fleetd #466 follow-up: pin main's choice of the escalating quarantine factory

BackendQuarantineTest proves BackendQuarantine.withEscalation itself
escalates, is ceilinged, and resets. Nothing proved Fleetd.main still
calls it: reverting main to the flat two-argument constructor compiled
with 0 errors and left the full 1608-test suite green, because every
other test builds its own BackendQuarantine directly.

Adds FleetdBackendQuarantineWiringTest, a source-text assertion on
Fleetd.java (same idiom as FleetdLeadSeatWiringTest and
FleetdCompletionResolverWiringTest) pinning that main's declaration is
built from BackendQuarantine.withEscalation(...), not `new
BackendQuarantine(...)`. Measured: reverting to the flat constructor
fails it (expected true, was false); renaming the anchored local
variable while keeping behaviour identical also fails it loudly, not
silently. The class doc states plainly this checks source text only,
not that the call executes or that escalation reaches a real backend.

fleetd/fleetd.example.yaml

@@ -452,6 +452,15 @@ placement: weighted
 # seconds, before a spawn may land on it again. Applies to every profile's effective credential
 # (its own name, or its credentialId if set above) — there is no per-profile override. Default
 # 1800 (30 minutes) when omitted or non-positive.
+#
+# fleetd #466: this is now only the BASE of an escalating backoff, not a flat retry rate. A
+# credential quarantined again within one base cooldown of the previous quarantine ending (still
+# reporting exhausted — e.g. a weekly subscription limit that hasn't reset) backs off further:
+# cooldown doubles each such time, capped at 12x this value (~6 hours at the 1800s default). A
+# quarantine that starts after a base-cooldown's worth of quiet resets back to this value. Not
+# configurable per se — the multiplier and ceiling are constants in BackendQuarantine, not new
+# YAML keys; see its class doc for the exact formula and why there is no automatic probe to clear
+# it early (the operator's own design constraint — a probe spends the quota it's measuring).
 # DEFERRED: baked once into the BackendQuarantine built at startup — a running quarantine keeps
 # its original cooldown regardless; a new value only applies to a quarantine that starts after a
 # restart. Editing this needs a daemon restart to take effect.

fleetd/src/main/java/dev/ltms/fleet/Fleetd.java

@@ -222,7 +222,13 @@ public final class Fleetd {
         // (checked at spawn) and the exhaustion sink wired in below (written on BACKEND_EXHAUSTED).
         // The cooldown is deferred (see FleetConfig#quarantineCooldownSeconds): it is read once
         // here, at startup, and a config reload only changes it for a daemon restart.
-        BackendQuarantine quarantine = new BackendQuarantine(System::nanoTime,
+        // fleetd #466: escalating, not flat — a credential that keeps reporting exhaustion (e.g. a
+        // weekly subscription limit, which would otherwise be retried on every ~30-minute cooldown,
+        // about 336 times across the week) backs off further each consecutive time, capped at
+        // BackendQuarantine.DEFAULT_MAX_COOLDOWN_MULTIPLE x the base cooldown. See BackendQuarantine's
+        // class doc for the mechanism, why this never fires on cooling-off (a separate, unescalated
+        // mechanism — BackendOutagePolicy below), and the reset.
+        BackendQuarantine quarantine = BackendQuarantine.withEscalation(System::nanoTime,
                 TimeUnit.SECONDS.toNanos(cfg.quarantineCooldownSeconds()));
         // fleetd #201 Unit 5: one outage-cool-off tracker for the whole daemon, shared between the
         // launcher (checked at spawn, like `quarantine` above) and the backend-error sink wired in

fleetd/src/main/java/dev/ltms/fleet/config/FleetConfig.java

@@ -70,12 +70,14 @@ import java.util.regex.PatternSyntaxException;
  *                      {@code fixed} (default), {@code round-robin}, or {@code weighted}
  * @param auth          API authentication mode ({@code null} → {@code loopback-trust}, the
  *                      historical behaviour), CB-501
- * @param quarantineCooldownSeconds  how long a credential stays quarantined after a
+ * @param quarantineCooldownSeconds  the BASE cooldown a credential is quarantined for after a
  *                      {@code BACKEND_EXHAUSTED} classification (CB-578 stage B); {@code null}/{@code
- *                      <=0} → {@link #DEFAULT_QUARANTINE_COOLDOWN_SECONDS}. Baked once into the
- *                      {@code BackendQuarantine} built at startup, so it is DEFERRED: changing it
- *                      needs a restart, and a quarantine already running keeps whatever cooldown was
- *                      live when it started.
+ *                      <=0} → {@link #DEFAULT_QUARANTINE_COOLDOWN_SECONDS}. Since fleetd #466 this is
+ *                      only the first occurrence's length — a credential quarantined again shortly
+ *                      after this cooldown ends backs off further, up to a ceiling; see {@code
+ *                      BackendQuarantine}'s class doc. Baked once into the {@code BackendQuarantine}
+ *                      built at startup, so it is DEFERRED: changing it needs a restart, and a
+ *                      quarantine already running keeps whatever cooldown was live when it started.
  * @param memberCredentials deny-by-default policy (CB-596) for which of the operator's own host
  *                      credentials a spawned member's pane inherits. {@code null} (the block
  *                      omitted) blocks nothing — see {@link MemberCredentials}.

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

@@ -21,46 +21,158 @@ import java.util.function.LongSupplier;
  * <p>The clock is injected ({@link LongSupplier}, conventionally {@code System::nanoTime} like
  * {@code FleetHealthMonitor}), never read inline, so a quarantine's expiry is testable without a
  * real sleep.
+ *
+ * <h2>Escalation (fleetd #466)</h2>
+ * A flat cooldown does not fit every exhaustion. A backend that reports "out of capacity for the
+ * rest of the hour" recovers in one cooldown; a weekly subscription limit does not — it keeps
+ * reporting exhausted on every attempt made before the window resets, so a flat 30-minute cooldown
+ * (the default {@code cooldownNanos}) means roughly 336 pointless spawn attempts across a week, one
+ * every cooldown.
+ *
+ * <p><strong>This class only ever sees the exhaustion signal.</strong> Its only production caller is
+ * {@code Fleetd.exhaustionSink}, wired to fire on a {@code BACKEND_EXHAUSTED} classification alone.
+ * The daemon's other outage state — a credential "cooling off" after repeated non-exhaustion
+ * backend errors (an HTTP 5xx storm, say) — is a separate mechanism, {@code BackendOutagePolicy},
+ * with its own short fixed 60s cooldown and no repeat tracking. The two are never merged: escalating
+ * on a cooling-off signal would turn a transient 5xx storm into a multi-hour backoff, which is
+ * exactly the failure this ticket is not asking for. Confirmed by reading every call site of
+ * {@link #quarantine} — {@code BackendOutagePolicy} has its own {@code coolOff} method and never
+ * calls this one.
+ *
+ * <p><strong>Mechanism</strong> — the {@link #withEscalation} constructors track, per credential, how
+ * many times in a row {@link #quarantine} has been called without an intervening "quiet" gap.
+ * Each call computes {@code cooldownNanos * backoffMultiplier ^ (repeatCount - 1)}, capped at
+ * {@code maxCooldownNanos}. A call counts as a continuation of the same streak — {@code repeatCount}
+ * increments — when it arrives no more than one base {@code cooldownNanos} after the previous
+ * quarantine's deadline (this covers both "still quarantined" and "quarantine just expired and it
+ * was exhausted again immediately"); otherwise the streak resets and this call is treated as a fresh
+ * first occurrence at the base cooldown.
+ *
+ * <p><strong>Reset, honestly stated.</strong> The ideal reset signal is "the cooldown expired and the
+ * next attempt succeeded" — but nothing in this codebase reports a spawn success back to this class
+ * (checked: {@code SessionManager} and {@code CompositePeerLauncher} never call any method here
+ * except {@link #quarantine}/{@link #isQuarantined}/{@link #remainingSeconds}, none of which is a
+ * success hook). Lacking that signal, the reset used here is a time-based proxy: a base-cooldown's
+ * worth of quiet — no exhaustion report for that credential — since the last quarantine ended. It is
+ * not proof the credential started working again, only the best available evidence without adding an
+ * active probe, which is out of scope by the operator's own design constraint (no automatic probing
+ * of a limited backend).
+ *
+ * <p><strong>Ceiling.</strong> {@code maxCooldownNanos} bounds the growth — an unbounded backoff is a
+ * permanent, unrecoverable-without-a-restart outage, which would be worse than the flat-rate bug this
+ * escalation fixes. {@link #withEscalation(LongSupplier, long)} defaults the ceiling to
+ * {@value #DEFAULT_MAX_COOLDOWN_MULTIPLE}x the base cooldown (12x the 1800s default ≈ 6 hours), so a
+ * chronically exhausted credential still gets re-tried roughly every 6 hours instead of every 30
+ * minutes — about a dozen attempts a week instead of ~336.
+ *
+ * <p><strong>Backward compatibility.</strong> The original two-argument {@link #BackendQuarantine(
+ * LongSupplier, long)} constructor is unchanged in behaviour: it is exactly {@code
+ * withEscalation}'s mechanism with {@code backoffMultiplier = 1.0} and {@code maxCooldownNanos =
+ * cooldownNanos}, which collapses the formula back to the original flat {@code now + cooldownNanos}
+ * on every call regardless of history. Every existing call site (roughly 20 across the test suite,
+ * plus {@link #none()}) keeps its current shape and behaviour unchanged.
  */
 public final class BackendQuarantine {
 
-    private final ConcurrentHashMap<String, Long> quarantinedUntilNanos = new ConcurrentHashMap<>();
+    /** Default growth per consecutive exhaustion streak — see the class doc's Mechanism section. */
+    static final double DEFAULT_BACKOFF_MULTIPLIER = 2.0;
+    /** Default ceiling, expressed as a multiple of the base cooldown — see the class doc's Ceiling section. */
+    static final long DEFAULT_MAX_COOLDOWN_MULTIPLE = 12;
+
+    private final ConcurrentHashMap<String, QuarantineState> quarantines = new ConcurrentHashMap<>();
     private final LongSupplier nowNanos;
     private final long cooldownNanos;
+    private final double backoffMultiplier;
+    private final long maxCooldownNanos;
     /** True only for {@link #none()}. See {@link #quarantine} for why this exists. */
     private final boolean inert;
 
+    /** How many consecutive exhaustion reports a credential is on, and when the resulting cooldown ends. */
+    private record QuarantineState(int repeatCount, long deadlineNanos) {
+    }
+
     /**
+     * Flat cooldown, unchanged from before fleetd #466 — every {@link #quarantine} call blocks the
+     * credential for exactly {@code cooldownNanos}, regardless of how many times it was called
+     * before. Equivalent to {@link #withEscalation} with no growth ({@code backoffMultiplier = 1.0})
+     * and a ceiling equal to the base cooldown, so it degrades to the identical {@code now +
+     * cooldownNanos} formula every call. Kept for the existing call sites that want a fixed cooldown
+     * (and for tests exercising the fixed-cooldown shape in isolation); production wiring uses
+     * {@link #withEscalation} instead.
+     *
      * @param nowNanos      monotonic clock, injected for testability
      * @param cooldownNanos how long a fresh {@link #quarantine} call blocks the credential for;
      *                      must be positive
      */
     public BackendQuarantine(LongSupplier nowNanos, long cooldownNanos) {
-        this(nowNanos, cooldownNanos, false);
+        this(nowNanos, cooldownNanos, 1.0, cooldownNanos, false);
     }
 
-    private BackendQuarantine(LongSupplier nowNanos, long cooldownNanos, boolean inert) {
+    /**
+     * Escalating cooldown (fleetd #466) — see the class doc's Mechanism/Reset/Ceiling sections.
+     *
+     * @param nowNanos          monotonic clock, injected for testability
+     * @param cooldownNanos     base cooldown, applied to a fresh (non-streak) exhaustion; must be
+     *                          positive
+     * @param backoffMultiplier growth per consecutive exhaustion; must be {@code >= 1.0} ({@code 1.0}
+     *                          disables growth and is exactly the flat two-argument constructor)
+     * @param maxCooldownNanos  ceiling on the escalated cooldown; must be {@code >= cooldownNanos}
+     */
+    public BackendQuarantine(LongSupplier nowNanos, long cooldownNanos, double backoffMultiplier,
+            long maxCooldownNanos) {
+        this(nowNanos, cooldownNanos, backoffMultiplier, maxCooldownNanos, false);
+    }
+
+    private BackendQuarantine(LongSupplier nowNanos, long cooldownNanos, double backoffMultiplier,
+            long maxCooldownNanos, boolean inert) {
         this.nowNanos = Objects.requireNonNull(nowNanos, "nowNanos");
         if (cooldownNanos <= 0) {
             throw new IllegalArgumentException("cooldownNanos must be positive: " + cooldownNanos);
         }
+        if (backoffMultiplier < 1.0) {
+            throw new IllegalArgumentException("backoffMultiplier must be >= 1.0: " + backoffMultiplier);
+        }
+        if (maxCooldownNanos < cooldownNanos) {
+            throw new IllegalArgumentException(
+                    "maxCooldownNanos must be >= cooldownNanos: " + maxCooldownNanos + " < " + cooldownNanos);
+        }
         this.cooldownNanos = cooldownNanos;
+        this.backoffMultiplier = backoffMultiplier;
+        this.maxCooldownNanos = maxCooldownNanos;
         this.inert = inert;
     }
 
+    /**
+     * Escalating cooldown with the fleetd #466 default shape: cooldown doubles
+     * ({@value #DEFAULT_BACKOFF_MULTIPLIER}x) per consecutive exhaustion streak, capped at
+     * {@value #DEFAULT_MAX_COOLDOWN_MULTIPLE}x the base cooldown. This is what production wiring
+     * ({@code Fleetd.main}) uses.
+     *
+     * @param nowNanos      monotonic clock, injected for testability
+     * @param cooldownNanos base cooldown, applied to a fresh (non-streak) exhaustion; must be positive
+     */
+    public static BackendQuarantine withEscalation(LongSupplier nowNanos, long cooldownNanos) {
+        return new BackendQuarantine(nowNanos, cooldownNanos, DEFAULT_BACKOFF_MULTIPLIER,
+                cooldownNanos * DEFAULT_MAX_COOLDOWN_MULTIPLE, false);
+    }
+
     /**
      * Inert quarantine — {@link #quarantine} does nothing on this instance, so nothing is ever
      * quarantined. The explicit stand-in a caller (or a test not exercising this feature) passes
      * instead of a defaulting overload, exactly like {@code ExhaustedPatternLookup.none()}.
      */
     public static BackendQuarantine none() {
-        return new BackendQuarantine(() -> 0L, 1, true);
+        return new BackendQuarantine(() -> 0L, 1, 1.0, 1, true);
     }
 
     /**
-     * Quarantine {@code credentialId} for the configured cooldown, starting now. A repeat call while
-     * already quarantined restarts the cooldown at full length — a fresh refusal is fresh evidence the
-     * account is still exhausted, not a reason to let an earlier, shorter wait stand.
+     * Quarantine {@code credentialId} starting now. On a flat instance (the two-argument
+     * constructor) this always blocks for exactly {@code cooldownNanos}, restarting the cooldown at
+     * full length on every call — a fresh refusal is fresh evidence the account is still exhausted,
+     * not a reason to let an earlier, shorter wait stand. On an escalating instance ({@link
+     * #withEscalation}) the cooldown grows with each call that arrives within one base cooldown of
+     * the previous deadline, and resets to the base cooldown once a call arrives after a longer gap
+     * — see the class doc.
      *
      * <p>On {@link #none()} this is a no-op. It has to be: that instance holds a clock frozen at 0,
      * so recording a deadline would produce a quarantine that never expires — a credential locked out
@@ -73,7 +185,13 @@ public final class BackendQuarantine {
         if (inert) {
             return;
         }
-        quarantinedUntilNanos.put(credentialId, nowNanos.getAsLong() + cooldownNanos);
+        long now = nowNanos.getAsLong();
+        quarantines.compute(credentialId, (id, prev) -> {
+            int repeatCount = (prev == null || now - prev.deadlineNanos() > cooldownNanos)
+                    ? 1
+                    : prev.repeatCount() + 1;
+            return new QuarantineState(repeatCount, now + escalatedCooldownNanos(repeatCount));
+        });
     }
 
     /** Whether {@code credentialId} is quarantined right now. */
@@ -95,8 +213,8 @@ public final class BackendQuarantine {
      */
     public Map<String, Long> activeRemainingSeconds() {
         Map<String, Long> out = new LinkedHashMap<>();
-        quarantinedUntilNanos.forEach((credentialId, deadline) -> {
-            long remaining = deadline - nowNanos.getAsLong();
+        quarantines.forEach((credentialId, state) -> {
+            long remaining = state.deadlineNanos() - nowNanos.getAsLong();
             if (remaining > 0) {
                 out.put(credentialId, toSecondsRoundedUp(remaining));
             }
@@ -105,8 +223,14 @@ public final class BackendQuarantine {
     }
 
     private long remainingNanos(String credentialId) {
-        Long deadline = quarantinedUntilNanos.get(credentialId);
-        return deadline == null ? 0L : deadline - nowNanos.getAsLong();
+        QuarantineState state = quarantines.get(credentialId);
+        return state == null ? 0L : state.deadlineNanos() - nowNanos.getAsLong();
+    }
+
+    /** {@code cooldownNanos * backoffMultiplier ^ (repeatCount - 1)}, capped at {@code maxCooldownNanos}. */
+    private long escalatedCooldownNanos(int repeatCount) {
+        double raw = cooldownNanos * Math.pow(backoffMultiplier, repeatCount - 1);
+        return raw >= (double) maxCooldownNanos ? maxCooldownNanos : (long) raw;
     }
 
     private static long toSecondsRoundedUp(long nanos) {

fleetd/src/test/java/dev/ltms/fleet/FleetdBackendQuarantineWiringTest.java

@@ -0,0 +1,65 @@
+package dev.ltms.fleet;
+
+import java.nio.file.Files;
+import java.nio.file.Path;
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+/**
+ * fleetd #466 follow-up: {@code Fleetd.main} builds the daemon's one {@code BackendQuarantine}
+ * from {@link dev.ltms.fleet.placement.BackendQuarantine#withEscalation(java.util.function.LongSupplier,
+ * long)} — the escalating factory — rather than the plain two-argument constructor, which is still a
+ * flat cooldown (kept for backward compatibility, see that class's doc). {@code
+ * BackendQuarantineTest} proves {@code withEscalation} itself escalates, is ceilinged, and resets;
+ * it says nothing about which one {@code main} actually calls.
+ *
+ * <p>Measured directly: reverting {@code main} to {@code new BackendQuarantine(System::nanoTime,
+ * TimeUnit.SECONDS.toNanos(cfg.quarantineCooldownSeconds()))} — the pre-#466 flat call — compiles
+ * with 0 errors and leaves the entire 1608-test suite (including every {@code BackendQuarantineTest}
+ * case) green, because no other test constructs its {@code BackendQuarantine} through {@code main};
+ * every one of them builds its own instance directly. That silent regression is exactly the shape
+ * {@link FleetdLeadSeatWiringTest} and {@link FleetdCompletionResolverWiringTest} already guard
+ * against for their own constructor arguments — this is the same class of gap for fleetd #466's
+ * factory choice, following their approach.
+ *
+ * <p><b>This test checks source text, not runtime behaviour.</b> It never constructs a {@code
+ * BackendQuarantine} and never runs {@code main} — a green result here proves only that the exact
+ * text {@code main} calls {@code BackendQuarantine.withEscalation(...)} rather than the flat
+ * constructor. It does not prove that call actually executes at startup (no test here starts the
+ * daemon), and it does not prove the escalation reaches a real backend or credential — only
+ * {@code BackendQuarantineTest} proves the factory's own behaviour, and only a live daemon proves
+ * the wiring runs.
+ */
+class FleetdBackendQuarantineWiringTest {
+
+    private static String fleetdSource() throws Exception {
+        return Files.readString(Path.of("src/main/java/dev/ltms/fleet/Fleetd.java"));
+    }
+
+    @Test
+    @DisplayName("[SOURCE TEXT] main's BackendQuarantine local is still built from BackendQuarantine.withEscalation(...)")
+    void mainStillWiresTheEscalatingQuarantineFactory() throws Exception {
+        String source = fleetdSource();
+        assertTrue(source.contains(
+                        "BackendQuarantine quarantine = BackendQuarantine.withEscalation(System::nanoTime,\n"
+                                + "                TimeUnit.SECONDS.toNanos(cfg.quarantineCooldownSeconds()));"),
+                "Fleetd.main's BackendQuarantine local must still be built from "
+                        + "BackendQuarantine.withEscalation(System::nanoTime, "
+                        + "TimeUnit.SECONDS.toNanos(cfg.quarantineCooldownSeconds())). Reverting to the flat "
+                        + "two-argument constructor (fleetd #466's measured regression) compiles with 0 errors "
+                        + "and leaves the whole suite green, including every BackendQuarantineTest case that "
+                        + "proves the escalation itself works — this source check is what must go red instead. "
+                        + "A reverted daemon would go back to retrying a weekly subscription limit on every "
+                        + "flat ~30-minute cooldown, about 336 times across the week.");
+
+        // Negative form of the same check: the pre-#466 flat call, if it ever reappears at this
+        // declaration, must not be mistaken for the escalating one by a looser positive-only check.
+        assertFalse(source.contains(
+                        "BackendQuarantine quarantine = new BackendQuarantine(System::nanoTime,\n"
+                                + "                TimeUnit.SECONDS.toNanos(cfg.quarantineCooldownSeconds()));"),
+                "main's BackendQuarantine local must never regress to the flat two-argument constructor");
+    }
+}

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

@@ -116,4 +116,117 @@ class BackendQuarantineTest {
         assertThrows(IllegalArgumentException.class, () -> new BackendQuarantine(() -> 0L, 0L));
         assertThrows(IllegalArgumentException.class, () -> new BackendQuarantine(() -> 0L, -1L));
     }
+
+    // --- fleetd #466: escalating cooldown -----------------------------------------------------
+    //
+    // Base cooldown 600s (10 min), multiplier 2.0, ceiling 2400s (4x base) — small round numbers
+    // chosen so every deadline is an exact assertion, not just "greater than before". Each call
+    // below lands at or before the previous deadline (a zero or negative gap), which is always
+    // "no more than one base cooldown after the previous deadline" — i.e. every call continues the
+    // same streak, matching a credential that keeps reporting exhausted with no lull.
+
+    @Test
+    void anInvalidBackoffMultiplierIsRejected() {
+        assertThrows(IllegalArgumentException.class,
+                () -> new BackendQuarantine(() -> 0L, TimeUnit.MINUTES.toNanos(30), 0.5, TimeUnit.HOURS.toNanos(6)));
+    }
+
+    @Test
+    void aCeilingBelowTheBaseCooldownIsRejected() {
+        assertThrows(IllegalArgumentException.class,
+                () -> new BackendQuarantine(() -> 0L, TimeUnit.MINUTES.toNanos(30), 2.0, TimeUnit.MINUTES.toNanos(10)));
+    }
+
+    @Test
+    void repeatedExhaustionEscalatesTheCooldownByExactAmounts() {
+        AtomicLong now = new AtomicLong(0L);
+        BackendQuarantine q = new BackendQuarantine(now::get, TimeUnit.SECONDS.toNanos(600), 2.0,
+                TimeUnit.SECONDS.toNanos(2400));
+
+        q.quarantine("shared-openai"); // 1st: base cooldown
+        assertEquals(OptionalLong.of(600L), q.remainingSeconds("shared-openai"));
+
+        now.set(TimeUnit.SECONDS.toNanos(100)); // still inside the 1st quarantine (deadline 600s)
+        q.quarantine("shared-openai"); // 2nd: 600 * 2^1 = 1200
+        assertEquals(OptionalLong.of(1200L), q.remainingSeconds("shared-openai"),
+                "a second consecutive exhaustion must double the cooldown, not just increase it");
+
+        now.set(TimeUnit.SECONDS.toNanos(1300)); // exactly the 2nd deadline (100 + 1200)
+        q.quarantine("shared-openai"); // 3rd: 600 * 2^2 = 2400 (exactly at the ceiling)
+        assertEquals(OptionalLong.of(2400L), q.remainingSeconds("shared-openai"));
+    }
+
+    @Test
+    void escalationStopsAtTheCeiling() {
+        AtomicLong now = new AtomicLong(0L);
+        BackendQuarantine q = new BackendQuarantine(now::get, TimeUnit.SECONDS.toNanos(600), 2.0,
+                TimeUnit.SECONDS.toNanos(2400));
+
+        q.quarantine("shared-openai"); // 1st: 600
+        now.set(TimeUnit.SECONDS.toNanos(600));
+        q.quarantine("shared-openai"); // 2nd: 1200, deadline 1800
+        now.set(TimeUnit.SECONDS.toNanos(1800));
+        q.quarantine("shared-openai"); // 3rd: 600 * 4 = 2400, at the ceiling, deadline 4200
+        now.set(TimeUnit.SECONDS.toNanos(4200));
+        q.quarantine("shared-openai"); // 4th: 600 * 8 = 4800 uncapped, must stay capped at 2400
+        assertEquals(OptionalLong.of(2400L), q.remainingSeconds("shared-openai"),
+                "the cooldown must never exceed the configured ceiling, however long the streak gets");
+
+        now.set(TimeUnit.SECONDS.toNanos(6600)); // 4th deadline
+        q.quarantine("shared-openai"); // 5th: still capped
+        assertEquals(OptionalLong.of(2400L), q.remainingSeconds("shared-openai"),
+                "pushing well past the ceiling must not budge it");
+    }
+
+    @Test
+    void aQuietGapLongerThanTheBaseCooldownResetsToTheBaseCooldown() {
+        AtomicLong now = new AtomicLong(0L);
+        BackendQuarantine q = new BackendQuarantine(now::get, TimeUnit.SECONDS.toNanos(600), 2.0,
+                TimeUnit.SECONDS.toNanos(2400));
+
+        q.quarantine("shared-openai"); // 1st: 600, deadline 600
+        now.set(TimeUnit.SECONDS.toNanos(600));
+        q.quarantine("shared-openai"); // 2nd: 1200, deadline 1800
+        now.set(TimeUnit.SECONDS.toNanos(1800));
+        q.quarantine("shared-openai"); // 3rd: 2400, deadline 4200
+        assertEquals(OptionalLong.of(2400L), q.remainingSeconds("shared-openai"));
+
+        // Quiet for well over one base cooldown (600s) past the 3rd deadline (4200s).
+        now.set(TimeUnit.SECONDS.toNanos(20_000));
+        q.quarantine("shared-openai"); // treated as a fresh occurrence
+        assertEquals(OptionalLong.of(600L), q.remainingSeconds("shared-openai"),
+                "a long quiet gap must reset the streak back to the base cooldown");
+    }
+
+    @Test
+    void escalatingOneCredentialDoesNotSlowAnother() {
+        AtomicLong now = new AtomicLong(0L);
+        BackendQuarantine q = new BackendQuarantine(now::get, TimeUnit.SECONDS.toNanos(600), 2.0,
+                TimeUnit.SECONDS.toNanos(2400));
+
+        q.quarantine("shared-openai"); // 1st: 600
+        now.set(TimeUnit.SECONDS.toNanos(600));
+        q.quarantine("shared-openai"); // 2nd: 1200
+        now.set(TimeUnit.SECONDS.toNanos(1800));
+        q.quarantine("shared-openai"); // 3rd: 2400 — three-in-a-row streak on this credential only
+
+        q.quarantine("another-credential"); // its first and only exhaustion
+        assertEquals(OptionalLong.of(600L), q.remainingSeconds("another-credential"),
+                "an unrelated credential's cooldown must stay at the base rate, unaffected by a sibling's streak");
+    }
+
+    @Test
+    void withEscalationDefaultsToDoublingCappedAtTwelveTimesTheBase() {
+        AtomicLong now = new AtomicLong(0L);
+        BackendQuarantine q = BackendQuarantine.withEscalation(now::get, TimeUnit.MINUTES.toNanos(30));
+
+        q.quarantine("shared-openai");
+        assertEquals(OptionalLong.of(1800L), q.remainingSeconds("shared-openai"),
+                "the first occurrence must still use the base cooldown");
+
+        now.set(TimeUnit.MINUTES.toNanos(30));
+        q.quarantine("shared-openai");
+        assertEquals(OptionalLong.of(3600L), q.remainingSeconds("shared-openai"),
+                "the default multiplier must be 2.0");
+    }
 }