Andrew Ginns

🧜‍♀️ Merbench - LLM Evaluation

Getting LLMs to consistently nail the Mermaid diagram syntax can be... an adventure.

Merbench evaluates an LLM's ability to autonomously write and debug Mermaid syntax. The agent can access an MCP server that validates its code and provides error feedback, guiding it towards a correct solution.

Each model is tested across three difficulty levels, with a limited number of five attempts per test case. Performance is measured by the final success rate, averaged over complete runs, reflecting both an understanding of Mermaid syntax and effective tool usage.

Evaluation Summary

879
Total Evaluation Runs
16
Models Evaluated
3
Test Cases

Providers Tested

AmazonGoogle
Data updated: Oct 14, 2025
Difficulty:
Provider:
What do these metrics mean?
Success Rate
The percentage of successful Mermaid diagram generations out of all runs.
Avg Cost/Run
The average cost in USD to generate one diagram, based on provider pricing.
Price/Success
The effective cost for each successful diagram, calculated as (Avg Cost / Success Rate).
Avg Duration
The average time in seconds taken to generate a diagram.
Avg Tokens
The average number of tokens (input + output) used per run.
Runs
The total number of times this model was run in the evaluation.

Model Leaderboard

Rank Model Success Rate Avg Cost/Run Price/Success Avg Duration Avg Tokens Runs Provider
1 gemini-2.5-flash-preview-09-2025
31.1%
$0.0206
$0.0661 32.33s 22,980.822 45 Google
2 gemini-2.5-pro-preview-06-05
29.4%
$0.0383
$0.1302 36.84s 8,111.882 51 Google
3 gemini-2.5-pro-preview-05-06
26.7%
$0.1308
$0.4904 49.85s 19,753.911 45 Google
4 gemini-2.5-pro-preview-03-25
22.9%
$0.1133
$0.4942 57.17s 16,393.313 48 Google
5 gemini-2.5-pro
20.0%
$0.0544
$0.2722 32.94s 14,255.511 45 Google
6 gemini-2.5-flash
13.3%
$0.0128
$0.0957 10.15s 6,990.467 45 Google
7 gemini-2.5-flash-lite-preview-06-17
5.0%
$0.0008
$0.0163 4.40s 4,974.583 60 Google
8 gemini-2.5-flash-preview-05-20
5.0%
$0.0101
$0.2014 9.75s 5,771.55 60 Google
9 gemini-2.5-flash-preview-04-17
4.4%
$0.0233
$0.5237 24.15s 10,492.711 45 Google
10 bedrock:us.amazon.nova-premier-v1:0
3.3%
$0.0356
$1.0692 63.19s 9,528.967 60 Amazon
11 gemini-2.5-flash-lite
3.3%
$0.0013
$0.0382 5.90s 9,506.689 90 Google
12 bedrock:us.amazon.nova-lite-v1:0
0.0%
$0.0002
N/A 24.54s 2,799.317 60 Amazon
13 bedrock:us.amazon.nova-micro-v1:0
0.0%
$0.0001
N/A 18.83s 1,783.85 60 Amazon
14 bedrock:us.amazon.nova-pro-v1:0
0.0%
$0.0008
N/A 49.53s 678.15 60 Amazon
15 gemini-2.0-flash
0.0%
$0.0003
N/A 4.21s 1,325.667 60 Google
16 gemini-2.5-flash-lite-preview-09-2025
0.0%
$0.0011
N/A 5.68s 5,687.822 45 Google

Performance vs Efficiency Trade-offs

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Performance by Difficulty Level

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Token Usage Breakdown

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Failure Analysis by Reason

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Last updated: October 16, 2025 at 01:15 AM UTC