mastra-ai/mastra · error · Error

Mapping step "${entry.id}" key "${key}" cannot be stored: so

Error message

Mapping step "${entry.id}" key "${key}" cannot be stored: source is a function.

What it means

Within a declarative mapping config, each key's source must be serializable data (value/template/requestContextPath/initData/step). A source written as `fn: (data, ctx) => ...` is a closure that cannot be persisted, so serialization of that specific key fails. The error identifies both the mapping step id and the offending key.

Source

Thrown at packages/core/src/workflows/dynamic/serialize.ts:141

    return {
      type: 'tool',
      id: entry.id,
      toolId: entry.toolId,
      description: entry.tool?.description,
      ...(options ? { options } : {}),
    };
  }
  if (entry.type === 'mapping') {
    if (typeof entry.mapConfig === 'function') {
      throw new Error(
        `Mapping step "${entry.id}" cannot be stored: the function form does not round-trip. Use the declarative form (template / step / initData / value).`,
      );
    }
    const serialized: Record<string, any> = {};
    for (const [key, mapping] of Object.entries(entry.mapConfig as Record<string, any>)) {
      const m: any = mapping;
      if (m.fn !== undefined) {
        throw new Error(`Mapping step "${entry.id}" key "${key}" cannot be stored: source is a function.`);
      }
      if (m.value !== undefined) {
        serialized[key] = { value: m.value };
      } else if (m.requestContextPath) {
        serialized[key] = { requestContextPath: m.requestContextPath };
      } else if (typeof m.template === 'string') {
        serialized[key] = { template: m.template };
      } else if (m.initData) {
        serialized[key] = { initData: m.initData?.id, path: m.path };
      } else if (m.step) {
        serialized[key] = {
          step: Array.isArray(m.step) ? m.step.map((s: any) => s?.id) : m.step?.id,
          path: m.path,
        };
      } else {
        serialized[key] = m;
      }
    }

View on GitHub (pinned to 75dd419e61)

Solutions

1. Replace the `fn` source with a declarative equivalent: `template`, `value`, `step` + `path`, `initData`, or `requestContextPath`.
2. Compute derived values in a preceding plain step and reference its output via a `step` source with a `path`.
3. Remove the computed key entirely if it can be derived downstream at consumption time.
4. Pre-validate each key: `Object.values(mapConfig).every(m => m.fn === undefined)`.

Example fix

// before
map({ total: { fn: (d) => d.price * d.qty } })

// after (compute in a prior step, then)
map({ total: { step: { step: 'computeTotal', path: 'total' } } })

for (const e of stepFlow) {
  if (e.type === 'mapping' && typeof e.mapConfig === 'object') {
    for (const [key, m] of Object.entries(e.mapConfig)) {
      if ((m as any)?.fn !== undefined) throw new Error(`mapping "${e.id}" key "${key}" uses a fn source`);
    }
  }
}

const isFnSource = (m: unknown): m is { fn: Function } =>
  !!m && typeof m === 'object' && typeof (m as any).fn === 'function';

try {
  storable = toStorableGraph(stepFlow);
} catch (e) {
  if (/cannot be stored: source is a function/.test(e.message)) {
    // replace the named key's fn source with a declarative source
  } else throw e;
}

When it happens

Trigger: Persisting a mapping entry whose declarative `mapConfig` object contains at least one key with a `fn` source, e.g. `map({ total: { fn: (d) => d.a + d.b } })`.

Common situations: Mixed usage: authors convert most keys to declarative sources but keep one computed field as `fn`, assuming partial closures are tolerated. Persistence then rejects the whole step.

Related errors

• Mapping step "${entry.id}" cannot be stored: the function fo
• SleepUntil step "${entry.id}" cannot be stored: dynamic date
• Conditional (branch) step cannot be stored: closure predicat
• Loop step "${getSingleStepEntryId(entry.step)}" cannot be st
• ${kind === 'agent' ? 'Agent' : 'Tool'} step "${entryId}" can

AI-assisted analysis of mastra-ai/mastra@75dd419e61 (2026-08-30). Data as JSON: /api/errors/d4d089a0946081f9. Report an issue: GitHub.