GLM-5.1 is Z.ai's April 2026 release, the step between GLM-5 and GLM-5.2, and it is the strongest reasoning model in the GLM line at its context tier. Inside Atlas, GLM-5.1 is the model to reach for on hard refactors and multi-hop debugging, where a plan pass has to actually be correct. GLM-5.1 gives Atlas a 200,000 token context window and a 131,072 output cap, and it bills at $1.40 per Mtok input and $4.40 per Mtok output.
What is GLM-5.1 best at inside Atlas?
GLM-5.1, released by Z.ai in April 2026, is the strongest reasoning model in the GLM line at its 200,000 token context tier, sitting a clear step above GLM-5's $1.00 and $3.20 capability. Inside Atlas, GLM-5.1 earns its $4.40 per Mtok output rate on refactors that have to be right the first time.
Atlas drafts a plan in a read-only plan agent and asks before switching to a build agent, and the plan pass is where GLM-5.1 separates from cheaper GLM tiers. A stronger reasoner produces a plan that survives contact with the codebase, which is worth far more than the price delta on a single session. GLM-5.1 also pairs well with Atlas's hybrid semantic and keyword retrieval fused by reciprocal rank fusion: better reasoning over retrieved context means fewer wasted tool calls chasing the wrong file. Every Atlas tool call is permission-gated against allow, ask, and deny rules before it runs, so even an aggressive GLM-5.1 session stays inside the boundary you set.
What does GLM-5.1 cost and what do you get for it?
GLM-5.1 is priced at $1.40 per Mtok input and $4.40 per Mtok output. Z.ai introduced that pricing with GLM-5.1 in April 2026, and GLM-5.2 later inherited the exact same $1.40 and $4.40 rates while adding a 1,000,000 token window on top of them.
The pricing story is the most important thing to understand about GLM-5.1. Because GLM-5.2 costs the identical $1.40 per Mtok input and $4.40 per Mtok output, an A/B between GLM-5.1 and GLM-5.2 is purely a context-window decision, not a budget decision. Nothing about the money changes when you flip the id in atlas.json. That also means GLM-5.1 has to be justified on its own reasoning quality at 200,000 tokens rather than on cost. Against the cheaper end of the line, $4.40 per Mtok output is double GLM-4.7's $2.20, and for a lot of routine agent work GLM-4.7 is genuinely sufficient.
How large is the GLM-5.1 context and output budget?
GLM-5.1 holds a 200,000 token context window and a 131,072 max output cap. The 131,072 output ceiling means a long chain of thought plus a complete implementation fits in one GLM-5.1 response, so Atlas does not need a continuation turn to finish a large edit.
The output budget is easy to overlook and it changes how a session feels. When Atlas computes a unified diff for every file edit and surfaces it for approval before writing, a 131,072 token ceiling on GLM-5.1 means the diff arrives whole rather than in fragments stitched across turns. On the input side, a 200,000 token window is enough to hold a substantial service and its tests at once, especially given that Atlas indexes code by AST declarations using tree-sitter, not blind line windows, so the tokens you spend are spent on complete declarations. Where 200,000 tokens is not enough, GLM-5.2's 1,000,000 token window at the same $1.40 and $4.40 is the obvious next stop.
GLM-5.1 vs GLM-5.2: which should you run in Atlas?
GLM-5.2 costs the same $1.40 per Mtok input and $4.40 per Mtok output as GLM-5.1 and ships a 1,000,000 token context, so on price alone there is little reason to choose GLM-5.1 over GLM-5.2 today. GLM-5.1 has to win on measured quality against your own repository, not on cost.
The right move is to test rather than assume. Atlas lets you switch the active model and provider on the fly with favorites and recents, so favorite both `zai/glm-5.1` and `zai/glm-5.2` in the TUI's `/models` dialog and run the same task on each. Because the two models cost the same, the only variables are reasoning quality and context headroom, and both are measurable on a real repo in an afternoon. If your work fits comfortably inside 200,000 tokens and GLM-5.1 plans better on your codebase, keep GLM-5.1. If you routinely need more context, GLM-5.2 gives you five times the window at no additional cost.
When should you pick a cheaper model than GLM-5.1?
Pick a cheaper model than GLM-5.1 for routine agent work. At $4.40 per Mtok output, GLM-5.1 is double GLM-4.7's $2.20 output rate, and a large share of what a coding agent does day to day is mechanical enough that GLM-4.7 handles it without a quality gap you would notice.
Cost discipline with GLM-5.1 comes down to routing. Atlas fans out work to subagents that can run in the foreground or in parallel background sessions, and those subagents are the fastest way to multiply a $4.40 per Mtok output rate into a real bill. Reserve `zai/glm-5.1` for the main agent loop on hard problems, and route titles, summaries, and background fan-out to a cheaper GLM tier. Unattended CI runs are another case where GLM-5.1 is hard to justify: no human is evaluating the reasoning in real time, and a cheaper model with Atlas's permission gating and unified diff review produces the same auditable output.
Setup
- 01Export ZHIPU_API_KEY, or run `atlas login` and select Z.ai to store the credential in Atlas.
- 02Run `atlas models zai` and confirm `glm-5.1` appears in the resolved model registry.
- 03Pin `"model": "zai/glm-5.1"` in atlas.json so every session starts on GLM-5.1.
- 04Benchmark GLM-5.1 head to head with `zai/glm-5.2` on your own repo using `/models` in the TUI, since they cost the same $1.40 and $4.40 and differ mainly in context.
- 05Route background subagents to a cheaper GLM tier so the $4.40 per Mtok output rate stays on the main agent loop.
Frequently asked questions
- how much does glm-5.1 cost
- GLM-5.1 is priced at $1.40 per Mtok input and $4.40 per Mtok output. Z.ai introduced that pricing with GLM-5.1 in April 2026 and later applied the same rates to GLM-5.2.
- what is the glm-5.1 context window
- GLM-5.1 has a 200,000 token context window and a 131,072 max output cap, which is enough for a long chain of thought plus a complete implementation in one response.
- glm-5.1 vs glm-5.2 which is better
- GLM-5.1 and GLM-5.2 cost the identical $1.40 per Mtok input and $4.40 per Mtok output. GLM-5.2 adds a 1,000,000 token context, so the choice between them is a context-window decision rather than a pricing one.
- how do i set glm-5.1 as the model in atlas
- Export ZHIPU_API_KEY or run `atlas login` and select Z.ai, run `atlas models zai` to confirm `glm-5.1` resolves, then pin `"model": "zai/glm-5.1"` in atlas.json.
- is glm-5.1 worth it over glm-4.7
- GLM-5.1 reasons better, but its $4.40 per Mtok output is double GLM-4.7's $2.20. For routine agent work GLM-4.7 is sufficient. Reserve GLM-5.1 for hard refactors and planning passes.
- does glm-5.1 support reasoning in a coding agent
- Yes. GLM-5.1 is a reasoning model with a 131,072 output cap, which is why it pairs well with Atlas's read-only plan agent that drafts a plan and asks before switching to a build agent.
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