bio-alignment-filtering

bio-alignment-filtering is a skill for Claude Code, Codex from GPTomics/bioSkills. It costs 44 tokens per session (4,295 once invoked), scanned A, original, MIT.

A guide to filtering DNA sequencing alignment files, which record where reads match a reference genome. It uses samtools or the Python library pysam to select reads by flags, quality, or genomic region.

In plain words
What is it for?
Use it to keep or exclude reads based on alignment status, mapping quality, or regions listed in a BED file.
Why use it?
It removes unwanted or unreliable reads so later analysis uses the intended data. It also helps extract reads from specific parts of a genome.

Skill for Claude CodeCodex

Install

Getting it into your agent

One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.

agentmods
npx agentmods add skills/gptomics/bioskills/alignment-filtering
Any agent
npx skills add GPTomics/bioSkills --skill alignment-filtering
Clone the repo
git clone --depth 1 https://github.com/GPTomics/bioSkills

Made for: Claude Code, Codex.

Wrote this? Show the measurements

A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.

agentmods badge for bio-alignment-filtering

README.md
[![agentmods](https://agentmods.dev/badge/skills/gptomics/bioskills/alignment-filtering.svg)](https://agentmods.dev/skills/gptomics/bioskills/alignment-filtering)
Your own site
<a href="https://agentmods.dev/skills/gptomics/bioskills/alignment-filtering"><img src="https://agentmods.dev/badge/skills/gptomics/bioskills/alignment-filtering.svg" alt="Measured on agentmods" height="20"></a>
Per session 44 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.
When invoked 4,295 The whole file, excluding the scripts and references it only reads on demand.
Security scan A 1 finding. Scan, not verified.
Origin original No closer match found in the catalogue.
Token cost

What it costs to keep this loaded

Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.

ModelPer sessionOnce invoked
Fable 5 $0.00044 $0.04295
Opus 5 $0.00022 $0.02148
Sonnet 5 $0.00009 $0.00859
Haiku 4.5 $0.00004 $0.00430

Measured 4d ago against content hash e24ea6628923, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

bio-alignment-filtering scanned grade A with 1 finding against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 4d ago.

The scan reads SKILL.md. This mod also ships 1 executable file (examples/filter_bam.py), listed below but not scanned — reading those needs a real analyzer, not pattern matching.

A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.

Makes network callslowCapability

Not a fault in itself. Listed so you know the mod talks to something, and to what.

for read in infile.fetch(chrom, start, end):
Origin

Copies of this mod

1 near-identical copy found in the catalogue:

alignment-files/alignment-filtering/SKILL.md · 411 lines

How it starts

The opening of the file, as written. The whole thing — 411 lines — stays where its author put it; the contents beside it link to each section on GitHub.

Version Compatibility

Reference examples tested with: pysam 0.22+, samtools 1.19+

Before using code patterns, verify installed versions match. If versions differ:

  • Python: pip show <package> then help(module.function) to check signatures
  • CLI: <tool> --version then <tool> --help to confirm flags

If code throws ImportError, AttributeError, or TypeError, introspect the installed package and adapt the example to match the actual API rather than retrying.

Alignment Filtering

"Filter my BAM file to keep only high-quality reads" -> Select reads by FLAG bits, mapping quality, and genomic regions using samtools view or pysam.

  • CLI: samtools view with -F/-f/-q/-L flags (samtools)
  • Python: pysam.AlignmentFile iteration with attribute filters (pysam)

Filter alignments by flags, quality, and regions using samtools and pysam.

Filter Flags

Option Description
-f FLAG Include reads with ALL bits set
-F FLAG Exclude reads with ANY bits set
-G FLAG Exclude reads with ALL bits set
-q MAPQ Minimum mapping quality
-L BED Include reads overlapping regions

Common FLAG Values

Flag Hex Meaning
1 0x1 Paired
2 0x2 Proper pair
4 0x4 Unmapped
8 0x8 Mate unmapped
16 0x10 Reverse strand
32 0x20 Mate reverse strand
64 0x40 First in pair (read1)
128 0x80 Second in pair (read2)
256 0x100 Secondary alignment
512 0x200 Failed QC
1024 0x400 Duplicate
2048 0x800 Supplementary

Filter by FLAG

Keep Only Mapped Reads

samtools view -F 4 -o mapped.bam input.bam

Keep Only Unmapped Reads

samtools view -f 4 -o unmapped.bam input.bam

Keep Only Properly Paired

samtools view -f 2 -o proper.bam input.bam

Remove Duplicates

samtools view -F 1024 -o nodup.bam input.bam

Remove Secondary and Supplementary

samtools view -F 2304 -o primary.bam input.bam

Read the full file on GitHub · 411 lines

Files

What ships with it

2 files beside SKILL.md in the same directory: the scripts, references and assets a skill reads on demand. Not counted in the per-session cost; read them before you install if any of them is executable.

Changes

What this file has done since we first saw it

Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.

  1. 4d ago First seen · 411 lines · 44 tokens per session scan A e24ea6628923

Subscribe to this mod's changes

bio-alignment-filtering is a skill published in the GitHub repository GPTomics/bioSkills (1,198 stars, last pushed 19d ago), licensed MIT. It adds 44 tokens to every session and 4,295 once invoked, about $0.0002 per session on Opus 5. A static security scan graded it A with 1 finding (makes network calls). No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-30.

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