5.15 fails to start in proxy mode

[scrocquesel-ml150]

When upgrading the docker image from 5.14.3 to 5.15, Prism throws an error at startup in proxy mode.

Context

I cannot run prism

Current Behavior

 TypeError: Cannot read properties of undefined (reading 'isPrimary')
     at createMultiProcessPrism (/usr/src/prism/packages/cli/dist/util/createServer.js:29:32)
     at runPrismAndSetupWatcher (/usr/src/prism/packages/cli/dist/util/runner.js:8:12)
     at Object.handler (/usr/src/prism/packages/cli/dist/commands/proxy.js:43:52)
     at Object.runCommand (/usr/src/prism/packages/cli/node_modules/yargs/build/index.cjs:446:48)
     at Object.parseArgs [as _parseArgs] (/usr/src/prism/packages/cli/node_modules/yargs/build/index.cjs:2697:57)
     at Object.get [as argv] (/usr/src/prism/packages/cli/node_modules/yargs/build/index.cjs:2651:25)
     at Object.<anonymous> (/usr/src/prism/packages/cli/dist/index.js:15:26)
     at Module._compile (node:internal/modules/cjs/loader:1812:14)
     at Object..js (node:internal/modules/cjs/loader:1943:10)
     at Module.load (node:internal/modules/cjs/loader:1533:32)

Expected Behavior

Upgrading a minor version should not break

Possible Workaround/Solution

Steps to Reproduce

1. docker run stoplight/prism:5.15 /app/openapi.yaml http://upstream/
   with whatever is needed as mounted volume

Environment

Docker images

[akshay-harale]

This is failing for me in normal mode as well.

[ceesvanegmond]

Same issue here

[rnagulapalle]

When upgrading the docker image from 5.14.3 to 5.15, Prism throws an error at startup in proxy mode.

Context

I cannot run prism

Current Behavior

 TypeError: Cannot read properties of undefined (reading 'isPrimary')
     at createMultiProcessPrism (/usr/src/prism/packages/cli/dist/util/createServer.js:29:32)
     at runPrismAndSetupWatcher (/usr/src/prism/packages/cli/dist/util/runner.js:8:12)
     at Object.handler (/usr/src/prism/packages/cli/dist/commands/proxy.js:43:52)
     at Object.runCommand (/usr/src/prism/packages/cli/node_modules/yargs/build/index.cjs:446:48)
     at Object.parseArgs [as _parseArgs] (/usr/src/prism/packages/cli/node_modules/yargs/build/index.cjs:2697:57)
     at Object.get [as argv] (/usr/src/prism/packages/cli/node_modules/yargs/build/index.cjs:2651:25)
     at Object.<anonymous> (/usr/src/prism/packages/cli/dist/index.js:15:26)
     at Module._compile (node:internal/modules/cjs/loader:1812:14)
     at Object..js (node:internal/modules/cjs/loader:1943:10)
     at Module.load (node:internal/modules/cjs/loader:1533:32)

Expected Behavior

Upgrading a minor version should not break

Possible Workaround/Solution

Steps to Reproduce

1. docker run stoplight/prism:5.15 /app/openapi.yaml http://upstream/
   with whatever is needed as mounted volume

Environment

Docker images

Minor-version breaks in dev/test tooling are especially painful because they stop work before anyone even gets to the integration bug they were trying to debug.

The expectation with a proxy/mock tool is that upgrades are boring. When a small version bump suddenly prevents startup in proxy mode, it turns the tool itself into the blocker.

The confirmations from multiple people here make this feel like more than a one-off local environment issue.

[JuicyLung91]

Same for me. Any solution? Did the cli command changed?

[zhe-sun-lyra]

Same thing for mock mode

[WilliamDEdwards]

Prism is no longer starting for me, in mock mode.

Version: 5.14.3 -> 5.15.6

Command: [ "mock", "-h", "0.0.0.0", "https://cluster-api-staging.cyberfusion.nl/openapi.json" ] (Docker)

Error:

2026-04-03T10:08:03.988751678Z prism mock <document>
2026-04-03T10:08:03.988796048Z 
2026-04-03T10:08:03.988801168Z Start a mock server with the given document file
2026-04-03T10:08:03.988805118Z 
2026-04-03T10:08:03.988808108Z Positionals:
2026-04-03T10:08:03.988811228Z   document  Path to a document file. Can be both a file or a fetchable resource on the web.  [string] [required]
2026-04-03T10:08:03.988814528Z 
2026-04-03T10:08:03.988817118Z Options:
2026-04-03T10:08:03.988820068Z       --version                           Show version number  [boolean]
2026-04-03T10:08:03.988823028Z       --help                              Show help  [boolean]
2026-04-03T10:08:03.988826138Z   -p, --port                              Port that Prism will run on.  [number] [required] [default: 4010]
2026-04-03T10:08:03.988829368Z   -h, --host                              Host that Prism will listen to.  [string] [required] [default: "127.0.0.1"]
2026-04-03T10:08:03.988832808Z       --cors                              Enables CORS headers.  [boolean] [default: true]
2026-04-03T10:08:03.988835968Z   -m, --multiprocess                      Forks the http server from the CLI for faster log processing.  [boolean] [default: true]
2026-04-03T10:08:03.988839318Z       --errors                            Specifies whether request/response violations marked as errors will produce an error response  [boolean] [required] [default: false]
2026-04-03T10:08:03.988842748Z   -v, --verboseLevel                      Turns on verbose logging.  [required] [choices: "trace", "debug", "info", "warn", "error", "fatal", "silent"] [default: "info"]
2026-04-03T10:08:03.988847018Z   -d, --dynamic                           Dynamically generate examples.  [boolean] [default: false]
2026-04-03T10:08:03.988850198Z       --json-schema-faker-fillProperties  Generate additional properties when using dynamic generation.  [boolean]
2026-04-03T10:08:03.988853638Z       --ignoreExamples                    Tell Prism to treat the spec as though it has no examples. When in static mode,
2026-04-03T10:08:03.988857048Z                                                                   returns an example that has not been generated using json-schema-faker, but was
2026-04-03T10:08:03.988860288Z                                                                   created by Prism. When in dynamic mode, this flag is ignored, since in dynamic mode,
2026-04-03T10:08:03.988863768Z                                                                   examples are not consulted and json-schema-faker is used to generate a response based
2026-04-03T10:08:03.988867578Z                                                                   on the schema defined in the spec  [boolean] [default: false]
2026-04-03T10:08:03.988871038Z       --seed                              Provide a seed so that Prism generates dynamic examples deterministically  [string] [required] [default: null]
2026-04-03T10:08:03.988975768Z 
2026-04-03T10:08:03.990843450Z TypeError: Cannot read properties of undefined (reading 'isPrimary')
2026-04-03T10:08:03.990866410Z     at createMultiProcessPrism (/usr/src/prism/packages/cli/dist/util/createServer.js:29:32)
2026-04-03T10:08:03.990870660Z     at runPrismAndSetupWatcher (/usr/src/prism/packages/cli/dist/util/runner.js:8:12)
2026-04-03T10:08:03.990874120Z     at Object.handler (/usr/src/prism/packages/cli/dist/commands/mock.js:60:52)
2026-04-03T10:08:03.990877360Z     at Object.runCommand (/usr/src/prism/packages/cli/node_modules/yargs/build/index.cjs:446:48)
2026-04-03T10:08:03.990880380Z     at Object.parseArgs [as _parseArgs] (/usr/src/prism/packages/cli/node_modules/yargs/build/index.cjs:2697:57)
2026-04-03T10:08:03.990883470Z     at Object.get [as argv] (/usr/src/prism/packages/cli/node_modules/yargs/build/index.cjs:2651:25)
2026-04-03T10:08:03.990886860Z     at Object.<anonymous> (/usr/src/prism/packages/cli/dist/index.js:15:26)
2026-04-03T10:08:03.990890550Z     at Module._compile (node:internal/modules/cjs/loader:1812:14)
2026-04-03T10:08:03.990893690Z     at Object..js (node:internal/modules/cjs/loader:1943:10)
2026-04-03T10:08:03.990896920Z     at Module.load (node:internal/modules/cjs/loader:1533:32)

[NishiwakiArashi-Sugi]

Possible cause

It seems that this issue started occurring after v5.14.3.

In the following code:
https://github.com/stoplightio/prism/blob/v5.14.3/packages/cli/src/util/createServer.ts#L37

cluster.isMaster was replaced with cluster.isPrimary, and this change might be related to the error.

---

Workarounds

The following workarounds have been confirmed:

1. Pinning the Docker image to stoplight/prism:5.14.3
2. Adding the --multiprocess=false option

Both approaches allow the container to start successfully.

[y-tajiri]

this is caused by this pr.
#2752

The import style changed from namespace import to default import.

Old
The TypeScript source was a namespace import (import * as cluster from 'cluster'). With module: commonjs and esModuleInterop OFF, a namespace import compiles to a plain require with
direct property access — no .default:

  const cluster = require("cluster");
  cluster.isMaster   // reads directly off the require() return value

Now.
The source was rewritten as a default import (import cluster from 'node:cluster'). With esModuleInterop still OFF, a default import compiles to a require plus a .default access:

  const node_cluster_1 = require("node:cluster");
  node_cluster_1.default.isPrimary   // reads .default first

But the CJS module has no .default property (require("node:cluster").default === undefined), so this throws Cannot read properties of undefined (reading 'isPrimary').

Why the two styles emit differently

Source form	Emit (esModuleInterop OFF)	Runtime
import * as cluster from 'cluster' (5.14.3)	const c = require("cluster"); c.isMaster	✅ members are on the object root
import cluster from 'node:cluster' (5.15.x)	const c = require("node:cluster"); c.default.isPrimary	❌ .default is undefined

Enabling esModuleInterop: true would be the ideal fix, but since it causes errors in other parts of the code, the following workaround is applied instead.

--- a/packages/cli/src/util/createServer.ts
+++ b/packages/cli/src/util/createServer.ts
@@ -2,7 +2,13 @@ import { createLogger } from '@stoplight/prism-core';
 import { IHttpConfig, IHttpRequest } from '@stoplight/prism-http';
 import { createServer as createHttpServer } from '@stoplight/prism-http-server';
 import * as chalk from 'chalk';
-import cluster from 'node:cluster';
+// The CJS `node:cluster` module exports the cluster singleton as `module.exports`, so at runtime
+// `require('node:cluster')` already is the instance. Because the build does not use esModuleInterop,
+// a default import (`import cluster from 'node:cluster'`) emits `cluster.default`, which is undefined
+// at runtime and crashes the multiprocess path. Import the namespace (the singleton at runtime) and
+// cast it to the default export's instance type so both types and runtime resolve correctly.
+import * as clusterImport from 'node:cluster';
+const cluster = clusterImport as unknown as typeof clusterImport.default;
 import * as E from 'fp-ts/Either';
 import { pipe } from 'fp-ts/function';
 import * as pino from 'pino';

[y-tajiri]

This issue will be fixed when this pr is merged.
#2807