Drop sampling params unsupported by the model

The common-pattern enrichment and tracker-mapping agents run on
reasoning models such as gpt-5-nano, which reject an explicit
temperature and fail the whole request with a 400 ("Unsupported
value: 'temperature' does not support 0.1 with this model"). The
model registry already records this capability, but nothing
consulted it before dispatch, and dated provider snapshots like
gpt-5-nano-2025-08-07 did not resolve in the registry.

Resolve dated snapshots to their undated base model in registry
Lookup, and sanitize each chat completion request in the LLM
client by omitting the sampling knobs the target model does not
accept (temperature, top_p, frequency/presence penalties, stop).
Unknown models are left untouched, so models absent from the
registry keep their current behavior.

Signed-off-by: Émile Ré <emile@probo.com>

pkg/llm/llm.go

@@ -17,6 +17,7 @@ package llm
 import (
 	"context"
 	"io"
+	"strings"
 	"time"
 
 	"go.gearno.de/kit/log"
@@ -72,6 +73,8 @@ func NewClient(provider Provider, system string, opts ...Option) *Client {
 }
 
 func (c *Client) ChatCompletion(ctx context.Context, req *ChatCompletionRequest) (*ChatCompletionResponse, error) {
+	req = sanitizeRequest(ctx, c.logger, req)
+
 	ctx, span := startChatSpan(ctx, c.tracer, c.system, req)
 
 	c.logger.InfoCtx(
@@ -115,6 +118,8 @@ func (c *Client) ChatCompletion(ctx context.Context, req *ChatCompletionRequest)
 }
 
 func (c *Client) ChatCompletionStream(ctx context.Context, req *ChatCompletionRequest) (ChatCompletionStream, error) {
+	req = sanitizeRequest(ctx, c.logger, req)
+
 	ctx, span := startChatSpan(ctx, c.tracer, c.system, req)
 
 	c.logger.InfoCtx(
@@ -140,3 +145,69 @@ func (c *Client) ChatCompletionStream(ctx context.Context, req *ChatCompletionRe
 
 	return newTracedStream(stream, span), nil
 }
+
+// sanitizeRequest drops sampling parameters the target model does not
+// accept, using the default model registry. Reasoning models (e.g. the
+// GPT-5 family) reject an explicit temperature, top_p, or penalties and
+// fail the whole request with a 400, so the safest behavior is to omit
+// the unsupported knobs rather than propagate a hard error. When the
+// model is unknown to the registry the request is left untouched.
+func sanitizeRequest(ctx context.Context, logger *log.Logger, req *ChatCompletionRequest) *ChatCompletionRequest {
+	if req == nil {
+		return req
+	}
+
+	model, ok := DefaultRegistry().Lookup(req.Model)
+	if !ok {
+		return req
+	}
+
+	supports := model.Supports
+
+	dropTemperature := req.Temperature != nil && !supports.Temperature
+	dropTopP := req.TopP != nil && !supports.TopP
+	dropFrequencyPenalty := req.FrequencyPenalty != nil && !supports.FrequencyPenalty
+	dropPresencePenalty := req.PresencePenalty != nil && !supports.PresencePenalty
+	dropStop := len(req.StopSequences) > 0 && !supports.Stop
+
+	if !dropTemperature && !dropTopP && !dropFrequencyPenalty && !dropPresencePenalty && !dropStop {
+		return req
+	}
+
+	sanitized := *req
+
+	dropped := make([]string, 0, 5)
+	if dropTemperature {
+		sanitized.Temperature = nil
+		dropped = append(dropped, "temperature")
+	}
+
+	if dropTopP {
+		sanitized.TopP = nil
+		dropped = append(dropped, "top_p")
+	}
+
+	if dropFrequencyPenalty {
+		sanitized.FrequencyPenalty = nil
+		dropped = append(dropped, "frequency_penalty")
+	}
+
+	if dropPresencePenalty {
+		sanitized.PresencePenalty = nil
+		dropped = append(dropped, "presence_penalty")
+	}
+
+	if dropStop {
+		sanitized.StopSequences = nil
+		dropped = append(dropped, "stop")
+	}
+
+	logger.WarnCtx(
+		ctx,
+		"dropping unsupported sampling parameters for model",
+		log.String("model", req.Model),
+		log.String("dropped", strings.Join(dropped, ",")),
+	)
+
+	return &sanitized
+}