How an AI Agent Diagnosed a Missed Trade Copier Fill

A short case study. Anonymized account names. The shape of the incident is the one most traders eventually hit if they run a copier across multiple accounts: leader fills cleanly, one of the followers does not.

Incident setup

Leader account: APEX-LDR. Follower accounts: APEX-FLW-A and APEX-FLW-B. Configured copier: 1-to-1 quantity, same instruments, no per-account override.

14:32 ET. The leader fired a buy 2 MES order. The leader filled at 5240.25. Within seconds, both followers should have mirrored. APEX-FLW-A did. APEX-FLW-B did not. The follower had no order, no rejection visible in the followers' broker UI, and no notification.

What the trader saw

Refreshing the broker UI showed APEX-LDR long 2 MES. APEX-FLW-A long 2 MES. APEX-FLW-B flat. The trader checked the connection status; both followers were marked connected. From the trader's point of view, the copier was silently failing.

What the agent inspected

Claude Code, connected to CrossTrade MCP with mcp:read, was given the prompt:

For leader APEX-LDR and follower APEX-FLW-B between 14:31 and 14:33 today, pull GetActivityLog for both. Pull ListOrders and ListExecutions for both. Build a chronological timeline. Identify the step where the follower path broke. State the verbatim rejection reason if any.

Timeline reconstruction

The agent produced the following timeline:

14:32:09.812 APEX-LDR (leader) order placed: buy 2 MES 06-26, Market
14:32:10.041 APEX-LDR (leader) execution: long 2 at 5240.25
14:32:10.044 copier leader fill received
14:32:10.046 copier follower APEX-FLW-A routing started
14:32:10.047 copier follower APEX-FLW-B routing started
14:32:10.051 APEX-FLW-A order placed: buy 2 MES 06-26, Market
14:32:10.094 APEX-FLW-A execution: long 2 at 5240.50
14:32:10.121 copier follower APEX-FLW-B rejected: max contracts (existing 4 + routed 2 > limit 5)
14:32:10.121 APEX-FLW-B no order placed

Root cause

The agent identified the cause in one line: the follower APEX-FLW-B already held 4 MES from an earlier session. The copier tried to add 2 more, which would have produced a total position of 6 contracts, above the account's configured 5-contract maximum. The copier correctly refused.

The trader had not noticed the existing 4-contract position because it was on a different chart she did not have open. The "silent" failure was an unsurfaced rejection in the follower account's per-account max-contracts setting.

Fix

Two fixes were possible:

1. Manually close the existing 4-contract position on APEX-FLW-B so future leader fills could replicate.
2. Raise the per-account max-contracts limit on APEX-FLW-B (if the firm's rules allow).

The trader chose option 1. She closed the orphaned position manually, then re-listed positions on all three accounts to confirm sync.

Lessons for funded traders

• Max contracts is per-account. A copier honors what is on each follower, not what is on the leader. Forgotten positions on followers cause the same surprise every time.
• Silent failures are real. Some copier rejections do not surface in the broker's UI. The CrossTrade Add-On activity log is the source of truth.
• The agent surfaces the verbatim rejection. "Max contracts (existing 4 + routed 2 > limit 5)" tells you exactly what fired and why. Hand-debugging the same incident usually takes 10 minutes of log scrolling.

Prompt template

Keep this in your prompt library. Replace the placeholders.

For leader <leader> and follower <follower> between <start> and <end>:

1. Pull GetActivityLog for both accounts.
2. Pull ListOrders for both accounts.
3. Pull ListExecutions for both accounts.
4. Build a single chronological timeline.
5. Identify the step where the follower path broke.
6. State the verbatim rejection reason if any.
7. Tell me the most likely root cause and a one-line preventive check.

Do not place, cancel, flatten, or modify any order.

Link to the diagnostics page

For more copier diagnostic patterns, see AI Trade Copier Diagnostics for NinjaTrader 8 and the trader-facing Learn page.

Anonymized, but representative. Most copier mysteries are not mysteries when the activity log is in the conversation.