diff --git a/src/evaluator/fallbackTreatmentsCalculator/__tests__/fallback-sanitizer.spec.ts b/src/evaluator/fallbackTreatmentsCalculator/__tests__/fallback-sanitizer.spec.ts
new file mode 100644
index 00000000..e063a570
--- /dev/null
+++ b/src/evaluator/fallbackTreatmentsCalculator/__tests__/fallback-sanitizer.spec.ts
@@ -0,0 +1,103 @@
+import { FallbacksSanitizer } from '../fallbackSanitizer';
+import { TreatmentWithConfig } from '../../../../types/splitio';
+
+describe('FallbacksSanitizer', () => {
+  const validTreatment: TreatmentWithConfig = { treatment: 'on', config: '{"color":"blue"}' };
+  const invalidTreatment: TreatmentWithConfig = { treatment: ' ', config: null };
+
+  beforeEach(() => {
+    jest.spyOn(console, 'error').mockImplementation(() => {});
+  });
+
+  afterEach(() => {
+    (console.error as jest.Mock).mockRestore();
+  });
+
+  describe('isValidFlagName', () => {
+    it('returns true for a valid flag name', () => {
+      // @ts-expect-private-access
+      expect((FallbacksSanitizer as any).isValidFlagName('my_flag')).toBe(true);
+    });
+
+    it('returns false for a name longer than 100 chars', () => {
+      const longName = 'a'.repeat(101);
+      expect((FallbacksSanitizer as any).isValidFlagName(longName)).toBe(false);
+    });
+
+    it('returns false if the name contains spaces', () => {
+      expect((FallbacksSanitizer as any).isValidFlagName('invalid flag')).toBe(false);
+    });
+  });
+
+  describe('isValidTreatment', () => {
+    it('returns true for a valid treatment string', () => {
+      expect((FallbacksSanitizer as any).isValidTreatment(validTreatment)).toBe(true);
+    });
+
+    it('returns false for null or undefined', () => {
+      expect((FallbacksSanitizer as any).isValidTreatment(null)).toBe(false);
+      expect((FallbacksSanitizer as any).isValidTreatment(undefined)).toBe(false);
+    });
+
+    it('returns false for a treatment longer than 100 chars', () => {
+      const long = { treatment: 'a'.repeat(101) };
+      expect((FallbacksSanitizer as any).isValidTreatment(long)).toBe(false);
+    });
+
+    it('returns false if treatment does not match regex pattern', () => {
+      const invalid = { treatment: 'invalid treatment!' };
+      expect((FallbacksSanitizer as any).isValidTreatment(invalid)).toBe(false);
+    });
+  });
+
+  describe('sanitizeGlobal', () => {
+    it('returns the treatment if valid', () => {
+      expect(FallbacksSanitizer.sanitizeGlobal(validTreatment)).toEqual(validTreatment);
+      expect(console.error).not.toHaveBeenCalled();
+    });
+
+    it('returns undefined and logs error if invalid', () => {
+      const result = FallbacksSanitizer.sanitizeGlobal(invalidTreatment);
+      expect(result).toBeUndefined();
+      expect(console.error).toHaveBeenCalledWith(
+        expect.stringContaining('Fallback treatments - Discarded fallback')
+      );
+    });
+  });
+
+  describe('sanitizeByFlag', () => {
+    it('returns a sanitized map with valid entries only', () => {
+      const input = {
+        valid_flag: validTreatment,
+        'invalid flag': validTreatment,
+        bad_treatment: invalidTreatment,
+      };
+
+      const result = FallbacksSanitizer.sanitizeByFlag(input);
+
+      expect(result).toEqual({ valid_flag: validTreatment });
+      expect(console.error).toHaveBeenCalledTimes(2); // invalid flag + bad_treatment
+    });
+
+    it('returns empty object if all invalid', () => {
+      const input = {
+        'invalid flag': invalidTreatment,
+      };
+
+      const result = FallbacksSanitizer.sanitizeByFlag(input);
+      expect(result).toEqual({});
+      expect(console.error).toHaveBeenCalled();
+    });
+
+    it('returns same object if all valid', () => {
+      const input = {
+        flag_one: validTreatment,
+        flag_two: { treatment: 'valid_2', config: null },
+      };
+
+      const result = FallbacksSanitizer.sanitizeByFlag(input);
+      expect(result).toEqual(input);
+      expect(console.error).not.toHaveBeenCalled();
+    });
+  });
+});
diff --git a/src/evaluator/fallbackTreatmentsCalculator/constants.ts b/src/evaluator/fallbackTreatmentsCalculator/constants.ts
new file mode 100644
index 00000000..292bae0f
--- /dev/null
+++ b/src/evaluator/fallbackTreatmentsCalculator/constants.ts
@@ -0,0 +1,4 @@
+export enum FallbackDiscardReason {
+  FlagName = 'Invalid flag name (max 100 chars, no spaces)',
+  Treatment = 'Invalid treatment (max 100 chars and must match pattern)',
+}
diff --git a/src/evaluator/fallbackTreatmentsCalculator/fallbackSanitizer/index.ts b/src/evaluator/fallbackTreatmentsCalculator/fallbackSanitizer/index.ts
new file mode 100644
index 00000000..a8e07e34
--- /dev/null
+++ b/src/evaluator/fallbackTreatmentsCalculator/fallbackSanitizer/index.ts
@@ -0,0 +1,63 @@
+import { TreatmentWithConfig } from '../../../../types/splitio';
+import { FallbackDiscardReason } from '../constants';
+
+
+export class FallbacksSanitizer {
+
+  private static readonly pattern = /^[0-9]+[.a-zA-Z0-9_-]*$|^[a-zA-Z]+[a-zA-Z0-9_-]*$/;
+
+  private static isValidFlagName(name: string): boolean {
+    return name.length <= 100 && !name.includes(' ');
+  }
+
+  private static isValidTreatment(t?: TreatmentWithConfig): boolean {
+    if (!t || !t.treatment) {
+      return false;
+    }
+
+    const { treatment } = t;
+
+    if (treatment.length > 100) {
+      return false;
+    }
+
+    return FallbacksSanitizer.pattern.test(treatment);
+  }
+
+  static sanitizeGlobal(treatment?: TreatmentWithConfig): TreatmentWithConfig | undefined {
+    if (!this.isValidTreatment(treatment)) {
+      console.error(
+        `Fallback treatments - Discarded fallback: ${FallbackDiscardReason.Treatment}`
+      );
+      return undefined;
+    }
+    return treatment!;
+  }
+
+  static sanitizeByFlag(
+    byFlagFallbacks: Record<string, TreatmentWithConfig>
+  ): Record<string, TreatmentWithConfig> {
+    const sanitizedByFlag: Record<string, TreatmentWithConfig> = {};
+
+    const entries = Object.entries(byFlagFallbacks);
+    entries.forEach(([flag, t]) => {
+      if (!this.isValidFlagName(flag)) {
+        console.error(
+          `Fallback treatments - Discarded flag '${flag}': ${FallbackDiscardReason.FlagName}`
+        );
+        return;
+      }
+
+      if (!this.isValidTreatment(t)) {
+        console.error(
+          `Fallback treatments - Discarded treatment for flag '${flag}': ${FallbackDiscardReason.Treatment}`
+        );
+        return;
+      }
+
+      sanitizedByFlag[flag] = t;
+    });
+
+    return sanitizedByFlag;
+  }
+}
diff --git a/types/splitio.d.ts b/types/splitio.d.ts
index ebeba4df..788029d3 100644
--- a/types/splitio.d.ts
+++ b/types/splitio.d.ts
@@ -621,6 +621,10 @@ declare namespace SplitIO {
      * User consent status if using in client-side. Undefined if using in server-side (Node.js).
      */
     readonly userConsent?: ConsentStatus;
+    /**
+     * Fallback treatments to be used when the SDK is not ready or the flag is not found.
+     */
+    readonly fallbackTreatments?: FallbackTreatmentOptions;
   }
   /**
    * Log levels.
@@ -1228,6 +1232,15 @@ declare namespace SplitIO {
    * User consent status.
    */
   type ConsentStatus = 'GRANTED' | 'DECLINED' | 'UNKNOWN';
+  /**
+   * Fallback treatments to be used when the SDK is not ready or the flag is not found.
+   */
+  type FallbackTreatmentOptions = {
+    global?: TreatmentWithConfig | Treatment,
+    byFlag: {
+      [key: string]: TreatmentWithConfig | Treatment
+    }
+  }
   /**
    * Logger. Its interface details are not part of the public API. It shouldn't be used directly.
    */