@@ -139,6 +139,17 @@ func convertMessagesWithCaps(ctx context.Context, messages []chat.Message, mc mo
 				}
 			}
 
+			// Replay stored reasoning back to OpenAI-compatible providers (e.g. Qwen via
+			// llama.cpp, vLLM, custom endpoints) that expect it on the assistant turn as
+			// reasoning_content. openai-go's typed ChatCompletionAssistantMessageParam has
+			// no such field (non-standard extension - see openai/openai-go#558), so it
+			// must go through SetExtraFields. See issue #4363.
+			if msg.ReasoningContent != "" {
+				assistantParam.SetExtraFields(map[string]any{
+					"reasoning_content": msg.ReasoningContent,
+				})
+			}
+
 			if msg.FunctionCall != nil {
 				assistantParam.FunctionCall.Name = msg.FunctionCall.Name
 				assistantParam.FunctionCall.Arguments = msg.FunctionCall.Arguments
 
@@ -0,0 +1,99 @@
+package oaistream
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/docker/docker-agent/pkg/chat"
+	"github.com/docker/docker-agent/pkg/modelsdev"
+	"github.com/docker/docker-agent/pkg/tools"
+)
+
+// TestReproIssue4363_ReasoningContentDroppedOnReplay reproduces
+// https://github.com/docker/docker-agent/issues/4363.
+//
+// For OpenAI-compatible custom providers (e.g. Qwen served via llama.cpp,
+// vLLM, or any other endpoint that speaks the DeepSeek-style
+// "reasoning_content" convention -- see the doc comment on
+// chat.Message.ReasoningContent), Docker Agent correctly captures streamed
+// reasoning, accumulates it, and stores it on the assistant chat.Message.
+//
+// However, convertMessagesWithCaps built the outgoing
+// ChatCompletionAssistantMessageParam from Content, FunctionCall, and
+// ToolCalls only -- msg.ReasoningContent was never read, so the reasoning
+// was silently dropped when the conversation was replayed on the next
+// request. A reasoning model then has no way to see its own prior
+// reasoning after a tool call, and must reconstruct it from scratch --
+// inflating reasoning tokens and latency on every subsequent turn.
+//
+// The fix attaches the stored reasoning to the outgoing assistant message
+// via SetExtraFields, since openai-go's typed
+// ChatCompletionAssistantMessageParam has no native field for it (this is
+// a non-standard, provider-specific extension; see openai/openai-go#558).
+func TestReproIssue4363_ReasoningContentDroppedOnReplay(t *testing.T) {
+	t.Parallel()
+
+	store := modelsdev.NewDatabaseStore(&modelsdev.Database{})
+
+	t.Run("bug: reasoning content is carried onto the replayed assistant message", func(t *testing.T) {
+		t.Parallel()
+
+		messages := []chat.Message{
+			{
+				Role:             chat.MessageRoleAssistant,
+				Content:          "The capital of France is Paris.",
+				ReasoningContent: "The user asked about France's capital; I recall it is Paris.",
+			},
+		}
+
+		result := ConvertMessages(t.Context(), messages, modelsdev.ID{}, store, nil)
+		require.Len(t, result, 1)
+		require.NotNil(t, result[0].OfAssistant, "expected an assistant message param")
+
+		extra := result[0].OfAssistant.ExtraFields()
+		require.NotNil(t, extra, "expected extra fields to carry reasoning_content")
+		assert.Equal(t, "The user asked about France's capital; I recall it is Paris.", extra["reasoning_content"],
+			"issue #4363: stored reasoning must be replayed to OpenAI-compatible providers, not silently dropped")
+	})
+
+	t.Run("no reasoning content: no extra field is added", func(t *testing.T) {
+		t.Parallel()
+
+		messages := []chat.Message{
+			{Role: chat.MessageRoleAssistant, Content: "Hi there!"},
+		}
+
+		result := ConvertMessages(t.Context(), messages, modelsdev.ID{}, store, nil)
+		require.Len(t, result, 1)
+		require.NotNil(t, result[0].OfAssistant)
+
+		extra := result[0].OfAssistant.ExtraFields()
+		assert.NotContains(t, extra, "reasoning_content",
+			"models with no stored reasoning should not gain a reasoning_content field")
+	})
+
+	t.Run("reasoning content survives alongside tool calls", func(t *testing.T) {
+		t.Parallel()
+
+		messages := []chat.Message{
+			{
+				Role:             chat.MessageRoleAssistant,
+				Content:          "",
+				ReasoningContent: "I should call the weather tool for this city.",
+				ToolCalls: []tools.ToolCall{
+					{ID: "call_1"},
+				},
+			},
+		}
+
+		result := ConvertMessages(t.Context(), messages, modelsdev.ID{}, store, nil)
+		require.Len(t, result, 1)
+		require.NotNil(t, result[0].OfAssistant)
+
+		extra := result[0].OfAssistant.ExtraFields()
+		assert.Equal(t, "I should call the weather tool for this city.", extra["reasoning_content"])
+		assert.Len(t, result[0].OfAssistant.ToolCalls, 1, "tool calls must still be converted alongside reasoning")
+	})
+}