diff --git a/backend/internal/service/openai_content_session_seed.go b/backend/internal/service/openai_content_session_seed.go
new file mode 100644
index 00000000..cb8fcb84
--- /dev/null
+++ b/backend/internal/service/openai_content_session_seed.go
@@ -0,0 +1,107 @@
+package service
+
+import (
+	"encoding/json"
+	"strings"
+
+	"github.com/tidwall/gjson"
+)
+
+// contentSessionSeedPrefix prevents collisions between content-derived seeds
+// and explicit session IDs (e.g. "sess-xxx" or "compat_cc_xxx").
+const contentSessionSeedPrefix = "compat_cs_"
+
+// deriveOpenAIContentSessionSeed builds a stable session seed from an
+// OpenAI-format request body. Only fields constant across conversation turns
+// are included: model, tools/functions definitions, system/developer prompts,
+// instructions (Responses API), and the first user message.
+// Supports both Chat Completions (messages) and Responses API (input).
+func deriveOpenAIContentSessionSeed(body []byte) string {
+	if len(body) == 0 {
+		return ""
+	}
+
+	var b strings.Builder
+
+	if model := gjson.GetBytes(body, "model").String(); model != "" {
+		b.WriteString("model=")
+		b.WriteString(model)
+	}
+
+	if tools := gjson.GetBytes(body, "tools"); tools.Exists() && tools.IsArray() && tools.Raw != "[]" {
+		b.WriteString("|tools=")
+		b.WriteString(normalizeCompatSeedJSON(json.RawMessage(tools.Raw)))
+	}
+
+	if funcs := gjson.GetBytes(body, "functions"); funcs.Exists() && funcs.IsArray() && funcs.Raw != "[]" {
+		b.WriteString("|functions=")
+		b.WriteString(normalizeCompatSeedJSON(json.RawMessage(funcs.Raw)))
+	}
+
+	if instr := gjson.GetBytes(body, "instructions").String(); instr != "" {
+		b.WriteString("|instructions=")
+		b.WriteString(instr)
+	}
+
+	firstUserCaptured := false
+
+	msgs := gjson.GetBytes(body, "messages")
+	if msgs.Exists() && msgs.IsArray() {
+		msgs.ForEach(func(_, msg gjson.Result) bool {
+			role := msg.Get("role").String()
+			switch role {
+			case "system", "developer":
+				b.WriteString("|system=")
+				if c := msg.Get("content"); c.Exists() {
+					b.WriteString(normalizeCompatSeedJSON(json.RawMessage(c.Raw)))
+				}
+			case "user":
+				if !firstUserCaptured {
+					b.WriteString("|first_user=")
+					if c := msg.Get("content"); c.Exists() {
+						b.WriteString(normalizeCompatSeedJSON(json.RawMessage(c.Raw)))
+					}
+					firstUserCaptured = true
+				}
+			}
+			return true
+		})
+	} else if inp := gjson.GetBytes(body, "input"); inp.Exists() {
+		if inp.Type == gjson.String {
+			b.WriteString("|input=")
+			b.WriteString(inp.String())
+		} else if inp.IsArray() {
+			inp.ForEach(func(_, item gjson.Result) bool {
+				role := item.Get("role").String()
+				switch role {
+				case "system", "developer":
+					b.WriteString("|system=")
+					if c := item.Get("content"); c.Exists() {
+						b.WriteString(normalizeCompatSeedJSON(json.RawMessage(c.Raw)))
+					}
+				case "user":
+					if !firstUserCaptured {
+						b.WriteString("|first_user=")
+						if c := item.Get("content"); c.Exists() {
+							b.WriteString(normalizeCompatSeedJSON(json.RawMessage(c.Raw)))
+						}
+						firstUserCaptured = true
+					}
+				}
+				if !firstUserCaptured && item.Get("type").String() == "input_text" {
+					b.WriteString("|first_user=")
+					if text := item.Get("text").String(); text != "" {
+						b.WriteString(text)
+					}
+					firstUserCaptured = true
+				}
+				return true
+			})
+		}
+	}
+
+	if b.Len() == 0 {
+		return ""
+	}
+	return contentSessionSeedPrefix + b.String()
+}
diff --git a/backend/internal/service/openai_gateway_service.go b/backend/internal/service/openai_gateway_service.go
index 0a959615..b9f42cd7 100644
--- a/backend/internal/service/openai_gateway_service.go
+++ b/backend/internal/service/openai_gateway_service.go
@@ -1044,6 +1044,7 @@ func (s *OpenAIGatewayService) ExtractSessionID(c *gin.Context, body []byte) str
 //  1. Header: session_id
 //  2. Header: conversation_id
 //  3. Body:   prompt_cache_key (opencode)
+//  4. Body:   content-based fallback (model + system + tools + first user message)
 func (s *OpenAIGatewayService) GenerateSessionHash(c *gin.Context, body []byte) string {
 	if c == nil {
 		return ""
@@ -1056,6 +1057,9 @@ func (s *OpenAIGatewayService) GenerateSessionHash(c *gin.Context, body []byte)
 	if sessionID == "" && len(body) > 0 {
 		sessionID = strings.TrimSpace(gjson.GetBytes(body, "prompt_cache_key").String())
 	}
+	if sessionID == "" && len(body) > 0 {
+		sessionID = deriveOpenAIContentSessionSeed(body)
+	}
 	if sessionID == "" {
 		return ""
 	}