Root cause: sequential execution of "messages.create"

3 "anthropic.messages.create" spans ran one after another — each started only after the previous one finished. Total: 5.8s, slowest single call: 2.5s. Running them concurrently saves 3.3s — a 2.4× speedup.

Now (sequential)After fix (parallel)

5.8s

→

~2.5s

Fix: run calls in parallel

fix.py

from concurrent.futures import ThreadPoolExecutor

# Before — sequential, each call blocks the next:
for item in items:
    result = process(item)

# After — all calls run concurrently:
with ThreadPoolExecutor(max_workers=8) as pool:
    results = list(pool.map(process, items))

# Cost unchanged. Output unchanged. Time = slowest single call.

Waterfall

workflow › execute

workflow.execute

user: u_heavy_27

6.57s

anthropic › messages › create

anthropic.messages.create

claude-opus-4-74.7k tok$0.183faithful 0.93user: u_heavy_27

1.68s

Input

Summarize the Q3 earnings report attached. Focus on revenue, margin, and guidance.

Output

(answer to: Summarize the Q3 earnings report attache…)

anthropic › messages › create

anthropic.messages.create

claude-opus-4-75.5k tok$0.214faithful 0.67user: u_heavy_27

2.48s

Input

Given the customer ticket below, draft a refund response that follows policy P-204.

Output

(answer to: Given the customer ticket below, draft a…)

anthropic › messages › create

anthropic.messages.create

claude-opus-4-75.1k tok$0.199faithful 0.95user: u_heavy_27

1.66s

Input

Translate the user manual section to Japanese, preserving the table structure.

Output

(answer to: Translate the user manual section to Jap…)

Tool: code_exec

tool.code_exec

user: u_heavy_27

219ms

Tool: web_fetch

tool.web_fetch

user: u_heavy_27

420ms

Faithfulness · anthropic.messages.create0.25

1 supported · 2 contradicted · 1 unsupported of 4 claims

contradicted
Q3 revenue was $4.2B.
contradicted
YoY growth was 38%.
unsupported
Operating margin expanded to 27%.
supported
Net income grew faster than revenue.