[BUG] postinstall in a package consumed as a workspace in a monorepo throws a "command not found" error when running `npm ci`

Is there an existing issue for this?

• I have searched the existing issues

This issue exists in the latest npm version

• I am using the latest npm

Current Behavior

When using npm workspaces, if a package (called consumer in my example) consumes another package (called consumed) as a workspace and THAT package uses a postinstall script that calls a third package (called topostinstall) then when you run npm ci on the first package (consumer) it will throw an error when it gets to the postinstall:

npm error code 127
npm error path /Users/bryce/Documents/LLK/experiments/workspacePostinstallReproduction/packages/consumed
npm error command failed
npm error command sh -c topostinstall
npm error sh: topostinstall: command not found
npm error A complete log of this run can be found in: /Users/bryce/.npm/_logs/2025-07-18T18_13_25_324Z-debug-0.log

Expected Behavior

running npm ci from packages/consumer installs the "consumed" package and successfully runs the postinstall command without errors.

This is similar to this issue https://github.com/npm/cli/issues/7692 which was closed without repro steps.

Steps To Reproduce

I created a simplified case of this in this repository: https://github.com/BryceLTaylor/workspacePostinstallReproduction

You can clone that repository and then

1. cd into packages/consumer
2. run npm ci
   You should see that error.

However, if you instead:

1. cd into packages/consumed
2. run npm ci OR npm install
3. cd into packages/consumer
4. run npm install

You will not get the error.

The README in that repo contains more context

Environment

• npm: 11.4.2
• Node.js: v22.17.0
• OS Name: macOS
• System Model Name:

Clone the linked workspacePostinstallReproduction repository, enter packages/consumer, and run npm ci to reproduce the missing topostinstall command. Then trace npm CLI handling for workspace installation and lifecycle scripts; done means the consumer install completes and the consumed package's postinstall command resolves successfully.