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.