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HTTP ステータスコード リファレンス

100 Continue から 511 Network Authentication Required まで 60 超の標準 HTTP 応答コード、各々に 1 行サマリ・RFC 参照・推奨用途・誤用されがちなコードへの「落とし穴」メモ（401 vs 403、301 が永遠にキャッシュ、400 vs 422 など）。クラス（1xx 情報・2xx 成功・3xx リダイレクト・4xx クライアント・5xx サーバ）でフィルタ、またはコード・名称・概要・メモ全文検索。

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件数: 61

  • 100Continue

    Server received headers; client should send the body.

    RFC: RFC 9110 §15.2.1

  • 101Switching Protocols

    Server agrees to switch protocols (e.g., HTTP/2, WebSocket).

    使い時: WebSocket upgrade handshakes.

    RFC: RFC 9110 §15.2.2

  • 102Processing

    WebDAV: request received but no response yet.

    RFC: RFC 2518

  • 103Early Hints

    Preload hints sent before the final response.

    使い時: Send Link: rel=preload for critical assets before the slow backend reply lands.

    RFC: RFC 8297

  • 200OK

    Standard success response with a body.

    RFC: RFC 9110 §15.3.1

  • 201Created

    Resource was created; include Location header pointing to it.

    使い時: POST that creates a new resource. Body is optional.

    RFC: RFC 9110 §15.3.2

  • 202Accepted

    Request accepted for async processing; no result yet.

    使い時: Long-running jobs — return a status URL for the client to poll.

    RFC: RFC 9110 §15.3.3

  • 203Non-Authoritative Information

    Response was transformed by a proxy.

    RFC: RFC 9110 §15.3.4

  • 204No Content

    Success but no body to send.

    使い時: DELETE success, PUT with no useful body, preflight OK.

    落とし穴: MUST NOT include a message body. Some libraries error if you do.

    RFC: RFC 9110 §15.3.5

  • 205Reset Content

    Success; client should reset the document view (clear form).

    RFC: RFC 9110 §15.3.6

  • 206Partial Content

    Range request was honored; partial body follows.

    使い時: Resumable downloads, video seek.

    RFC: RFC 9110 §15.3.7

  • 207Multi-Status

    WebDAV multi-resource batched results.

    RFC: RFC 4918

  • 208Already Reported

    WebDAV: bindings already enumerated earlier in the response.

    RFC: RFC 5842

  • 226IM Used

    Delta encoding response.

    RFC: RFC 3229

  • 300Multiple Choices

    Multiple resources match; client should pick one.

    RFC: RFC 9110 §15.4.1

  • 301Moved Permanently

    Resource has a new permanent URL. Caches forever.

    使い時: Domain migration, URL restructure.

    落とし穴: Browsers cache 301 aggressively — fixing a wrong one is painful. Test with 302 first.

    RFC: RFC 9110 §15.4.2

  • 302Found

    Temporary redirect (legacy semantics, method may change).

    落とし穴: Historically rewrote POST→GET. Use 303 or 307 for explicit method-preservation semantics.

    RFC: RFC 9110 §15.4.3

  • 303See Other

    Redirect that always becomes a GET.

    使い時: POST/Redirect/GET pattern after form submit.

    RFC: RFC 9110 §15.4.4

  • 304Not Modified

    Conditional request: cached copy is still fresh.

    使い時: Response to If-None-Match / If-Modified-Since.

    落とし穴: MUST NOT include a body.

    RFC: RFC 9110 §15.4.5

  • 307Temporary Redirect

    Temporary; method and body must be preserved.

    RFC: RFC 9110 §15.4.8

  • 308Permanent Redirect

    Permanent; method and body must be preserved.

    使い時: POSTs that must survive a permanent move. Prefer over 301 for API endpoints.

    RFC: RFC 9110 §15.4.9

  • 400Bad Request

    Generic malformed request the server cannot parse.

    落とし穴: Don't use 400 for valid syntax with bad semantics — use 422 instead.

    RFC: RFC 9110 §15.5.1

  • 401Unauthorized

    Authentication required or failed. Must include WWW-Authenticate.

    落とし穴: Misnamed historically — means "unauthenticated". Use 403 for "authenticated but not allowed".

    RFC: RFC 9110 §15.5.2

  • 402Payment Required

    Reserved for future use; some APIs use for billing failures.

    RFC: RFC 9110 §15.5.3

  • 403Forbidden

    Authenticated but the action is not allowed for this principal.

    使い時: Authorization failure, geo-block, or "you cannot do this regardless of auth".

    RFC: RFC 9110 §15.5.4

  • 404Not Found

    Resource does not exist (or you are hiding it).

    使い時: Often returned instead of 403 to avoid disclosing existence.

    RFC: RFC 9110 §15.5.5

  • 405Method Not Allowed

    Resource exists but does not support this verb. MUST include Allow header.

    RFC: RFC 9110 §15.5.6

  • 406Not Acceptable

    No representation matches the Accept headers.

    RFC: RFC 9110 §15.5.7

  • 407Proxy Authentication Required

    Like 401 but for proxies. Must include Proxy-Authenticate.

    RFC: RFC 9110 §15.5.8

  • 408Request Timeout

    Server gave up waiting for the request to finish.

    RFC: RFC 9110 §15.5.9

  • 409Conflict

    Request collides with current state (e.g., edit conflict, duplicate).

    使い時: Unique-key violations on creation, version mismatch on update.

    RFC: RFC 9110 §15.5.10

  • 410Gone

    Resource permanently removed with no forwarding address.

    使い時: Use over 404 when you want crawlers/clients to drop the URL permanently.

    RFC: RFC 9110 §15.5.11

  • 411Length Required

    Server requires Content-Length and the request omitted it.

    RFC: RFC 9110 §15.5.12

  • 412Precondition Failed

    If-Match / If-Unmodified-Since check failed.

    使い時: Optimistic concurrency control on updates.

    RFC: RFC 9110 §15.5.13

  • 413Content Too Large

    Body exceeds the server limit. Formerly "Payload Too Large".

    RFC: RFC 9110 §15.5.14

  • 414URI Too Long

    URL exceeds server limit (commonly ~8KB).

    RFC: RFC 9110 §15.5.15

  • 415Unsupported Media Type

    Content-Type is not supported by this endpoint.

    RFC: RFC 9110 §15.5.16

  • 416Range Not Satisfiable

    Range header asks for bytes outside the resource.

    RFC: RFC 9110 §15.5.17

  • 417Expectation Failed

    Server cannot meet the Expect: header requirements.

    RFC: RFC 9110 §15.5.18

  • 418I'm a teapot

    April-fools status (RFC 2324). Returned by some debugging or honeypot endpoints.

    RFC: RFC 2324

  • 421Misdirected Request

    Request was routed to a server that cannot produce a response (e.g., wrong SNI).

    RFC: RFC 9110 §15.5.20

  • 422Unprocessable Content

    Syntactically valid but semantically incorrect (e.g., validation failure).

    使い時: Form-level validation errors on a JSON API.

    RFC: RFC 9110 §15.5.21

  • 423Locked

    WebDAV: resource is locked.

    RFC: RFC 4918

  • 424Failed Dependency

    WebDAV: a previous request in the chain failed.

    RFC: RFC 4918

  • 425Too Early

    Server refuses replay-vulnerable early data (TLS 1.3 0-RTT).

    RFC: RFC 8470

  • 426Upgrade Required

    Client must upgrade to a different protocol. Include Upgrade header.

    RFC: RFC 9110 §15.5.22

  • 428Precondition Required

    Server requires the request to be conditional (If-Match etc).

    使い時: APIs that demand optimistic-concurrency tokens to prevent lost updates.

    RFC: RFC 6585

  • 429Too Many Requests

    Rate-limited. Include Retry-After header.

    RFC: RFC 6585

  • 431Request Header Fields Too Large

    Sum of headers exceeds the server limit.

    RFC: RFC 6585

  • 451Unavailable For Legal Reasons

    Blocked due to legal demand (Fahrenheit 451 reference).

    RFC: RFC 7725

  • 500Internal Server Error

    Unhandled exception or generic server crash.

    落とし穴: Never expose stack traces — log them server-side only.

    RFC: RFC 9110 §15.6.1

  • 501Not Implemented

    Server does not support the requested method at all.

    RFC: RFC 9110 §15.6.2

  • 502Bad Gateway

    Upstream returned an invalid response to the gateway.

    使い時: Load balancer cannot reach origin; origin returned junk.

    RFC: RFC 9110 §15.6.3

  • 503Service Unavailable

    Server temporarily down or overloaded. Should include Retry-After.

    RFC: RFC 9110 §15.6.4

  • 504Gateway Timeout

    Gateway gave up waiting for upstream.

    RFC: RFC 9110 §15.6.5

  • 505HTTP Version Not Supported

    Server does not support the HTTP version in the request.

    RFC: RFC 9110 §15.6.6

  • 506Variant Also Negotiates

    Transparent content negotiation is misconfigured.

    RFC: RFC 2295

  • 507Insufficient Storage

    WebDAV: server cannot store the representation.

    RFC: RFC 4918

  • 508Loop Detected

    WebDAV: infinite loop while processing.

    RFC: RFC 5842

  • 510Not Extended

    Further extensions required to fulfill the request.

    RFC: RFC 2774

  • 511Network Authentication Required

    Captive portal: log in to the network first.

    使い時: Public Wi-Fi splash pages.

    RFC: RFC 6585

使い方

  1. 検索バーにコード・名前・任意の語を入力（例：「リダイレクト」「rate limit」「422」）。
  2. 色付きチップでクラス絞り込み（任意）。
  3. 各ヒットの概要・使い時・落とし穴・RFC 参照を読む。

よくある質問

なぜ一部に「落とし穴」を表示？
頻繁に誤用されるコードがあります。401 は「未認証」ですが「unauthorized」と読めて紛らわしく、301 はブラウザで実質永久キャッシュ、400 vs 422 は議論が絶えない。コードレビューやポストモーテムで本当に効く罠だけ明示。
WebDAV コードも？
含めています — 102・207・208・423・424・507・508 と RFC。WebDAV を直接使わなくともライブラリやプロキシが返すことがあり、意味を知っておくと便利。
ベンダー独自コード（Cloudflare 520-527 など）は？
ベンダー専用（Cloudflare 520-527・AWS 460/463・IIS 440）は除外 — RFC になく、ベンダーごとに意味が違う。各ベンダーのドキュメントを参照。
なぜ「使い時」欄が空のコードがある？
名前で自明な場合（200 OK・500 Internal Server Error）や特殊コンテキストでしか出ない場合は概要のみで十分。本当に語るべきストーリーがある時のみ when/pitfall を表示。

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