diff --git a/README.md b/README.md
index d35b3fd..0fee48a 100644
--- a/README.md
+++ b/README.md
@@ -86,6 +86,38 @@ This command should automatically bind your current working directory into the c
 this repository's code and data which it needs run scAbsolute. If errors are generated related to lacking access to needed code or scripts this problem
 could be resolved by appending ` -B ${PWD}:${PWD}` to `--singularity-args` in the above command.
 
+We also recommend passing `--keep-going` so that a single-cell failure mid-run
+does not abort the whole batch.
+
+### Failure handling and the failed_cells.csv report
+
+The workflow runs a pre-flight `samtools quickcheck` against every BAM listed
+in your sample sheet before any copy-number calling starts. If any BAM is
+truncated or otherwise unreadable, the workflow aborts immediately with the
+full list of offending files, so you can fix them all in one pass (typically
+by re-copying from the source filesystem) instead of discovering failures one
+cell at a time deep into a multi-hour run.
+
+All cell failures across the whole pipeline are recorded in a single canonical
+CSV:
+
+```
+results/<binSize>/<sampleName>_<binSize>_failed_cells.csv
+```
+
+Schema: `name,failure_reason`. Possible `failure_reason` values are:
+
+| Value | Stage | Meaning |
+|---|---|---|
+| `truncated_bam` | pre-flight `validate_bams` | `samtools quickcheck` rejected the BAM (most often an incomplete copy) |
+| `missing_output` | `combine` | The `.rds` for this cell was never produced |
+| `process_crash` | `combine` | The `.rds` exists but is empty / unreadable |
+| (other) | `scAbsolute` | Per-cell QC failures recorded by `scAbsolute` (low coverage, fit failure, etc.) |
+
+After a validation failure, the workflow stops with no `.rds` results. After a
+successful validation but partial downstream failure, the CSV lists only the
+cells that failed in scAbsolute / combine — everything else has a result.
+
 ### QC analysis
 
 Please take the time to analyze the data (the qc script to be used for this step is available at scripts/qc-script.R)
diff --git a/workflow/Snakefile_absolute b/workflow/Snakefile_absolute
index 69120f7..824a12a 100644
--- a/workflow/Snakefile_absolute
+++ b/workflow/Snakefile_absolute
@@ -35,15 +35,16 @@ def helper_dirname(wildcards):
     return os.path.join("results", str(config["binSize"]), filename)
 
 
+include: "rules/validate.smk"
 include: "rules/qc.smk"
 include: "rules/scale_scAbsolute.smk"
 include: "rules/combine.smk"
 
 if config["estimateReadDensity"]:
     include: "rules/density.smk"
-    ruleorder: qc > density > scale_scAbsolute > combine
+    ruleorder: validate_bams > qc > density > scale_scAbsolute > combine
 else:
-    ruleorder: qc > scale_scAbsolute > combine
+    ruleorder: validate_bams > qc > scale_scAbsolute > combine
 
 
 rule all:
diff --git a/workflow/rules/qc.smk b/workflow/rules/qc.smk
index cf1e4cd..bd8095d 100644
--- a/workflow/rules/qc.smk
+++ b/workflow/rules/qc.smk
@@ -1,6 +1,7 @@
 rule qc:
     input:
-        bam="data/aligned/" + str(config["sampleName"]) + "/{sample}.bam"
+        bam="data/aligned/" + str(config["sampleName"]) + "/{sample}.bam",
+        validated="data/aligned/" + str(config["sampleName"]) + "/.bam_validation.passed"
     output:
         flagstat="data/aligned/" + str(config["sampleName"]) + "/{sample}.flagstat",
         bai="data/aligned/" + str(config["sampleName"]) + "/{sample}.bam.bai"
diff --git a/workflow/rules/validate.smk b/workflow/rules/validate.smk
new file mode 100644
index 0000000..d6cdf5a
--- /dev/null
+++ b/workflow/rules/validate.smk
@@ -0,0 +1,70 @@
+rule validate_bams:
+    """Pre-flight BAM integrity check. Runs samtools quickcheck on every BAM
+    listed in the sample sheet and fails fast with a full list of bad files
+    before any qc / scale_scAbsolute job is scheduled. Common failure mode
+    this catches: BAMs that were truncated during copy from the source path.
+
+    On failure, writes the bad cells into the SAME failed_cells.csv that the
+    combine / merge step uses downstream, with failure_reason = "truncated_bam".
+    This gives a single canonical place to look for any failed cell across the
+    whole pipeline (truncated input, missing output, process crash, or QC
+    failure during scAbsolute).
+    """
+    input:
+        bams=expand("data/aligned/" + str(config["sampleName"]) + "/{sample}.bam",
+                    sample=SAMPLE_FILES)
+    output:
+        report="data/aligned/" + str(config["sampleName"]) + "/.bam_validation.passed"
+    params:
+        failed_csv="results/" + str(config["binSize"]) + "/" + str(config["sampleName"]) + "_" + str(config["binSize"]) + "_failed_cells.csv"
+    container:
+        config["qc_img"]
+    message:
+        "Pre-flight BAM integrity check (samtools quickcheck)"
+    threads:
+        1
+    shell:
+        r"""
+        set -uo pipefail
+        bad_tmp=$(mktemp)
+        n_total=0
+        n_bad=0
+        for bam in {input.bams}; do
+            n_total=$((n_total+1))
+            if ! samtools quickcheck "$bam" 2>/dev/null; then
+                # Strip .bam extension to match cell-name convention used downstream
+                base=$(basename "$bam" .bam)
+                echo "$base" >> "$bad_tmp"
+                n_bad=$((n_bad+1))
+            fi
+        done
+
+        if [ "$n_bad" -gt 0 ]; then
+            # Write failed cells into the canonical failed_cells.csv (same file
+            # merge.R uses) so the user has ONE place to look for failures.
+            mkdir -p "$(dirname {params.failed_csv})"
+            {{
+                echo '"name","failure_reason"'
+                while IFS= read -r cell; do
+                    printf '"%s","truncated_bam"\n' "$cell"
+                done < "$bad_tmp"
+            }} > {params.failed_csv}
+
+            echo "" >&2
+            echo "============================================================" >&2
+            echo "ERROR: $n_bad / $n_total BAM file(s) failed integrity check:" >&2
+            echo "============================================================" >&2
+            cat "$bad_tmp" >&2
+            echo "============================================================" >&2
+            echo "Full failure report written to:" >&2
+            echo "  {params.failed_csv}" >&2
+            echo "(same CSV that downstream failures use; failure_reason = truncated_bam)" >&2
+            echo "Common cause: incomplete copy from source filesystem." >&2
+            echo "Action: re-copy listed files from source and re-run." >&2
+            echo "============================================================" >&2
+            rm -f "$bad_tmp"
+            exit 1
+        fi
+        rm -f "$bad_tmp"
+        echo "All $n_total BAMs passed samtools quickcheck on $(date)" > {output.report}
+        """