diff --git a/pkg/agent/hooks.go b/pkg/agent/hooks.go
index 69336b89f..2d60e8120 100644
--- a/pkg/agent/hooks.go
+++ b/pkg/agent/hooks.go
@@ -409,6 +409,7 @@ func systemMessageFingerprints(messages []providers.Message) []systemMessageFing
 		if msg.Role != "system" {
 			continue
 		}
+		msg = providerVisibleMessage(msg)
 		fingerprints = append(fingerprints, systemMessageFingerprint{
 			Index:   i,
 			Message: cloneProviderMessages([]providers.Message{msg})[0],
@@ -418,7 +419,32 @@ func systemMessageFingerprints(messages []providers.Message) []systemMessageFing
 }
 
 func llmHookToolDefinitionsUnchanged(before, after []providers.ToolDefinition) bool {
-	return reflect.DeepEqual(cloneToolDefinitions(before), cloneToolDefinitions(after))
+	return reflect.DeepEqual(providerVisibleToolDefinitions(before), providerVisibleToolDefinitions(after))
+}
+
+func providerVisibleMessage(msg providers.Message) providers.Message {
+	msg.PromptLayer = ""
+	msg.PromptSlot = ""
+	msg.PromptSource = ""
+	if len(msg.SystemParts) > 0 {
+		msg.SystemParts = append([]providers.ContentBlock(nil), msg.SystemParts...)
+		for i := range msg.SystemParts {
+			msg.SystemParts[i].PromptLayer = ""
+			msg.SystemParts[i].PromptSlot = ""
+			msg.SystemParts[i].PromptSource = ""
+		}
+	}
+	return msg
+}
+
+func providerVisibleToolDefinitions(defs []providers.ToolDefinition) []providers.ToolDefinition {
+	cloned := cloneToolDefinitions(defs)
+	for i := range cloned {
+		cloned[i].PromptLayer = ""
+		cloned[i].PromptSlot = ""
+		cloned[i].PromptSource = ""
+	}
+	return cloned
 }
 
 func (hm *HookManager) BeforeTool(
diff --git a/pkg/agent/hooks_test.go b/pkg/agent/hooks_test.go
index a936296dd..65400e4b8 100644
--- a/pkg/agent/hooks_test.go
+++ b/pkg/agent/hooks_test.go
@@ -2,6 +2,7 @@ package agent
 
 import (
 	"context"
+	"encoding/json"
 	"errors"
 	"os"
 	"strings"
@@ -186,6 +187,46 @@ func (h *llmUserAppendHook) AfterLLM(
 	return resp.Clone(), HookDecision{Action: HookActionContinue}, nil
 }
 
+type llmJSONRoundTripUserAppendHook struct{}
+
+type jsonRoundTripLLMHookRequest struct {
+	Model    string                     `json:"model"`
+	Messages []providers.Message        `json:"messages,omitempty"`
+	Tools    []providers.ToolDefinition `json:"tools,omitempty"`
+}
+
+func (h *llmJSONRoundTripUserAppendHook) BeforeLLM(
+	ctx context.Context,
+	req *LLMHookRequest,
+) (*LLMHookRequest, HookDecision, error) {
+	payload := jsonRoundTripLLMHookRequest{
+		Model:    req.Model,
+		Messages: req.Messages,
+		Tools:    req.Tools,
+	}
+	data, err := json.Marshal(payload)
+	if err != nil {
+		return nil, HookDecision{}, err
+	}
+	var decoded jsonRoundTripLLMHookRequest
+	if err := json.Unmarshal(data, &decoded); err != nil {
+		return nil, HookDecision{}, err
+	}
+	next := req.Clone()
+	next.Model = decoded.Model
+	next.Messages = decoded.Messages
+	next.Tools = decoded.Tools
+	next.Messages = append(next.Messages, providers.Message{Role: "user", Content: "json extra user context"})
+	return next, HookDecision{Action: HookActionModify}, nil
+}
+
+func (h *llmJSONRoundTripUserAppendHook) AfterLLM(
+	ctx context.Context,
+	resp *LLMHookResponse,
+) (*LLMHookResponse, HookDecision, error) {
+	return resp.Clone(), HookDecision{Action: HookActionContinue}, nil
+}
+
 type llmToolRewriteHook struct{}
 
 func (h *llmToolRewriteHook) BeforeLLM(
@@ -274,6 +315,58 @@ func TestHookManager_BeforeLLMAllowsNonSystemMessageMutation(t *testing.T) {
 	}
 }
 
+func TestHookManager_BeforeLLMAllowsJSONRoundTripNonSystemMessageMutation(t *testing.T) {
+	hm := NewHookManager(nil)
+	if err := hm.Mount(NamedHook("json-append-user", &llmJSONRoundTripUserAppendHook{})); err != nil {
+		t.Fatalf("Mount() error = %v", err)
+	}
+
+	req := &LLMHookRequest{
+		Model: "model",
+		Messages: []providers.Message{
+			{
+				Role:         "system",
+				Content:      "system",
+				PromptLayer:  string(PromptLayerKernel),
+				PromptSlot:   string(PromptSlotIdentity),
+				PromptSource: string(PromptSourceKernel),
+				SystemParts: []providers.ContentBlock{
+					{
+						Type:         "text",
+						Text:         "system",
+						CacheControl: &providers.CacheControl{Type: "ephemeral"},
+						PromptLayer:  string(PromptLayerKernel),
+						PromptSlot:   string(PromptSlotIdentity),
+						PromptSource: string(PromptSourceKernel),
+					},
+				},
+			},
+			{Role: "user", Content: "hello"},
+		},
+		Tools: []providers.ToolDefinition{
+			{
+				Type: "function",
+				Function: providers.ToolFunctionDefinition{
+					Name:        "mcp_github_create_issue",
+					Description: "create issue",
+					Parameters:  map[string]any{"type": "object"},
+				},
+				PromptLayer:  string(PromptLayerCapability),
+				PromptSlot:   string(PromptSlotMCP),
+				PromptSource: "mcp:github",
+			},
+		},
+	}
+
+	got, _ := hm.BeforeLLM(context.Background(), req)
+	if len(got.Messages) != 3 {
+		t.Fatalf("messages len = %d, want 3", len(got.Messages))
+	}
+	if got.Messages[2].Role != "user" || got.Messages[2].Content != "json extra user context" {
+		t.Fatalf("appended message = %#v, want json extra user context", got.Messages[2])
+	}
+}
+
 func TestHookManager_BeforeLLMControlsToolDefinitionMutation(t *testing.T) {
 	hm := NewHookManager(nil)
 	if err := hm.Mount(NamedHook("rewrite-tool", &llmToolRewriteHook{})); err != nil {
diff --git a/pkg/agent/prompt_test.go b/pkg/agent/prompt_test.go
index b3f609610..b76b0040d 100644
--- a/pkg/agent/prompt_test.go
+++ b/pkg/agent/prompt_test.go
@@ -135,8 +135,14 @@ func TestBuildMessagesFromPrompt_AttachesInternalPromptMetadata(t *testing.T) {
 		switch part.PromptSource {
 		case string(PromptSourceRuntime):
 			hasRuntime = true
+			if part.CacheControl != nil {
+				t.Fatalf("runtime cache control = %#v, want nil", part.CacheControl)
+			}
 		case string(PromptSourceSummary):
 			hasSummary = true
+			if part.CacheControl != nil {
+				t.Fatalf("summary cache control = %#v, want nil", part.CacheControl)
+			}
 		}
 	}
 	if !hasRuntime {
@@ -181,6 +187,9 @@ func TestContextBuilder_CollectsToolDiscoveryContributor(t *testing.T) {
 			if part.PromptLayer != string(PromptLayerCapability) || part.PromptSlot != string(PromptSlotTooling) {
 				t.Fatalf("tool discovery metadata = %#v, want capability/tooling", part)
 			}
+			if part.CacheControl == nil || part.CacheControl.Type != "ephemeral" {
+				t.Fatalf("tool discovery cache control = %#v, want ephemeral", part.CacheControl)
+			}
 		}
 	}
 	if !found {
@@ -213,6 +222,9 @@ func TestContextBuilder_CollectsMCPServerContributor(t *testing.T) {
 			if part.PromptLayer != string(PromptLayerCapability) || part.PromptSlot != string(PromptSlotMCP) {
 				t.Fatalf("mcp metadata = %#v, want capability/mcp", part)
 			}
+			if part.CacheControl == nil || part.CacheControl.Type != "ephemeral" {
+				t.Fatalf("mcp cache control = %#v, want ephemeral", part.CacheControl)
+			}
 		}
 	}
 	if !found {
diff --git a/pkg/agent/prompt_turn.go b/pkg/agent/prompt_turn.go
index 7b7d295c1..588a8f00f 100644
--- a/pkg/agent/prompt_turn.go
+++ b/pkg/agent/prompt_turn.go
@@ -49,6 +49,9 @@ func promptOverlaysForOptions(opts processOptions) []PromptPart {
 }
 
 func promptContentBlock(part PromptPart, cache *providers.CacheControl) providers.ContentBlock {
+	if cache == nil {
+		cache = cacheControlForPromptPart(part)
+	}
 	return providers.ContentBlock{
 		Type:         "text",
 		Text:         part.Content,
@@ -59,6 +62,15 @@ func promptContentBlock(part PromptPart, cache *providers.CacheControl) provider
 	}
 }
 
+func cacheControlForPromptPart(part PromptPart) *providers.CacheControl {
+	switch part.Cache {
+	case PromptCacheEphemeral:
+		return &providers.CacheControl{Type: "ephemeral"}
+	default:
+		return nil
+	}
+}
+
 func promptMessageWithMetadata(
 	msg providers.Message,
 	layer PromptLayer,