Materials Project API: bounded queries, provenance, and computed-data limits

This reference targets mp-api==0.46.5 (released 2026-08-18) with pymatgen==2026.9.24 and pymatgen-core==2026.9.23. mp-api requires Python 3.11+ and depends on pymatgen>2024.2.20.

Use the separate official client:

from mp_api.client import MPRester

Do not use a legacy Materials Project client import from older pymatgen examples.

Verification on 2026-09-30 used current official guides, the current OpenAPI schema captured during this review, installed 0.46.5 SDK source and mocked transports. A repeat OpenAPI request returned HTTP 403; protected endpoints were not exercised with credentials. Network examples in this reference are illustrative, not authenticated end-to-end tests.

Installation

Pin the tested client and materials stack:

uv add "pymatgen==2026.9.24" "pymatgen-core==2026.9.23" "mp-api==0.46.5"
uv lock
uv sync --frozen

The 0.46.5 package metadata declares direct dependencies including pymatgen>2024.2.20, monty>=2024.12.10, emmet-core>=0.87.1, requests>=2.23.0, orjson>=3.10,<4, pyarrow>=20, and deltalake>=1.4,<1.6, plus boto3 and typing extensions. A lockfile is needed to freeze transitive artifacts.

Authentication: one named secret

An API key is required and is available from the logged-in Materials Project dashboard.

The approved pattern is:

from mp_api.client import MPRester

# Inject MP_API_KEY before execution; keep this workflow on the official host.
with MPRester(endpoint="https://api.materialsproject.org/") as rester:
    pass

Operational rules:

• use only the environment variable MP_API_KEY
• inject it through the user's shell/session secret manager
• never accept it as a command-line argument
• never embed it in code, notebooks, URLs, cache files, or manifests
• never traverse dot-env files or dump the environment
• never print the key or unredacted exceptions that might contain it
• do not send it anywhere except the official Materials Project API endpoint

The bundled query CLI follows these rules and reads the variable only after --execute.

Query contract before network

Before any request, disclose:

1. endpoint and mp-api version
2. all filters
3. exact response fields
4. num_chunks, chunk_size, and maximum serialized bytes
5. cache reads/writes
6. output path and no-overwrite behavior
7. API-key source by name, never value
8. data license, citation, provenance, and scientific limitations

The dry-run planner:

python scripts/mp_query.py \
  --chemsys Li-Fe-O \
  --energy-above-hull 0 0.05 \
  --fields formula_pretty,energy_above_hull,band_gap,origins \
  --limit 25

Only an explicit execution permits the disclosed network workflow:

python scripts/mp_query.py \
  --material-id mp-149 \
  --fields formula_pretty,structure,origins,last_updated \
  --limit 1 --output mp-149.json --execute

The CLI has no implicit result cache. The explicit bounded JSON output is the reusable artifact. MPRester initialization performs compatibility and heartbeat/database-version metadata requests before the bounded summary search. The CLI discloses those requests, disables the platform-detail user agent and local database-version notification log, and records the server's database version.

The CLI pins endpoint explicitly so SDK endpoint configuration cannot redirect its credential. Initialization makes separate unauthenticated GET /heartbeat calls for server/emmet version, database version and controlled-dataset metadata. Those calls do not use the session retry/timeout mechanism; apply a process deadline for automation. If heartbeat is denied, the SDK may report an empty database version; retain that limitation rather than invent a release.

Summary searches

The official docs identify summary data as the main property overview for a material:

from mp_api.client import MPRester

with MPRester() as rester:
    docs = rester.materials.summary.search(
        material_ids=["mp-149", "mp-13"],
        fields=[
            "material_id",
            "formula_pretty",
            "energy_above_hull",
            "band_gap",
            "origins",
            "last_updated",
        ],
        all_fields=False,
        num_chunks=1,
        chunk_size=25,
    )

material_ids accepts one ID or a list in the current signature. Both numeric IDs (mp-149) and alphabetical AlphaIDs (mp-aaaaaaft) are supported. The SDK normalizes them through emmet; these two examples identify the same record. Preserve the user's query and returned identifier instead of enforcing a digits-only pattern. The result is a list of Pydantic document objects based on SummaryDoc; selected-field results can use a reduced MPDataDoc model.

Property filters

Verified public SummaryRester.search parameters include:

with MPRester() as rester:
    docs = rester.materials.summary.search(
        chemsys="Li-Fe-O",
        elements=["Li", "O"],
        exclude_elements=["F"],
        energy_above_hull=(0.0, 0.05),
        band_gap=(0.5, 3.0),
        is_stable=None,
        fields=["material_id", "formula_pretty", "energy_above_hull", "band_gap"],
        all_fields=False,
        num_chunks=1,
        chunk_size=25,
    )

Other current filters include formula, crystal system, density, deprecation, dielectric ranges, elastic ranges, metal/direct-gap flags, property availability, magnetic ordering, element/site counts, space group, theoretical status, energy ranges, volume, and surface-property ranges. Consult the exact installed signature instead of passing guessed keywords.

exclude_elements is a list of element symbols; it is not a Boolean flag.

available_fields lists fields the endpoint can return. It does not mean every field is a valid filter:

with MPRester() as rester:
    returnable_fields = rester.materials.summary.available_fields

Requesting all fields is the documented default and can be expensive. Always pass a short fields list and bound chunks.

The REST search is a GET /materials/summary/ with x-api-key authentication and URL query parameters, not a JSON POST. The SDK joins lists (for example material_ids, elements, exclude_elements) with commas and expands range tuples to endpoint-specific _min/_max parameters. Responses contain a data array and meta.total_doc; the SDK returns the document list. num_chunks=None retrieves all pages; num_chunks=1, chunk_size=25 bounds this workflow. Pagination uses _limit/_skip; current search() also exposes _page and _sort_fields. List splitting or retries can make one SDK search more than one HTTP request. A full page does not prove the query is exhaustive. The saved-byte cap applies after receipt and serialization.

For direct REST requests, _limit defaults to 100 and is capped at 1000. _page/_per_page take precedence over _limit/_skip; do not combine both pagination styles. id_format=legacy|alpha changes response ID formatting, not which input IDs are accepted. This REST parameter is not a documented SummaryRester.search() argument in 0.46.5.

Serialization

Use the public Pydantic model interface:

payload = [document.model_dump(mode="json") for document in docs]

Then write strict JSON with allow_nan=False, a byte bound, and a new output path. Include query, fields, retrieval time, endpoint, client versions, and license/citation. Do not use deprecated generic dictionary shims, pickle, or a general object decoder on untrusted cached data.

Structures

from mp_api.client import MPRester

with MPRester() as rester:
    structure = rester.get_structure_by_material_id(
        "mp-149",
        final=True,
        conventional_unit_cell=False,
    )

Alternatively request structure as an explicit summary field. Preserve:

• material ID
• whether the final/initial and conventional/primitive representation was requested
• origins and task IDs
• retrieval and database release
• parser/API warnings

Validate the returned structure locally. A Materials Project structure is a computed relaxed representation, not necessarily the experimental setting, lattice parameters, disorder, temperature, or composition model.

This convenience method searches /materials/core/ for structure when final=True, or initial_structures when false. The former returns one Structure, the latter a list; unavailable data can return None.

Entries and phase diagrams

from mp_api.client import MPRester

with MPRester() as rester:
    entries = rester.get_entries_in_chemsys(
        "Li-Fe-O",
        compatible_only=True,
        conventional_unit_cell=False,
    )

Version 0.46.5 changed this workflow: the default thermotype is the joint GGA_GGA+U_R2SCAN hull. An unfiltered full-system query with compatible_only=True normally returns entries from MP's precomputed phase diagram, keeping all subsystems on one energy scale. If that diagram is missing, or property_data/conventional_unit_cell require reshaping, the client runs MaterialsProjectDFTMixingScheme locally; it warns about possible hull changes and dropped entries. This can query/cache Delta-table data, unlike the bundled summary-only CLI.

To explicitly select the earlier GGA/GGA+U correction convention:

with MPRester() as rester:
    entries = rester.get_entries_in_chemsys(
        "Co-N",
        additional_criteria={
            "thermo_types": ["GGA_GGA+U"]
        },
    )

Extra filters on mixed thermotypes or compatible_only=False bypass the consistent full-system path; those entries are not immediately hull-ready. Retrieve the full consistent set before post-filtering for reporting, and keep all competitors for hull construction. Do not pool all three thermotypes into one hull. The non-mixed path uses /materials/thermo/; the prebuilt mixed path uses the materialsproject-build Delta table objects/phase-diagrams, selected by chemical system, thermotype and database version. These are SDK-managed data locations, not REST endpoints to invent. Record all arguments, correction data, origins and database release; cache and analyze the resulting entries offline.

use_gibbs requests a GibbsComputedStructureEntry model estimate at 300–2000 K, not a measured free energy. property_data selects supported thermo fields. The convenience method has no summary-CLI result cap; bound the chemical system and expected dataset independently.

Band structures and DOS

Official convenience methods include:

with MPRester() as rester:
    bands = rester.get_bandstructure_by_material_id("mp-149", load_projections=False)
    dos = rester.get_dos_by_material_id("mp-149", load_projections=False)

In 0.46.5 missing metadata or task data raises MPRestError; catch it without turning absence into a zero gap or DOS. The client first searches /materials/electronic_structure/ for the task, then queries SDK-managed Delta tables under materialsproject-parsed/core/electronic-structure/ for bandstructures or total-dos; projections use additional tables only when requested. Default DOS retrieval returns Dos, not necessarily CompleteDos. Preserve task origins, spin/SOC, path convention, functional and database release. These convenience calls have different network/cache costs from a summary property query.

Provenance with origins

The official query guide recommends requesting origins to connect a summary property to a calculation task:

with MPRester() as rester:
    summaries = rester.materials.summary.search(
        material_ids=["mp-149"],
        fields=["material_id", "structure", "origins"],
        all_fields=False,
        num_chunks=1,
        chunk_size=1,
    )

An origin can identify the task used for a property. A corresponding thermo document's run_type distinguishes categories such as GGA, GGA+U, or r2SCAN. Do not assume all properties on one summary document came from one calculation or functional.

Other routes

The client exposes endpoint-specific resters under rester.materials, with routes documented for thermo, electronic structure, elasticity, dielectric, magnetism, phonons, surfaces, XAS, synthesis-related data, and others. Signatures and document models vary. Inspect the current route documentation and request only supported fields.

Do not copy old endpoint examples or invent filter names. Endpoint availability and schema can evolve independently of the Python wrapper.

Errors, retries, and rate handling

Use the current exception:

from mp_api.client.core.exceptions import MPRestError

try:
    with MPRester() as rester:
        docs = rester.materials.summary.search(
            material_ids=["mp-149"],
            fields=["material_id"],
            all_fields=False,
            num_chunks=1,
            chunk_size=1,
        )
except MPRestError:
    # Report a bounded, credential-redacted failure.
    raise

Official 0.46.5 client source configures retries for HTTP 429, 502, and 504 and respects Retry-After for its API session (heartbeat is separate). It wraps request failures as MPRestError and advises smaller requests on connection timeout.

Safety rules:

• rely on the pinned client's bounded retry behavior
• do not add an unbounded retry loop
• do not claim a numeric service quota unless current official documentation publishes one
• reduce fields/chunk size on timeout or oversized responses
• stop after a persistent authorization, schema, or validation error
• redact MP_API_KEY from any exception text

The current large-download guide says rate limiting starts at 25 requests/second. Treat HTTP 429 and Retry-After as authoritative, not that number as a guaranteed quota.

Cache policy

Caching can improve reproducibility but creates a data-governance obligation. Before using a cache, disclose:

• exact path, schema, size limit, and retention
• cache key: endpoint, filters, fields, client version, and database release
• whether stale data is acceptable
• license/citation metadata
• whether structures or contributed data are stored

Never cache credentials. Never treat a cache as current without checking its retrieval time and database version. mp-api exposes a local full-dataset cache path for bulk/delta-table workflows; the bundled summary-query CLI does not request those downloads and intentionally uses no hidden result cache.

License, attribution, and citation

Materials Project states that its data is licensed under CC BY 4.0 and that contributed data is owned by its respective contributors.

The official FAQ says citations are appropriate wherever Materials Project data, methods, or output are used. Preserve:

• canonical Materials Project citation
• property/tool-specific citations from the material/citation page
• database release citation
• material IDs and task/property origins
• retrieval date and query

See How to Cite and Database Versions.

Computed-data limitations

Materials Project documents that:

• core properties are calculated in-house using simulation methods
• typical/systematic errors must be assessed from property publications
• PBE lattice parameters often show systematic overestimation, with larger interlayer errors where van der Waals interactions are poorly described
• PBE band gaps are systematically underestimated
• summary/aggregated values can change as calculations and database releases are updated
• space groups depend on symprec; the MP pipeline commonly uses 0.1 Å

Therefore:

• computed stability is not experimental stability or synthesizability
• predicted structures are not proof of existence
• missing data is not zero
• a material ID does not guarantee one immutable property record
• cite the database version and property methodology

Minimal provenance envelope

{
  "retrieved_at_utc": "2026-09-30T00:00:00Z",
  "endpoint": "https://api.materialsproject.org/materials/summary/",
  "filters": {"material_ids": ["mp-149"]},
  "fields": ["material_id", "formula_pretty", "origins", "last_updated"],
  "limit": 1,
  "client": {
    "mp-api": "0.46.5",
    "pymatgen": "2026.9.24",
    "pymatgen-core": "2026.9.23"
  },
  "database_version": "record from current MP release metadata",
  "license": "CC BY 4.0",
  "citation": "https://materialsproject.org/about/cite"
}

Do not include the API key.

Sources (verified 2026-09-30)

• mp-api 0.46.5 on PyPI
• Official mp-api repository
• Current REST schema
• mp-api 0.46.5 source
• Materials Project identifier conventions
• Getting started
• Querying data
• mp-api route reference
• Materials Project FAQ
• Materials Project calculation details
• How to Cite
• Database versions
• Materials Project home and CC BY statement