hubspot engagements API 'Invalid input JSON' from agent's malformed note payload

TL;DR

The agent built the note payload by string concatenation and produced invalid JSON. Serialize the payload with a real JSON library, validate the string before sending, and log the raw body on failure so the next malformed payload is diagnosable in seconds.

hubspot engagements API 'Invalid input JSON' from agent's malformed note payload

Steps

1. Capture the exact request body the agent sent and run it through a JSON parser locally. Find the position the parser reports. Expected: the parser points at the exact broken spot (a stray quote, a missing comma, an unescaped newline).

2. Replace any hand-built JSON strings in the agent's code with the standard JSON serializer for its language. Never concatenate JSON by hand. Expected: the serializer produces valid output for the same note content.

3. Add a pre-send validation step: parse the serialized string and confirm the required engagement fields (type, body, associations) are present before the HTTP call. Expected: malformed payloads are caught before they reach HubSpot.

4. Resend the note with the serialized payload. Expected: the engagements API returns a 200 with an engagement id.

5. Add the raw request body to the error log whenever a JSON parse error comes back, so future failures carry their evidence. Expected: the next malformed payload is fixed from the log alone.

Use this when

• The engagements API returns 'Invalid input JSON' on note, email, call, or task creates.
• The payload includes user-generated text (note bodies with quotes or newlines).
• The agent assembles request bodies with string templates.

Not for this skill when

• The error is a validation error on field values rather than a JSON parse error.
• The request fails with 401, 403, or 429.
• Other endpoints accept the agent's payloads fine (then it is endpoint-specific schema, not JSON).

Variant phrasings

• "hubspot note create invalid json"
• "engagements api json parse error from agent"
• "malformed engagement payload hubspot"

Why it happens

Note bodies are free text full of quotes, backslashes, and newlines. When the agent interpolates that text into a JSON template string instead of letting a serializer escape it, the result is not valid JSON. HubSpot's parser rejects it before any business logic runs.

Edge cases

• Emoji and other multi-byte characters are fine for serializers but break naive byte-level truncation. Truncate by character, then serialize.
• Some serializers escape forward slashes or unicode by default; that is still valid JSON and HubSpot accepts it.
• If the payload is valid JSON but HubSpot still complains, the problem is schema (missing required association), not syntax.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_b-4WEtby2iNGdjlVcFEe-Q