RhysSullivan · Aug 17, 2026 · Aug 17, 2026
diff --git a/.changeset/mcp-client-span-attribution.md b/.changeset/mcp-client-span-attribution.md
@@ -0,0 +1,8 @@
+---
+"@executor-js/host-mcp": patch
+"@executor-js/cloudflare": patch
+---
+
+**MCP execution spans now carry the client identity (`mcp.client.*`)**
+
+The `clientInfo` a client self-reports at `initialize` (or in a modern request's `_meta`) previously existed only on the initialize request itself, which has no session id yet, so execution telemetry could not be segmented by client. Execute, execute-action, and resume spans (and their descendants) now carry `mcp.client.name` / `mcp.client.version` / `mcp.client.title` alongside the existing session join keys. Cloudflare session Durable Objects persist the reported identity in session meta, so attribution survives cold restores; it feeds telemetry only, never behavior.
diff --git a/apps/cloud/src/mcp/session-durable-object.ts b/apps/cloud/src/mcp/session-durable-object.ts
@@ -379,6 +379,10 @@ export class McpSessionDOSqlite extends McpAgentSessionDOBase<Env, CloudSessionD
         // the negotiated apps support comes back from storage instead.
         restoredAppsEnabled: sessionMeta.appsEnabled ?? false,
         onAppsEnabledChange: (appsEnabled) => self.persistAppsEnabled(appsEnabled),
+        // Same restore contract for the client identity that keys the
+        // `mcp.client.*` span attribution on execution spans.
+        ...(sessionMeta.clientInfo ? { restoredClientInfo: sessionMeta.clientInfo } : {}),
+        onClientInfoChange: (clientInfo) => self.persistClientInfo(clientInfo),
         appsEnabled: false,
         sessionful: true,
         requestStateSigningKey: self.modernRequestStateSigningKey(),

diff --git a/packages/hosts/cloudflare/src/mcp/agent-session-durable-object.ts b/packages/hosts/cloudflare/src/mcp/agent-session-durable-object.ts
@@ -24,11 +24,13 @@ import {
 import {
   appsEnabledForClientCapabilities,
   clientCapabilitiesFromRequestBody,
+  clientInfoFromRequestBody,
   mcpRequestStateBindingFromBody,
   PAUSED_APPROVAL_TIMEOUT_MS,
   formatMcpExecutionOutcome,
   mcpRequestStatePrincipal,
   requestBodyFromRequest,
+  type McpClientInfo,
   type PausedExecutionHooks,
   type ResumeFallbackOutcome,
 } from "@executor-js/host-mcp/tool-server";
@@ -147,6 +149,14 @@ export interface SessionMeta {
    * unknown, which behaves as disabled until the next `initialize`.
    */
   readonly appsEnabled?: boolean;
+  /**
+   * The client identity (`clientInfo`) self-reported at `initialize` or in a
+   * modern request's `_meta`. Persisted for the same reason as
+   * {@link appsEnabled}: a cold-restored server never sees an `initialize`,
+   * and without this the execution spans' `mcp.client.*` attribution vanishes
+   * mid-conversation. Telemetry/display only, never behavior.
+   */
+  readonly clientInfo?: McpClientInfo;
   /** Creation time of this session, retained across isolate eviction. */
   readonly createdAtMs?: number;
 }
@@ -164,6 +174,8 @@ export interface ModernMcpServerRequestOptions {
   readonly requestStateSigningKey: Uint8Array | string;
   readonly requestStatePrincipal: string;
   readonly requestStateBinding?: string;
+  /** Client identity for span attribution: this request's `_meta`, else the session's persisted copy. */
+  readonly restoredClientInfo?: McpClientInfo;
 }
 
 /** Long-lived DO execution runtime shared by per-request MCP servers. */
@@ -515,6 +527,34 @@ export abstract class McpAgentSessionDOBase<
     );
   }
 
+  /**
+   * Persist the client identity self-reported at `initialize` (or in a modern
+   * request's `_meta`), so a cold restore keeps span attribution. Subclasses
+   * hand this to `buildMcpServer` as `onClientInfoChange`; the modern request
+   * path calls it directly. Same no-op-before-meta contract as
+   * {@link persistAppsEnabled}.
+   */
+  protected persistClientInfo(clientInfo: McpClientInfo): Effect.Effect<void> {
+    const self = this;
+    return Effect.gen(function* () {
+      const stored = yield* self.loadSessionMeta();
+      if (
+        !stored ||
+        (stored.clientInfo?.name === clientInfo.name &&
+          stored.clientInfo?.version === clientInfo.version &&
+          stored.clientInfo?.title === clientInfo.title)
+      ) {
+        return;
+      }
+      yield* Effect.promise(() => self.saveSessionMeta({ ...stored, clientInfo }));
+    }).pipe(
+      Effect.withSpan("mcp.session.persist_client_info", {
+        attributes: { "mcp.client.name": clientInfo.name },
+      }),
+      Effect.ignoreCause({ log: false }),
+    );
+  }
+
   private async markActivity(now = Date.now()): Promise<void> {
     this.lastActivityMs = now;
     const key =
@@ -606,6 +646,7 @@ export abstract class McpAgentSessionDOBase<
         ...resolved,
         ...(token.webOrigin ? { webOrigin: token.webOrigin } : {}),
         appsEnabled: stored?.appsEnabled ?? false,
+        ...(stored?.clientInfo ? { clientInfo: stored.clientInfo } : {}),
         createdAtMs: stored?.createdAtMs ?? Date.now(),
       };
       yield* Effect.promise(() => self.saveSessionMeta(sessionMeta)).pipe(
@@ -816,6 +857,7 @@ export abstract class McpAgentSessionDOBase<
         const parsedBody = self.modernRequestBodies.get(request);
         const propagation = self.modernRequestPropagation.get(request);
         const capabilities = clientCapabilitiesFromRequestBody(parsedBody);
+        const clientInfo = clientInfoFromRequestBody(parsedBody) ?? sessionMeta.clientInfo;
         return Effect.runPromise(
           Effect.gen(function* () {
             const requestStatePrincipal = mcpRequestStatePrincipal({
@@ -834,6 +876,7 @@ export abstract class McpAgentSessionDOBase<
               requestStateSigningKey: self.modernRequestStateSigningKey(),
               requestStatePrincipal,
               ...(requestStateBinding === null ? {} : { requestStateBinding }),
+              ...(clientInfo === undefined ? {} : { restoredClientInfo: clientInfo }),
             });
           }).pipe(
             (effect) => self.withTelemetry(effect, propagation),
@@ -1282,6 +1325,17 @@ export abstract class McpAgentSessionDOBase<
       });
     }
 
+    // Modern clients self-report identity per request (`_meta` clientInfo) or
+    // at `initialize`; persist it so meta-less requests and cold restores keep
+    // their span attribution. Best-effort by construction (persistClientInfo
+    // swallows failures) and a storage no-op unless the identity changed.
+    const reportedClientInfo = clientInfoFromRequestBody(parsedBody);
+    if (reportedClientInfo) {
+      await Effect.runPromise(
+        this.withTelemetry(this.persistClientInfo(reportedClientInfo), props.propagation),
+      );
+    }
+
     this.modernRequestBodies.set(request, parsedBody);
     this.modernRequestPropagation.set(request, props.propagation);
     this.modernRunningRequestCount += 1;

diff --git a/packages/hosts/mcp/src/tool-server-core.ts b/packages/hosts/mcp/src/tool-server-core.ts
@@ -196,6 +196,30 @@ type SharedMcpServerConfig = {
    * restore. Best-effort: failures are swallowed and never affect the session.
    */
   readonly onAppsEnabledChange?: (appsEnabled: boolean) => Effect.Effect<void>;
+  /**
+   * The client identity self-reported at a previous `initialize` (or in a
+   * modern request's `_meta`), restored for the same reason as
+   * {@link restoredAppsEnabled}. Feeds only the `mcp.client.*` span
+   * attributes on execution spans, never behavior or security decisions.
+   */
+  readonly restoredClientInfo?: McpClientInfo;
+  /**
+   * Called when `initialize` reports the client identity, so the host can
+   * persist it for {@link restoredClientInfo} on a later cold restore.
+   * Best-effort: failures are swallowed and never affect the session.
+   */
+  readonly onClientInfoChange?: (clientInfo: McpClientInfo) => Effect.Effect<void>;
+};
+
+/**
+ * Client software identity as self-reported over MCP (`clientInfo` at
+ * `initialize`, `_meta` on modern requests). Display and telemetry vocabulary
+ * only: the spec forbids relying on it for behavior or security.
+ */
+export type McpClientInfo = {
+  readonly name: string;
+  readonly version?: string;
+  readonly title?: string;
 };
 
 /**
@@ -340,6 +364,8 @@ export type ExecutorMcpAssembly<Server, RequestContext extends McpRequestJoinKey
   readonly server: Server;
   readonly initialAppsEnabled: boolean;
   readonly getClientCapabilities: () => unknown | null;
+  /** The live `initialize`-reported client identity, when the SDK has one. */
+  readonly getClientInfo: () => McpClientInfo | null;
   readonly getElicitationSupport: () => { readonly form: boolean; readonly url: boolean };
   readonly getUiCapability: () => { readonly mimeTypes?: readonly string[] } | undefined;
   readonly onInitialized: (callback: () => void) => void;
@@ -782,6 +808,21 @@ const joinKeyAttributes = (joinKeys: McpRequestJoinKeys): Record<string, unknown
   "mcp.request.session_id": joinKeys.sessionId ?? "",
 });
 
+// Client identity uses the same `mcp.client.*` vocabulary as the cloud
+// worker's `initialize` fingerprint (`annotateMcpRequest`), so execution spans
+// segment by client directly instead of through a session join that `initialize`
+// (which has no session id yet) cannot satisfy. Absent means no key at all, not
+// an empty string, when no `initialize` was seen and nothing was restored:
+// unknown is real state here, unlike the always-present session key above.
+const clientInfoAttributes = (clientInfo: McpClientInfo | null): Record<string, unknown> =>
+  clientInfo === null
+    ? {}
+    : {
+        "mcp.client.name": clientInfo.name,
+        ...(clientInfo.version !== undefined ? { "mcp.client.version": clientInfo.version } : {}),
+        ...(clientInfo.title !== undefined ? { "mcp.client.title": clientInfo.title } : {}),
+      };
+
 const startMarker = (name: string, attributes: Record<string, unknown>): Effect.Effect<void> =>
   Effect.void.pipe(Effect.withSpan(name, { attributes }));
 
@@ -1110,6 +1151,15 @@ export const buildExecutorMcpTools = <
     );
     const server = assembly.server;
 
+    // Seeded from the host's persisted copy; a live `initialize` on this
+    // instance replaces it via `syncClientInfo` below. Read lazily at each
+    // tool call so spans always carry the newest identity.
+    let clientInfo: McpClientInfo | null = config.restoredClientInfo ?? null;
+    const requestSpanAttributes = (joinKeys: McpRequestJoinKeys): Record<string, unknown> => ({
+      ...joinKeyAttributes(joinKeys),
+      ...clientInfoAttributes(clientInfo),
+    });
+
     const executeWithNativeElicitation = (
       code: string,
       extra: RequestContext,
@@ -1179,7 +1229,7 @@ export const buildExecutorMcpTools = <
             "mcp.execute.code_length": code.length,
           },
         }),
-        Effect.annotateSpans(joinKeyAttributes(extra)),
+        Effect.annotateSpans(requestSpanAttributes(extra)),
       );
 
     /** What the caller could bind an unresolved role to. Best effort: the
@@ -1340,7 +1390,7 @@ export const buildExecutorMcpTools = <
             "mcp.execute.execution_id": executionId,
           },
         }),
-        Effect.annotateSpans(joinKeyAttributes(extra)),
+        Effect.annotateSpans(requestSpanAttributes(extra)),
       );
 
     const requireUserResumeApproval = (executionId: string): Effect.Effect<McpToolResult> =>
@@ -1419,7 +1469,7 @@ export const buildExecutorMcpTools = <
             "mcp.execute.execution_id": executionId,
           },
         }),
-        Effect.annotateSpans(joinKeyAttributes(extra)),
+        Effect.annotateSpans(requestSpanAttributes(extra)),
       );
 
     // --- tools ---
@@ -2169,9 +2219,35 @@ export const buildExecutorMcpTools = <
       });
     };
 
+    // Client identity arrives with the same `initialize` that carries the
+    // capabilities above. An absent live value (cold restore, construction
+    // time) is not evidence the client changed, so the restored value stands,
+    // the same asymmetry `syncToolAvailability` documents for capabilities.
+    const syncClientInfo = () => {
+      const live = assembly.getClientInfo();
+      if (live === null) return;
+      const changed =
+        live.name !== clientInfo?.name ||
+        live.version !== clientInfo?.version ||
+        live.title !== clientInfo?.title;
+      if (!changed) return;
+      clientInfo = live;
+      const onClientInfoChange = config.onClientInfoChange;
+      if (onClientInfoChange) {
+        // oxlint-disable-next-line executor/no-effect-escape-hatch -- boundary: `oninitialized` is a sync SDK hook; persistence is fire-and-forget and its failure must not fail the session
+        void Effect.runPromiseWith(context)(
+          onClientInfoChange(live).pipe(Effect.ignoreCause({ log: false })),
+        );
+      }
+    };
+
     yield* Effect.sync(() => {
       syncToolAvailability();
-      assembly.onInitialized(syncToolAvailability);
+      syncClientInfo();
+      assembly.onInitialized(() => {
+        syncToolAvailability();
+        syncClientInfo();
+      });
     }).pipe(Effect.withSpan("mcp.host.sync_tool_availability"));
 
     return server;

diff --git a/packages/hosts/mcp/src/tool-server.test.ts b/packages/hosts/mcp/src/tool-server.test.ts
@@ -17,8 +17,10 @@ import type { ExecutionEngine, ExecutionResult } from "@executor-js/execution";
 
 import {
   buildMcpServer,
+  clientInfoFromRequestBody,
   formatMcpExecutionOutcome,
   type ExecutorMcpServerConfig,
+  type McpClientInfo,
 } from "./tool-server";
 
 // ---------------------------------------------------------------------------
@@ -71,6 +73,7 @@ type TestServerConfig<E extends Cause.YieldableError> = Pick<
   | "pausedExecutionHooks"
   | "pausedExecutionLeaseMs"
   | "resumeFallback"
+  | "onClientInfoChange"
 >;
 
 /** Connect a real MCP Client to our executor MCP server over in-memory transports. */
@@ -1610,6 +1613,77 @@ describe("MCP host server — skills tool", () => {
   });
 });
 
+describe("MCP host server — client attribution", () => {
+  it("stamps the initialize-reported client identity on execution spans", async () => {
+    await withTracedClient(makeStubEngine({}), async (client, spans) => {
+      await client.callTool({ name: "execute", arguments: { code: "1+1" } });
+
+      const execute = spans.find((span) => span.name === "mcp.host.tool.execute");
+      expectDefined(execute);
+      expect(execute.attributes.get("mcp.client.name")).toBe("test-client");
+      expect(execute.attributes.get("mcp.client.version")).toBe("1.0.0");
+    });
+  });
+
+  it("reports the initialize-reported identity to onClientInfoChange once", async () => {
+    const reported: McpClientInfo[] = [];
+    await withClient(
+      makeStubEngine({}),
+      NO_CAPS,
+      async (client) => {
+        await client.callTool({ name: "execute", arguments: { code: "1+1" } });
+        expect(reported).toEqual([{ name: "test-client", version: "1.0.0" }]);
+      },
+      {
+        onClientInfoChange: (clientInfo) =>
+          Effect.sync(() => {
+            reported.push(clientInfo);
+          }),
+      },
+    );
+  });
+});
+
+describe("clientInfoFromRequestBody", () => {
+  const META_KEY = "io.modelcontextprotocol/clientInfo";
+
+  it("prefers the per-request _meta identity", () => {
+    expect(
+      clientInfoFromRequestBody({
+        method: "tools/call",
+        params: { _meta: { [META_KEY]: { name: "meta-client", version: "2.0.0" } } },
+      }),
+    ).toEqual({ name: "meta-client", version: "2.0.0" });
+  });
+
+  it("falls back to the initialize body's native clientInfo", () => {
+    expect(
+      clientInfoFromRequestBody({
+        method: "initialize",
+        params: { clientInfo: { name: "init-client", version: "1.2.3", title: "Init Client" } },
+      }),
+    ).toEqual({ name: "init-client", version: "1.2.3", title: "Init Client" });
+  });
+
+  it("decodes a malformed or absent identity to null", () => {
+    expect(clientInfoFromRequestBody(null)).toBeNull();
+    expect(clientInfoFromRequestBody({ method: "tools/call", params: {} })).toBeNull();
+    expect(
+      clientInfoFromRequestBody({
+        method: "tools/call",
+        params: { _meta: { [META_KEY]: { version: "no-name" } } },
+      }),
+    ).toBeNull();
+    // Native clientInfo is only trusted on `initialize`, where the spec puts it.
+    expect(
+      clientInfoFromRequestBody({
+        method: "tools/call",
+        params: { clientInfo: { name: "misplaced" } },
+      }),
+    ).toBeNull();
+  });
+});
+
 describe("MCP host server — hang-visibility tracing", () => {
   it("execute emits a start marker and stamps the JSON-RPC id on execution spans", async () => {
     const engine = makeStubEngine({});