---
title: "Band traceback (where the extra band came from)"
method: POST
path: "/band_traceback"
tags: ["Primers & oligos"]
---

# Band traceback (where the extra band came from)

`POST /band_traceback`

## Request

- Base URL: `https://seqbench.com/api/v1`
- URL: `https://seqbench.com/api/v1/band_traceback`
- Auth: none required

Explain a band you measured on a gel. Given the template, both primers and the observed size, it enumerates every pair of priming sites — including a single primer priming both strands — that would give a product that size, and ranks them by how much of each primer's 3' end matches without interruption, which is what decides whether a mispriming event can extend at all. Reports no yield and assigns no share of the band: the band is the input, not the output. Says plainly when nothing on this template explains the size, and what that points to instead.

## Request body

- object
  - `template` string, required: The template you amplified, raw or FASTA. Max 200,000 bp.
  - `forwardPrimer` string, required: Forward primer, 5'→3'.
  - `reversePrimer` string, required: Reverse primer, 5'→3'.
  - `observedSize` number, required: The band size you measured off the gel, in bp.
  - `tolerancePercent` number: How far off the size read may be, as a percentage. A percentage rather than a fixed number of bases because ±10% of 200 bp is not the same reading precision as ±10% of 5 kb.
  - `toleranceBp` number: Override the tolerance with a fixed window in bp.
  - `circular` boolean: Template is circular (a plasmid).
  - `maxMismatches` integer: Mismatches tolerated per primer while looking for sites (max 6). Higher finds more explanations and more coincidences.
  - `maxCandidates` integer: Cap on returned candidates, best-anchored first.

## Response `200`

Tool ran successfully.

- SuccessResponse: Standard success envelope returned by every single-tool call.
  - `ok` unknown, required
  - `tool` string, required: The tool slug that ran.
  - `result` object, required: Tool-specific output object. Its shape depends on the tool — call `GET /{tool}` or see the docs for each tool's fields.
  - `gate` GateResult
    - `pass` boolean, required: True if and only if every hard check passed.
    - `checks` GateCheck[], required
      - `id` string, required
      - `label` string, required
      - `severity` 'hard' | 'soft', required
      - `pass` boolean, required
      - `message` string, required
      - `value` number
      - `threshold` string
    - `notChecked` string[], required: Honest list of what this gate does NOT verify.
  - `provenance` Provenance, required
    - `apiVersion` string, required
    - `tool` union, required: Tool slug, or an array of slugs (one per step) for a workflow.
      - string
      - string[]
    - `generatedAt` string, date-time, required: ISO 8601 timestamp of when the result was generated.

## Other responses

- `400`: Invalid argument or the tool could not run.
- `404`: Unknown tool.
- `429`: Rate limited (metered tools only). Body carries code `rate_limited` and retryable `true`; honor the Retry-After header.
- `503`: A metered tool is momentarily at capacity. The identical request should succeed after Retry-After; body carries code `upstream_unavailable`.

---

[API](https://skmtc.dev/seqbench/apis/seqbench-api.md) · [All operations](https://skmtc.dev/seqbench/apis/seqbench-api/llms.txt) · [OpenAPI document](https://skmtc.dev/seqbench/apis/seqbench-api/revisions/132575216865?raw)