fleetd #422 follow-up: gate FixedPlacementPolicy on modelOff too

FixedPlacementPolicy is the DEFAULT placement policy (PlacementPolicies.fromName
returns it for an absent/blank name) and it built its own inline candidate
filter instead of calling PlacementPolicyUtil.available(). That filter checked
quarantined/coolingOff/unreachable/excluded() but never modelOff(), so an
unqualified fleet_spawn on any fleet without an explicit placement: policy
could still land on a profile whose model the operator turned off.

- Add ctx.modelOff() to both filter sites: the default-profile fast path and
  the fallback walk over ctx.candidates().
- Add a modelOff refusal reason to the default-profile reasons list, worded as
  an operator decision ("turned off in models.allow"), matching
  enforceModelEnabled. Quarantine and cooling off still take priority when a
  profile is also model-off, matching CompositePeerLauncher's explicit-spawn
  check order.
- Update the two stale "excluded from automatic selection" messages to name
  model-off, consistent with PlacementPolicyUtil.emptyException.
- Update the class javadoc: four exceptions -> five, with a new bullet for
  model-off (fleetd #422).

Tests: PlacementPolicyTest gains fixedSkipsModelOffDefault (fast-path),
fixedFallbackWalkSkipsModelOffCandidate (fallback walk),
fixedThrowsWhenDefaultAndEveryCandidateModelOff (all-off refusal wording), and
fixedReportsQuarantineNotModelOffWhenBothApply (priority). CompositePeerLauncherTest
gains fixedPlacementSkipsAnOffModelProfileToo, an integration-level mirror of
the existing placementSkipsAnOffModelProfileAndRoutesToAnotherOne but under
PlacementPolicies.fixed(). The two existing weighted()-based tests are
untouched.

fleetd/src/main/java/dev/ltms/fleet/placement/FixedPlacementPolicy.java

@@ -12,7 +12,7 @@ import java.util.List;
  * checked for reachability up front, only skipped once it has already failed in <em>this same</em>
  * spawn call's retry loop — see the unreachable case below.
  *
- * <p>Four exceptions walk past the default instead of returning it unconditionally:
+ * <p>Five exceptions walk past the default instead of returning it unconditionally:
  * <ul>
  *   <li>Quarantine (CB-578 stage B): a quarantined default is a credential that just refused on
  *       a usage limit, not a transient capacity or reachability concern.
@@ -20,6 +20,14 @@ import java.util.List;
  *       ({@code BackendOutagePolicy}) — a separate, shorter-lived source from quarantine. When a
  *       profile is both quarantined and cooling off, only the quarantine reason is reported
  *       (exhaustion takes priority), matching {@code CompositePeerLauncher}'s explicit-spawn order.
+ *   <li>Model off (fleetd #422): a profile whose {@code model:} the operator has turned off in
+ *       {@code models.allow:} — an operator decision, never a backend-reported outage, so it is a
+ *       FOURTH, independent source from both quarantine and cooling off (never merged with either),
+ *       exactly as {@code CompositePeerLauncher.enforceModelEnabled} and {@link
+ *       PlacementPolicyUtil#available} treat it. When a profile is model-off <em>and</em> quarantined
+ *       or cooling off, only the quarantine/cooling-off reason is reported — those still take
+ *       priority, matching {@code CompositePeerLauncher}'s explicit-spawn check order (quarantine,
+ *       then cooling off, then max load, then model-off).
  *   <li>Unreachable (fleetd #315): {@code CompositePeerLauncher.spawn} retries a failed candidate
  *       on the next one and rebuilds the {@link PlacementContext} so {@code ctx.unreachable()}
  *       names every profile that already failed with {@code PeerUnreachableException} in this same
@@ -32,9 +40,10 @@ import java.util.List;
  *       {@code weighted}/{@code round-robin} skip it — an explicit {@code fleet_spawn} naming
  *       the profile is unaffected, only this automatic fallback walk.
  * </ul>
- * A fleet where nothing is ever quarantined, cooling off, unreachable, or weight-0 never exercises
- * any of these paths, so today's behaviour is unchanged — in particular, the very first selection
- * of a spawn call always sees an empty {@code unreachable} set, so the first choice is untouched.
+ * A fleet where nothing is ever quarantined, cooling off, model-off, unreachable, or weight-0 never
+ * exercises any of these paths, so today's behaviour is unchanged — in particular, the very first
+ * selection of a spawn call always sees an empty {@code unreachable} set, so the first choice is
+ * untouched.
  */
 final class FixedPlacementPolicy implements PlacementPolicy {
 
@@ -42,12 +51,13 @@ final class FixedPlacementPolicy implements PlacementPolicy {
     public PlacementCandidate select(PlacementContext ctx) {
         String d = ctx.defaultProfile();
         if (d != null && !d.isBlank() && !ctx.quarantined().contains(d) && !ctx.coolingOff().contains(d)
-                && !ctx.unreachable().contains(d) && !weightExcluded(ctx, d)) {
+                && !ctx.modelOff().contains(d) && !ctx.unreachable().contains(d) && !weightExcluded(ctx, d)) {
             return new PlacementCandidate(d, null, 1.0f, null);
         }
         for (PlacementCandidate c : ctx.candidates()) {
             if (!ctx.quarantined().contains(c.profile()) && !ctx.coolingOff().contains(c.profile())
-                    && !ctx.unreachable().contains(c.profile()) && !c.excluded()) {
+                    && !ctx.modelOff().contains(c.profile()) && !ctx.unreachable().contains(c.profile())
+                    && !c.excluded()) {
                 return new PlacementCandidate(c.profile(), null, c.weight(), c.maxLoad());
             }
         }
@@ -56,9 +66,14 @@ final class FixedPlacementPolicy implements PlacementPolicy {
             // Exhaustion quarantine takes priority: reported only when quarantine is absent, so the
             // message never claims "cooling off" for a profile that is really backend-exhausted.
             boolean dCoolingOff = !dQuarantined && ctx.coolingOff().contains(d);
+            // fleetd #422: model-off is a fourth, independent source (an operator decision) — but
+            // quarantine/cooling-off still take priority when more than one applies, matching
+            // CompositePeerLauncher's explicit-spawn check order (quarantine, cooling off, max load,
+            // then model-off).
+            boolean dModelOff = !dQuarantined && !dCoolingOff && ctx.modelOff().contains(d);
             boolean dUnreachable = ctx.unreachable().contains(d);
             boolean dWeightExcluded = weightExcluded(ctx, d);
-            if (dQuarantined || dCoolingOff || dUnreachable || dWeightExcluded) {
+            if (dQuarantined || dCoolingOff || dModelOff || dUnreachable || dWeightExcluded) {
                 List<String> reasons = new ArrayList<>();
                 if (dQuarantined) {
                     reasons.add("is quarantined (backend exhausted)");
@@ -66,6 +81,9 @@ final class FixedPlacementPolicy implements PlacementPolicy {
                 if (dCoolingOff) {
                     reasons.add("is cooling off after repeated backend errors");
                 }
+                if (dModelOff) {
+                    reasons.add("names a model the operator has turned off in models.allow");
+                }
                 if (dUnreachable) {
                     reasons.add("is unreachable");
                 }
@@ -78,7 +96,7 @@ final class FixedPlacementPolicy implements PlacementPolicy {
         }
         if (!ctx.candidates().isEmpty()) {
             throw new PlacementException("all worker profiles are excluded from automatic "
-                    + "selection (quarantined, cooling off, unreachable, or weight-0)");
+                    + "selection (quarantined, cooling off, model-off, unreachable, or weight-0)");
         }
         throw new PlacementException("no worker profiles configured");
     }

fleetd/src/test/java/dev/ltms/fleet/member/CompositePeerLauncherTest.java

@@ -1353,6 +1353,31 @@ class CompositePeerLauncherTest {
                         + e.getMessage());
     }
 
+    /**
+     * fleetd #422 follow-up: {@code fixed} is the DEFAULT placement policy ({@code
+     * PlacementPolicies.fromName} returns it for an absent/blank name), and it built its own inline
+     * candidate filter instead of calling {@code PlacementPolicyUtil.available()} — so it never
+     * checked {@code modelOff()}. Mirrors {@code placementSkipsAnOffModelProfileAndRoutesToAnotherOne}
+     * above with only the policy swapped, to prove the gate now fires on the path most fleets use.
+     */
+    @Test
+    void fixedPlacementSkipsAnOffModelProfileToo() {
+        FakeHerdr herdr = new FakeHerdr();
+        Map<String, FleetConfig.Profile> profiles = ordered(
+                "local", stubWorkerModel("local", "deepseek-v4-flash"),
+                "sonnet", stubWorkerModel("sonnet", "claude-sonnet-5"));
+        StubLauncher adapter = new StubLauncher("claude", herdr, profiles, "local", Set.of());
+        FleetConfig.Models models = new FleetConfig.Models(List.of(
+                new FleetConfig.Models.ModelEntry("deepseek-v4-flash", false)));
+        CompositePeerLauncher composite = new CompositePeerLauncher(List.of(adapter), "local", profiles,
+                PlacementPolicies.fixed(), _ -> 0, null, BackendQuarantine.none(), NO_OUTAGE,
+                () -> models);
+
+        PeerHandle h = composite.spawn(new SpawnRequest(null, null, null));
+        assertEquals("sonnet", h.profile(), "local's model is off, so an unqualified spawn must land on sonnet");
+        assertEquals(0, adapter.spawnCount("local"));
+    }
+
     /**
      * Criterion 4: turning a model off/on is HOT — no restart — proven through a REAL
      * {@code ConfigRef.reload()}, not a hand-rolled supplier swap. Also proves {@code models} is

fleetd/src/test/java/dev/ltms/fleet/placement/PlacementPolicyTest.java

@@ -90,6 +90,63 @@ class PlacementPolicyTest {
         assertTrue(e.getMessage().contains("quarantined"), e.getMessage());
     }
 
+    // --- fleetd #422: FixedPlacementPolicy must consult modelOff too, at BOTH filter sites ------
+
+    /** The default-profile fast path (:44) must skip a default whose model is off. */
+    @Test
+    void fixedSkipsModelOffDefault() {
+        PlacementPolicy policy = PlacementPolicies.fixed();
+        PlacementContext ctx = new PlacementContext("b",
+                List.of(PlacementCandidate.profile("a"), PlacementCandidate.profile("b")),
+                noSessions(), Set.of(), Set.of(), Set.of(), Set.of("b"));
+        assertEquals("a", policy.select(ctx).profile(),
+                "the default 'b' names an off model, so fixed falls through to the first available candidate");
+    }
+
+    /**
+     * The fallback walk (:48-53) must skip an off-model candidate too — exercised independently of
+     * the default-profile fast path by using no default at all, so this is the only filter that runs.
+     */
+    @Test
+    void fixedFallbackWalkSkipsModelOffCandidate() {
+        PlacementPolicy policy = PlacementPolicies.fixed();
+        PlacementContext ctx = new PlacementContext(null,
+                List.of(PlacementCandidate.profile("a"), PlacementCandidate.profile("b")),
+                noSessions(), Set.of(), Set.of(), Set.of(), Set.of("a"));
+        assertEquals("b", policy.select(ctx).profile(),
+                "candidate 'a' names an off model, so the fallback walk skips it and picks 'b'");
+    }
+
+    @Test
+    void fixedThrowsWhenDefaultAndEveryCandidateModelOff() {
+        PlacementPolicy policy = PlacementPolicies.fixed();
+        PlacementContext ctx = new PlacementContext("b",
+                List.of(PlacementCandidate.profile("a"), PlacementCandidate.profile("b")),
+                noSessions(), Set.of(), Set.of(), Set.of(), Set.of("a", "b"));
+        PlacementException e = assertThrows(PlacementException.class, () -> policy.select(ctx));
+        assertTrue(e.getMessage().contains("turned off"),
+                "message names model-off as the cause: " + e.getMessage());
+        assertFalse(e.getMessage().contains("quarantined"), "must not read like quarantine: " + e.getMessage());
+        assertFalse(e.getMessage().contains("cooling off"), "must not read like cool-off: " + e.getMessage());
+        assertFalse(e.getMessage().contains("weight 0"), "must not read like weight-0: " + e.getMessage());
+    }
+
+    /**
+     * Quarantine still wins when a profile is both quarantined and model-off (mirrors {@code
+     * fixedThrowsWhenDefaultAndEveryCandidateQuarantined}'s priority over cooling off).
+     */
+    @Test
+    void fixedReportsQuarantineNotModelOffWhenBothApply() {
+        PlacementPolicy policy = PlacementPolicies.fixed();
+        PlacementContext ctx = new PlacementContext("b",
+                List.of(PlacementCandidate.profile("a"), PlacementCandidate.profile("b")),
+                noSessions(), Set.of(), Set.of("a", "b"), Set.of(), Set.of("a", "b"));
+        PlacementException e = assertThrows(PlacementException.class, () -> policy.select(ctx));
+        assertTrue(e.getMessage().contains("quarantined"), e.getMessage());
+        assertFalse(e.getMessage().contains("turned off"),
+                "quarantine takes priority over model-off in the message: " + e.getMessage());
+    }
+
     @Test
     void roundRobinCyclesThroughAvailableProfiles() {
         PlacementPolicy policy = PlacementPolicies.roundRobin();