Merge pull request #226 from christianhaberl/fix/input-loss-on-failed-delivery

fix(api,ws): an input whose delivery fails can be retried instead of being lost

src/session.ts

@@ -2620,20 +2620,23 @@ export class Session extends EventEmitter {
    * For interactive sessions, this is how you send user input to Claude.
    * Remember to include `\r` (carriage return) to simulate pressing Enter.
    *
-   * @param data - The input data to send (text, escape sequences, etc.)
-   *
    * @example
    * ```typescript
    * session.write('hello world');  // Text only, no Enter
    * session.write('\r');           // Enter key
    * session.write('ls -la\r');     // Command with Enter
    * ```
+   *
+   * @param data - The input data to send (text, escape sequences, etc.)
+   * @returns true if the data reached a PTY. A session whose PTY is gone still
+   * discards the data, but it used to do so with no signal at all — which is how
+   * input could disappear while the caller believed it had been delivered.
    */
-  write(data: string): void {
+  write(data: string): boolean {
     this._trackSubmit(data);
-    if (this.ptyProcess) {
-      this.ptyProcess.write(data);
-    }
+    if (!this.ptyProcess) return false;
+    this.ptyProcess.write(data);
+    return true;
   }
 
   // ── Conversation tracking ─────────────────────────────────────────────
@@ -2691,6 +2694,23 @@ export class Session extends EventEmitter {
     return true;
   }
 
+  /**
+   * Undo the bookkeeping of {@link shouldApplyInput} for a delivery that failed.
+   *
+   * Without this, the reliable-delivery layer guarantees exactly-once delivery of
+   * something that may never have been delivered: the seq is recorded as applied
+   * BEFORE the write is attempted, so a client retry — the very mechanism the seq
+   * exists for — is rejected as a duplicate and the input is lost for good.
+   *
+   * Only rolls back if `seq` is still the newest recorded one; a later input has
+   * already superseded it and must not be re-opened.
+   */
+  forgetInputSeq(clientId: string, seq: number): void {
+    if (this._appliedInputSeq.get(clientId) === seq) {
+      this._appliedInputSeq.set(clientId, seq - 1);
+    }
+  }
+
   /**
    * Sends input via the terminal multiplexer's direct input mechanism.
    *

src/web/routes/session-routes.ts

@@ -879,28 +879,41 @@ export function registerSessionRoutes(
     // Reliable delivery (POST fallback when the WebSocket is down): a 2xx IS the
     // client's ACK, so a tagged duplicate redelivery must still return 200 but
     // skip the write. Untagged requests (curl/legacy) always apply.
-    if (typeof clientId === 'string' && typeof seq === 'number' && !session.shouldApplyInput(clientId, seq)) {
+    const tagged = typeof clientId === 'string' && typeof seq === 'number';
+    if (tagged && !session.shouldApplyInput(clientId as string, seq as number)) {
       return {};
     }
 
     // Write input to PTY. Direct write is synchronous; writeViaMux
     // (tmux send-keys) is fire-and-forget to avoid blocking the HTTP response.
     if (useMux) {
-      // Fire-and-forget: don't block HTTP response on tmux child process.
-      // Fallback to direct write on failure.
+      // Fire-and-forget: don't block the HTTP response on a tmux child process.
+      // Fallback to a direct write on failure.
+      //
+      // Because the response has already been sent by then, a failure here is the
+      // one case the caller can never learn about — so the dedup bookkeeping is
+      // rolled back. Otherwise the seq stays recorded as applied and a retry, the
+      // very mechanism reliable delivery exists for, is rejected as a duplicate.
+      const undoOnFailure = () => {
+        if (tagged) session.forgetInputSeq(clientId as string, seq as number);
+      };
       session
         .writeViaMux(inputStr)
         .then((ok) => {
-          if (!ok) {
-            console.warn(`[Server] writeViaMux failed for session ${id}, falling back to direct write`);
-            session.write(inputStr);
-          }
+          if (ok) return;
+          console.warn(`[Server] writeViaMux failed for session ${id}, falling back to direct write`);
+          if (!session.write(inputStr)) undoOnFailure();
         })
         .catch(() => {
-          session.write(inputStr);
+          if (!session.write(inputStr)) undoOnFailure();
         });
     } else {
-      session.write(inputStr);
+      // Same rollback. NOT an error response, deliberately: a session can
+      // legitimately have no PTY yet (created but not started), and callers have
+      // always been able to write to one without a 4xx.
+      if (!session.write(inputStr) && tagged) {
+        session.forgetInputSeq(clientId as string, seq as number);
+      }
     }
     return {};
   });

src/web/routes/ws-routes.ts

@@ -180,13 +180,19 @@ export function registerWsRoutes(app: FastifyInstance, ctx: SessionPort, getHost
             const cid = typeof msg.cid === 'string' ? msg.cid : null;
             const seq = Number.isInteger(msg.seq) ? (msg.seq as number) : null;
             const apply = cid && seq !== null ? session.shouldApplyInput(cid, seq) : true;
+            let delivered = true;
             if (apply) {
               // Typed input from a claim-holding desktop keeps the claim "hot"
               // and re-asserts the desktop layout after a mobile override.
               if (holdsDesktopClaim) session.noteDesktopActivity();
-              session.write(msg.d);
+              delivered = session.write(msg.d);
+              // A session whose PTY is gone swallows the write. ACKing anyway told
+              // the client to drop the frame from its durable queue and left the seq
+              // burnt, so the retry that reliable delivery exists for was rejected as
+              // a duplicate: the input was lost for good.
+              if (!delivered && cid && seq !== null) session.forgetInputSeq(cid, seq);
             }
-            if (seq !== null && socket.readyState === 1) {
+            if (delivered && seq !== null && socket.readyState === 1) {
               socket.send(`{"t":"ia","seq":${seq}}`);
             }
           } else if (