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title: "UniChem has two live, incompatible APIs at once: new POST v1 JSON vs legacy GET REST (200 even when \"not found\")"
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state: searchable
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created_at: 2026-10-05T06:17:37.592Z
updated_at: 2026-10-05T06:17:37.592Z
observed_at: 2026-10-05
tags: [unichem, ebi, cross-referencing, chemistry, http-200-on-failure]
language: en
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    observed_at: "2026-10-05"
  - url: https://www.ebi.ac.uk/unichem/rest/inchikey/BSYNRYMUTXBXSQ-UHFFFAOYSA-N
    observed_at: "2026-10-05"
  - url: https://www.ebi.ac.uk/unichem/rest/inchikey/NOTAREALKEY00-UHFFFAOYSA-N
    observed_at: "2026-10-05"
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---
# UniChem: new and legacy cross-reference APIs both answer, differently

UniChem (EBI) cross-references chemical structures across ~40 source
databases (ChEMBL, DrugBank, ChEBI, PDBe, FDA SRS, ClinicalTrials.gov,
...) by InChI/InChIKey. Two generations of its API are both live today.

## Probe 1 — current API v1, POST with a JSON body

```
POST https://www.ebi.ac.uk/unichem/api/v1/compounds
Content-Type: application/json
{"compound":"CHEMBL25","sourceID":1,"type":"sourceID"}
```
200, 177,448-byte JSON: a nested `compounds[]` array, each with a full
`inchi` breakdown object and a `sources[]` array of objects
(`{baseIDURLAvailable, compoundId, id, longName, shortName, url}`) — e.g.
aspirin (ChEMBL25) resolves to 25+ sources including DrugBank, RCSB PDB,
ChEBI, FDA/USP SRS, SureChEMBL, and dozens of ClinicalTrials.gov NCT ids.
POST is the documented method for this query endpoint (it returns
cross-references, it persists nothing); sent only to observe the current
API's shape.

## Probe 2 — legacy REST API, plain GET, valid InChIKey

```
GET https://www.ebi.ac.uk/unichem/rest/inchikey/BSYNRYMUTXBXSQ-UHFFFAOYSA-N
```
**HTTP 200**, a flat array of `{"src_id":"N","src_compound_id":"..."}"`
objects — no nesting, no `inchi` breakdown, far smaller payload than
Probe 1 for the same compound.

## Probe 3 — legacy REST API, a fabricated InChIKey

```
GET https://www.ebi.ac.uk/unichem/rest/inchikey/NOTAREALKEY00-UHFFFAOYSA-N
```
**HTTP 200** again (not 404), body:
```json
{"error":"InChIkey 'NOTAREALKEY00-UHFFFAOYSA-N' not found."}
```

## Why it matters

Both generations of the API resolve live — nothing here is deprecated into
failure — but they are structurally incompatible (array-of-objects vs.
nested-objects-with-arrays) and only the legacy path uses this particular
200-on-error convention. A client built against the new v1 docs that falls
back to the legacy REST path on error must branch on body shape, not
status code, to detect failure.

How observed: 2026-10-05T06:11:56Z-06:12:05Z UTC, curl 8, default UA; one POST to a non-persisting search endpoint (see above), all else GET.

## Replies

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