Haiku agent confirms successful tool calls but silently omits required fields in multi-step agentic workflows

Preflight Checklist

• [x] I have searched existing issues for similar behavior reports
• [x] This report does NOT contain sensitive information (API keys, passwords, etc.)

Type of Behavior Issue

Subagent behaved unexpectedly

What You Asked Claude to Do

Model: claude-haiku-4-5-20251001 (spawned as a sub-agent via the Claude Code Agent tool with model: "haiku")

When Haiku is used as an agent in a multi-step agentic workflow involving sequential Notion API writes, it confirms
successful completion and returns valid output (e.g. Notion page links) — but the underlying API calls are executed
with incomplete payloads. Required fields are silently omitted with no error, no warning, and no deviation from the
reported success state.

This is not a tool failure. The API calls succeed and real pages are created. The problem is that the model does not
execute the full field set specified in its instructions, and then misrepresents the outcome as complete.

Additional context

What Claude Actually Did

The agent writes partial records, reports success, returns valid page links as confirmation, and produces no
indication that any fields were skipped. The session log is indistinguishable from a correct run.

Expected Behavior

All fields specified in the workflow instructions are included in the API call. If a field is omitted, the agent
either flags it before writing or raises it in the output summary.

Files Affected

Permission Mode

Accept Edits was ON (auto-accepting changes)

Can You Reproduce This?

Sometimes (intermittent)

Steps to Reproduce

1. Run a multi-step agentic workflow using Haiku that involves writing structured records to Notion (or any structured

API) with 8–10 fields per record

1. The workflow includes a skill file that specifies mandatory fields, including a template_id parameter explicitly

marked "always pass this"

1. Haiku executes the write calls, returns page URLs, and reports success
2. On manual inspection of the created records, multiple fields are missing across entries — including the

template_id, relation fields (Tags), and body text

Claude Model

Haiku

Relevant Conversation

# 3-2-1 Session — April 30, 2026

**Newsletter:** https://jamesclear.com/3-2-1/april-30-2026

## Quotes Added to Notion

| # | Header | Author | Notion Link |
|---|--------|--------|-------------|
| 1 | improvement = you vs your past self, not you vs others... | James Clear | https://app.notion.com/p/35302043385881589b37f649964d822d |
| 2 | pick a direction and get moving; the aim adjusts as you go... | James Clear | https://app.notion.com/p/3530204338588140809febc136a7a391 |
| 3 | optimised perception = confident blindness... | Alan Kay | https://app.notion.com/p/353020433858813e8756e71102b44d35 |

## Interesting Question Added

- Are you building or maintaining? → https://app.notion.com/p/35302043385881a2b20cc8e5cfa2649f

Impact

Critical - Data loss or corrupted project

Claude Code Version

Claude Code version: 2.1.126

Platform

Anthropic API

Additional Context

• This is a recurring pattern, not a one-off. We have observed Haiku confirming task completion without fully

executing the specified instructions across multiple sessions.

• The failure mode is particularly problematic in agentic workflows because the confirmation output (e.g. a valid page

link) is used downstream as proof of completion — there is no natural point at which the gap surfaces unless the user
manually inspects every output.

• The workflow does have a read-back validation step designed to catch exactly this, but it is currently disabled for

this workflow path.

• Suspected cause: under token or complexity pressure in long multi-step sessions, Haiku appears to compress or

shortcut tool call construction while maintaining the appearance of full execution in its text output.