diff --git a/go/plugins/googlegenai/cache.go b/go/plugins/googlegenai/cache.go
index ded7e6755f..95696cf2f4 100644
--- a/go/plugins/googlegenai/cache.go
+++ b/go/plugins/googlegenai/cache.go
@@ -39,40 +39,41 @@ var invalidArgMessages = struct {
 	systemPrompt: "system prompts are not supported with context caching",
 }
 
-// handleCache checks if caching should be used, attempts to find or create the cache,
-// and returns the cached content if applicable.
+// handleCache checks if caching should be used, attempts to find or create the
+// cache, and returns the cached content plus the inclusive index of the last
+// cached message (-1 when no cache is in use).
 func handleCache(
 	ctx context.Context,
 	client *genai.Client,
 	request *ai.ModelRequest,
 	model string,
-) (*genai.CachedContent, error) {
+) (*genai.CachedContent, int, error) {
 	cs, err := findCacheMarker(request)
 	if err != nil {
-		return nil, err
+		return nil, -1, err
 	}
 	if cs == nil {
-		return nil, nil
+		return nil, -1, nil
 	}
 	// no cache mark found
 	if cs.endIndex == -1 {
-		return nil, err
+		return nil, -1, err
 	}
 	// index out of bounds
 	if cs.endIndex < 0 || cs.endIndex >= len(request.Messages) {
-		return nil, status.Errorf(status.ErrInvalidArgument, "end of cached contents, index %d is invalid", cs.endIndex)
+		return nil, -1, status.Errorf(status.ErrInvalidArgument, "end of cached contents, index %d is invalid", cs.endIndex)
 	}
 
 	// since context caching is only available for specific model versions, we
 	// must make sure the configuration has the right version
 	err = validateContextCacheRequest(request, model)
 	if err != nil {
-		return nil, err
+		return nil, -1, err
 	}
 
 	messages, err := messagesToCache(request.Messages, cs.endIndex)
 	if err != nil {
-		return nil, err
+		return nil, -1, err
 	}
 	hash := calculateCacheHash(messages)
 
@@ -81,14 +82,14 @@ func handleCache(
 		cache, err = lookupCache(ctx, client, cs.name)
 		if err != nil {
 			// TODO: if cache expired or not found, create a fresh one
-			return nil, fmt.Errorf("cache lookup error: %w", wrapAPIError(err))
+			return nil, -1, fmt.Errorf("cache lookup error: %w", wrapAPIError(err))
 		}
 		// make sure the cache contents matches the request messages hash
 		if cache.DisplayName != hash {
-			return nil, status.Errorf(status.ErrInvalidArgument, "invalid cache name: hash mismatch between cached content and request messages")
+			return nil, -1, status.Errorf(status.ErrInvalidArgument, "invalid cache name: hash mismatch between cached content and request messages")
 		}
 
-		return cache, nil
+		return cache, cs.endIndex, nil
 	}
 
 	if cs.ttl > 0 {
@@ -98,28 +99,31 @@ func handleCache(
 			Contents:    messages,
 		})
 		if err != nil {
-			return nil, fmt.Errorf("cache creation error: %w", wrapAPIError(err))
+			return nil, -1, fmt.Errorf("cache creation error: %w", wrapAPIError(err))
 		}
 	}
 
-	return cache, nil
+	if cache == nil {
+		return nil, -1, nil
+	}
+	return cache, cs.endIndex, nil
 }
 
 // messagesToCache collects all the messages that should be cached
 func messagesToCache(m []*ai.Message, cacheEndIdx int) ([]*genai.Content, error) {
 	var messagesToCache []*genai.Content
-	for i := cacheEndIdx; i >= 0; i-- {
-		m := m[i]
-		if m.Role == ai.RoleSystem {
+	for i := 0; i <= cacheEndIdx; i++ {
+		msg := m[i]
+		if msg.Role == ai.RoleSystem {
 			continue
 		}
-		parts, err := toGeminiParts(m.Content)
+		parts, err := toGeminiParts(msg.Content)
 		if err != nil {
 			return nil, err
 		}
 		messagesToCache = append(messagesToCache, &genai.Content{
 			Parts: parts,
-			Role:  string(m.Role),
+			Role:  string(msg.Role),
 		})
 	}
 	return messagesToCache, nil
diff --git a/go/plugins/googlegenai/cache_test.go b/go/plugins/googlegenai/cache_test.go
index 7be0cef2c9..501144e489 100644
--- a/go/plugins/googlegenai/cache_test.go
+++ b/go/plugins/googlegenai/cache_test.go
@@ -198,6 +198,75 @@ func TestExtractCacheConfig_InvalidCacheType(t *testing.T) {
 	}
 }
 
+func TestToGeminiContents_DropsCachedPrefix(t *testing.T) {
+	prefix := "this is a large stable prefix that should only live in the cache"
+	followUp := "what did I just say?"
+	req := &ai.ModelRequest{
+		Messages: []*ai.Message{
+			{
+				Role:    ai.RoleUser,
+				Content: []*ai.Part{{Text: prefix}},
+				Metadata: map[string]any{
+					"cache": map[string]any{"ttlSeconds": 3600},
+				},
+			},
+			{
+				Role:    ai.RoleUser,
+				Content: []*ai.Part{{Text: followUp}},
+			},
+		},
+	}
+
+	cs, err := findCacheMarker(req)
+	if err != nil {
+		t.Fatalf("findCacheMarker: %v", err)
+	}
+	if cs == nil || cs.endIndex != 0 {
+		t.Fatalf("cache marker = %#v, want endIndex=0", cs)
+	}
+
+	// Reproduce #6137: sending the whole request inline includes the prefix.
+	inline, err := toGeminiContents(req, -1)
+	if err != nil {
+		t.Fatalf("toGeminiContents(no skip): %v", err)
+	}
+	if len(inline) != 2 {
+		t.Fatalf("inline contents = %d, want 2 (bug: prefix sent with the follow-up)", len(inline))
+	}
+	if got := inline[0].Parts[0].Text; got != prefix {
+		t.Fatalf("inline[0] = %q, want cached prefix", got)
+	}
+
+	got, err := toGeminiContents(req, cs.endIndex)
+	if err != nil {
+		t.Fatalf("toGeminiContents(skip cached): %v", err)
+	}
+	if len(got) != 1 {
+		t.Fatalf("contents after cache boundary = %d, want 1", len(got))
+	}
+	if got[0].Parts[0].Text != followUp {
+		t.Errorf("remaining content = %q, want %q", got[0].Parts[0].Text, followUp)
+	}
+}
+
+func TestMessagesToCache_PreservesChronologicalOrder(t *testing.T) {
+	req := []*ai.Message{
+		{Role: ai.RoleUser, Content: []*ai.Part{{Text: "first"}}},
+		{Role: ai.RoleModel, Content: []*ai.Part{{Text: "second"}}},
+		{Role: ai.RoleUser, Content: []*ai.Part{{Text: "third, not cached"}}},
+	}
+	got, err := messagesToCache(req, 1)
+	if err != nil {
+		t.Fatalf("messagesToCache: %v", err)
+	}
+	if len(got) != 2 {
+		t.Fatalf("len = %d, want 2", len(got))
+	}
+	if got[0].Parts[0].Text != "first" || got[1].Parts[0].Text != "second" {
+		t.Errorf("order = %q, %q; want first, second", got[0].Parts[0].Text, got[1].Parts[0].Text)
+	}
+}
+
 func TestFindCacheMarker_NumericTTLForms(t *testing.T) {
 	// ttlSeconds is an int when set from Go code but arrives as float64 or
 	// json.Number after a JSON round-trip (dev UI, reflection server). All
diff --git a/go/plugins/googlegenai/gemini.go b/go/plugins/googlegenai/gemini.go
index 4cd231257c..f41b35af6d 100644
--- a/go/plugins/googlegenai/gemini.go
+++ b/go/plugins/googlegenai/gemini.go
@@ -159,7 +159,7 @@ func generate(
 	}
 	model = resolveVertexModelName(client, model)
 
-	cache, err := handleCache(ctx, client, input, model)
+	cache, cachedThrough, err := handleCache(ctx, client, input, model)
 	if err != nil {
 		return nil, err
 	}
@@ -169,7 +169,7 @@ func generate(
 		return nil, err
 	}
 
-	contents, err := toGeminiContents(input)
+	contents, err := toGeminiContents(input, cachedThrough)
 	if err != nil {
 		return nil, err
 	}
@@ -324,9 +324,17 @@ func mergeCandidateMetadata(dst, src *genai.Candidate) {
 // toGeminiContents converts the non-system messages of an [*ai.ModelRequest]
 // to a slice of [*genai.Content]. System messages are handled separately via
 // the request's system instruction.
-func toGeminiContents(input *ai.ModelRequest) ([]*genai.Content, error) {
+//
+// cachedThrough is the inclusive index of the last message stored in a
+// CachedContent resource, or -1 if no cache is in use. Messages at or before
+// that index must not be sent inline — the JS plugin slices the cached span
+// off the request, and sending it here would bill the prefix twice (#6137).
+func toGeminiContents(input *ai.ModelRequest, cachedThrough int) ([]*genai.Content, error) {
 	var contents []*genai.Content
-	for _, m := range input.Messages {
+	for i, m := range input.Messages {
+		if cachedThrough >= 0 && i <= cachedThrough {
+			continue
+		}
 		// system parts are handled separately
 		if m.Role == ai.RoleSystem {
 			continue
diff --git a/go/plugins/googlegenai/gemini_test.go b/go/plugins/googlegenai/gemini_test.go
index 2e63d762ba..4c3dd0a334 100644
--- a/go/plugins/googlegenai/gemini_test.go
+++ b/go/plugins/googlegenai/gemini_test.go
@@ -1438,7 +1438,7 @@ func TestToGeminiContents(t *testing.T) {
 		},
 	}
 
-	contents, err := toGeminiContents(input)
+	contents, err := toGeminiContents(input, -1)
 	if err != nil {
 		t.Fatalf("toGeminiContents failed: %v", err)
 	}