diff --git a/app/src/main/java/com/m57/hermescontrol/ui/chat/ChatViewModel.kt b/app/src/main/java/com/m57/hermescontrol/ui/chat/ChatViewModel.kt
index 5a7b873e..0bce0c20 100644
--- a/app/src/main/java/com/m57/hermescontrol/ui/chat/ChatViewModel.kt
+++ b/app/src/main/java/com/m57/hermescontrol/ui/chat/ChatViewModel.kt
@@ -4,6 +4,7 @@ import android.app.Application
 import android.util.Log
 import androidx.lifecycle.AndroidViewModel
 import androidx.lifecycle.viewModelScope
+import com.google.gson.Gson
 import com.m57.hermescontrol.data.local.AuthManager
 import com.m57.hermescontrol.data.local.HermesDatabase
 import com.m57.hermescontrol.data.local.toEntity
@@ -371,10 +372,11 @@ class ChatViewModel(
         var orphanToPersist: ChatMessage? = null
         val sessionId = _uiState.value.currentSessionId
 
+        val contentJson = event.data?.let { Gson().toJson(it) } ?: ""
         val toolMessage =
             ChatMessage(
                 role = MessageRole.TOOL,
-                content = event.data.toString(),
+                content = contentJson,
                 toolName = event.name,
                 toolStatus = ToolStatus.RUNNING,
             )
@@ -418,10 +420,11 @@ class ChatViewModel(
                         it.toolStatus == ToolStatus.RUNNING
                 }
             if (toolIdx >= 0) {
+                val contentJson = event.data?.let { Gson().toJson(it) } ?: ""
                 val updated =
                     messages[toolIdx].copy(
                         toolStatus = ToolStatus.COMPLETED,
-                        content = event.data.toString(),
+                        content = contentJson,
                     )
                 messages[toolIdx] = updated
                 completedTool = updated
diff --git a/app/src/test/java/com/m57/hermescontrol/ui/chat/ChatViewModelTest.kt b/app/src/test/java/com/m57/hermescontrol/ui/chat/ChatViewModelTest.kt
index afb79907..23d9223c 100644
--- a/app/src/test/java/com/m57/hermescontrol/ui/chat/ChatViewModelTest.kt
+++ b/app/src/test/java/com/m57/hermescontrol/ui/chat/ChatViewModelTest.kt
@@ -562,4 +562,52 @@ class ChatViewModelTest {
             assertEquals("Second response segment", state.streamingMessage?.content)
             assertTrue(state.streamingMessage?.isStreaming == true)
         }
+
+    @Test
+    fun testToolExecution_serializesDataAsJson() =
+        runTest {
+            val viewModel = ChatViewModel(app, startCleanup = false)
+            advanceUntilIdle()
+
+            // Setup active session
+            var createReqId = ""
+            every { HermesWsClient.send(WsMethods.SESSION_CREATE, any(), any()) } answers {
+                createReqId = "create-req-tool-json-test"
+                val onSent = arg<((String) -> Unit)?>(2)
+                onSent?.invoke(createReqId)
+                createReqId
+            }
+            mockEventsFlow.emit(WsEvent.GatewayReady(null))
+            advanceUntilIdle()
+            mockEventsFlow.emit(WsEvent.RpcResult(createReqId, mapOf("session_id" to "session-123")))
+            advanceUntilIdle()
+
+            // Start tool call
+            mockEventsFlow.emit(
+                WsEvent.ToolStart(
+                    name = "calculator",
+                    data =
+                        mapOf(
+                            "input" to "2+2",
+                            "nested" to mapOf("key" to "value"),
+                        ),
+                ),
+            )
+            advanceUntilIdle()
+
+            var state = viewModel.uiState.value
+            assertEquals(2, state.messages.size)
+            assertEquals(MessageRole.TOOL, state.messages[1].role)
+            assertEquals("{\"input\":\"2+2\",\"nested\":{\"key\":\"value\"}}", state.messages[1].content)
+            assertEquals(ToolStatus.RUNNING, state.messages[1].toolStatus)
+
+            // Complete tool call
+            mockEventsFlow.emit(WsEvent.ToolComplete("calculator", mapOf("result" to "4", "exit_code" to 0)))
+            advanceUntilIdle()
+
+            state = viewModel.uiState.value
+            assertEquals(2, state.messages.size)
+            assertEquals("{\"result\":\"4\",\"exit_code\":0}", state.messages[1].content)
+            assertEquals(ToolStatus.COMPLETED, state.messages[1].toolStatus)
+        }
 }