diff --git a/apps/cloud/src/server.ts b/apps/cloud/src/server.ts
index e941de78d..0ac72c93f 100644
--- a/apps/cloud/src/server.ts
+++ b/apps/cloud/src/server.ts
@@ -22,11 +22,12 @@ const sentryOptions = (env: Env) => ({
   enableLogs: true,
   sendDefaultPii: true,
   skipOpenTelemetrySetup: true,
-  // Our DO methods (init/handleRequest/alarm) live on the prototype, not on
-  // the instance. Sentry's default DO auto-wrap only visits own properties,
-  // which misses prototype methods — so errors thrown inside init() never
-  // reach Sentry. This flag opts into prototype-method instrumentation.
-  instrumentPrototypeMethods: true,
+  // NOTE: do NOT enable `instrumentPrototypeMethods`. It walks the DO prototype
+  // and reads every property — including accessors — to find methods to wrap,
+  // which invokes the `sessionId` getter with `this` bound to the prototype
+  // (where `ctx` is undefined) and throws during construction, 500ing every
+  // session create / cold restore. The DO captures its own errors via the
+  // `captureCause` seam (→ Sentry) instead.
 });
 
 // ---------------------------------------------------------------------------
diff --git a/packages/hosts/cloudflare/src/mcp/session-durable-object.ts b/packages/hosts/cloudflare/src/mcp/session-durable-object.ts
index 943368ac6..250911f31 100644
--- a/packages/hosts/cloudflare/src/mcp/session-durable-object.ts
+++ b/packages/hosts/cloudflare/src/mcp/session-durable-object.ts
@@ -628,6 +628,10 @@ export abstract class McpSessionDOBase<
       Effect.tapCause((cause) =>
         Effect.gen(function* () {
           console.error("[mcp-session] init failed:", Cause.pretty(cause));
+          // Report to the host's error sink (cloud → Sentry). init() runs on
+          // the prototype, which Sentry's auto-wrap doesn't cover, so capture
+          // here explicitly rather than relying on prototype instrumentation.
+          self.captureCause(cause);
           // Annotate `McpSessionDO.init` (the active span here — `doInit` opens
           // none of its own) so the surviving, flushed span names the frame.
           yield* self.recordCauseOnSpan(cause);