Node '${node.id}': failed to serialize structured_output to

Error message

Node '${node.id}': failed to serialize structured_output to JSON: ${err.message}

What it means

When a node declares output_format and the provider's structured output passes schema validation, the executor converts it to canonical JSON via canonicalValueText. If that serialization throws (e.g. non-JSON-serializable values like BigInt, circular structures), the node fails with this error wrapping the underlying message.

Source

Thrown at packages/workflows/src/dag-executor.ts:3036

        );
        if (schemaCompileError !== undefined) {
          getLog().warn(
            { nodeId: node.id, workflowRunId: workflowRun.id, compileMsg: schemaCompileError },
            'dag.structured_output_schema_uncompilable'
          );
          await safeSendMessage(
            platform,
            conversationId,
            `⚠️ Node '${node.id}': its \`output_format\` schema could not be compiled (${schemaCompileError}), so the structured output was NOT validated against it. Fix the schema to enforce it.`,
            nodeContext
          );
        }
        if (validation.valid) {
          try {
            nodeOutputText = canonicalValueText(structuredOutput);
          } catch (serializeErr) {
            const err = serializeErr as Error;
            throw new Error(
              `Node '${node.id}': failed to serialize structured_output to JSON: ${err.message}`
            );
          }
          getLog().debug({ nodeId: node.id, streamingMode }, 'dag.structured_output_override');
          break;
        }
        // Invalid payload.
        getLog().warn(
          { nodeId: node.id, workflowRunId: workflowRun.id, errors: validation.errors },
          'dag.structured_output_invalid'
        );
        if (canReask) {
          await scheduleReask(validation.errors);
          continue;
        }
        throw new Error(
          `Node '${node.id}': output_format declared but the provider's structured output failed schema validation: ${validation.errors.join('; ')}`
        );

View on GitHub (pinned to 0773b97458)

Solutions

Inspect err.message in the error to see which value failed serialization.
Tighten the output_format schema so only JSON-serializable types are allowed (no exotic types).
Check/fix the provider adapter that produced the structured output value.
Report/fix a canonicalValueText bug if the value is ordinary JSON data.

Example fix

// before
schema allows: { id: "integer-as-bigint" }  // adapter yields BigInt
// after
schema: { id: { type: "string" } }  // or convert BigInt in adapter

Defensive patterns

Strategy: type-guard

Validate before calling

function isJsonSerializable(v: unknown, seen = new Set()): boolean {
  if (v === null || ['string','number','boolean'].includes(typeof v)) return true;
  if (typeof v === 'bigint') return false;
  if (typeof v !== 'object') return false;
  if (seen.has(v)) return false;
  seen.add(v);
  return Object.values(v).every(x => isJsonSerializable(x, seen));
}

Type guard

function isPlainJsonObject(v: unknown): v is Record<string, unknown> {
  return typeof v === 'object' && v !== null && !Array.isArray(v) && Object.getPrototypeOf(v) === Object.prototype;
}

Try / catch

try {
  await runNode(node);
} catch (e) {
  if (String(e).includes('failed to serialize structured_output')) console.error('Non-JSON value in structured output — inspect adapter');
  else throw e;
}

Prevention

Restrict output_format schemas to plain JSON types.
Audit custom provider adapters for exotic value types.
Round-trip structured outputs through JSON.parse(JSON.stringify()) in adapters.

When it happens

Trigger: structuredOutput validated against the schema but contains values canonicalValueText cannot serialize to JSON — typically unusual value types produced by a provider adapter or a schema allowing exotic types.

Common situations: A custom provider adapter returning non-standard JS values (BigInt, undefined-in-arrays, circular refs); a schema that validated loosely enough to admit unserializable data; bug in structured-output parsing.

Understand the failure class

Background: "JSON serialization failed", "not JSON serializable", "Failed to serialize": why JSON marshaling errors happen and how to fix them — this error's family across 46 libraries.

Related errors

AI-assisted analysis of coleam00/Archon@0773b97458 (2026-09-01). Data as JSON: /api/errors/dfb06f6b4f524be4. Report an issue: GitHub.