FactReason

---

Stop an agent shipping a broken dependency upgrade

Your agent wants to bump chalk from 4.1.2 to 5.0.0. Before it edits package.json, it asks what breaks:

GET /api/v1/packages/npm/chalk/advisory?from=4.1.2&to=5.0.0
Authorization: Bearer <key>

{
  "found": true,
  "packageName": "chalk",
  "fromVersion": "4.1.2",
  "toVersion": "5.0.0",
  "breakingCount": 2,
  "advisories": [
    {
      "severity": "breaking",
      "changeType": "MODULE_FORMAT_CHANGED",
      "description": "chalk 5.0.0 changed module format from \"commonjs (undeclared)\" to \"module\". CommonJS require() of this package will now fail.",
      "evidenceField": "type",
      "evidenceBefore": null,
      "evidenceAfter": "module",
      "migrationHint": "Convert the call site to import(), or pin to the last CommonJS release."
    },
    {
      "severity": "breaking",
      "changeType": "RUNTIME_REQUIREMENT_RAISED",
      "description": "chalk 5.0.0 raised its runtime requirement from \">=10\" to \"^12.17.0 || ^14.13 || >=16.0.0\". Older runtimes are no longer supported.",
      "evidenceField": "engines.node",
      "evidenceBefore": ">=10",
      "evidenceAfter": "^12.17.0 || ^14.13 || >=16.0.0",
      "migrationHint": "Confirm the deployment runtime satisfies the new floor before upgrading."
    }
  ]
}

Via MCP, the same question is one tool call:

{
  "name": "factreason_package_upgrade_advisory",
  "arguments": { "registry": "npm", "name": "chalk", "from": "4.1.2", "to": "5.0.0" }
}

Set responseFormat: "compact" on supported heavy MCP tools to remove repeated fields and prose while retaining the facts an agent needs. Package advisories preserve evidenceField, evidenceBefore, evidenceAfter, and migrationHint. Each compact response includes tokenMetrics counted with o200k_base. The default remains full for compatibility, and compact output never bypasses authorization or changes the query price.

Token savings vary with the response. Metrics count one exact JSON structuredContent payload and include the tokenMetrics object itself; the matching text block remains for MCP clients that do not consume structured content yet. The private application repository also carries a repeatable npm run benchmark:tokens fixture so published claims can be checked before release.

Every finding carries evidenceField, evidenceBefore and evidenceAfter — the exact registry metadata field and its values either side of the change. An agent can verify the claim against npm or PyPI directly rather than taking our word for it.

An agent can call factreason_create_key to get its own key instantly — no human, no card, no signup. The agent can use the returned key straight away, in the same session, by passing it as the apiKey argument of any paid tool; no reconnect needed. A client that can set headers can put it on the MCP connection as an Authorization: Bearer header instead. Eligible new users may receive up to $0.05 of one-off trial credit, subject to grant budgets and spent at the published per-tier prices — up to about two advisory lookups, or fifty component lookups. Successful advisory lookups after that cost $0.02 per query. Misses are never billed and never consume trial credit. Wallet-equipped agents can pay the 402 challenge directly with x402 on Base.

---

Connect

Streamable HTTP. No install, no package to pull.

{
  "mcpServers": {
    "factreason": {
      "type": "http",
      "url": "https://factreason.com/mcp"
    }
  }
}

Manifest: /.well-known/mcp.json

Human quickstart and workflow guidance: /docs. Agents can fetch the same material as Markdown from /docs.md or use the complete index at /llms-full.txt.

The container/stdio bridge accepts an optional FACTREASON_API_KEY environment variable and forwards it as a Bearer credential to the same live endpoint. Tool discovery works without a key; metered lookups return HTTP 402 when neither a key nor a valid x402 payment is supplied.

Get a key — no signup, no card:

curl -X POST https://factreason.com/api/v1/keys/create \
  -H 'Content-Type: application/json' \
  -d '{"email":"[email protected]"}'

---

Tools

Tool	What it answers
factreason_package_upgrade_advisory	What breaks between two published npm/PyPI versions
factreason_create_key	Create a key; pass it as the apiKey argument of paid tools (or as a Bearer header)
factreason_api_schema	Exact request/response shape of an API endpoint at a version
factreason_api_breaking_changes	What breaks between two versions of a third-party HTTP API
factreason_integration_brief	Everything needed to write one working call to an API
factreason_discover_api	Which of 1,097 services can do X
factreason_deprecation_scan	Which endpoints are deprecated, sunset dates, replacements
factreason_auth_playbook	Auth schemes, scopes, rate-limit headers, error-code table
factreason_component_spec	Electronics component pinouts, voltages, packages
factreason_subscribe_spec_changes	Webhook or polling callback when a spec changes
factreason_create_topup_link	Stripe checkout link when credit runs out

---

Proof-of-Fact Signatures

Successful REST data responses and MCP tool-call responses are signed by our Ed25519 private key:

• X-Warehouse-Proof-Signature: Ed25519 signature of the response payload
• X-Fact-Public-Key-Id: Stable signing key identifier
• X-Fact-Signature-Alg: EdDSA

Public keys are published and rotated via JWKS at https://factreason.com/.well-known/jwks.json.

---

Licence & Contact

MIT. Attribution requirement for APIs.guru source data: CC-BY 4.0. Questions, support, or data reports: [email protected].