diff --git a/src/servers/api/chat/types.ts b/src/servers/api/chat/types.ts
index feb38dd2..b383e37d 100644
--- a/src/servers/api/chat/types.ts
+++ b/src/servers/api/chat/types.ts
@@ -115,6 +115,12 @@ export type ServerMessage =
       type: 'result';
       sessionId: string;
       cost: MessageCost;
+      // Claude's OWN session uuid — the name of the transcript file under ~/.claude/projects/<slug>/.
+      // Distinct from `sessionId`, which is the key officer made up. Until this crossed the wire the
+      // platform had no way to address a session it had started: the mapping was captured inside the
+      // sidecar (claude-manager's setClaudeSession) and never left it. Absent if the harness didn't
+      // report one.
+      claudeSessionId?: string;
     }
   | {
       type: 'sync:messages';
@@ -177,6 +183,7 @@ export type ChatEvent =
   | {
       type: 'result';
       cost: MessageCost;
+      claudeSessionId?: string;
     }
   | { type: 'error'; message: string }
   | { type: 'stopped' }
diff --git a/src/servers/sidecar/claude/stream-parser.ts b/src/servers/sidecar/claude/stream-parser.ts
index 2790ff4a..ae35bb5c 100644
--- a/src/servers/sidecar/claude/stream-parser.ts
+++ b/src/servers/sidecar/claude/stream-parser.ts
@@ -115,7 +115,9 @@ function handleResult(
     callbacks.onSessionId(sessionId);
   }
 
-  callbacks.onEvent({ type: 'result', cost });
+  // Carried on the event as well as into sidecar-local state: the callback only feeds `--resume` here,
+  // so without this the platform never learns which transcript file a session it started ended up in.
+  callbacks.onEvent({ type: 'result', cost, claudeSessionId: sessionId });
 }
 
 function handleSystem(msg: Record<string, unknown>, callbacks: StreamParserCallbacks): void {
diff --git a/src/servers/sidecar/claude/turn-stream.ts b/src/servers/sidecar/claude/turn-stream.ts
index 51bd79b9..31032324 100644
--- a/src/servers/sidecar/claude/turn-stream.ts
+++ b/src/servers/sidecar/claude/turn-stream.ts
@@ -67,7 +67,13 @@ export function createTurnStream(sessionId: string): TurnStream {
         ];
 
       case 'result':
-        return [...flush(), { msg: { type: 'result', sessionId, cost: event.cost }, durable: true }];
+        return [
+          ...flush(),
+          {
+            msg: { type: 'result', sessionId, cost: event.cost, claudeSessionId: event.claudeSessionId },
+            durable: true,
+          },
+        ];
 
       case 'error':
         return [{ msg: { type: 'error', message: event.message }, durable: true }];