fleetd #435: make FixedPlacementPolicy honor maxLoad

FixedPlacementPolicy (the default placement policy) never consulted
maxLoad, so an at-cap default was chosen anyway on every unqualified
spawn -- the cap was advisory, not enforced, for the one policy every
config uses by default. weighted/round-robin already gated on it via
PlacementPolicyUtil.available().

Extract the "at cap" predicate into PlacementPolicyUtil.atCap(ctx, c)
so all three policies share one definition, and consult it at both of
FixedPlacementPolicy's filter sites (the default fast path and the
candidate walk), mirroring the existing weightExcluded pattern. An
at-cap default now falls through to the next candidate instead of
refusing the spawn -- only when every candidate is unusable does the
policy still throw, naming the cap in the message. Update the class
javadoc (five exceptions -> six) and the reason-priority comments to
match CompositePeerLauncher's explicit-spawn order (quarantine,
cooling off, max load, model-off).

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

@@ -5,14 +5,12 @@ import java.util.List;
 
 /**
  * Backward-compatible placement: an unqualified spawn always resolves to the configured default
- * profile, exactly as {@code CompositePeerLauncher} did before CB-518. This ignores caps
- * ({@code maxLoad}) so that a pre-existing config behaves identically after upgrade — capacity
- * gating for automatic placement is deliberately out of scope for {@code fixed}, exactly as it
- * always has been. Reachability is a narrower exception (fleetd #315, below): a profile is never
- * 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.
+ * profile, exactly as {@code CompositePeerLauncher} did before CB-518. Reachability is a narrower
+ * exception (fleetd #315, below): a profile is never 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>Five exceptions walk past the default instead of returning it unconditionally:
+ * <p>Six 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.
@@ -20,14 +18,24 @@ 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>At cap (fleetd #435): a profile whose live count has reached its {@code maxLoad}
+ *       ({@link PlacementPolicyUtil#atCap}) — a documented, unconditional capacity limit (see
+ *       {@code FleetConfig.Profile#maxLoad}), so {@code fixed} must gate on it exactly as {@code
+ *       weighted}/{@code round-robin} already do via {@link PlacementPolicyUtil#available}. Before
+ *       this fix {@code fixed} built its own {@link PlacementCandidate} for the default with {@code
+ *       maxLoad} forced to {@code null}, so a capped default was chosen anyway on every unqualified
+ *       spawn — the cap was advisory, not enforced, for the one placement policy every config uses
+ *       by default. Reported only when quarantine and cooling off are both absent, matching {@code
+ *       CompositePeerLauncher}'s explicit-spawn check order (quarantine, then cooling off, then max
+ *       load, then model-off).
  *   <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).
+ *       fifth, independent source from quarantine, cooling off, and at-cap (never merged with any
+ *       of them), exactly as {@code CompositePeerLauncher.enforceModelEnabled} and {@link
+ *       PlacementPolicyUtil#available} treat it. When a profile is model-off <em>and</em> quarantined,
+ *       cooling off, or at cap, only the higher-priority reason is reported, 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
@@ -40,10 +48,10 @@ 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, 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.
+ * A fleet where nothing is ever quarantined, cooling off, at cap, 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 {
 
@@ -51,13 +59,14 @@ 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.modelOff().contains(d) && !ctx.unreachable().contains(d) && !weightExcluded(ctx, d)) {
+                && !ctx.modelOff().contains(d) && !ctx.unreachable().contains(d) && !weightExcluded(ctx, d)
+                && !capExcluded(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.modelOff().contains(c.profile()) && !ctx.unreachable().contains(c.profile())
-                    && !c.excluded()) {
+                    && !c.excluded() && !PlacementPolicyUtil.atCap(ctx, c)) {
                 return new PlacementCandidate(c.profile(), null, c.weight(), c.maxLoad());
             }
         }
@@ -66,14 +75,18 @@ 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
+            // fleetd #435: at-cap sits between cooling off and model-off, matching
+            // CompositePeerLauncher's explicit-spawn check order (quarantine, cooling off, max load,
+            // then model-off) — reported only when quarantine/cooling-off are both absent.
+            boolean dAtCap = !dQuarantined && !dCoolingOff && capExcluded(ctx, d);
+            // fleetd #422: model-off is a fifth, independent source (an operator decision) — but
+            // quarantine/cooling-off/at-cap 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 dModelOff = !dQuarantined && !dCoolingOff && !dAtCap && ctx.modelOff().contains(d);
             boolean dUnreachable = ctx.unreachable().contains(d);
             boolean dWeightExcluded = weightExcluded(ctx, d);
-            if (dQuarantined || dCoolingOff || dModelOff || dUnreachable || dWeightExcluded) {
+            if (dQuarantined || dCoolingOff || dAtCap || dModelOff || dUnreachable || dWeightExcluded) {
                 List<String> reasons = new ArrayList<>();
                 if (dQuarantined) {
                     reasons.add("is quarantined (backend exhausted)");
@@ -81,6 +94,11 @@ final class FixedPlacementPolicy implements PlacementPolicy {
                 if (dCoolingOff) {
                     reasons.add("is cooling off after repeated backend errors");
                 }
+                if (dAtCap) {
+                    PlacementCandidate c = candidateFor(ctx, d);
+                    int live = ctx.liveCount().apply(d);
+                    reasons.add("is at maxLoad (" + live + " live >= " + c.maxLoad() + " cap)");
+                }
                 if (dModelOff) {
                     reasons.add("names a model the operator has turned off in models.allow");
                 }
@@ -96,18 +114,38 @@ final class FixedPlacementPolicy implements PlacementPolicy {
         }
         if (!ctx.candidates().isEmpty()) {
             throw new PlacementException("all worker profiles are excluded from automatic "
-                    + "selection (quarantined, cooling off, model-off, unreachable, or weight-0)");
+                    + "selection (quarantined, cooling off, at cap, model-off, unreachable, or weight-0)");
         }
         throw new PlacementException("no worker profiles configured");
     }
 
     /** Whether {@code profile} carries {@code weight <= 0} (CB-554) among {@code ctx}'s candidates. */
     private static boolean weightExcluded(PlacementContext ctx, String profile) {
+        PlacementCandidate c = candidateFor(ctx, profile);
+        return c != null && c.excluded();
+    }
+
+    /**
+     * Whether {@code profile} has reached its {@code maxLoad} cap (fleetd #435), using the shared
+     * {@link PlacementPolicyUtil#atCap} definition — the same one {@code weighted}/{@code
+     * round-robin} already consult via {@link PlacementPolicyUtil#available}. Looked up by name,
+     * the same way {@link #weightExcluded} is: the default fast path above builds its own {@link
+     * PlacementCandidate} with {@code maxLoad} forced to {@code null} (it carries no cap of its
+     * own), so the candidate actually configured for {@code profile} has to be found in {@code
+     * ctx.candidates()} first.
+     */
+    private static boolean capExcluded(PlacementContext ctx, String profile) {
+        PlacementCandidate c = candidateFor(ctx, profile);
+        return c != null && PlacementPolicyUtil.atCap(ctx, c);
+    }
+
+    /** The configured candidate named {@code profile} in {@code ctx}, or {@code null} if none. */
+    private static PlacementCandidate candidateFor(PlacementContext ctx, String profile) {
         for (PlacementCandidate c : ctx.candidates()) {
             if (c.profile().equals(profile)) {
-                return c.excluded();
+                return c;
             }
         }
-        return false;
+        return null;
     }
 }

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

@@ -11,14 +11,29 @@ final class PlacementPolicyUtil {
     private PlacementPolicyUtil() {
     }
 
+    /**
+     * True when {@code c} has reached its {@code maxLoad} cap: {@code liveCount(c.profile()) >=
+     * c.maxLoad()}. A {@code null} maxLoad means unlimited, so it is never at cap.
+     *
+     * <p>Extracted as the single shared definition of "at cap" (fleetd #435): before this fix it
+     * was computed inline in both {@link #available} and {@link #emptyException}, and {@code
+     * FixedPlacementPolicy} — not a caller of either — quietly kept its own {@code select} free of
+     * any cap check at all, so a capped default profile was chosen anyway under the default
+     * placement policy. Every automatic policy must call this, not re-derive it.
+     */
+    static boolean atCap(PlacementContext ctx, PlacementCandidate c) {
+        Integer cap = c.maxLoad();
+        return cap != null && ctx.liveCount().apply(c.profile()) >= cap;
+    }
+
     /**
      * Candidates that are not weight-excluded (CB-554: explicit {@code weight <= 0}, checked
      * first because it is a static config choice rather than transient state), not
      * known-unreachable, not quarantined (CB-578 stage B), not cooling off after repeated backend
      * errors (fleetd #201 Unit 5 — a separate, shorter-lived source from quarantine), not naming a
      * model the operator has turned off (fleetd #422 — a third, independent source: an operator
-     * decision, never a backend-reported outage), and have not reached their maxLoad. A {@code
-     * null} maxLoad means unlimited.
+     * decision, never a backend-reported outage), and have not reached their maxLoad (see {@link
+     * #atCap}). A {@code null} maxLoad means unlimited.
      */
     static List<PlacementCandidate> available(PlacementContext ctx) {
         List<PlacementCandidate> out = new ArrayList<>();
@@ -26,16 +41,10 @@ final class PlacementPolicyUtil {
             if (c.excluded() || ctx.unreachable().contains(c.profile())
                     || ctx.quarantined().contains(c.profile())
                     || ctx.coolingOff().contains(c.profile())
-                    || ctx.modelOff().contains(c.profile())) {
+                    || ctx.modelOff().contains(c.profile())
+                    || atCap(ctx, c)) {
                 continue;
             }
-            Integer cap = c.maxLoad();
-            if (cap != null) {
-                int live = ctx.liveCount().apply(c.profile());
-                if (live >= cap) {
-                    continue;
-                }
-            }
             out.add(c);
         }
         return out;
@@ -59,7 +68,6 @@ final class PlacementPolicyUtil {
         int coolingOff = 0;
         int modelOff = 0;
         for (PlacementCandidate c : ctx.candidates()) {
-            Integer cap = c.maxLoad();
             if (c.excluded()) {
                 weightExcluded++;
             } else if (ctx.quarantined().contains(c.profile())) {
@@ -70,7 +78,7 @@ final class PlacementPolicyUtil {
                 modelOff++;
             } else if (ctx.unreachable().contains(c.profile())) {
                 unreachable++;
-            } else if (cap != null && ctx.liveCount().apply(c.profile()) >= cap) {
+            } else if (atCap(ctx, c)) {
                 atCap++;
             }
         }

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

@@ -536,6 +536,31 @@ class CompositePeerLauncherTest {
         assertEquals("claude", h.profile(), "the returned handle carries the resolved default profile");
     }
 
+    /**
+     * fleetd #435: {@code FixedPlacementPolicy} — the default placement policy every config uses
+     * unless {@code placement:} is set — never consulted {@code maxLoad}, so an unqualified spawn
+     * (a blank profile, the normal delegation path) landed on a capped default anyway. Measured on
+     * 7667727: a single dev profile at {@code maxLoad: 1} with 1 live, under {@code fixed()},
+     * returned "SPAWNED on profile=a". This test goes through {@code CompositePeerLauncher.spawn}
+     * with a blank profile, not the policy in isolation, so it proves the caller actually reaches
+     * the fixed default's new cap check rather than only the {@code select} method.
+     */
+    @Test
+    void fixedPolicyGatesDefaultProfileAtMaxLoadOnUnqualifiedSpawn() {
+        FakeHerdr herdr = new FakeHerdr();
+        Map<String, FleetConfig.Profile> profiles = ordered(
+                "a", stubWorker("a", 1.0f, 1),
+                "b", stubWorker("b", 1.0f, null));
+        StubLauncher adapter = new StubLauncher("claude", herdr, profiles, "a", Set.of());
+        CompositePeerLauncher composite = new CompositePeerLauncher(
+                List.of(adapter), "a", profiles, PlacementPolicies.fixed(), name -> "a".equals(name) ? 1 : 0);
+
+        PeerHandle h = composite.spawn(new SpawnRequest(null, null, null));
+        assertEquals("b", h.profile(),
+                "the default profile a is at maxLoad, so fixed placement must fall through to b");
+        assertEquals(0, adapter.spawnCount("a"), "a is never spawned — it is already at cap");
+    }
+
     @Test
     void weightedPolicyGatesProfileAtMaxLoad() {
         FakeHerdr herdr = new FakeHerdr();

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

@@ -461,6 +461,93 @@ class PlacementPolicyTest {
         assertTrue(e.getMessage().contains("weight 0"), e.getMessage());
     }
 
+    // --- fleetd #435: FixedPlacementPolicy must consult maxLoad too, at BOTH filter sites -------
+
+    /**
+     * The default-profile fast path must skip a capped default. Measured on 7667727 before this
+     * fix: a single dev profile at {@code maxLoad: 1} with 1 live, under {@code fixed()}, still
+     * returned "SPAWNED on profile=a" — the cap was advisory for every unqualified spawn.
+     */
+    @Test
+    void fixedSkipsCappedDefault() {
+        PlacementPolicy policy = PlacementPolicies.fixed();
+        PlacementContext ctx = new PlacementContext("b",
+                List.of(PlacementCandidate.profile("a", 1.0f, null),
+                        PlacementCandidate.profile("b", 1.0f, 1)),
+                name -> "b".equals(name) ? 1 : 0, Set.of(), Set.of(), Set.of());
+        assertEquals("a", policy.select(ctx).profile(),
+                "the default 'b' is at its maxLoad cap, so fixed falls through to the free candidate 'a'");
+    }
+
+    /**
+     * The fallback walk must skip a capped 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 fixedFallbackWalkSkipsCappedCandidate() {
+        PlacementPolicy policy = PlacementPolicies.fixed();
+        PlacementContext ctx = new PlacementContext(null,
+                List.of(PlacementCandidate.profile("a", 1.0f, 1),
+                        PlacementCandidate.profile("b", 1.0f, null)),
+                name -> "a".equals(name) ? 1 : 0, Set.of(), Set.of(), Set.of());
+        assertEquals("b", policy.select(ctx).profile(),
+                "candidate 'a' is at its maxLoad cap, so the fallback walk skips it and picks 'b'");
+    }
+
+    @Test
+    void fixedThrowsWhenDefaultAndEveryCandidateAtCap() {
+        PlacementPolicy policy = PlacementPolicies.fixed();
+        PlacementContext ctx = new PlacementContext("b",
+                List.of(PlacementCandidate.profile("a", 1.0f, 1),
+                        PlacementCandidate.profile("b", 1.0f, 1)),
+                _ -> 1, Set.of(), Set.of(), Set.of());
+        PlacementException e = assertThrows(PlacementException.class, () -> policy.select(ctx));
+        assertTrue(e.getMessage().contains("maxLoad"), "message names the cap: " + e.getMessage());
+        assertTrue(e.getMessage().contains("1 cap"), "message names the cap value: " + e.getMessage());
+        assertFalse(e.getMessage().contains("quarantined"), "must not read like quarantine: " + e.getMessage());
+        assertFalse(e.getMessage().contains("turned off"), "must not read like model-off: " + e.getMessage());
+    }
+
+    /** The mirror: an uncapped default is still chosen, so the new term cannot exclude everything. */
+    @Test
+    void fixedStillReturnsUncappedDefault() {
+        PlacementPolicy policy = PlacementPolicies.fixed();
+        PlacementContext ctx = new PlacementContext("b",
+                List.of(PlacementCandidate.profile("a", 1.0f, null),
+                        PlacementCandidate.profile("b", 1.0f, null)),
+                noSessions(), Set.of(), Set.of(), Set.of());
+        assertEquals("b", policy.select(ctx).profile(), "an uncapped default is returned unconditionally");
+    }
+
+    /** CB-585: an explicit {@code maxLoad: 0} on the default caps it at zero live members. */
+    @Test
+    void fixedSkipsMaxLoadZeroDefaultEvenWithZeroLiveWorkers() {
+        PlacementPolicy policy = PlacementPolicies.fixed();
+        PlacementContext ctx = new PlacementContext("b",
+                List.of(PlacementCandidate.profile("a", 1.0f, null),
+                        PlacementCandidate.profile("b", 1.0f, 0)),
+                _ -> 0, Set.of(), Set.of(), Set.of());
+        assertEquals("a", policy.select(ctx).profile(),
+                "the default 'b' has maxLoad 0, so it is already at its cap with nobody live");
+    }
+
+    /**
+     * Quarantine still wins when a profile is both quarantined and at cap (mirrors {@code
+     * fixedReportsQuarantineNotModelOffWhenBothApply}'s priority over model-off).
+     */
+    @Test
+    void fixedReportsQuarantineNotAtCapWhenBothApply() {
+        PlacementPolicy policy = PlacementPolicies.fixed();
+        PlacementContext ctx = new PlacementContext("b",
+                List.of(PlacementCandidate.profile("a", 1.0f, 1),
+                        PlacementCandidate.profile("b", 1.0f, 1)),
+                _ -> 1, Set.of(), Set.of("a", "b"), Set.of());
+        PlacementException e = assertThrows(PlacementException.class, () -> policy.select(ctx));
+        assertTrue(e.getMessage().contains("quarantined"), e.getMessage());
+        assertFalse(e.getMessage().contains("maxLoad"),
+                "quarantine takes priority over at-cap in the message: " + e.getMessage());
+    }
+
     @Test
     void unknownPolicyNameThrows() {
         assertThrows(IllegalArgumentException.class, () -> PlacementPolicies.fromName("random"));