diff --git a/app/src/main/java/com/m57/hermescontrol/data/ws/SubagentRepository.kt b/app/src/main/java/com/m57/hermescontrol/data/ws/SubagentRepository.kt
index 1cda13ec..65ca1cb5 100644
--- a/app/src/main/java/com/m57/hermescontrol/data/ws/SubagentRepository.kt
+++ b/app/src/main/java/com/m57/hermescontrol/data/ws/SubagentRepository.kt
@@ -55,17 +55,14 @@ object SubagentRepository {
     suspend fun tailSubagent(
         sessionId: String,
         subagentId: String,
-        maxBytes: Int = 16384,
     ): SubagentTailResponse? {
-        if (subagentId.isBlank()) return null
+        // Contract (SubagentIdParams): session_id and subagent_id are required, extra keys forbidden (#1379).
+        if (subagentId.isBlank() || sessionId.isBlank()) return null
         val params =
-            mutableMapOf<String, Any>(
+            mapOf<String, Any>(
+                "session_id" to sessionId,
                 "subagent_id" to subagentId,
-                "max_bytes" to maxBytes,
             )
-        if (sessionId.isNotBlank()) {
-            params["session_id"] = sessionId
-        }
         val result =
             HermesWsClient
                 .request(
diff --git a/app/src/main/java/com/m57/hermescontrol/ui/chat/ChatSubagentsDelegate.kt b/app/src/main/java/com/m57/hermescontrol/ui/chat/ChatSubagentsDelegate.kt
index eaa56386..0685c0ea 100644
--- a/app/src/main/java/com/m57/hermescontrol/ui/chat/ChatSubagentsDelegate.kt
+++ b/app/src/main/java/com/m57/hermescontrol/ui/chat/ChatSubagentsDelegate.kt
@@ -27,7 +27,6 @@ class ChatSubagentsDelegate(
 ) {
     companion object {
         private const val TAG = "ChatSubagentsDelegate"
-        const val SUBAGENT_TAIL_MAX_BYTES = 16384
     }
 
     private var pollingJob: Job? = null
@@ -130,7 +129,6 @@ class ChatSubagentsDelegate(
                     subagentRepository.tailSubagent(
                         sessionId = sessionId,
                         subagentId = subagentId,
-                        maxBytes = SUBAGENT_TAIL_MAX_BYTES,
                     )
 
                 if (tailResponse != null) {
diff --git a/app/src/test/java/com/m57/hermescontrol/data/ws/SubagentRepositoryTest.kt b/app/src/test/java/com/m57/hermescontrol/data/ws/SubagentRepositoryTest.kt
index 10fc76d9..f1559c81 100644
--- a/app/src/test/java/com/m57/hermescontrol/data/ws/SubagentRepositoryTest.kt
+++ b/app/src/test/java/com/m57/hermescontrol/data/ws/SubagentRepositoryTest.kt
@@ -3,6 +3,14 @@ package com.m57.hermescontrol.data.ws
 import com.m57.hermescontrol.data.model.SubagentListItem
 import com.m57.hermescontrol.data.model.SubagentListResponse
 import com.m57.hermescontrol.data.model.SubagentTailResponse
+import io.mockk.every
+import io.mockk.mockkObject
+import io.mockk.slot
+import io.mockk.unmockkAll
+import io.mockk.verify
+import kotlinx.coroutines.CompletableDeferred
+import kotlinx.coroutines.runBlocking
+import org.junit.After
 import org.junit.Assert.assertEquals
 import org.junit.Assert.assertFalse
 import org.junit.Assert.assertNotNull
@@ -11,6 +19,34 @@ import org.junit.Assert.assertTrue
 import org.junit.Test
 
 class SubagentRepositoryTest {
+    @After
+    fun tearDown() {
+        unmockkAll()
+    }
+
+    @Test
+    fun tailSubagentSendsExactlySessionIdAndSubagentId() {
+        // #1379: the contract forbids extra keys (max_bytes) and requires session_id.
+        val captured = slot<Map<String, Any>>()
+        mockkObject(HermesWsClient)
+        every { HermesWsClient.request(WsMethods.SUBAGENT_TAIL, capture(captured), any(), any()) } returns
+            CompletableDeferred<Any?>(mapOf("subagent_id" to "sub-1"))
+
+        runBlocking { SubagentRepository.tailSubagent("s1", "sub-1") }
+
+        assertEquals(mapOf<String, Any>("session_id" to "s1", "subagent_id" to "sub-1"), captured.captured)
+    }
+
+    @Test
+    fun tailSubagentSkipsTheCallWhenAnIdIsBlank() {
+        mockkObject(HermesWsClient)
+        runBlocking {
+            assertNull(SubagentRepository.tailSubagent("", "sub-1"))
+            assertNull(SubagentRepository.tailSubagent("s1", " "))
+        }
+        verify(exactly = 0) { HermesWsClient.request(any(), any(), any(), any()) }
+    }
+
     @Test
     fun testWsMethodsConstants() {
         assertEquals("subagent.list", WsMethods.SUBAGENT_LIST)
diff --git a/app/src/test/java/com/m57/hermescontrol/ui/chat/ChatSubagentsDelegateTest.kt b/app/src/test/java/com/m57/hermescontrol/ui/chat/ChatSubagentsDelegateTest.kt
index a3179f5d..cf4a55ab 100644
--- a/app/src/test/java/com/m57/hermescontrol/ui/chat/ChatSubagentsDelegateTest.kt
+++ b/app/src/test/java/com/m57/hermescontrol/ui/chat/ChatSubagentsDelegateTest.kt
@@ -448,7 +448,7 @@ class ChatSubagentsDelegateTest {
                         ),
                 )
 
-            coEvery { mockRepository.tailSubagent("session-123", "sub-1", 16384) } returnsMany
+            coEvery { mockRepository.tailSubagent("session-123", "sub-1") } returnsMany
                 listOf(
                     SubagentTailResponse(subagentId = "sub-1", text = "Line 1\n", truncated = false),
                     SubagentTailResponse(subagentId = "sub-1", text = "Line 1\nLine 2\n", truncated = false),
@@ -485,7 +485,7 @@ class ChatSubagentsDelegateTest {
                         ),
                 )
 
-            coEvery { mockRepository.tailSubagent("session-123", "sub-1", 16384) } returns
+            coEvery { mockRepository.tailSubagent("session-123", "sub-1") } returns
                 SubagentTailResponse(subagentId = "sub-1", text = "Finished output", truncated = false)
 
             delegate.inspectSubagentTranscript("sub-1")
@@ -507,7 +507,7 @@ class ChatSubagentsDelegateTest {
 
             advanceTimeBy(3000)
             // Verify tailSubagent was called at most once or twice, not indefinitely
-            coVerify(atMost = 2) { mockRepository.tailSubagent("session-123", "sub-1", 16384) }
+            coVerify(atMost = 2) { mockRepository.tailSubagent("session-123", "sub-1") }
             delegate.closeSubagentTranscript()
         }
 
@@ -526,7 +526,7 @@ class ChatSubagentsDelegateTest {
                         ),
                 )
 
-            coEvery { mockRepository.tailSubagent("session-123", "sub-1", 16384) } returns
+            coEvery { mockRepository.tailSubagent("session-123", "sub-1") } returns
                 SubagentTailResponse(subagentId = "sub-1", text = "Output")
 
             delegate.inspectSubagentTranscript("sub-1")
@@ -553,7 +553,7 @@ class ChatSubagentsDelegateTest {
                         ),
                 )
 
-            coEvery { mockRepository.tailSubagent("session-123", "sub-1", 16384) } throws
+            coEvery { mockRepository.tailSubagent("session-123", "sub-1") } throws
                 RuntimeException("Gateway connection error")
 
             delegate.inspectSubagentTranscript("sub-1")
@@ -565,7 +565,7 @@ class ChatSubagentsDelegateTest {
             assertFalse(transcript?.isLoading == true)
 
             // Now recover on retry
-            coEvery { mockRepository.tailSubagent("session-123", "sub-1", 16384) } returns
+            coEvery { mockRepository.tailSubagent("session-123", "sub-1") } returns
                 SubagentTailResponse(subagentId = "sub-1", text = "Recovered logs")
 
             delegate.retryTranscript()