TopP

TopP implements nucleus sampling. At each generation step, tokens are sorted by probability and the engine keeps only the smallest set whose cumulative probability is at least TopP. Tokens outside that nucleus are discarded before sampling.

Quick reference
Type float
Default 0.9
Range 0.0 – 1.0 (values ≥ 1.0 disable the filter)
Category Core sampling
Field on SamplerParameters.TopP

What it does

After Temperature scales the distribution, sort tokens by probability descending. Walk the sorted list accumulating probability. Stop when the running sum reaches TopP. Every token past that cutoff is removed; the remaining tokens are renormalized and sampled from.

  • At TopP = 1.0, no tokens are removed — the filter is effectively off.
  • At TopP = 0.9 (default), the engine keeps ~90 % of the probability mass. On a peaked distribution this is 2-5 tokens; on a flat distribution it can be 50+.
  • At TopP = 0.5, only the dominant half of the mass survives — tighter, more deterministic output.

TopP is probability-aware: on a confident step (one token at 0.95 probability) it keeps only that one token; on an uncertain step (many comparable tokens) it keeps a larger set.

When to change it

Scenario Value
Disabled (rely on TopK + MinP) 1.0
Creative output with some variety 0.95
Balanced general-purpose chat (default) 0.9
Conservative, precise output 0.7 – 0.8
Very tight, near-greedy 0.5 – 0.6

Lower TopP excludes more of the tail — output is more predictable but less varied. The default 0.9 is a broadly accepted balance.

Example

var preset = new Qwen25Preset();
preset.SamplerParameters.TopP = 0.95f; // keep a bit more of the tail for creativity

using var api = AsposeLLMApi.Create(preset);
string reply = await api.SendMessageAsync("Suggest a name for a coffee shop.");
Console.WriteLine(reply);

For precision work:

preset.SamplerParameters.Temperature = 0.2f;
preset.SamplerParameters.TopP = 0.8f;
// Narrow both dimensions — precise and deterministic.

Interactions

  • Temperature — applied before TopP. Very low Temperature + loose TopP still produces near-greedy output.
  • TopK — stacks with TopP. The final candidate set is the intersection.
  • MinP — complements TopP on the tail; both can be active.
  • TypicalP — alternative to TopP based on local typicality.
  • MinKeep — floor on candidate count; TopP never cuts below MinKeep.
  • Mirostat — bypasses TopP when active.

What’s next

Temperature TopK