diff --git a/internal/tools/edit.go b/internal/tools/edit.go
index 9cd0414e..aa2fc76a 100644
--- a/internal/tools/edit.go
+++ b/internal/tools/edit.go
@@ -137,7 +137,7 @@ func (t *EditTool) Execute(ctx context.Context, args map[string]any) *Result {
 			if result != nil {
 				return result
 			}
-			mwr, err := t.memIntc.WriteFile(ctx, path, newContent)
+			mwr, err := t.memIntc.WriteFile(ctx, path, newContent, false)
 			if err != nil {
 				return ErrorResult(fmt.Sprintf("failed to write memory file: %v", err))
 			}
diff --git a/internal/tools/filesystem_write.go b/internal/tools/filesystem_write.go
index 0f80df9b..17bb1d62 100644
--- a/internal/tools/filesystem_write.go
+++ b/internal/tools/filesystem_write.go
@@ -121,7 +121,7 @@ func (t *WriteFileTool) Execute(ctx context.Context, args map[string]any) *Resul
 
 	// Virtual FS: route memory files to DB
 	if t.memIntc != nil {
-		if mwr, err := t.memIntc.WriteFile(ctx, path, content); mwr.Handled {
+		if mwr, err := t.memIntc.WriteFile(ctx, path, content, appendMode); mwr.Handled {
 			if err != nil {
 				return ErrorResult(fmt.Sprintf("failed to write memory file: %v", err))
 			}
@@ -129,6 +129,18 @@ func (t *WriteFileTool) Execute(ctx context.Context, args map[string]any) *Resul
 			if mwr.KGTriggered {
 				msg += "\n\n[Knowledge graph extraction triggered in background. The knowledge system may take a moment to fully update with new entities and relationships.]"
 			}
+			if mwr.PreviousContent != "" {
+				prev := mwr.PreviousContent
+				prevRunes := []rune(prev)
+				if len(prevRunes) > 4000 {
+					prev = string(prevRunes[:4000]) + "\n... (truncated)"
+				}
+				msg += fmt.Sprintf("\n\n⚠️ WARNING: This file had existing content (%d chars) that was replaced. "+
+					"If the old content below contains information not present in your new version, "+
+					"please re-write the file to merge both.\n\n"+
+					"--- PREVIOUS CONTENT ---\n%s\n--- END PREVIOUS CONTENT ---",
+					len([]rune(mwr.PreviousContent)), prev)
+			}
 			return SilentResult(msg)
 		}
 	}
diff --git a/internal/tools/memory_interceptor.go b/internal/tools/memory_interceptor.go
index f7bc2d3f..e9f78e1e 100644
--- a/internal/tools/memory_interceptor.go
+++ b/internal/tools/memory_interceptor.go
@@ -127,15 +127,16 @@ func (m *MemoryInterceptor) ReadFile(ctx context.Context, path string) (string,
 
 // MemoryWriteResult holds the outcome of a memory write operation.
 type MemoryWriteResult struct {
-	Handled     bool
-	KGTriggered bool
+	Handled         bool
+	KGTriggered     bool
+	PreviousContent string // non-empty if an existing document was overwritten (non-append)
 }
 
 // WriteFile attempts to write a memory file to the DB (+ re-index chunks for .md files).
-// Non-.md files are stored but NOT indexed/chunked/embedded,
-// matching TS behavior where only .md files are indexed.
-// Returns MemoryWriteResult with Handled=true if this was a memory path, KGTriggered=true if KG extraction was started.
-func (m *MemoryInterceptor) WriteFile(ctx context.Context, path, content string) (MemoryWriteResult, error) {
+// When appendMode is true, new content is appended to the existing document with a separator.
+// When appendMode is false and an existing document is overwritten with different content,
+// PreviousContent is populated in the result to allow callers to warn the agent.
+func (m *MemoryInterceptor) WriteFile(ctx context.Context, path, content string, appendMode bool) (MemoryWriteResult, error) {
 	ws := effectiveWorkspace(ctx, m.workspace)
 	if !isMemoryPath(path, ws) {
 		return MemoryWriteResult{}, nil
@@ -152,13 +153,28 @@ func (m *MemoryInterceptor) WriteFile(ctx context.Context, path, content string)
 	userID := store.MemoryUserID(ctx)
 	agentStr := agentID.String()
 
+	var previousContent string
+
+	if appendMode {
+		// Append: read existing content and merge with separator.
+		existing, err := m.memStore.GetDocument(ctx, agentStr, userID, relPath)
+		if err == nil && existing != "" {
+			content = existing + "\n\n---\n\n" + content
+		}
+	} else {
+		// Replace: capture previous content for overwrite warning.
+		oldContent, err := m.memStore.GetDocument(ctx, agentStr, userID, relPath)
+		if err == nil && oldContent != "" && oldContent != content {
+			previousContent = oldContent
+		}
+	}
+
 	// Write document to DB
 	if err := m.memStore.PutDocument(ctx, agentStr, userID, relPath, content); err != nil {
 		return MemoryWriteResult{Handled: true}, err
 	}
 
-	// Only index .md files (chunk + embed). Non-.md files (JSON, etc.) are stored
-	// as key-value documents but not searchable via memory_search.
+	// Only index .md files (chunk + embed).
 	if strings.HasSuffix(relPath, ".md") {
 		if err := m.memStore.IndexDocument(ctx, agentStr, userID, relPath); err != nil {
 			slog.Warn("memory interceptor: index failed after write", "path", path, "error", err)
@@ -173,7 +189,7 @@ func (m *MemoryInterceptor) WriteFile(ctx context.Context, path, content string)
 		kgTriggered = true
 	}
 
-	return MemoryWriteResult{Handled: true, KGTriggered: kgTriggered}, nil
+	return MemoryWriteResult{Handled: true, KGTriggered: kgTriggered, PreviousContent: previousContent}, nil
 }
 
 // ListFiles lists memory documents from the DB when path is the memory directory.