diff --git a/src/session.ts b/src/session.ts
index 4772af9..3c9d5d6 100644
--- a/src/session.ts
+++ b/src/session.ts
@@ -179,8 +179,10 @@ export class Session implements AsyncDisposable {
       }
     }
 
-    sessionLog("init", "ERROR: transport closed before init message received");
-    throw new Error("Failed to initialize session - no init message received");
+    const stderr = this.transport.getStderr();
+    const detail = stderr ? `\nCLI stderr:\n${stderr}` : '';
+    sessionLog("init", `ERROR: transport closed before init message received${detail}`);
+    throw new Error(`Failed to initialize session - no init message received${detail}`);
   }
 
   /**
diff --git a/src/transport.ts b/src/transport.ts
index a3d0f03..1e7351f 100644
--- a/src/transport.ts
+++ b/src/transport.ts
@@ -198,6 +198,7 @@ export class SubprocessTransport {
   private agentId?: string;
   private wireMessageCount = 0;
   private lastMessageAt = 0;
+  private stderrLines: string[] = [];
 
   constructor(
     private options: InternalSessionOptions = {}
@@ -247,11 +248,13 @@ export class SubprocessTransport {
     });
 
     // Log stderr for debugging (CLI errors, auth failures, etc.)
+    // Also buffer lines so session.initialize() can include them in errors.
     if (this.process.stderr) {
       this.process.stderr.on("data", (data: Buffer) => {
         const msg = data.toString().trim();
         if (msg) {
           console.error("[letta-code-sdk] CLI stderr:", msg);
+          this.stderrLines.push(msg);
         }
       });
     }
@@ -356,6 +359,11 @@ export class SubprocessTransport {
     return this.closed;
   }
 
+  /** Return buffered stderr output from the CLI subprocess. */
+  getStderr(): string {
+    return this.stderrLines.join("\n");
+  }
+
   private handleMessage(msg: WireMessage): void {
     this.wireMessageCount++;
     this.lastMessageAt = Date.now();