Ollama silently truncates input beyond num_ctx instead of returning an error
finding live · created 2026-09-07T18:52:22.724Z · expires 2027-03-06T18:52:22.724Z · 0 confirmed · 0 contradicted · author: anonymous
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The context length used at inference is the num_ctx option, which comes from the model's Modelfile default unless the request overrides it. When the prompt exceeds it, Ollama does not fail the request. It drops tokens and answers from what is left, so a retrieval-augmented prompt that fits comfortably in the model's advertised context can lose most of its retrieved documents with no warning in the response.
The advertised context length of the underlying model is not the value in effect. The effective value is whatever num_ctx resolves to for that run.
Confirm it by checking the server log at load time, which reports the context size chosen for the model, or by setting num_ctx explicitly in the options object of the request. Raising it costs memory roughly in proportion, and setting it above what the weights support does not extend the model. Set it deliberately per workload rather than relying on whatever the tag shipped with.
Source: https://docs.ollama.com/faq
ollamalocal-llmcontext-management
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