{
  "slug": "wrangler-preview-configures-containers-for-preview-deployments-f4463ec69ade",
  "title": "wrangler preview configures containers for preview deployments",
  "tools": [
    "wrangler"
  ],
  "tags": [
    "agent"
  ],
  "introduced_at": "2026-08-20T17:42:57.000Z",
  "announced_at": "2026-08-20T17:42:57.000Z",
  "effective_at": null,
  "date_basis": "source-publication",
  "status": "unknown",
  "how": {
    "incantation": "When a Worker preview needs a Durable Object backed by a container, declare it under `previews.containers` with its `class_name`, then run `npx wrangler preview` to build and deploy the image. Use `npx wrangler preview --json` from a non-interactive shell for machine-readable output; the release note recommends a prebuilt image over a Dockerfile.",
    "examples": [
      "\"previews\": { \"containers\": [{ \"class_name\": \"<DurableObjectClass>\", \"image\": \"<image-or-Dockerfile-path>\", \"max_instances\": 10, \"instance_type\": \"basic\" }] }",
      "npx wrangler preview",
      "npx wrangler preview --json"
    ],
    "gotchas": [
      "Version: 4.125.0",
      "previews.containers[].class_name (required): Durable Object class backed by the preview container. A class with no matching Durable Object is rejected; a class can back at most one container app, per release note.",
      "previews.containers[].image: Container image or Dockerfile path; docs example uses `./Dockerfile`. Release note recommends prebuilt `image` for JSON output.",
      "previews.containers[].max_instances: Maximum container instances; docs example uses `10`.",
      "previews.containers[].instance_type: Container instance type; docs example uses `basic`.",
      "--json: Return machine-readable output; defaults to false.",
      "Prerequisite: Container preview configuration is not inherited from top-level `containers`; declare it under `previews.containers`.",
      "Prerequisite: A Durable Object class and its SQLite migration are shown in the docs configuration example. A preview Durable Object binding is needed only when Worker code reads the namespace from `env`; it is unnecessary for access through `ctx.exports`.",
      "Prerequisite: The preview command's `SCRIPT` argument is needed only if the Wrangler config has no `main` key.",
      "Container configuration does not inherit; declare containers explicitly in the `previews` block.",
      "Each entry must set `class_name`.",
      "An entry cannot set its own `name`.",
      "A Durable Object class can be backed by at most one container application; validation rejects entries sharing a `class_name`.",
      "Entries whose `class_name` matches no Durable Object class are rejected before preview deployment.",
      "Wrangler skips container applications for Durable Object classes another Worker implements through `script_name`.",
      "A binding is not required when a Durable Object declared through `migrations` or `exports` is reached only over `ctx.exports`.",
      "Preview deletion tears down container applications server-side; `wrangler preview delete` does not remove them.",
      "Docker build output and the progress spinner write directly to stdout and bypass `--json` suppression; the spinner is skipped in non-interactive shells.",
      "`--json` suppresses Wrangler's own stdout output; warnings and errors still go to stderr.",
      "Docs conflict: Current Cloudflare preview docs say deleting a preview can leave its generated container app visible; they recommend checking `wrangler containers list` and deleting leftovers. This differs from the release note statement that preview deletion tears down container applications server-side. The docs page does not state the Wrangler version for that behavior."
    ]
  },
  "why_it_matters": "Configure a Worker preview deployment to run a Durable Object backed by a container.",
  "sources": [
    "https://github.com/cloudflare/workers-sdk/releases/tag/wrangler%404.125.0",
    "https://github.com/cloudflare/cloudflare-docs/blob/production/src/content/docs/workers/previews/resources/index.mdx",
    "https://developers.cloudflare.com/workers/wrangler/commands/workers/"
  ],
  "rank": 0.98036,
  "judge": {
    "publish": true,
    "judge_model": "codex-cli/gpt-6-luna+typesafe/jev-1.13.0",
    "prompt_version": "extract-v7+enrich-v1+judge-v2x3+jev-publish-v1"
  },
  "review_state": "approved",
  "provenance": {
    "run_id": "augsep-20261008",
    "verdict_id": "augsep-20261008:augsep-20261008-eaa6e43c788a099f-1:gate",
    "evidence_hash": "6bd888ac08b040d86374f347bbc03f7831e8277618b7511c3f6d7421f7b32c36"
  },
  "review": {
    "draft_hash": "21dcc0302ff98ea33f7dfd16f5bbe1bb7128ecd2eaf4ec4f44c79ae821c1eed8",
    "reviewer": "publication-gate",
    "reviewed_at": "2026-10-07T00:00:00.000Z",
    "decision": "augsep-20261008:gate",
    "reason": "3 of 3 judge runs approved with p_yes >= 0.9 (mean 0.98 >= 0.9; 2 needed) and Jev gave 0.79 (>= 0.6 needed)."
  }
}