diff --git a/apps/cloud/src/mcp/session-durable-object.ts b/apps/cloud/src/mcp/session-durable-object.ts
index de2ef2a58..70d2f29ce 100644
--- a/apps/cloud/src/mcp/session-durable-object.ts
+++ b/apps/cloud/src/mcp/session-durable-object.ts
@@ -50,7 +50,7 @@ import {
   type DbServiceShape,
 } from "../db/db";
 import { CloudExecutionStackLayer, makeExecutionStack } from "../engine/execution-stack";
-import { DoTelemetryLive } from "../observability/telemetry";
+import { DoTelemetryLive, flushTracerProvider } from "../observability/telemetry";
 import { captureCause as reportCause } from "../observability";
 
 // Re-export the shared types so existing cloud importers
@@ -196,6 +196,9 @@ export class McpSessionDO extends McpSessionDOBase<CloudSessionDbHandle> {
       );
       // Build the description here so the postgres query it runs
       // (`executor.sources.list`) lands as a child of `McpSessionDO.createRuntime`.
+      // It also tags the span with this org's source/connector inventory (ids,
+      // kinds, plugin ids, connection counts) — see `buildExecuteDescription` —
+      // so a failing init names *what* it was resolving without re-listing.
       // host-mcp would otherwise call `Effect.runPromise(engine.getDescription)`
       // at its async MCP-SDK boundary and orphan the sub-span.
       const description = yield* buildExecuteDescription(executor);
@@ -240,4 +243,13 @@ export class McpSessionDO extends McpSessionDOBase<CloudSessionDbHandle> {
   protected override captureCause(cause: Cause.Cause<unknown>): void {
     reportCause(cause);
   }
+
+  // Force-export the DO isolate's buffered spans before the RPC settles, so a
+  // dying init/handleRequest still ships its own spans (and the exception +
+  // stack recorded on them) — not just the worker-side `mcp.do.*` span. The
+  // base wraps each entrypoint's outermost effect in an `ensuring` that awaits
+  // this after the span has ended and the SimpleSpanProcessor fired its export.
+  protected override flushTelemetry(): Promise<void> {
+    return flushTracerProvider();
+  }
 }
diff --git a/packages/core/execution/src/description.ts b/packages/core/execution/src/description.ts
index 97ad689ff..4b2e5939a 100644
--- a/packages/core/execution/src/description.ts
+++ b/packages/core/execution/src/description.ts
@@ -25,6 +25,24 @@ export const buildExecuteDescription = (executor: Executor): Effect.Effect<strin
     yield* Effect.annotateCurrentSpan({
       "executor.source_count": sources.length,
       "schema.kind": "execute",
+      // Source/connector inventory so a failing session build (which runs this
+      // during init) names *what* it was resolving: empty/OpenAPI-only scopes
+      // build cleanly, scopes with remote MCP connectors are the ones that fail.
+      "executor.source_ids": sources
+        .map((source) => source.id)
+        .slice(0, 50)
+        .join(","),
+      "executor.source_kinds": [...new Set(sources.map((source) => source.kind))].join(","),
+      "executor.source_plugin_ids": [...new Set(sources.map((source) => source.pluginId))].join(
+        ",",
+      ),
+      "executor.connection_count": sources.reduce(
+        (total, source) => total + source.connectionIds.length,
+        0,
+      ),
+      "executor.sources_with_connection": sources.filter(
+        (source) => source.connectionIds.length > 0,
+      ).length,
     });
 
     return description;
diff --git a/packages/hosts/cloudflare/src/mcp/session-durable-object.ts b/packages/hosts/cloudflare/src/mcp/session-durable-object.ts
index 8f12c9ffb..943368ac6 100644
--- a/packages/hosts/cloudflare/src/mcp/session-durable-object.ts
+++ b/packages/hosts/cloudflare/src/mcp/session-durable-object.ts
@@ -225,6 +225,47 @@ export abstract class McpSessionDOBase<
   /** Optional error seam: report a fatal request cause (cloud → Sentry). */
   protected captureCause(_cause: Cause.Cause<unknown>): void {}
 
+  /** Optional flush seam: force-export buffered spans before the DO RPC
+   *  settles. Default is a no-op; cloud overrides it with the tracer
+   *  provider's `forceFlush`. Without this, a DO whose Effect dies tears the
+   *  isolate down with the SimpleSpanProcessor's in-flight export `fetch`
+   *  still pending — so the failing method's OWN spans (and the exception +
+   *  stack the Effect tracer records on them) never reach the collector, and
+   *  only the worker-side `mcp.do.*` span (which carries just the RPC-boundary
+   *  message, no real stack) survives. Flushing in an `ensuring` finalizer —
+   *  placed OUTSIDE the span so the span has already ended and the
+   *  SimpleSpanProcessor has fired its export — lets the flush await that
+   *  export before the RPC rejects. */
+  protected flushTelemetry(): Promise<void> {
+    return Promise.resolve();
+  }
+
+  /** Mirror an error cause onto the active span as top-level attributes. The
+   *  Effect OTel tracer already records the cause as an `exception` span
+   *  EVENT (`exception.stacktrace` = the full `Cause.pretty` rendering), but
+   *  span events are awkward to query in Axiom and the worker-side spans never
+   *  see them. Copying type/message/stack onto plain span attributes makes the
+   *  failing frame a one-field APL lookup on the span row itself. */
+  private recordCauseOnSpan(cause: Cause.Cause<unknown>): Effect.Effect<void> {
+    const errors = Cause.prettyErrors(cause);
+    // No `Error` reasons means a pure interruption (or empty cause) — nothing
+    // worth annotating, and we don't want to flag interrupts as failures.
+    if (errors.length === 0) return Effect.void;
+    const first = errors[0];
+    return Effect.annotateCurrentSpan({
+      "exception.type": first?.name ?? "Error",
+      "exception.message": first?.message ?? "unknown",
+      "exception.stacktrace": Cause.pretty(cause),
+    });
+  }
+
+  /** Flush DO-side spans once the wrapped method (incl. its span) has ended,
+   *  whether it succeeded or died. Apply as the OUTERMOST pipe step. */
+  private withSpanFlush<A, E>(effect: Effect.Effect<A, E>): Effect.Effect<A, E> {
+    const self = this;
+    return effect.pipe(Effect.ensuring(Effect.promise(() => self.flushTelemetry())));
+  }
+
   /** The session id — equal to this DO's id. */
   protected get sessionId(): string {
     return this.ctx.id.toString();
@@ -546,6 +587,7 @@ export abstract class McpSessionDOBase<
         (eff) => this.withTelemetry(eff, incoming),
         // oxlint-disable-next-line executor/no-effect-escape-hatch -- boundary: Durable Object init method can only reject its Promise
         Effect.orDie,
+        (eff) => self.withSpanFlush(eff),
       ),
     );
   }
@@ -584,8 +626,11 @@ export abstract class McpSessionDOBase<
       );
     }).pipe(
       Effect.tapCause((cause) =>
-        Effect.sync(() => {
-          console.error("[mcp-session] init failed:", cause);
+        Effect.gen(function* () {
+          console.error("[mcp-session] init failed:", Cause.pretty(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);
         }),
       ),
       Effect.catchCause((cause) =>
@@ -637,6 +682,11 @@ export abstract class McpSessionDOBase<
         ),
       );
     }).pipe(
+      // Cold-restore failures (`restoreRuntimeFromStorage` is `orDie`'d and is
+      // NOT under the handled `dispatchAuthorizedRequest` branch) die straight
+      // through here — annotate the handleRequest span with their stack before
+      // it ends so the flushed span names the frame.
+      Effect.tapCause((cause) => self.recordCauseOnSpan(cause)),
       Effect.withSpan("McpSessionDO.handleRequest", {
         attributes: {
           "mcp.request.method": request.method,
@@ -644,6 +694,7 @@ export abstract class McpSessionDOBase<
         },
       }),
       (eff) => this.withTelemetry(eff, incoming),
+      (eff) => self.withSpanFlush(eff),
     );
     return Effect.runPromise(program);
   }
@@ -678,6 +729,7 @@ export abstract class McpSessionDOBase<
         (eff) => this.withTelemetry(eff, incoming),
         // oxlint-disable-next-line executor/no-effect-escape-hatch -- boundary: DO RPC exposes Promise results
         Effect.orDie,
+        (eff) => self.withSpanFlush(eff),
       ),
     );
   }
@@ -752,6 +804,7 @@ export abstract class McpSessionDOBase<
         (eff) => this.withTelemetry(eff, incoming),
         // oxlint-disable-next-line executor/no-effect-escape-hatch -- boundary: DO RPC exposes Promise results
         Effect.orDie,
+        (eff) => self.withSpanFlush(eff),
       ),
     );
   }
@@ -814,9 +867,10 @@ export abstract class McpSessionDOBase<
       return response;
     }).pipe(
       Effect.catchCause((cause) =>
-        Effect.sync(() => {
+        Effect.gen(function* () {
           console.error("[mcp-session] handleRequest error:", Cause.pretty(cause));
           self.captureCause(cause);
+          yield* self.recordCauseOnSpan(cause);
           return jsonRpcError(500, -32603, "Internal error");
         }),
       ),
@@ -827,6 +881,7 @@ export abstract class McpSessionDOBase<
     const program = Effect.promise(() => this.runAlarm()).pipe(
       Effect.withSpan("McpSessionDO.alarm"),
       (eff) => this.withTelemetry(eff),
+      (eff) => this.withSpanFlush(eff),
     );
     return Effect.runPromise(program);
   }
@@ -836,6 +891,7 @@ export abstract class McpSessionDOBase<
       Effect.promise(() => this.cleanup()).pipe(
         Effect.withSpan("McpSessionDO.clearSession"),
         (eff) => this.withTelemetry(eff, incoming),
+        (eff) => this.withSpanFlush(eff),
       ),
     );
   }