diff --git a/fleetd/src/main/java/dev/ltms/fleet/config/FleetConfig.java b/fleetd/src/main/java/dev/ltms/fleet/config/FleetConfig.java
index 65440ff..f751599 100644
--- a/fleetd/src/main/java/dev/ltms/fleet/config/FleetConfig.java
+++ b/fleetd/src/main/java/dev/ltms/fleet/config/FleetConfig.java
@@ -2680,10 +2680,13 @@ public record FleetConfig(
 
     /**
      * Reject a member tab-label template that could render as a configured lead tab or match a
-     * lead-tab naming convention.
+     * lead-tab naming convention, and reject two {@code fleet.leaders} entries that share one exact
+     * tab.
      *
      * @throws IllegalStateException when the fleet template or a profile {@code tabLabel} override
-     *                               can render as a configured lead tab or match a lead-tab prefix
+     *                               can render as a configured lead tab or match a lead-tab prefix,
+     *                               or when two {@code fleet.leaders} entries carry the same exact
+     *                               {@code tab} (case-insensitively)
      */
     public void validateLeadTabPrefixes() {
         if (fleet == null || fleet.leaders().isEmpty()) {
@@ -2719,13 +2722,40 @@ public record FleetConfig(
                         }
                     });
         });
-        if (bad.isEmpty()) {
+        if (!bad.isEmpty()) {
+            throw new IllegalStateException("refusing to start: " + String.join("; ", bad)
+                    + ". Every member labelled that way would be read back as a lead and granted "
+                    + "spawn/stop/send on the whole fleet. Change one of the two so member tabs "
+                    + "and lead tabs cannot be confused.");
+        }
+
+        List<String> collisions = new ArrayList<>();
+        List<String> leadNames = fleet.leaders().keySet().stream().sorted().toList();
+        for (int i = 0; i < leadNames.size(); i++) {
+            String nameA = leadNames.get(i);
+            Leader a = fleet.leaders().get(nameA);
+            if (a == null || a.tab() == null || a.tab().isBlank()) {
+                continue;
+            }
+            for (int j = i + 1; j < leadNames.size(); j++) {
+                String nameB = leadNames.get(j);
+                Leader b = fleet.leaders().get(nameB);
+                if (b == null || b.tab() == null || b.tab().isBlank()) {
+                    continue;
+                }
+                if (a.tab().equalsIgnoreCase(b.tab())) {
+                    collisions.add("lead '" + nameA + "' and lead '" + nameB + "' both use tab \""
+                            + a.tab() + "\"");
+                }
+            }
+        }
+        if (collisions.isEmpty()) {
             return;
         }
-        throw new IllegalStateException("refusing to start: " + String.join("; ", bad)
-                + ". Every member labelled that way would be read back as a lead and granted "
-                + "spawn/stop/send on the whole fleet. Change one of the two so member tabs and "
-                + "lead tabs cannot be confused.");
+        throw new IllegalStateException("refusing to start: " + String.join("; ", collisions)
+                + ". Tab identity is matched exactly, so only one of two leads sharing a tab can "
+                + "ever be found — the other is silently unreachable. Give each lead its own "
+                + "exact tab.");
     }
 
     private static boolean templateCanRenderAs(String template, String tab) {
diff --git a/fleetd/src/test/java/dev/ltms/fleet/config/FleetConfigTest.java b/fleetd/src/test/java/dev/ltms/fleet/config/FleetConfigTest.java
index 379858a..47e93b2 100644
--- a/fleetd/src/test/java/dev/ltms/fleet/config/FleetConfigTest.java
+++ b/fleetd/src/test/java/dev/ltms/fleet/config/FleetConfigTest.java
@@ -774,6 +774,51 @@ class FleetConfigTest {
                 "a label that collides with a convention nobody reads is not a problem");
     }
 
+    /**
+     * fleetd #677: identity is matched on a lead's exact {@code tab} alone, so two leads sharing
+     * one tab means only one of them is ever found — the guard must catch this independently of
+     * the member-template checks above.
+     */
+    @Test
+    void twoLeadsSharingTheSameExactTabRefusesToStart(@TempDir Path dir) throws Exception {
+        Path f = dir.resolve("shared-tab.yaml");
+        Files.writeString(f, """
+                bind:
+                  port: 8080
+                fleet:
+                  leaders:
+                    opus:
+                      tab: "shared tab"
+                    sonnet:
+                      tab: "shared tab"
+                """);
+        FleetConfig cfg = FleetConfig.load(f);
+
+        IllegalStateException e =
+                assertThrows(IllegalStateException.class, cfg::validateLeadTabPrefixes);
+        assertTrue(e.getMessage().contains("opus"), "the message must name one offending lead");
+        assertTrue(e.getMessage().contains("sonnet"), "the message must name the other offending lead");
+    }
+
+    /** Control for {@link #twoLeadsSharingTheSameExactTabRefusesToStart}: distinct tabs load cleanly. */
+    @Test
+    void twoLeadsWithDistinctExactTabsAreAllowed(@TempDir Path dir) throws Exception {
+        Path f = dir.resolve("distinct-tabs.yaml");
+        Files.writeString(f, """
+                bind:
+                  port: 8080
+                fleet:
+                  leaders:
+                    opus:
+                      tab: "opus tab"
+                    sonnet:
+                      tab: "sonnet tab"
+                """);
+        FleetConfig cfg = FleetConfig.load(f);
+
+        assertDoesNotThrow(cfg::validateLeadTabPrefixes);
+    }
+
     // ── validatePanePlacementAgainstLeadTabs ────────────────────────────────────────────────────
 
     /**