~= from then return end
   if vim.api.nvim_win_get_config(0).relative ~= "" then return end
diff --git a/lua/plugins/pane-nav.lua b/lua/plugins/pane-nav.lua
index 6562b87..355b249 100644
--- a/lua/plugins/pane-nav.lua
+++ b/lua/plugins/pane-nav.lua
@@ -1,10 +1,11 @@
 --- <C-h/j/k/l> pane navigation that follows whatever multiplexer Neovim is
 --- running under (herdr, tmux, or bare kitty) — see config.pane_nav.
 ---
---- This replaces AstroNvim's smart-splits mappings (which only know
---- tmux/wezterm/kitty and would move the wrong layer inside herdr). The plugin
---- is disabled in plugins/user.lua; its <C-Up/Down/Left/Right> resize binds
---- are plain :resize here.
+--- These entries overwrite AstroNvim's smart-splits mappings, which only know
+--- tmux/wezterm/kitty and would move the wrong layer inside herdr. Under tmux
+--- they hand back to smart-splits: navigation through config.pane_nav, resize
+--- directly, plus <M-h/j/k/l> resize, the keys tmux.conf forwards to an
+--- @pane-is-vim pane. Elsewhere <C-Up/Down/Left/Right> are plain :resize.
 
 local nav = require "config.pane_nav"
 
@@ -18,11 +19,20 @@ local moves = {
 }
 
 --- AstroNvim's key spelling again, overriding the smart-splits resize entries.
+--- dir is the smart-splits direction under tmux; cmd the plain fallback.
 local resizes = {
-  ["<C-Up>"] = { "resize +2", "Resize split taller" },
-  ["<C-Down>"] = { "resize -2", "Resize split shorter" },
-  ["<C-Left>"] = { "vertical resize -2", "Resize split narrower" },
-  ["<C-Right>"] = { "vertical resize +2", "Resize split wider" },
+  ["<C-Up>"] = { dir = "up", cmd = "resize +2", desc = "Resize split taller" },
+  ["<C-Down>"] = { dir = "down", cmd = "resize -2", desc = "Resize split shorter" },
+  ["<C-Left>"] = { dir = "left", cmd = "vertical resize -2", desc = "Resize split narrower" },
+  ["<C-Right>"] = { dir = "right", cmd = "vertical resize +2", desc = "Resize split wider" },
+}
+
+--- tmux.conf's vim-aware resize keys; only mapped under tmux.
+local tmux_resizes = {
+  ["<M-h>"] = "left",
+  ["<M-j>"] = "down",
+  ["<M-k>"] = "up",
+  ["<M-l>"] = "right",
 }
 
 return {
@@ -36,8 +46,18 @@ return {
       maps.x[key] = maps.n[key]
       maps.t[key] = { nav.terminal(dir), desc = desc }
     end
+    local tmux = nav.env() == "tmux"
     for key, spec in pairs(resizes) do
-      maps.n[key] = { "<Cmd>" .. spec[1] .. "<CR>", desc = spec[2] }
+      if tmux then
+        maps.n[key] = { function() require("smart-splits")["resize_" .. spec.dir]() end, desc = "Resize split " .. spec.dir }
+      else
+        maps.n[key] = { "<Cmd>" .. spec.cmd .. "<CR>", desc = spec.desc }
+      end
+    end
+    if tmux then
+      for key, dir in pairs(tmux_resizes) do
+        maps.n[key] = { function() require("smart-splits")["resize_" .. dir]() end, desc = "Resize split " .. dir .. " (tmux aware)" }
+      end
     end
   end,
 }
diff --git a/lua/plugins/user.lua b/lua/plugins/user.lua
index 78cf7f2..7c034bd 100644
--- a/lua/plugins/user.lua
+++ b/lua/plugins/user.lua
@@ -53,8 +53,21 @@ return {
   { "nvimtools/none-ls.nvim", enabled = false },
   { "jay-babu/mason-null-ls.nvim", enabled = false },
 
-  -- <C-h/j/k/l> is config.pane_nav; resize binds are native (plugins/pane-nav.lua)
-  { "mrjones2014/smart-splits.nvim", enabled = false },
+  -- Under tmux, smart-splits drives <C-h/j/k/l> and resizing (config.pane_nav
+  -- and plugins/pane-nav.lua delegate to it) and flags the pane @pane-is-vim
+  -- for tmux.conf. herdr and kitty stay on config.pane_nav, so its env() picks
+  -- the integration instead of smart-splits' own env-var guess. Loaded at
+  -- startup under tmux so the flag is set before the first keypress
+  -- (AstroNvim's early load keys off TERM_PROGRAM, which tmux.conf passes
+  -- through from the outer terminal).
+  {
+    "mrjones2014/smart-splits.nvim",
+    lazy = require("config.pane_nav").env() ~= "tmux",
+    opts = function(_, opts)
+      opts.at_edge = "wrap"
+      opts.multiplexer_integration = require("config.pane_nav").env() == "tmux" and "tmux" or false
+    end,
+  },
 
   -- lsp_signature.nvim is gone; astrolsp's own signature help replaces it
   { "AstroNvim/astrolsp", opts = { features = { signature_help = true } } },