What causes "Could not find the implementation for builder " in angular/angular-cli?

Add the 'implementation' field pointing to the builder's JS module in the builders.json entry.

Could not find the implementation for builder

Error message

Could not find the implementation for builder

What it means

A manifest entry for a builder must specify an 'implementation' path (the JS module implementing the builder). resolveBuilder throws when the entry lacks one, so no builder function could be loaded.

Solutions

Add the 'implementation' field pointing to the builder's JS module in the builders.json entry.
Publish a corrected version of the builder package.
Use a different builder that is fully defined.

Example fix

// before (builders.json)
"my-builder": {"schema": "./schema.json"}
// after
"my-builder": {"implementation": "./my-builder.impl", "schema": "./schema.json"}

Defensive patterns

Strategy: validation

Validate before calling

const entry = require(`${packageName}/builders.json`).builders?.[builderName];
if (typeof entry === 'object' && !entry.implementation) throw new Error(`Builder "${packageName}:${builderName}" is missing an implementation path.`);

Type guard

const hasImplementation = (b) => typeof b === 'object' && b !== null && typeof b.implementation === 'string' && b.implementation.length > 0;

Try / catch

try { await host.resolveBuilder(builderStr); } catch (e) { if (String(e.message).includes('Could not find the implementation for builder')) { /* fix the manifest entry's 'implementation' field */ } else throw e; }

Prevention

Validate builder manifests against the BuilderSchema in CI.
Include implementation, schema, and description in every manifest entry.
Test `ng builder` discovery locally before publishing.

When it happens

Trigger: A builders.json entry like {"schema": "./schema.json"} with no 'implementation' key, and code resolves that builder.

Common situations: Authoring a custom builder and forgetting the implementation field; a manifest entry left partially written; string alias chains resolving to an incomplete object entry.

Related errors

AI-assisted analysis of angular/angular-cli@bb72145f9a (2026-08-30). Data as JSON: /api/errors/628880ad5a98548f. Report an issue: GitHub.

Appendix: source

Thrown at packages/angular_devkit/architect/node/node-modules-architect-host.ts:193

    // Attempt to locate an entry for the specified builder by name
    const builder = buildersManifest.builders?.[builderName];
    if (!builder) {
      throw new Error(`Cannot find builder ${JSON.stringify(builderStr)}.`);
    }

    // Resolve alias reference if entry is a string
    if (typeof builder === 'string') {
      return this.resolveBuilder(
        builder,
        path.dirname(packageJsonPath),
        (seenBuilders ?? new Set()).add(builderStr),
      );
    }

    // Determine builder implementation path (relative within package only)
    const implementationPath = builder.implementation && path.normalize(builder.implementation);
    if (!implementationPath) {
      throw new Error('Could not find the implementation for builder ' + builderStr);
    }
    if (path.isAbsolute(implementationPath) || implementationPath.startsWith('..')) {
      throw new Error(
        `Package "${packageName}" has an invalid builder implementation path: "${builderName}" --> "${builder.implementation}"`,
      );
    }

    // Determine builder option schema path (relative within package only)
    let schemaPath = builder.schema;
    if (!schemaPath) {
      throw new Error('Could not find the schema for builder ' + builderStr);
    }
    if (path.isAbsolute(schemaPath) || path.normalize(schemaPath).startsWith('..')) {
      throw new Error(
        `Package "${packageName}" has an invalid builder schema path: "${builderName}" --> "${builder.schema}"`,
      );
    }

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