fleetd #302: require content in MessageService.reply so a REST reply with no content cannot silently resolve a waiter

fleetd/src/main/java/dev/ltms/fleet/msg/MessageService.java

@@ -408,10 +408,28 @@ public final class MessageService {
      * as the zero-candidate case does, and let {@link #abandon} apply the eventual recovery
      * deterministically instead.
      *
+     * <p><strong>{@code content} is required (fleetd #302).</strong> Both doors that reach this
+     * method must reject a missing/blank reply the same way, so the check lives here rather than in
+     * either caller: {@code FleetMcp.reply} already refuses a {@code null} content before it ever
+     * calls this method (its own required-arg guard), and no test or production call site anywhere
+     * in the codebase relies on replying with empty content — confirmed by searching every call site
+     * of this method before adding the check, not assumed. Without this guard, a REST {@code
+     * POST /sessions/{id}/reply} whose body omits {@code content} (or a client library that maps a
+     * missing field to {@code ""}) used to reach {@link Rendezvous#resolve} with an empty string,
+     * silently completing the lead's blocking wait with nothing — indistinguishable from a worker
+     * that genuinely replied with nothing, which is worse than a loud failure because it destroys the
+     * information that the reply never arrived.
+     *
+     * @throws IllegalArgumentException if {@code content} is {@code null} or blank — the caller must
+     *         report this as a client error (REST: 400 {@code bad_request}) rather than resolve
+     *         anything
      * @return always {@code true} — the reply resolved a live send, completed a parked ticket, or
      *         was queued
      */
     public boolean reply(String session, String content) {
+        if (content == null || content.isBlank()) {
+            throw new IllegalArgumentException("content is required");
+        }
         if (rendezvous.resolve(session, content)) {
             count(FleetMetrics.REPLIES, "path", "rendezvous");
             return true;                       // a live send took it — unchanged fast path

fleetd/src/main/java/dev/ltms/fleet/rest/FleetApp.java

@@ -684,7 +684,19 @@ public final class FleetApp {
             ctx.status(400).json(Map.of("error", "bad_request", "detail", "body must be JSON"));
             return;
         }
-        messages.reply(id, content);
+        // fleetd #302: content is required. `.path("content").asText("")` above turns a missing key
+        // into "" rather than throwing, so without this check an empty/blank reply used to reach
+        // messages.reply(...) and silently resolve the lead's waiter — the same class of bug as the
+        // sibling "content is required" guards on sendMessage/askMessage below, except this one wrote
+        // a WRONG value instead of failing loudly. The check lives in MessageService.reply so both
+        // this door and FleetMcp.reply inherit the same rule; this catch only translates it into the
+        // {error, detail} envelope this file uses everywhere else.
+        try {
+            messages.reply(id, content);
+        } catch (IllegalArgumentException e) {
+            ctx.status(400).json(Map.of("error", "bad_request", "detail", e.getMessage()));
+            return;
+        }
         ctx.status(200).json(Map.of("sessionId", id, "delivered", true));
     }
 

fleetd/src/test/java/dev/ltms/fleet/rest/FleetAppTest.java

@@ -532,6 +532,53 @@ class FleetAppTest {
         assertEquals("orphan", body.get("replies").get(0).get("content").asText());
     }
 
+    @Test
+    void replyWithMissingContentIsRejectedAndDoesNotResolveTheWaiter() throws Exception {
+        // fleetd #302: `.path("content").asText("")` used to turn a missing "content" key into an
+        // empty string that reached rendezvous.resolve, silently completing the lead's blocking wait
+        // with nothing. Prove the fix two ways: the bad call is rejected with 400, AND the real send
+        // it would have wrongly resolved is still open afterwards — a real reply completes it.
+        FakeHerdr herdr = new FakeHerdr().agentStatus("idle");
+        int port = start(herdr, "http://gx00.gw:8000", Set.of("gx00.gw"));
+
+        var send = java.util.concurrent.CompletableFuture.supplyAsync(() -> {
+            try { return postMessage(port, "{\"content\":\"review this\",\"timeoutMs\":4000}"); }
+            catch (Exception e) { throw new RuntimeException(e); }
+        });
+
+        Thread.sleep(200); // let the background send open its rendezvous waiter
+
+        HttpResponse<String> badReply = postJson(port, "/sessions/term_a/reply", "{}");
+        assertEquals(400, badReply.statusCode());
+        JsonNode err = mapper.readTree(badReply.body());
+        assertEquals("bad_request", err.get("error").asText());
+        assertTrue(err.has("detail"));
+
+        // The waiter must still be open — a real reply now completes the ORIGINAL send.
+        HttpResponse<String> goodReply = postJson(port, "/sessions/term_a/reply", "{\"content\":\"LGTM ship it\"}");
+        assertEquals(200, goodReply.statusCode());
+
+        HttpResponse<String> res = send.get(6, java.util.concurrent.TimeUnit.SECONDS);
+        assertEquals(200, res.statusCode());
+        assertEquals("LGTM ship it", mapper.readTree(res.body()).get("reply").asText());
+    }
+
+    @Test
+    void replyWithEmptyOrWhitespaceContentIsRejectedSameAsMissing() throws Exception {
+        // fleetd #302 sibling case: present-but-blank content is treated the same as a missing key —
+        // FleetMcp's own required-content guard (fleet_reply's "content is required") makes no
+        // distinction between the two either, so diverging here would be a new asymmetry.
+        int port = startHealthy();
+
+        HttpResponse<String> empty = postJson(port, "/sessions/term_a/reply", "{\"content\":\"\"}");
+        assertEquals(400, empty.statusCode());
+        assertEquals("bad_request", mapper.readTree(empty.body()).get("error").asText());
+
+        HttpResponse<String> whitespace = postJson(port, "/sessions/term_a/reply", "{\"content\":\"   \"}");
+        assertEquals(400, whitespace.statusCode());
+        assertEquals("bad_request", mapper.readTree(whitespace.body()).get("error").asText());
+    }
+
     @Test
     void drainRepliesReturnsEmptyForNoReplies() throws Exception {
         int port = startHealthy();