diff --git a/workflow/envs/cyvcf.yaml b/workflow/envs/cyvcf.yaml
new file mode 100644
index 0000000..bf54b6e
--- /dev/null
+++ b/workflow/envs/cyvcf.yaml
@@ -0,0 +1,5 @@
+channels:
+  - conda-forge
+  - bioconda
+dependencies:
+  - cyvcf2
\ No newline at end of file
diff --git a/workflow/rules/common.smk b/workflow/rules/common.smk
index b45b183..52e1261 100644
--- a/workflow/rules/common.smk
+++ b/workflow/rules/common.smk
@@ -520,6 +520,8 @@ def get_vaf_status(wildcards):
     vaf_benchmark = benchmarks[wildcards.benchmark].get("vaf-field")
     if vaf_benchmark is None:
         return False
+    if vaf_benchmark == "tbc":
+        return True
     else:
         callsets = get_benchmark_callsets(wildcards.benchmark)
         vaf_callsets = [
diff --git a/workflow/rules/eval.smk b/workflow/rules/eval.smk
index 9385639..d60fb04 100644
--- a/workflow/rules/eval.smk
+++ b/workflow/rules/eval.smk
@@ -109,6 +109,19 @@ rule remove_non_pass:
         "v3.3.6/bio/bcftools/view"
 
 
+rule calculate_vaf:
+    input:
+        "results/filtered-variants/{callset}.bcf",
+    output:
+        "results/calculate-vaf/{callset}.added-vaf.bcf",
+    log:
+        "logs/calculate-vaf/{callset}.log",
+    conda:
+        "../envs/cyvcf.yaml"
+    script:
+        "../scripts/calc-vaf.py"
+
+
 rule intersect_calls_with_target_regions:
     input:
         bcf="results/filtered-variants/{callset}.bcf",
diff --git a/workflow/scripts/calc-vaf.py b/workflow/scripts/calc-vaf.py
new file mode 100644
index 0000000..fe528aa
--- /dev/null
+++ b/workflow/scripts/calc-vaf.py
@@ -0,0 +1,145 @@
+from cyvcf2 import VCF, Writer
+import sys
+import argparse
+import numpy as np
+                                  
+def get_snv_allele_freq(variant):
+    refCounts = variant.format(variant.REF + "U")
+    altCounts = variant.format(variant.ALT[0] + "U")
+
+    # TODO: check which value is the correct one from the matrix (this leads to many zero VAF)
+    tier1RefCounts = refCounts[0, 0]
+    tier1AltCounts = altCounts[0, 0]
+
+    vaf = tier1AltCounts / (tier1AltCounts + tier1RefCounts)
+
+    return vaf
+
+def get_indel_allele_freq(variant):
+    tier1RefCounts = variant.format("TAR")[0,0]
+    tier1AltCounts = variant.format("TIR")[0,0]
+
+    vaf = tier1AltCounts / (tier1AltCounts + tier1RefCounts)
+    return vaf
+
+def calculate_vaf(variant, samples):
+    """
+    Calculates the Variant Allele Frequency (VAF) for each sample and
+    each alternate allele based on the AD (Allelic Depth) FORMAT field.
+
+    Args:
+        variant (cyvcf2.Variant): The variant object.
+        samples (list): List of sample names.
+
+    Returns:
+        numpy.ndarray: A numpy array of shape (n_samples, n_alt_alleles)
+                       containing the VAFs. Returns None if AD field is missing.
+                       Missing values or divisions by zero result in np.nan.
+    """
+    try:
+        # Get Allelic Depths (AD) - shape: (n_samples, n_alleles)
+        # n_alleles includes the reference allele
+        ad = variant.format('AD')
+    except KeyError:
+        # AD field is not present for this variant
+        print(f"Warning: AD field missing for variant at {variant.CHROM}:{variant.POS}. Skipping VAF calculation.", file=sys.stderr)
+        return None
+
+    n_samples = len(samples)
+    n_alleles = len(variant.alleles) # Includes reference
+    n_alt_alleles = n_alleles - 1
+
+    # Initialize VAF array with NaNs
+    # Shape: (n_samples, n_alt_alleles)
+    vaf_values = np.full((n_samples, n_alt_alleles), np.nan, dtype=np.float32)
+
+    for i in range(n_samples):
+        sample_ad = ad[i]
+
+        # Check for missing AD data for the sample (represented by negative values or could be None depending on VCF)
+        # cyvcf2 often uses negative numbers for missing integers in FORMAT fields like AD
+        if np.any(sample_ad < 0):
+             # Keep VAF as NaN if AD is missing for this sample
+            continue
+
+        # Calculate total depth for this sample
+        total_depth = np.sum(sample_ad)
+
+        if total_depth == 0:
+            # Avoid division by zero, keep VAFs as NaN
+            continue
+
+        # Calculate VAF for each alternate allele
+        for j in range(n_alt_alleles):
+            alt_depth = sample_ad[j + 1] # AD format is [ref_depth, alt1_depth, alt2_depth, ...]
+            vaf = alt_depth / total_depth
+            vaf_values[i, j] = vaf
+
+    return vaf_values
+
+def add_vaf_to_vcf(input_vcf_path, output_vcf_path):
+    """
+    Reads an input VCF, calculates VAF for each variant/sample, adds it
+    as a new FORMAT field 'VAF', and writes to an output VCF file.
+    """
+    # Open the input VCF file
+    try:
+        vcf_reader = VCF(input_vcf_path)
+    except Exception as e:
+        print(f"Error opening input VCF file '{input_vcf_path}': {e}", file=sys.stderr)
+        sys.exit(1)
+
+
+    # Add the new VAF FORMAT field definition to the header
+    # Number='A' means one value per alternate allele
+    # Type='Float' for the frequency value
+    try:
+        vcf_reader.add_format_to_header({
+            'ID': 'VAF',
+            'Description': 'Variant Allele Frequency calculated from AD field (Alt Depth / Total Depth)',
+            'Type': 'Float',
+            'Number': 'A' # One value per alternate allele
+        })
+    except ValueError as e:
+         # Catch error if the field already exists
+         print(f"Warning: FORMAT field 'VAF' might already exist in header: {e}", file=sys.stderr)
+
+
+    # Create a VCF writer object using the modified header
+    try:
+        vcf_writer = Writer(output_vcf_path, vcf_reader)
+    except Exception as e:
+        print(f"Error creating output VCF file '{output_vcf_path}': {e}", file=sys.stderr)
+        vcf_reader.close()
+        sys.exit(1)
+
+    print(f"Processing VCF: {input_vcf_path}")
+    print(f"Writing output to: {output_vcf_path}")
+
+    processed_count = 0
+    # Iterate through each variant in the VCF
+    for variant in vcf_reader:
+        # Calculate VAFs for all samples for the current variant
+        vaf_array = calculate_vaf(variant, vcf_reader.samples)
+
+        # Add the calculated VAFs to the variant's FORMAT fields
+        # The vaf_array must have shape (n_samples, n_alt_alleles)
+        if vaf_array is not None:
+            try:
+                # Use set_format to add/update the VAF field for all samples
+                variant.set_format('VAF', vaf_array)
+            except Exception as e:
+                 print(f"Error setting VAF format for variant at {variant.CHROM}:{variant.POS}: {e}", file=sys.stderr)
+                 # Decide if you want to skip writing this variant or write without VAF
+                 # Here, we'll still write the variant but VAF might be missing/incorrect
+
+        # Write the (potentially modified) variant record to the output file
+        vcf_writer.write_record(variant)
+        processed_count += 1
+        if processed_count % 1000 == 0:
+            print(f"Processed {processed_count} variants...", file=sys.stderr)
+
+    # Close the VCF reader and writer
+    vcf_reader.close()
+    vcf_writer.close()
+    print(f"Finished processing. Total variants processed: {processed_count}")
\ No newline at end of file