GLM-5.1 versus GLM-5.3-Flash

What are we comparing?

This comparison pits two open-weight models that you can install on your own servers against each other: GLM-5.1 and GLM-5.3-Flash, both built by Zhipu AI (Z.ai). The first dates from April 7, 2026, the second from September 18, 2026.

All figures below come from public sources checked on October 22, 2026: models.dev and OpenRouter for prices and capabilities, Epoch AI for scores.

What the benchmarks say

On our capability index, GLM-5.1 scores 70.9 out of 100 and GLM-5.3-Flash 71.9. GLM-5.3-Flash leads by 1 points.

On the 4 benchmarks reported for both, GLM-5.1 wins 0 times and GLM-5.3-Flash 4:

• GPQA Diamond (PhD-level science questions): GLM-5.1 89.9%, GLM-5.3-Flash 90.2%
• OTIS Mock AIME (competition mathematics problems): GLM-5.1 93.3%, GLM-5.3-Flash 93.9%
• FrontierMath (very hard research-level mathematics): GLM-5.1 36.8%, GLM-5.3-Flash 55.8%
• APEX-Agents (professional tasks carried out by an agent): GLM-5.1 40.9%, GLM-5.3-Flash 52.8%

Pricing

GLM-5.1 costs $0.90 per million input tokens and $3.59 per million output tokens; GLM-5.3-Flash costs $0.30 and $1.05.

For a typical agent task (read 20,000 tokens, write 2,000), expect $0.025 with GLM-5.1 versus $0.008 with GLM-5.3-Flash. GLM-5.3-Flash is 3.1 times cheaper, a gap that adds up quickly over 1,000 tasks: $17.03 difference.

Run the numbers with your own volumes in the agent cost calculator.

Features and limits

GLM-5.3-Flash reads more in one go: 1,000,000 tokens of context (roughly 1,500 pages), versus 200,000 for GLM-5.1.

Reasoning mode: GLM-5.1 yes, GLM-5.3-Flash yes. Tool calling: GLM-5.1 yes, GLM-5.3-Flash yes. Image input: GLM-5.1 no, GLM-5.3-Flash yes.

Both publish their weights: you can host them yourself.

In short

Our reading of the data:

• Choose GLM-5.3-Flash for a higher capability index, a lower cost, a larger context window, a more recent release and image input.

This verdict relies on public figures; it does not replace a trial on your own tasks.