# Cross-provider comparisons — OpenAI ↔ Anthropic ↔ xAI ↔ Gemini

Status: synthesis layer over the atlas, extended from two to four providers on 2026-09-18/19. Nothing on these pages was measured independently: every cell points back to a record in generated/*.json (statuses copied verbatim) or to a domain page under docs/openai/, docs/anthropic/, docs/xai/, docs/gemini/, docs/tools/, docs/models/, docs/errors/. When two sources disagree the disagreement is stated, not resolved. Generators: scripts/generators/synth/{features,endpoints,pricing,faq}.py (re-runnable; the other pages are hand-written from the same data). Sources: generated/{models,endpoints,parameters,tools,streaming-events,errors,headers,pricing,deprecations,objects,sdks,webhook-events,rate-limits}.json, generated/compatibility/*.json, generated/fragments/headers/*.json, generated/examples-manifest.json; vendor documentation as cited on each domain page (https://developers.openai.com/api/docs/… · https://platform.claude.com/docs/en/… · https://docs.x.ai/developers/… · https://ai.google.dev/gemini-api/docs/…). Last verified: 2026-09-18

# Pages

# Feature coverage summary (from cross-provider-feature-matrix.json, 125 features)

Per-provider supported rows: OpenAI 103 · Anthropic 83 · xAI 77 · Gemini 77.

# Methodology

1. Objective differences only. Pages describe what each API accepts, returns and charges. No "winner", no quality judgement about model outputs. Where a capability exists on a subset of providers it is marked "— not offered" on the others.
2. Statuses are inherited, never upgraded. A cell says LIVE_VERIFIED only if the underlying record was called successfully with this atlas's keys on 2026-09-18/19. DOCUMENTED = in current official docs, not tested; ACCOUNT_RESTRICTED = documented but our key was refused — for xAI that means the Management API (separate key), the Skills API, /v1/embeddings and alpha tool_search; for Gemini it means paid-tier-only models and features probed with a free-tier key (Pro models, image/video/music generation, cachedContents, Batch, Google Search grounding quota). A 403/404/429-with-limit-0 never means "does not exist". BETA/PREVIEW = gated by header (Anthropic, OpenAI), by /v1beta path or -preview id (Gemini), or by product stage (xAI multi-agent, Grok Build). GA appears only on Gemini fragments.
3. Current models only in the model table. OpenAI: GPT-6 Astra, GPT-5.6 Sol/Terra/Luna, GPT-5.5(-pro), GPT-5.4(-pro/-mini/-nano), GPT-5.3-codex, GPT-5.2, GPT-5.1, GPT-5 family, o3/o3-pro/o4-mini (deprecated), GPT-4.1/4o lines. Anthropic: Fable 5.1 / 5, Mythos 5.1 / 5 (invite), Opus 5 / 4.8 / 4.7 / 4.6 / 4.5, Sonnet 5 / 4.6 / 4.5, Haiku 4.5. xAI: grok-4.6, grok-4.5, grok-4.3, grok-4.20-0309-reasoning / -non-reasoning / multi-agent (beta), grok-build-0.1 (preview), Imagine image/video, voice models. Gemini: 3.8 / 3.7 / 3.6 / 3.5 Flash, 3.5 Flash-Lite, 3.1 Pro Preview, 3.1 Flash-Lite (deprecated), 3 Flash Preview, 2.5 family (blocked for new users), Gemma 4, Live / TTS / transcribe / image / Veo / Lyria / embedding models, Interactions agents. Retired ids appear only in the lifecycle sections.
4. Equivalence ≠ identity. "Portable" means the same task can be expressed with an equivalent parameter on every provider that offers the feature; a single-provider feature is never portable. The per-topic pages give the exact parameter mapping and a JSON quad ("the same task on all four providers"); the differences column lists what does not carry over (defaults, limits, billing, gating). xAI's Responses/Chat/Realtime surfaces reuse OpenAI's wire formats, so OpenAI↔xAI mappings are often literal; Gemini's generateContent (camelCase parts) and Interactions (snake_case steps) are distinct shapes.
5. Prices come from generated/pricing.json and the pricing block of generated/models.json (vendor pricing and model pages retrieved 2026-09-18; xAI also from the live catalogue price ticks). Cost models are arithmetic on those list prices; no invoice was reconciled. Provider-specific rules that change the arithmetic are stated next to each table: xAI bills reasoning tokens on every call and applies its ≥200k-token tier to the whole request; Gemini 3.6–3.8 Flash prices are introductory until 2026-12-31 and Gemini has a $0 tier; only Anthropic and GPT-5.6+ charge cache writes.
6. Traceability. The JSON twin of the feature matrix carries a ref per row (file or docs page). The FAQ shows the jq used for each computed table. Endpoint pages are rendered from generated/endpoints.json without manual edits.
7. Known data inconsistencies found while synthesising are listed at the end of models.md (29 items, 13 new for xAI/Gemini) so the fragment owners can fix them at the source (generated/fragments/**), never in the merged files.

# Vocabulary map (the same idea, four names)