diff --git a/docs/API.md b/docs/API.md
index d27cb70..717f09c 100644
--- a/docs/API.md
+++ b/docs/API.md
@@ -1485,7 +1485,7 @@ GET    /api/run/:id                   Run handle (returns live run state)
 DELETE /api/run/:id                   Kill (SIGTERM → SIGKILL after 5 s)
 ```
 
-**`POST /api/run` (start/attach):** Requires `laneId` (the lane this run belongs to). Creates or attaches an existing tmux session named `ccam-lane-<id>` in the lane's working directory. Optionally accepts `initialPrompt` to immediately type/send into the session (if empty or omitted, the session is created/attached with no initial input). Returns `{ id, laneId, status, cwd, model, permissionMode, effort, resumeSessionId, sessionId, startedAt, promptPreview }` where `id` is the tmux session name. The dashboard self-heals a lane's `run_id`/`status` on every read if the tmux session has been killed externally.
+**`POST /api/run` (start/attach):** Requires `laneId` (the lane this run belongs to). Creates or attaches an existing tmux session named `ccam-lane-<id>` in the lane's working directory. If that session already exists and its pane is idling at a shell prompt, the `claude` command line (including `--resume` and any initial prompt) is typed into that pane instead of being dropped; if the pane is running a program, the request adopts the session unchanged. Optionally accepts `initialPrompt` to immediately type/send into the session (if empty or omitted, the session is created/attached with no initial input). Returns `{ id, laneId, status, cwd, model, permissionMode, effort, resumeSessionId, sessionId, startedAt, promptPreview }` where `id` is the tmux session name. The dashboard self-heals a lane's `run_id`/`status` on every read if the tmux session has been killed externally.
 
 **PTY streaming:** Frames from the tmux pane are streamed to the client over `/ws-pty/:runId` as binary WebSocket frames (not JSON). The Workspace page's TerminalView component feeds these frames to xterm.js for live rendering. Simultaneously, `ccam lanes shell` can attach the same session via a real local terminal, staying in sync with the browser view.
 
diff --git a/docs/LANES.md b/docs/LANES.md
index 35bc60d..bc9cb42 100644
--- a/docs/LANES.md
+++ b/docs/LANES.md
@@ -327,6 +327,8 @@ An external session is deduped against a dashboard run with the same `session_id
 
 External rows have **no Attach action**: the dashboard owns no tmux session for them, so there is no PTY to bridge. Their action is **Resume**, which does what resuming from history does — `POST /api/lanes/ensure` for the session's `cwd`, then `POST /api/lanes/:id/start` with `resumeSessionId` — spawning a *new* tmux-backed `claude --resume <session>` in that folder. The original terminal keeps running; resuming gives you a second Claude Code process on the same transcript, not a view of the first one.
 
+**Start and Resume are create-or-reuse, and never silently swallow the request.** When the lane's `ccam-lane-<id>` tmux session does not exist, it is created with the full argv. When it exists but its pane is sitting at a **shell prompt** (a `ccam lanes shell` you opened, or a `claude` that has since exited), the argv is typed into that pane — so `--resume` really runs, and an initial prompt really lands, in the session you are already looking at. Only when the pane is running something (a live `claude`, an editor, a build) is the request adopted as-is: attaching shows you what is running rather than typing over it. Before this, an existing session was always adopted silently, so the first Resume after a `ccam lanes shell` answered `200` while doing nothing at all.
+
 The UI operates on a working directory (`cwd`), not a lane id. Starting a run in a `cwd` that no lane owns calls `POST /api/lanes/ensure` first, to idempotently find or adopt a lane for that path; a `cwd` an existing lane already owns is matched from the loaded lane list without a round trip. Either way the run is then started through `POST /api/lanes/:id/start` rather than directly through `POST /api/run`.
 
 ### Finding or adopting a lane: `POST /api/lanes/ensure`
diff --git a/server/__tests__/pty-run.test.js b/server/__tests__/pty-run.test.js
index 4bbfe9f..a9095c5 100644
--- a/server/__tests__/pty-run.test.js
+++ b/server/__tests__/pty-run.test.js
@@ -58,15 +58,46 @@ describe("pty-run", () => {
     assert.ok(newSessionCall.includes("opus"));
   });
 
-  it("spawnRun is a no-op (adopts) when the tmux session already exists", () => {
+  it("spawnRun is a no-op (adopts) when the existing session's pane runs claude", () => {
     const calls = [];
     tmux.__setExecImpl((args) => {
       calls.push(args);
+      if (args[0] === "display-message") return "claude\n";
       return ""; // has-session succeeds → already running
     });
     const handle = pty.spawnRun({ laneId: 7, cwd: "/tmp/repo" });
     assert.equal(handle.id, "ccam-lane-7");
     assert.ok(!calls.some((c) => c[0] === "new-session"), "must not create a duplicate session");
+    assert.ok(!calls.some((c) => c[0] === "send-keys"), "must not type over a live agent");
+  });
+
+  it("spawnRun types the argv into an existing session idling at a shell prompt", () => {
+    const calls = [];
+    tmux.__setExecImpl((args) => {
+      calls.push(args);
+      if (args[0] === "display-message") return "bash\n";
+      return ""; // has-session succeeds → session exists, pane is a shell
+    });
+    pty.spawnRun({ laneId: 8, cwd: "/tmp/repo", resumeSessionId: "abc12345" });
+    assert.ok(!calls.some((c) => c[0] === "new-session"), "must not create a duplicate session");
+    const literal = calls.find((c) => c[0] === "send-keys" && c[3] === "-l");
+    assert.ok(literal, "expected a literal send-keys with the command line");
+    assert.match(literal[4], /^'claude' .*'--resume' 'abc12345'$/);
+    assert.ok(
+      calls.some((c) => c[0] === "send-keys" && c[3] === "Enter"),
+      "expected the command to be submitted"
+    );
+  });
+
+  it("spawnRun single-quotes an initial prompt typed into an existing shell pane", () => {
+    let literal = null;
+    tmux.__setExecImpl((args) => {
+      if (args[0] === "display-message") return "zsh\n";
+      if (args[0] === "send-keys" && args[3] === "-l") literal = args[4];
+      return "";
+    });
+    pty.spawnRun({ laneId: 9, cwd: "/tmp/repo", initialPrompt: "don't; rm -rf /" });
+    assert.ok(literal.endsWith(`'don'\\''t; rm -rf /'`), literal);
   });
 
   it("spawnRun with resumeSessionId passes --resume in argv", () => {
diff --git a/server/lib/pty-run.js b/server/lib/pty-run.js
index 3cc3f19..627ab3c 100644
--- a/server/lib/pty-run.js
+++ b/server/lib/pty-run.js
@@ -9,6 +9,10 @@
  * out-of-band (crash, manual `tmux kill-session`, host reboot) self-corrects
  * on the next read instead of leaving a ghost "running" row.
  *
+ * Start/Resume is create-or-reuse: when the lane's tmux session already
+ * exists but its pane sits at a shell prompt, the argv is typed into that
+ * pane instead of being dropped on the floor by a silent adopt.
+ *
  * Every session is named `ccam-lane-<laneId>` so a real terminal can attach
  * to the exact same session (`tmux attach -t ccam-lane-<id>`, or
  * `ccam lanes shell`) — that's the whole point: the dashboard both creates
@@ -30,6 +34,8 @@ try {
 const RUN_ID_RE = /^ccam-lane-(\d+)$/;
 const EFFORT_LEVELS = new Set(["low", "medium", "high", "xhigh", "max"]);
 const ALLOWED_PERMISSION_MODES = new Set(["acceptEdits", "default", "plan", "bypassPermissions"]);
+// Pane commands that mean "idle shell prompt, safe to type a command into".
+const SHELL_COMMANDS = new Set(["sh", "bash", "zsh", "fish", "dash", "ksh", "csh", "tcsh"]);
 
 function runIdForLane(laneId) {
   return `ccam-lane-${laneId}`;
@@ -99,31 +105,44 @@ function spawnRun(args) {
 
   const id = runIdForLane(laneId);
   const startedAt = Date.now();
+  const argv = buildArgv({ model, permissionMode, effort, resumeSessionId, initialPrompt });
+
+  const record = () => {
+    if (!dashboardRuns) return;
+    dashboardRuns.recordRun({
+      id,
+      sessionId: resumeSessionId || null,
+      mode: null,
+      cwd,
+      model: model || null,
+      permissionMode: permissionMode || "acceptEdits",
+      effort: effort || null,
+      resumeSessionId: resumeSessionId || null,
+      prompt: initialPrompt || "",
+      status: "running",
+      startedAt,
+      endedAt: null,
+      exitCode: null,
+      laneId,
+    });
+  };
 
   if (!tmux.hasSession(id)) {
-    const argv = buildArgv({ model, permissionMode, effort, resumeSessionId, initialPrompt });
     tmux.newSession({ name: id, cwd, argv });
-    if (dashboardRuns) {
-      dashboardRuns.recordRun({
-        id,
-        sessionId: resumeSessionId || null,
-        mode: null,
-        cwd,
-        model: model || null,
-        permissionMode: permissionMode || "acceptEdits",
-        effort: effort || null,
-        resumeSessionId: resumeSessionId || null,
-        prompt: initialPrompt || "",
-        status: "running",
-        startedAt,
-        endedAt: null,
-        exitCode: null,
-        laneId,
-      });
-    }
+    record();
+  } else if (SHELL_COMMANDS.has(tmux.paneCommand(id) || "")) {
+    // The session exists but its pane is sitting at a bare shell prompt — a
+    // `ccam lanes shell`, or a `claude` that already exited. Adopting it
+    // silently here would swallow the whole request: a Resume would spawn no
+    // `--resume` and an initial prompt would never be typed, while the API
+    // still answered 200. Run the argv in the pane the user already sees
+    // instead of erroring or opening a second session.
+    tmux.sendCommand(id, argv);
+    record();
   }
-  // Already running: adopt silently, same convention as this repo's server
-  // port-adoption logic — no error, no duplicate session.
+  // Pane is running something (a live `claude`, an editor, a build): adopt
+  // silently, same convention as this repo's server port-adoption logic — no
+  // error, no duplicate session. Attaching shows the user what is running.
 
   return getRun(id);
 }
diff --git a/server/lib/tmux.js b/server/lib/tmux.js
index 26e91fb..9f1336c 100644
--- a/server/lib/tmux.js
+++ b/server/lib/tmux.js
@@ -6,6 +6,9 @@
  * `tmux attach -t ccam-lane-<id>` (or `ccam lanes shell`). Never builds a
  * shell string — every call is `execFileSync("tmux", [...argv])` with an
  * explicit argument array (matches this repo's rule for git in worktree.js).
+ * The one place a command line is composed is `sendCommand`, which types into
+ * an existing pane's shell: there the shell IS the consumer, so every argument
+ * is POSIX single-quoted first and sent with `send-keys -l` (literal).
  * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
  */
 
@@ -39,6 +42,36 @@ function newSession({ name, cwd, argv }) {
   execImpl(["new-session", "-d", "-s", name, "-c", cwd, "--", ...argv]);
 }
 
+/**
+ * The command currently running in the session's active pane (`bash`, `zsh`,
+ * `claude`, …). Null when tmux can't answer — callers treat that as "unknown,
+ * don't touch the pane".
+ */
+function paneCommand(name) {
+  try {
+    return (
+      execImpl(["display-message", "-p", "-t", name, "#{pane_current_command}"]).trim() || null
+    );
+  } catch {
+    return null;
+  }
+}
+
+/** POSIX single-quote escaping — the pane is a shell, so argv must be quoted. */
+function shellQuote(arg) {
+  return `'${String(arg).replace(/'/g, `'\\''`)}'`;
+}
+
+/**
+ * Type `argv` into an existing session's pane and press Enter. Only ever
+ * called when the pane sits at a shell prompt (see `paneCommand`); `-l` sends
+ * the string literally so no character is read as a tmux key name.
+ */
+function sendCommand(name, argv) {
+  execImpl(["send-keys", "-t", name, "-l", argv.map(shellQuote).join(" ")]);
+  execImpl(["send-keys", "-t", name, "Enter"]);
+}
+
 /** Idempotent — a session that's already gone is not an error. */
 function killSession(name) {
   try {
@@ -74,6 +107,8 @@ function isTmuxAvailable() {
 module.exports = {
   hasSession,
   newSession,
+  paneCommand,
+  sendCommand,
   killSession,
   listSessions,
   isTmuxAvailable,