diff --git a/apps/orchestrator/src/app.module.ts b/apps/orchestrator/src/app.module.ts
index 20eb134..55b7e24 100644
--- a/apps/orchestrator/src/app.module.ts
+++ b/apps/orchestrator/src/app.module.ts
@@ -4,6 +4,7 @@ import { BullModule } from "@nestjs/bullmq";
 import { HealthModule } from "./api/health/health.module";
 import { AgentsModule } from "./api/agents/agents.module";
 import { CoordinatorModule } from "./coordinator/coordinator.module";
+import { BudgetModule } from "./budget/budget.module";
 import { orchestratorConfig } from "./config/orchestrator.config";
 
 @Module({
@@ -21,6 +22,7 @@ import { orchestratorConfig } from "./config/orchestrator.config";
     HealthModule,
     AgentsModule,
     CoordinatorModule,
+    BudgetModule,
   ],
 })
 export class AppModule {}
diff --git a/apps/orchestrator/src/budget/budget.service.spec.ts b/apps/orchestrator/src/budget/budget.service.spec.ts
index 40b1f2d..da742c0 100644
--- a/apps/orchestrator/src/budget/budget.service.spec.ts
+++ b/apps/orchestrator/src/budget/budget.service.spec.ts
@@ -2,7 +2,7 @@
  * BudgetService Unit Tests
  *
  * Tests usage budget tracking, enforcement, and reporting.
- * Covers issue #329 (ORCH-135)
+ * Covers issue #329 (ORCH-135) including security hardening.
  */
 import { describe, it, expect, beforeEach, vi } from "vitest";
 import { BudgetService } from "./budget.service";
@@ -12,7 +12,7 @@ describe("BudgetService", () => {
   let service: BudgetService;
 
   const mockConfigService = {
-    get: vi.fn((_key: string, defaultValue?: unknown) => defaultValue),
+    get: vi.fn((_key: string, _defaultValue?: unknown) => undefined),
   };
 
   beforeEach(() => {
@@ -53,6 +53,46 @@ describe("BudgetService", () => {
       expect(budget.maxTaskDurationMinutes).toBe(60);
       expect(budget.enforceHardLimits).toBe(true);
     });
+
+    it("should clamp negative config values to defaults", () => {
+      const badConfig = {
+        get: vi.fn((key: string) => {
+          const config: Record<string, unknown> = {
+            "orchestrator.budget.dailyTokenLimit": -100,
+            "orchestrator.budget.perAgentTokenLimit": -50,
+            "orchestrator.budget.maxConcurrentAgents": -1,
+            "orchestrator.budget.maxTaskDurationMinutes": 0,
+          };
+          return config[key];
+        }),
+      };
+
+      const badService = new BudgetService(badConfig as unknown as ConfigService);
+      const budget = badService.getBudget();
+
+      expect(budget.dailyTokenLimit).toBe(10_000_000);
+      expect(budget.perAgentTokenLimit).toBe(2_000_000);
+      expect(budget.maxConcurrentAgents).toBe(10);
+      expect(budget.maxTaskDurationMinutes).toBe(120);
+    });
+
+    it("should clamp NaN config values to defaults", () => {
+      const nanConfig = {
+        get: vi.fn((key: string) => {
+          const config: Record<string, unknown> = {
+            "orchestrator.budget.dailyTokenLimit": NaN,
+            "orchestrator.budget.maxConcurrentAgents": Infinity,
+          };
+          return config[key];
+        }),
+      };
+
+      const nanService = new BudgetService(nanConfig as unknown as ConfigService);
+      const budget = nanService.getBudget();
+
+      expect(budget.dailyTokenLimit).toBe(10_000_000);
+      expect(budget.maxConcurrentAgents).toBe(10);
+    });
   });
 
   describe("recordUsage", () => {
@@ -84,6 +124,42 @@ describe("BudgetService", () => {
       expect(agent1?.totalTokens).toBe(1500);
       expect(agent2?.totalTokens).toBe(4500);
     });
+
+    it("should clamp negative token values to 0", () => {
+      service.recordUsage("agent-1", "task-1", -500, -200);
+
+      const summary = service.getUsageSummary();
+      expect(summary.dailyTokensUsed).toBe(0);
+    });
+
+    it("should clamp NaN token values to 0", () => {
+      service.recordUsage("agent-1", "task-1", NaN, NaN);
+
+      const summary = service.getUsageSummary();
+      expect(summary.dailyTokensUsed).toBe(0);
+    });
+
+    it("should clamp Infinity token values to 0", () => {
+      service.recordUsage("agent-1", "task-1", Infinity, -Infinity);
+
+      const summary = service.getUsageSummary();
+      expect(summary.dailyTokensUsed).toBe(0);
+    });
+
+    it("should skip recording when agentId is empty", () => {
+      service.recordUsage("", "task-1", 1000, 500);
+
+      const summary = service.getUsageSummary();
+      expect(summary.dailyTokensUsed).toBe(0);
+      expect(summary.agentUsage).toHaveLength(0);
+    });
+
+    it("should skip recording when taskId is empty", () => {
+      service.recordUsage("agent-1", "", 1000, 500);
+
+      const summary = service.getUsageSummary();
+      expect(summary.dailyTokensUsed).toBe(0);
+    });
   });
 
   describe("canSpawnAgent", () => {
@@ -94,7 +170,7 @@ describe("BudgetService", () => {
 
     it("should block spawning when at max concurrent agents", () => {
       for (let i = 0; i < 10; i++) {
-        service.agentStarted();
+        service.agentStarted(`agent-${String(i)}`);
       }
 
       const result = service.canSpawnAgent();
@@ -104,12 +180,12 @@ describe("BudgetService", () => {
 
     it("should allow spawning after agent stops", () => {
       for (let i = 0; i < 10; i++) {
-        service.agentStarted();
+        service.agentStarted(`agent-${String(i)}`);
       }
 
       expect(service.canSpawnAgent().allowed).toBe(false);
 
-      service.agentStopped();
+      service.agentStopped("agent-0");
 
       expect(service.canSpawnAgent().allowed).toBe(true);
     });
@@ -141,6 +217,43 @@ describe("BudgetService", () => {
     });
   });
 
+  describe("trySpawnAgent", () => {
+    it("should atomically check and reserve slot", () => {
+      const result = service.trySpawnAgent("agent-1");
+      expect(result.allowed).toBe(true);
+
+      const summary = service.getUsageSummary();
+      expect(summary.activeAgents).toBe(1);
+    });
+
+    it("should block when at max concurrent and not reserve slot", () => {
+      for (let i = 0; i < 10; i++) {
+        service.trySpawnAgent(`agent-${String(i)}`);
+      }
+
+      const result = service.trySpawnAgent("agent-11");
+      expect(result.allowed).toBe(false);
+      expect(result.reason).toContain("Maximum concurrent agents");
+
+      const summary = service.getUsageSummary();
+      expect(summary.activeAgents).toBe(10);
+    });
+
+    it("should reject empty agent ID", () => {
+      const result = service.trySpawnAgent("");
+      expect(result.allowed).toBe(false);
+      expect(result.reason).toContain("Agent ID is required");
+    });
+
+    it("should not double-count same agent ID", () => {
+      service.trySpawnAgent("agent-1");
+      service.trySpawnAgent("agent-1");
+
+      const summary = service.getUsageSummary();
+      expect(summary.activeAgents).toBe(1);
+    });
+  });
+
   describe("isAgentOverBudget", () => {
     it("should return false when agent is within budget", () => {
       service.recordUsage("agent-1", "task-1", 100_000, 50_000);
@@ -166,31 +279,38 @@ describe("BudgetService", () => {
   });
 
   describe("agentStarted / agentStopped", () => {
-    it("should track active agent count", () => {
-      service.agentStarted();
-      service.agentStarted();
-      service.agentStarted();
+    it("should track active agent count by ID", () => {
+      service.agentStarted("agent-1");
+      service.agentStarted("agent-2");
+      service.agentStarted("agent-3");
 
       const summary = service.getUsageSummary();
       expect(summary.activeAgents).toBe(3);
     });
 
     it("should decrement active count on stop", () => {
-      service.agentStarted();
-      service.agentStarted();
-      service.agentStopped();
+      service.agentStarted("agent-1");
+      service.agentStarted("agent-2");
+      service.agentStopped("agent-1");
 
       const summary = service.getUsageSummary();
       expect(summary.activeAgents).toBe(1);
     });
 
-    it("should not go below zero", () => {
-      service.agentStopped();
-      service.agentStopped();
+    it("should handle stopping non-existent agent gracefully", () => {
+      service.agentStopped("non-existent");
 
       const summary = service.getUsageSummary();
       expect(summary.activeAgents).toBe(0);
     });
+
+    it("should not double-count same agent ID on start", () => {
+      service.agentStarted("agent-1");
+      service.agentStarted("agent-1");
+
+      const summary = service.getUsageSummary();
+      expect(summary.activeAgents).toBe(1);
+    });
   });
 
   describe("getUsageSummary", () => {
@@ -290,7 +410,7 @@ describe("BudgetService", () => {
       const budget2 = service.getBudget();
 
       expect(budget1).toEqual(budget2);
-      expect(budget1).not.toBe(budget2); // Different reference
+      expect(budget1).not.toBe(budget2);
     });
   });
 });
diff --git a/apps/orchestrator/src/budget/budget.service.ts b/apps/orchestrator/src/budget/budget.service.ts
index 4bd16e6..7c8e82e 100644
--- a/apps/orchestrator/src/budget/budget.service.ts
+++ b/apps/orchestrator/src/budget/budget.service.ts
@@ -15,31 +15,21 @@ import type {
 } from "./budget.types";
 import { DEFAULT_BUDGET } from "./budget.types";
 
+/** Sanitize strings for safe log output */
+function sanitizeForLog(value: string): string {
+  return value.replace(/[\n\r\t]/g, "_").slice(0, 128);
+}
+
 @Injectable()
 export class BudgetService {
   private readonly logger = new Logger(BudgetService.name);
   private readonly budget: UsageBudget;
-  private readonly records: UsageRecord[] = [];
-  private readonly activeAgentCount = { value: 0 };
+  private records: UsageRecord[] = [];
+  private readonly activeAgents = new Set<string>();
+  private lastPurgeDate = "";
 
   constructor(private readonly configService: ConfigService) {
-    this.budget = {
-      dailyTokenLimit:
-        this.configService.get<number>("orchestrator.budget.dailyTokenLimit") ??
-        DEFAULT_BUDGET.dailyTokenLimit,
-      perAgentTokenLimit:
-        this.configService.get<number>("orchestrator.budget.perAgentTokenLimit") ??
-        DEFAULT_BUDGET.perAgentTokenLimit,
-      maxConcurrentAgents:
-        this.configService.get<number>("orchestrator.budget.maxConcurrentAgents") ??
-        DEFAULT_BUDGET.maxConcurrentAgents,
-      maxTaskDurationMinutes:
-        this.configService.get<number>("orchestrator.budget.maxTaskDurationMinutes") ??
-        DEFAULT_BUDGET.maxTaskDurationMinutes,
-      enforceHardLimits:
-        this.configService.get<boolean>("orchestrator.budget.enforceHardLimits") ??
-        DEFAULT_BUDGET.enforceHardLimits,
-    };
+    this.budget = this.loadAndValidateConfig();
 
     this.logger.log(
       `BudgetService initialized: daily=${String(this.budget.dailyTokenLimit)} tokens, ` +
@@ -49,29 +39,68 @@ export class BudgetService {
   }
 
   /**
-   * Record token usage for an agent
+   * Record token usage for an agent.
+   * Negative and NaN values are clamped to 0.
    */
   recordUsage(agentId: string, taskId: string, inputTokens: number, outputTokens: number): void {
+    if (!agentId || !taskId) {
+      this.logger.warn("recordUsage called with empty agentId or taskId — skipping");
+      return;
+    }
+
+    const safeInput = Math.max(0, Number.isFinite(inputTokens) ? inputTokens : 0);
+    const safeOutput = Math.max(0, Number.isFinite(outputTokens) ? outputTokens : 0);
+
+    this.purgeStaleRecords();
+
     const record: UsageRecord = {
       agentId,
       taskId,
-      inputTokens,
-      outputTokens,
-      timestamp: new Date().toISOString(),
+      inputTokens: safeInput,
+      outputTokens: safeOutput,
+      timestamp: new Date(),
     };
 
     this.records.push(record);
 
     this.logger.debug(
-      `Usage recorded: agent=${agentId} input=${String(inputTokens)} output=${String(outputTokens)}`
+      `Usage recorded: agent=${sanitizeForLog(agentId)} input=${String(safeInput)} output=${String(safeOutput)}`
     );
   }
 
   /**
-   * Check if an agent can be spawned (concurrency and budget check)
+   * Check if an agent can be spawned (concurrency and budget check).
+   * When allowed, atomically increments the active agent count.
+   */
+  trySpawnAgent(agentId: string): { allowed: boolean; reason?: string } {
+    if (!agentId) {
+      return { allowed: false, reason: "Agent ID is required" };
+    }
+
+    if (this.activeAgents.size >= this.budget.maxConcurrentAgents) {
+      return {
+        allowed: false,
+        reason: `Maximum concurrent agents reached (${String(this.budget.maxConcurrentAgents)})`,
+      };
+    }
+
+    const dailyUsed = this.getDailyTokensUsed();
+    if (this.budget.enforceHardLimits && dailyUsed >= this.budget.dailyTokenLimit) {
+      return {
+        allowed: false,
+        reason: `Daily token budget exceeded (${String(dailyUsed)}/${String(this.budget.dailyTokenLimit)})`,
+      };
+    }
+
+    this.activeAgents.add(agentId);
+    return { allowed: true };
+  }
+
+  /**
+   * Check if an agent can be spawned without reserving a slot.
    */
   canSpawnAgent(): { allowed: boolean; reason?: string } {
-    if (this.activeAgentCount.value >= this.budget.maxConcurrentAgents) {
+    if (this.activeAgents.size >= this.budget.maxConcurrentAgents) {
       return {
         allowed: false,
         reason: `Maximum concurrent agents reached (${String(this.budget.maxConcurrentAgents)})`,
@@ -90,10 +119,13 @@ export class BudgetService {
   }
 
   /**
-   * Check if an agent has exceeded its per-task budget
+   * Check if an agent has exceeded its per-task budget (today only).
    */
   isAgentOverBudget(agentId: string): { overBudget: boolean; totalTokens: number } {
-    const agentRecords = this.records.filter((r) => r.agentId === agentId);
+    const todayStart = this.getTodayStart();
+    const agentRecords = this.records.filter(
+      (r) => r.agentId === agentId && r.timestamp >= todayStart
+    );
     const totalTokens = agentRecords.reduce((sum, r) => sum + r.inputTokens + r.outputTokens, 0);
 
     return {
@@ -103,23 +135,24 @@ export class BudgetService {
   }
 
   /**
-   * Notify that an agent has started (increment active count)
+   * Notify that an agent has started (track by ID).
    */
-  agentStarted(): void {
-    this.activeAgentCount.value++;
+  agentStarted(agentId: string): void {
+    this.activeAgents.add(agentId);
   }
 
   /**
-   * Notify that an agent has stopped (decrement active count)
+   * Notify that an agent has stopped (remove by ID).
    */
-  agentStopped(): void {
-    this.activeAgentCount.value = Math.max(0, this.activeAgentCount.value - 1);
+  agentStopped(agentId: string): void {
+    this.activeAgents.delete(agentId);
   }
 
   /**
    * Get comprehensive usage summary
    */
   getUsageSummary(): UsageSummary {
+    this.purgeStaleRecords();
     const dailyTokensUsed = this.getDailyTokensUsed();
     const dailyUsagePercent =
       this.budget.dailyTokenLimit > 0 ? (dailyTokensUsed / this.budget.dailyTokenLimit) * 100 : 0;
@@ -129,7 +162,7 @@ export class BudgetService {
       dailyTokenLimit: this.budget.dailyTokenLimit,
       dailyUsagePercent: Math.round(dailyUsagePercent * 100) / 100,
       agentUsage: this.getAgentUsageSummaries(),
-      activeAgents: this.activeAgentCount.value,
+      activeAgents: this.activeAgents.size,
       maxConcurrentAgents: this.budget.maxConcurrentAgents,
       budgetStatus: this.getBudgetStatus(dailyUsagePercent),
     };
@@ -143,25 +176,98 @@ export class BudgetService {
   }
 
   /**
-   * Get total tokens used today
+   * Load configuration with validation. Clamps invalid values to defaults.
    */
-  private getDailyTokensUsed(): number {
-    const todayStart = new Date();
-    todayStart.setHours(0, 0, 0, 0);
-    const todayIso = todayStart.toISOString();
+  private loadAndValidateConfig(): UsageBudget {
+    const raw = {
+      dailyTokenLimit:
+        this.configService.get<number>("orchestrator.budget.dailyTokenLimit") ??
+        DEFAULT_BUDGET.dailyTokenLimit,
+      perAgentTokenLimit:
+        this.configService.get<number>("orchestrator.budget.perAgentTokenLimit") ??
+        DEFAULT_BUDGET.perAgentTokenLimit,
+      maxConcurrentAgents:
+        this.configService.get<number>("orchestrator.budget.maxConcurrentAgents") ??
+        DEFAULT_BUDGET.maxConcurrentAgents,
+      maxTaskDurationMinutes:
+        this.configService.get<number>("orchestrator.budget.maxTaskDurationMinutes") ??
+        DEFAULT_BUDGET.maxTaskDurationMinutes,
+      enforceHardLimits:
+        this.configService.get<boolean>("orchestrator.budget.enforceHardLimits") ??
+        DEFAULT_BUDGET.enforceHardLimits,
+    };
 
-    return this.records
-      .filter((r) => r.timestamp >= todayIso)
-      .reduce((sum, r) => sum + r.inputTokens + r.outputTokens, 0);
+    return {
+      dailyTokenLimit: this.clampPositive(raw.dailyTokenLimit, DEFAULT_BUDGET.dailyTokenLimit),
+      perAgentTokenLimit: this.clampPositive(
+        raw.perAgentTokenLimit,
+        DEFAULT_BUDGET.perAgentTokenLimit
+      ),
+      maxConcurrentAgents: this.clampPositiveInt(
+        raw.maxConcurrentAgents,
+        DEFAULT_BUDGET.maxConcurrentAgents
+      ),
+      maxTaskDurationMinutes: this.clampPositiveInt(
+        raw.maxTaskDurationMinutes,
+        DEFAULT_BUDGET.maxTaskDurationMinutes
+      ),
+      enforceHardLimits: raw.enforceHardLimits,
+    };
+  }
+
+  private clampPositive(value: number, fallback: number): number {
+    return Number.isFinite(value) && value > 0 ? value : fallback;
+  }
+
+  private clampPositiveInt(value: number, fallback: number): number {
+    return Number.isFinite(value) && value > 0 ? Math.floor(value) : fallback;
   }
 
   /**
-   * Get per-agent usage summaries
+   * Purge records from previous days to prevent unbounded memory growth.
+   */
+  private purgeStaleRecords(): void {
+    const todayStr = new Date().toISOString().slice(0, 10);
+    if (this.lastPurgeDate === todayStr) return;
+
+    const todayStart = this.getTodayStart();
+    const before = this.records.length;
+    this.records = this.records.filter((r) => r.timestamp >= todayStart);
+
+    if (before > this.records.length) {
+      this.logger.log(
+        `Purged ${String(before - this.records.length)} stale usage records from previous days`
+      );
+    }
+    this.lastPurgeDate = todayStr;
+  }
+
+  /**
+   * Get total tokens used today using proper Date comparison.
+   */
+  private getDailyTokensUsed(): number {
+    const todayStart = this.getTodayStart();
+
+    return this.records
+      .filter((r) => r.timestamp >= todayStart)
+      .reduce((sum, r) => sum + r.inputTokens + r.outputTokens, 0);
+  }
+
+  private getTodayStart(): Date {
+    const todayStart = new Date();
+    todayStart.setHours(0, 0, 0, 0);
+    return todayStart;
+  }
+
+  /**
+   * Get per-agent usage summaries (today only).
    */
   private getAgentUsageSummaries(): AgentUsageSummary[] {
+    const todayStart = this.getTodayStart();
+    const todayRecords = this.records.filter((r) => r.timestamp >= todayStart);
     const agentMap = new Map<string, { taskId: string; input: number; output: number }>();
 
-    for (const record of this.records) {
+    for (const record of todayRecords) {
       const existing = agentMap.get(record.agentId);
       if (existing) {
         existing.input += record.inputTokens;
@@ -175,7 +281,7 @@ export class BudgetService {
       }
     }
 
-    return Array.from(agentMap.entries()).map(([agentId, data]) => {
+    return [...agentMap.entries()].map(([agentId, data]) => {
       const totalTokens = data.input + data.output;
       const usagePercent =
         this.budget.perAgentTokenLimit > 0
diff --git a/apps/orchestrator/src/budget/budget.types.ts b/apps/orchestrator/src/budget/budget.types.ts
index 44678f6..86a0c57 100644
--- a/apps/orchestrator/src/budget/budget.types.ts
+++ b/apps/orchestrator/src/budget/budget.types.ts
@@ -28,7 +28,7 @@ export interface UsageRecord {
   /** Number of output tokens used */
   outputTokens: number;
   /** Timestamp of usage */
-  timestamp: string;
+  timestamp: Date;
 }
 
 export interface UsageSummary {