test(gateway): add tests for content-based session hash fallback

- 20 unit tests for deriveOpenAIContentSessionSeed covering:
  - Empty/nil inputs, model-only, stable across turns
  - Different model/system/first-user produce different seeds
  - Tools, functions, developer role, structured content
  - Responses API: input string, input array, instructions, input_text typed items
  - JSON canonicalization (whitespace/key-order insensitive)
  - Prefix presence, empty tools ignored, messages preferred over input
- 3 integration tests for GenerateSessionHash content fallback:
  - Content fallback produces stable hash
  - Explicit signals override content fallback
  - Empty body still returns empty hash

backend/internal/service/openai_content_session_seed_test.go

+package service
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestDeriveOpenAIContentSessionSeed_EmptyInputs(t *testing.T) {
+	require.Empty(t, deriveOpenAIContentSessionSeed(nil))
+	require.Empty(t, deriveOpenAIContentSessionSeed([]byte{}))
+	require.Empty(t, deriveOpenAIContentSessionSeed([]byte(`{}`)))
+}
+
+func TestDeriveOpenAIContentSessionSeed_ModelOnly(t *testing.T) {
+	seed := deriveOpenAIContentSessionSeed([]byte(`{"model":"gpt-5.4"}`))
+	require.Contains(t, seed, contentSessionSeedPrefix)
+	require.Contains(t, seed, "model=gpt-5.4")
+}
+
+func TestDeriveOpenAIContentSessionSeed_ChatCompletions_StableAcrossTurns(t *testing.T) {
+	turn1 := []byte(`{
+		"model": "gpt-5.4",
+		"messages": [
+			{"role": "system", "content": "You are helpful."},
+			{"role": "user", "content": "Hello"}
+		]
+	}`)
+	turn2 := []byte(`{
+		"model": "gpt-5.4",
+		"messages": [
+			{"role": "system", "content": "You are helpful."},
+			{"role": "user", "content": "Hello"},
+			{"role": "assistant", "content": "Hi there!"},
+			{"role": "user", "content": "How are you?"}
+		]
+	}`)
+	s1 := deriveOpenAIContentSessionSeed(turn1)
+	s2 := deriveOpenAIContentSessionSeed(turn2)
+	require.Equal(t, s1, s2, "seed should be stable across later turns")
+	require.NotEmpty(t, s1)
+}
+
+func TestDeriveOpenAIContentSessionSeed_ChatCompletions_DifferentFirstUserDiffers(t *testing.T) {
+	req1 := []byte(`{"model":"gpt-5.4","messages":[{"role":"user","content":"Question A"}]}`)
+	req2 := []byte(`{"model":"gpt-5.4","messages":[{"role":"user","content":"Question B"}]}`)
+	s1 := deriveOpenAIContentSessionSeed(req1)
+	s2 := deriveOpenAIContentSessionSeed(req2)
+	require.NotEqual(t, s1, s2)
+}
+
+func TestDeriveOpenAIContentSessionSeed_ChatCompletions_DifferentSystemDiffers(t *testing.T) {
+	req1 := []byte(`{"model":"gpt-5.4","messages":[{"role":"system","content":"A"},{"role":"user","content":"Hi"}]}`)
+	req2 := []byte(`{"model":"gpt-5.4","messages":[{"role":"system","content":"B"},{"role":"user","content":"Hi"}]}`)
+	s1 := deriveOpenAIContentSessionSeed(req1)
+	s2 := deriveOpenAIContentSessionSeed(req2)
+	require.NotEqual(t, s1, s2)
+}
+
+func TestDeriveOpenAIContentSessionSeed_ChatCompletions_DifferentModelDiffers(t *testing.T) {
+	req1 := []byte(`{"model":"gpt-5.4","messages":[{"role":"user","content":"Hi"}]}`)
+	req2 := []byte(`{"model":"gpt-4o","messages":[{"role":"user","content":"Hi"}]}`)
+	s1 := deriveOpenAIContentSessionSeed(req1)
+	s2 := deriveOpenAIContentSessionSeed(req2)
+	require.NotEqual(t, s1, s2)
+}
+
+func TestDeriveOpenAIContentSessionSeed_ChatCompletions_WithTools(t *testing.T) {
+	withTools := []byte(`{
+		"model": "gpt-5.4",
+		"tools": [{"type":"function","function":{"name":"get_weather"}}],
+		"messages": [{"role": "user", "content": "Hello"}]
+	}`)
+	withoutTools := []byte(`{
+		"model": "gpt-5.4",
+		"messages": [{"role": "user", "content": "Hello"}]
+	}`)
+	s1 := deriveOpenAIContentSessionSeed(withTools)
+	s2 := deriveOpenAIContentSessionSeed(withoutTools)
+	require.NotEqual(t, s1, s2, "tools should affect the seed")
+	require.Contains(t, s1, "|tools=")
+}
+
+func TestDeriveOpenAIContentSessionSeed_ChatCompletions_WithFunctions(t *testing.T) {
+	body := []byte(`{
+		"model": "gpt-5.4",
+		"functions": [{"name":"get_weather","parameters":{}}],
+		"messages": [{"role": "user", "content": "Hello"}]
+	}`)
+	seed := deriveOpenAIContentSessionSeed(body)
+	require.Contains(t, seed, "|functions=")
+}
+
+func TestDeriveOpenAIContentSessionSeed_ChatCompletions_DeveloperRole(t *testing.T) {
+	body := []byte(`{
+		"model": "gpt-5.4",
+		"messages": [
+			{"role": "developer", "content": "You are helpful."},
+			{"role": "user", "content": "Hello"}
+		]
+	}`)
+	seed := deriveOpenAIContentSessionSeed(body)
+	require.Contains(t, seed, "|system=")
+	require.Contains(t, seed, "|first_user=")
+}
+
+func TestDeriveOpenAIContentSessionSeed_ChatCompletions_StructuredContent(t *testing.T) {
+	body := []byte(`{
+		"model": "gpt-5.4",
+		"messages": [
+			{"role": "user", "content": [{"type":"text","text":"Hello"}]}
+		]
+	}`)
+	seed := deriveOpenAIContentSessionSeed(body)
+	require.NotEmpty(t, seed)
+	require.Contains(t, seed, "|first_user=")
+}
+
+func TestDeriveOpenAIContentSessionSeed_ResponsesAPI_InputString(t *testing.T) {
+	body := []byte(`{"model":"gpt-5.4","input":"Hello, how are you?"}`)
+	seed := deriveOpenAIContentSessionSeed(body)
+	require.Contains(t, seed, "|input=Hello, how are you?")
+}
+
+func TestDeriveOpenAIContentSessionSeed_ResponsesAPI_InputArray(t *testing.T) {
+	body := []byte(`{
+		"model": "gpt-5.4",
+		"input": [
+			{"role": "system", "content": "You are helpful."},
+			{"role": "user", "content": "Hello"}
+		]
+	}`)
+	seed := deriveOpenAIContentSessionSeed(body)
+	require.Contains(t, seed, "|system=")
+	require.Contains(t, seed, "|first_user=")
+}
+
+func TestDeriveOpenAIContentSessionSeed_ResponsesAPI_WithInstructions(t *testing.T) {
+	body := []byte(`{
+		"model": "gpt-5.4",
+		"instructions": "You are a coding assistant.",
+		"input": "Write a hello world"
+	}`)
+	seed := deriveOpenAIContentSessionSeed(body)
+	require.Contains(t, seed, "|instructions=You are a coding assistant.")
+	require.Contains(t, seed, "|input=Write a hello world")
+}
+
+func TestDeriveOpenAIContentSessionSeed_Deterministic(t *testing.T) {
+	body := []byte(`{
+		"model": "gpt-5.4",
+		"messages": [
+			{"role": "system", "content": "You are helpful."},
+			{"role": "user", "content": "Hello"}
+		]
+	}`)
+	s1 := deriveOpenAIContentSessionSeed(body)
+	s2 := deriveOpenAIContentSessionSeed(body)
+	require.Equal(t, s1, s2, "seed must be deterministic")
+}
+
+func TestDeriveOpenAIContentSessionSeed_PrefixPresent(t *testing.T) {
+	body := []byte(`{"model":"gpt-5.4","messages":[{"role":"user","content":"Hi"}]}`)
+	seed := deriveOpenAIContentSessionSeed(body)
+	require.True(t, len(seed) > len(contentSessionSeedPrefix))
+	require.Equal(t, contentSessionSeedPrefix, seed[:len(contentSessionSeedPrefix)])
+}
+
+func TestDeriveOpenAIContentSessionSeed_EmptyToolsIgnored(t *testing.T) {
+	body := []byte(`{"model":"gpt-5.4","tools":[],"messages":[{"role":"user","content":"Hi"}]}`)
+	seed := deriveOpenAIContentSessionSeed(body)
+	require.NotContains(t, seed, "|tools=")
+}
+
+func TestDeriveOpenAIContentSessionSeed_MessagesPreferredOverInput(t *testing.T) {
+	body := []byte(`{
+		"model": "gpt-5.4",
+		"messages": [{"role": "user", "content": "from messages"}],
+		"input": "from input"
+	}`)
+	seed := deriveOpenAIContentSessionSeed(body)
+	require.Contains(t, seed, "|first_user=")
+	require.NotContains(t, seed, "|input=")
+}
+
+func TestDeriveOpenAIContentSessionSeed_JSONCanonicalisation(t *testing.T) {
+	compact := []byte(`{"model":"gpt-5.4","tools":[{"type":"function","function":{"name":"get_weather","description":"Get weather"}}],"messages":[{"role":"user","content":"Hi"}]}`)
+	spaced := []byte(`{
+		"model": "gpt-5.4",
+		"tools": [
+			{ "type" : "function", "function": { "description": "Get weather", "name": "get_weather" } }
+		],
+		"messages": [ { "role": "user", "content": "Hi" } ]
+	}`)
+	s1 := deriveOpenAIContentSessionSeed(compact)
+	s2 := deriveOpenAIContentSessionSeed(spaced)
+	require.Equal(t, s1, s2, "different formatting of identical JSON should produce the same seed")
+}
+
+func TestDeriveOpenAIContentSessionSeed_ResponsesAPI_InputTextTypedItem(t *testing.T) {
+	body := []byte(`{
+		"model": "gpt-5.4",
+		"input": [{"type": "input_text", "text": "Hello world"}]
+	}`)
+	seed := deriveOpenAIContentSessionSeed(body)
+	require.Contains(t, seed, "|first_user=")
+	require.Contains(t, seed, "Hello world")
+}
+
+func TestDeriveOpenAIContentSessionSeed_ResponsesAPI_TypedMessageItem(t *testing.T) {
+	body := []byte(`{
+		"model": "gpt-5.4",
+		"input": [{"type": "message", "role": "user", "content": "Hello from typed message"}]
+	}`)
+	seed := deriveOpenAIContentSessionSeed(body)
+	require.Contains(t, seed, "|first_user=")
+	require.Contains(t, seed, "Hello from typed message")
+}

backend/internal/service/openai_gateway_service_test.go

@@ -237,6 +237,60 @@ func TestOpenAIGatewayService_GenerateSessionHashWithFallback(t *testing.T) {
 	require.Equal(t, "", empty)
 }
 
+func TestOpenAIGatewayService_GenerateSessionHash_ContentFallback(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	rec := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(rec)
+	c.Request = httptest.NewRequest(http.MethodPost, "/openai/v1/chat/completions", nil)
+
+	svc := &OpenAIGatewayService{}
+
+	body := []byte(`{"model":"gpt-5.4","messages":[{"role":"system","content":"You are helpful."},{"role":"user","content":"Hello"}]}`)
+
+	hash := svc.GenerateSessionHash(c, body)
+	require.NotEmpty(t, hash, "content-based fallback should produce a hash")
+
+	hash2 := svc.GenerateSessionHash(c, body)
+	require.Equal(t, hash, hash2, "same content should produce same hash")
+
+	bodyExtended := []byte(`{"model":"gpt-5.4","messages":[{"role":"system","content":"You are helpful."},{"role":"user","content":"Hello"},{"role":"assistant","content":"Hi!"},{"role":"user","content":"How are you?"}]}`)
+	hashExtended := svc.GenerateSessionHash(c, bodyExtended)
+	require.Equal(t, hash, hashExtended, "hash should be stable across later turns")
+
+	bodyDifferent := []byte(`{"model":"gpt-5.4","messages":[{"role":"user","content":"Different question"}]}`)
+	hashDifferent := svc.GenerateSessionHash(c, bodyDifferent)
+	require.NotEqual(t, hash, hashDifferent, "different content should produce different hash")
+}
+
+func TestOpenAIGatewayService_GenerateSessionHash_ExplicitSignalWinsOverContent(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	rec := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(rec)
+	c.Request = httptest.NewRequest(http.MethodPost, "/openai/v1/chat/completions", nil)
+
+	svc := &OpenAIGatewayService{}
+	body := []byte(`{"model":"gpt-5.4","messages":[{"role":"user","content":"Hello"}]}`)
+
+	contentHash := svc.GenerateSessionHash(c, body)
+	require.NotEmpty(t, contentHash)
+
+	c.Request.Header.Set("session_id", "explicit-session")
+	explicitHash := svc.GenerateSessionHash(c, body)
+	require.NotEmpty(t, explicitHash)
+	require.NotEqual(t, contentHash, explicitHash, "explicit session_id should override content fallback")
+}
+
+func TestOpenAIGatewayService_GenerateSessionHash_EmptyBodyStillEmpty(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	rec := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(rec)
+	c.Request = httptest.NewRequest(http.MethodPost, "/openai/v1/chat/completions", nil)
+
+	svc := &OpenAIGatewayService{}
+	require.Empty(t, svc.GenerateSessionHash(c, []byte(`{}`)))
+	require.Empty(t, svc.GenerateSessionHash(c, nil))
+}
+
 func (c stubConcurrencyCache) GetAccountWaitingCount(ctx context.Context, accountID int64) (int, error) {
 	if c.waitCounts != nil {
 		if count, ok := c.waitCounts[accountID]; ok {