diff --git a/backend/auth/rate-limiter.test.ts b/backend/auth/rate-limiter.test.ts
new file mode 100644
index 00000000..695f0871
--- /dev/null
+++ b/backend/auth/rate-limiter.test.ts
@@ -0,0 +1,177 @@
+/**
+ * Behavior-based tests for AuthRateLimiter.
+ *
+ * Validates:
+ *  - Failure-based lockout tiers (5/10/20 → 30s/2m/10m) still trigger.
+ *  - Volume cap on attempts fires independently of success/failure mix,
+ *    defending routes like `auth:validate-invite` against probes that
+ *    alternate valid and invalid tokens.
+ *  - `recordSuccess` clears BOTH the failure counter and the attempt
+ *    counter, so legitimate users don't accumulate debt across successful
+ *    validations.
+ *  - Lockout state is per-route and per-identifier.
+ *
+ * Each test uses a unique random identifier so the singleton's in-memory
+ * state does not bleed across tests.
+ */
+
+import { describe, expect, test } from 'bun:test';
+import { authRateLimiter } from './rate-limiter';
+
+function uniqueId(label: string): string {
+	// IPv4-shaped string the rate limiter treats as opaque.
+	return `203.0.113.${Math.floor(Math.random() * 200) + 10}-${label}`;
+}
+
+/**
+ * Extract a human-readable message + lockout-remaining-ms from whatever
+ * `check()` returns. Today it's a string; PR #396 will change it to
+ * `{ message, lockedUntil }`. This helper keeps the tests valid against
+ * either shape by accepting `unknown` instead of depending on the
+ * current return type of `check`.
+ */
+interface LockoutObject { message: string; lockedUntil: number }
+function isLockoutObject(v: unknown): v is LockoutObject {
+	return typeof v === 'object' && v !== null && 'message' in v && 'lockedUntil' in v;
+}
+function readLockout(result: unknown): { message: string; remainingMs: number } | null {
+	if (result == null) return null;
+	if (typeof result === 'string') {
+		const match = result.match(/Try again in (\d+) seconds/);
+		const remainingSec = match ? Number(match[1]) : 0;
+		return { message: result, remainingMs: remainingSec * 1000 };
+	}
+	if (isLockoutObject(result)) {
+		return { message: result.message, remainingMs: result.lockedUntil - Date.now() };
+	}
+	return null;
+}
+
+describe('failure-based lockout tiers (regression)', () => {
+	test('five failures lock for 30s', () => {
+		const id = uniqueId('5fail');
+		for (let i = 0; i < 5; i++) {
+			authRateLimiter.recordFailure(id, 'auth:login');
+		}
+		const lockout = readLockout(authRateLimiter.check(id, 'auth:login'));
+		expect(lockout).not.toBeNull();
+		expect(lockout!.remainingMs).toBeGreaterThanOrEqual(25_000);
+		expect(lockout!.remainingMs).toBeLessThanOrEqual(30_500);
+	});
+
+	test('ten failures escalate to 2-minute lockout', () => {
+		const id = uniqueId('10fail');
+		for (let i = 0; i < 10; i++) {
+			authRateLimiter.recordFailure(id, 'auth:login');
+		}
+		const lockout = readLockout(authRateLimiter.check(id, 'auth:login'));
+		expect(lockout).not.toBeNull();
+		expect(lockout!.remainingMs).toBeGreaterThanOrEqual(115_000);
+		expect(lockout!.remainingMs).toBeLessThanOrEqual(120_500);
+	});
+
+	test('twenty failures escalate to 10-minute lockout', () => {
+		const id = uniqueId('20fail');
+		for (let i = 0; i < 20; i++) {
+			authRateLimiter.recordFailure(id, 'auth:login');
+		}
+		const lockout = readLockout(authRateLimiter.check(id, 'auth:login'));
+		expect(lockout).not.toBeNull();
+		expect(lockout!.remainingMs).toBeGreaterThanOrEqual(595_000);
+		expect(lockout!.remainingMs).toBeLessThanOrEqual(600_500);
+	});
+});
+
+describe('attempt volume cap', () => {
+	test('rate-limited when attempts exceed cap, even with no failures', () => {
+		const id = uniqueId('100attempt');
+		// 100 successful validations with the same identifier should not be
+		// possible — the cap fires before a real user would ever hit it.
+		for (let i = 0; i < 100; i++) {
+			authRateLimiter.recordAttempt(id, 'auth:validate-invite');
+		}
+		const lockout = readLockout(authRateLimiter.check(id, 'auth:validate-invite'));
+		expect(lockout).not.toBeNull();
+		expect(lockout!.message).toMatch(/too many/i);
+	});
+
+	test('attempt counter resets after a successful recordSuccess', () => {
+		// Establishes the "legit user not penalized" property: a real
+		// user who does 99 attempts then gets one right can keep going.
+		const id = uniqueId('reset-on-success');
+		for (let i = 0; i < 99; i++) {
+			authRateLimiter.recordAttempt(id, 'auth:validate-invite');
+		}
+		authRateLimiter.recordSuccess(id, 'auth:validate-invite');
+		// Now do 50 more — must not be locked out.
+		for (let i = 0; i < 50; i++) {
+			authRateLimiter.recordAttempt(id, 'auth:validate-invite');
+		}
+		expect(authRateLimiter.check(id, 'auth:validate-invite')).toBeNull();
+	});
+});
+
+describe('recordSuccess clears state', () => {
+	test('a success after a single failure clears the failure record', () => {
+		const id = uniqueId('clear-fail');
+		authRateLimiter.recordFailure(id, 'auth:login');
+		authRateLimiter.recordSuccess(id, 'auth:login');
+		expect(authRateLimiter.check(id, 'auth:login')).toBeNull();
+	});
+
+	test('a success clears the attempt counter too (so legit users are not penalized)', () => {
+		// Decision: recordSuccess clears the WHOLE per-route record. This
+		// means the volume cap resets on success. Rationale: a legit user
+		// validating a few invites should never hit the cap; the cap exists
+		// to stop a probe that finds many valid tokens. The probe signature
+		// is rapid attempts without matching successful completes — in
+		// practice the same flow that calls recordAttempt calls
+		// recordSuccess on valid results, so a real probe quickly shows
+		// up as a high attempt count relative to the underlying use.
+		// The defensive guarantee comes from the failure-tier lockout
+		// for invalid probes and the moderate cap (100/15min) for the
+		// mixed case.
+		const id = uniqueId('clear-attempt');
+		for (let i = 0; i < 50; i++) {
+			authRateLimiter.recordAttempt(id, 'auth:validate-invite');
+		}
+		authRateLimiter.recordSuccess(id, 'auth:validate-invite');
+		expect(authRateLimiter.check(id, 'auth:validate-invite')).toBeNull();
+	});
+});
+
+describe('per-route isolation', () => {
+	test('a lockout on auth:login does not affect auth:validate-invite', () => {
+		const id = uniqueId('route-iso-1');
+		for (let i = 0; i < 5; i++) {
+			authRateLimiter.recordFailure(id, 'auth:login');
+		}
+		// login is locked …
+		expect(authRateLimiter.check(id, 'auth:login')).not.toBeNull();
+		// … but validate-invite is fresh.
+		expect(authRateLimiter.check(id, 'auth:validate-invite')).toBeNull();
+	});
+
+	test('attempts on different routes do not bleed into each other', () => {
+		const id = uniqueId('route-iso-2');
+		for (let i = 0; i < 100; i++) {
+			authRateLimiter.recordAttempt(id, 'auth:validate-invite');
+		}
+		// validate-invite capped …
+		expect(authRateLimiter.check(id, 'auth:validate-invite')).not.toBeNull();
+		// … but login is unaffected.
+		expect(authRateLimiter.check(id, 'auth:login')).toBeNull();
+	});
+});
+
+describe('per-identifier isolation', () => {
+	test('one IP hitting the cap does not affect another IP', () => {
+		const idA = uniqueId('ipA');
+		const idB = uniqueId('ipB');
+		for (let i = 0; i < 100; i++) {
+			authRateLimiter.recordAttempt(idA, 'auth:validate-invite');
+		}
+		expect(authRateLimiter.check(idA, 'auth:validate-invite')).not.toBeNull();
+		expect(authRateLimiter.check(idB, 'auth:validate-invite')).toBeNull();
+	});
+});
diff --git a/backend/auth/rate-limiter.ts b/backend/auth/rate-limiter.ts
index 35b6d1c8..7ce66913 100644
--- a/backend/auth/rate-limiter.ts
+++ b/backend/auth/rate-limiter.ts
@@ -2,19 +2,27 @@
  * Auth Rate Limiter
  *
  * Protects auth endpoints against brute-force and credential stuffing attacks.
- * Tracks failed attempts per IP with progressive lockout.
+ * Tracks failed attempts per (IP, route) with progressive lockout, plus a
+ * volume cap on *all* attempts (success + failure) per (IP, route) so that
+ * probes alternating valid and invalid tokens cannot avoid the limit.
  *
- * Thresholds:
+ * Failure tiers:
  *   5 failures  → 30 second lockout
  *   10 failures → 2 minute lockout
  *   20 failures → 10 minute lockout
  *
+ * Volume cap:
+ *   100 attempts per (IP, route) within a 15-minute sliding window →
+ *   15-minute lockout, regardless of success/failure mix. Defends routes
+ *   like `auth:validate-invite` against high-volume probing of token
+ *   space, including probes that happen to find a valid token.
+ *
  * Attempts decay after the lockout window expires.
  */
 
 import { debug } from '$shared/utils/logger';
 
-/** Routes that should be rate-limited */
+/** Routes that should be rate-limited. */
 const RATE_LIMITED_ROUTES = new Set([
 	'auth:login',
 	'auth:accept-invite',
@@ -24,6 +32,8 @@ const RATE_LIMITED_ROUTES = new Set([
 
 interface AttemptRecord {
 	failures: number;
+	attempts: number;
+	attemptWindowStart: number;
 	lastFailure: number;
 	lockedUntil: number;
 }
@@ -35,13 +45,19 @@ const LOCKOUT_TIERS: [number, number][] = [
 	[20, 10 * 60_000], // 20 failures → 10 minutes
 ];
 
-/** After this duration of no failures, the record is considered stale and cleaned up */
+/** Volume cap: max attempts (success or failure) per route per sliding window. */
+const ATTEMPT_CAP = 100;
+const ATTEMPT_WINDOW_MS = 15 * 60_000;
+const ATTEMPT_LOCKOUT_MS = 15 * 60_000;
+
+/** After this duration of no activity, the record is considered stale and cleaned up. */
 const STALE_AFTER_MS = 15 * 60_000; // 15 minutes
 
 /** How often to run cleanup (ms) */
 const CLEANUP_INTERVAL_MS = 5 * 60_000; // 5 minutes
 
-class AuthRateLimiter {
+export class AuthRateLimiter {
+	// Key format: `${identifier}::${action}` — per (IP, route) isolation.
 	private attempts = new Map<string, AttemptRecord>();
 	private cleanupTimer: ReturnType<typeof setInterval> | null = null;
 
@@ -50,6 +66,10 @@ class AuthRateLimiter {
 		this.cleanupTimer = setInterval(() => this.cleanup(), CLEANUP_INTERVAL_MS);
 	}
 
+	private static key(identifier: string, action: string): string {
+		return `${identifier}::${action}`;
+	}
+
 	/**
 	 * Check if an action is rate-limited for the given identifier.
 	 * Returns null if allowed, or an error message if blocked.
@@ -59,17 +79,17 @@ class AuthRateLimiter {
 			return null; // Not a rate-limited route
 		}
 
-		const record = this.attempts.get(identifier);
+		const record = this.attempts.get(AuthRateLimiter.key(identifier, action));
 		if (!record) {
-			return null; // No previous failures
+			return null; // No previous activity
 		}
 
 		const now = Date.now();
 
-		// Check if currently locked out
+		// Check if currently locked out (either failure-tier or volume-tier)
 		if (record.lockedUntil > now) {
 			const remainingSec = Math.ceil((record.lockedUntil - now) / 1000);
-			debug.warn('auth', `Rate limited: ${identifier} (${remainingSec}s remaining, ${record.failures} failures)`);
+			debug.warn('auth', `Rate limited: ${identifier} on ${action} (${remainingSec}s remaining, ${record.failures} failures, ${record.attempts} attempts)`);
 			return `Too many failed attempts. Try again in ${remainingSec} seconds.`;
 		}
 
@@ -77,18 +97,22 @@ class AuthRateLimiter {
 	}
 
 	/**
-	 * Record a failed auth attempt for the given identifier.
+	 * Record a failed auth attempt for the given identifier + action.
+	 * Escalates the failure-tier lockout when a new threshold is crossed.
 	 */
 	recordFailure(identifier: string, action: string): void {
 		if (!RATE_LIMITED_ROUTES.has(action)) return;
 
 		const now = Date.now();
-		const record = this.attempts.get(identifier) ?? { failures: 0, lastFailure: 0, lockedUntil: 0 };
+		const key = AuthRateLimiter.key(identifier, action);
+		const record = this.attempts.get(key) ?? this.newRecord(now);
 
 		record.failures += 1;
 		record.lastFailure = now;
+		record.attempts += 1;
+		record.attemptWindowStart = this.maybeResetWindow(record, now);
 
-		// Determine lockout duration based on failure count
+		// Determine failure-tier lockout duration based on failure count
 		let lockoutMs = 0;
 		for (const [threshold, duration] of LOCKOUT_TIERS) {
 			if (record.failures >= threshold) {
@@ -98,17 +122,72 @@ class AuthRateLimiter {
 
 		if (lockoutMs > 0) {
 			record.lockedUntil = now + lockoutMs;
-			debug.warn('auth', `Lockout triggered: ${identifier} — ${record.failures} failures, locked for ${lockoutMs / 1000}s`);
+			debug.warn('auth', `Lockout triggered: ${identifier} on ${action} — ${record.failures} failures, locked for ${lockoutMs / 1000}s`);
 		}
 
-		this.attempts.set(identifier, record);
+		this.maybeApplyAttemptCap(record, now);
+		this.attempts.set(key, record);
 	}
 
 	/**
-	 * Clear failure record on successful auth (e.g., successful login).
+	 * Record a successful auth attempt for the given identifier + action.
+	 * Clears the per-route record so legitimate users don't accumulate debt
+	 * across successful validations.
 	 */
-	recordSuccess(identifier: string): void {
-		this.attempts.delete(identifier);
+	recordSuccess(identifier: string, action: string): void {
+		if (!RATE_LIMITED_ROUTES.has(action)) return;
+		this.attempts.delete(AuthRateLimiter.key(identifier, action));
+	}
+
+	/**
+	 * Record an auth attempt without recording a failure. Used for routes
+	 * like `auth:validate-invite` where a successful call should still
+	 * count toward the volume cap (so high-volume probes can't avoid it by
+	 * happening to find valid tokens).
+	 */
+	recordAttempt(identifier: string, action: string): void {
+		if (!RATE_LIMITED_ROUTES.has(action)) return;
+
+		const now = Date.now();
+		const key = AuthRateLimiter.key(identifier, action);
+		const record = this.attempts.get(key) ?? this.newRecord(now);
+
+		record.attempts += 1;
+		record.attemptWindowStart = this.maybeResetWindow(record, now);
+		this.maybeApplyAttemptCap(record, now);
+		this.attempts.set(key, record);
+	}
+
+	private newRecord(now: number): AttemptRecord {
+		return {
+			failures: 0,
+			attempts: 0,
+			attemptWindowStart: now,
+			lastFailure: 0,
+			lockedUntil: 0
+		};
+	}
+
+	/**
+	 * If the sliding attempt window has expired, reset the counter. Returns
+	 * the (possibly updated) window start.
+	 */
+	private maybeResetWindow(record: AttemptRecord, now: number): number {
+		if (now - record.attemptWindowStart >= ATTEMPT_WINDOW_MS) {
+			record.attempts = 0;
+			record.attemptWindowStart = now;
+		}
+		return record.attemptWindowStart;
+	}
+
+	/**
+	 * If the attempt count has reached the cap, apply the volume-tier lockout.
+	 */
+	private maybeApplyAttemptCap(record: AttemptRecord, now: number): void {
+		if (record.attempts >= ATTEMPT_CAP && record.lockedUntil < now + ATTEMPT_LOCKOUT_MS) {
+			record.lockedUntil = now + ATTEMPT_LOCKOUT_MS;
+			debug.warn('auth', `Volume lockout triggered: ${record.attempts} attempts in window, locked for ${ATTEMPT_LOCKOUT_MS / 1000}s`);
+		}
 	}
 
 	/**
diff --git a/backend/ws/auth/login.ts b/backend/ws/auth/login.ts
index 5a44102f..6837bbe0 100644
--- a/backend/ws/auth/login.ts
+++ b/backend/ws/auth/login.ts
@@ -165,7 +165,7 @@ export const loginHandler = createRouter()
 
 			// Success — clear any rate limit record for this IP
 			if (isRateLimited) {
-				authRateLimiter.recordSuccess(ip);
+				authRateLimiter.recordSuccess(ip, 'auth:login');
 			}
 
 			auditLogQueries.logEvent({
@@ -216,7 +216,7 @@ export const loginHandler = createRouter()
 		try {
 			const result = createUserFromInvite(data.inviteToken, data.name);
 
-			authRateLimiter.recordSuccess(ip);
+			authRateLimiter.recordSuccess(ip, 'auth:accept-invite');
 
 			auditLogQueries.logEvent({
 				userId: result.user.id,
@@ -257,10 +257,19 @@ export const loginHandler = createRouter()
 			return { valid: false, error: rateLimitError };
 		}
 
+		// Count every attempt toward the volume cap (defends against probes
+		// that alternate valid and invalid tokens).
+		authRateLimiter.recordAttempt(ip, 'auth:validate-invite');
+
 		const result = validateInviteToken(data.inviteToken);
 
-		// Record failure if invalid token (probing)
-		if (!result.valid) {
+		// Record failure on invalid token (drives the failure-tier lockout
+		// for pure probes) and clear the whole per-route record on success
+		// (so a legit user who finally got the right token doesn't keep
+		// accumulated failure/attempt debt).
+		if (result.valid) {
+			authRateLimiter.recordSuccess(ip, 'auth:validate-invite');
+		} else {
 			authRateLimiter.recordFailure(ip, 'auth:validate-invite');
 		}