diff --git a/src/core/tdai-core.ts b/src/core/tdai-core.ts
index 977d4a2..b2f66c0 100644
--- a/src/core/tdai-core.ts
+++ b/src/core/tdai-core.ts
@@ -68,6 +68,37 @@ export interface TdaiCoreOptions {
   instanceId?: string;
 }
 
+export interface TdaiDiagnostics {
+  dataDir: string;
+  store: {
+    vectorStore: boolean;
+    embeddingService: boolean;
+    l0Count: number | null;
+    l1Count: number | null;
+    countError?: string;
+  };
+  scheduler: {
+    enabled: boolean;
+    started: boolean;
+    queues: ReturnType<MemoryPipelineManager["getQueueSizes"]> | null;
+  };
+  checkpoint: {
+    totalProcessed: number;
+    l0ConversationsCount: number;
+    totalMemoriesExtracted: number;
+    memoriesSinceLastPersona: number;
+    scenesProcessed: number;
+  };
+  sessions: Array<{
+    sessionKey: string;
+    trackedInMemory: boolean;
+    bufferedMessages: number;
+    schedulerState: ReturnType<MemoryPipelineManager["getSessionState"]> | null;
+    checkpointRunnerState: unknown;
+    checkpointPipelineState: unknown;
+  }>;
+}
+
 // ============================
 // TdaiCore
 // ============================
@@ -363,6 +394,74 @@ export class TdaiCore {
     await this.scheduler.flushSession(sessionKey);
   }
 
+  /**
+   * Read-only diagnostics for Gateway/CLI observability.
+   *
+   * This intentionally avoids starting the scheduler by itself: diagnostics
+   * should report current state, not create new pipeline timers.
+   */
+  async getDiagnostics(): Promise<TdaiDiagnostics> {
+    await this.storeReady?.catch(() => {});
+
+    const checkpoint = new CheckpointManager(this.dataDir, this.logger);
+    const cp = await checkpoint.read();
+    const scheduler = this.scheduler;
+
+    let l0Count: number | null = null;
+    let l1Count: number | null = null;
+    let countError: string | undefined;
+    if (this.vectorStore) {
+      try {
+        l0Count = await this.vectorStore.countL0();
+        l1Count = await this.vectorStore.countL1();
+      } catch (err) {
+        countError = err instanceof Error ? err.message : String(err);
+      }
+    }
+
+    const sessionKeys = new Set<string>([
+      ...Object.keys(cp.runner_states ?? {}),
+      ...Object.keys(cp.pipeline_states ?? {}),
+      ...(scheduler?.getSessionKeys() ?? []),
+    ]);
+
+    const sessions = Array.from(sessionKeys).sort().map((sessionKey) => {
+      const schedulerState = scheduler?.getSessionState(sessionKey) ?? null;
+      return {
+        sessionKey,
+        trackedInMemory: schedulerState !== null,
+        bufferedMessages: scheduler?.getBufferedMessageCount(sessionKey) ?? 0,
+        schedulerState,
+        checkpointRunnerState: cp.runner_states?.[sessionKey] ?? null,
+        checkpointPipelineState: cp.pipeline_states?.[sessionKey] ?? null,
+      };
+    });
+
+    return {
+      dataDir: this.dataDir,
+      store: {
+        vectorStore: !!this.vectorStore,
+        embeddingService: !!this.embeddingService,
+        l0Count,
+        l1Count,
+        ...(countError ? { countError } : {}),
+      },
+      scheduler: {
+        enabled: !!scheduler,
+        started: this.isSchedulerStarted(),
+        queues: scheduler?.getQueueSizes() ?? null,
+      },
+      checkpoint: {
+        totalProcessed: cp.total_processed,
+        l0ConversationsCount: cp.l0_conversations_count,
+        totalMemoriesExtracted: cp.total_memories_extracted,
+        memoriesSinceLastPersona: cp.memories_since_last_persona,
+        scenesProcessed: cp.scenes_processed,
+      },
+      sessions,
+    };
+  }
+
   // ============================
   // Accessors (for migration bridge)
   // ============================
diff --git a/src/gateway/server.ts b/src/gateway/server.ts
index b4c66ec..5769622 100644
--- a/src/gateway/server.ts
+++ b/src/gateway/server.ts
@@ -9,6 +9,7 @@
  *   POST /search/conversations — L0 conversation search
  *   POST /session/end         — Session end + flush
  *   POST /seed               — Batch seed historical conversations (L0 → L1)
+ *   GET  /diagnostics/sessions — Read-only session/pipeline diagnostics
  *
  * Built with Node.js native `http` module — no Express/Fastify dependency.
  * Designed to run as a managed sidecar alongside Hermes.
@@ -37,6 +38,7 @@ import type {
   SessionEndResponse,
   SeedRequest,
   SeedResponse,
+  DiagnosticsResponse,
   GatewayErrorResponse,
 } from "./types.js";
 import type { Logger } from "../core/types.js";
@@ -272,6 +274,8 @@ export class TdaiGateway {
           return await this.handleSessionEnd(req, res);
         case "POST /seed":
           return await this.handleSeed(req, res);
+        case "GET /diagnostics/sessions":
+          return await this.handleDiagnosticsSessions(res);
         default:
           sendError(res, 404, `Not found: ${method} ${pathname}`);
       }
@@ -368,6 +372,11 @@ export class TdaiGateway {
     sendJson(res, 200, response);
   }
 
+  private async handleDiagnosticsSessions(res: http.ServerResponse): Promise<void> {
+    const response: DiagnosticsResponse = await this.core.getDiagnostics();
+    sendJson(res, 200, response);
+  }
+
   private async handleRecall(req: http.IncomingMessage, res: http.ServerResponse): Promise<void> {
     const body = await parseJsonBody<RecallRequest>(req);
 
diff --git a/src/gateway/types.ts b/src/gateway/types.ts
index 50b2ff4..53dbf66 100644
--- a/src/gateway/types.ts
+++ b/src/gateway/types.ts
@@ -25,6 +25,41 @@ export interface HealthResponse {
   };
 }
 
+// ============================
+// /diagnostics/sessions
+// ============================
+
+export interface DiagnosticsResponse {
+  dataDir: string;
+  store: {
+    vectorStore: boolean;
+    embeddingService: boolean;
+    l0Count: number | null;
+    l1Count: number | null;
+    countError?: string;
+  };
+  scheduler: {
+    enabled: boolean;
+    started: boolean;
+    queues: unknown;
+  };
+  checkpoint: {
+    totalProcessed: number;
+    l0ConversationsCount: number;
+    totalMemoriesExtracted: number;
+    memoriesSinceLastPersona: number;
+    scenesProcessed: number;
+  };
+  sessions: Array<{
+    sessionKey: string;
+    trackedInMemory: boolean;
+    bufferedMessages: number;
+    schedulerState: unknown;
+    checkpointRunnerState: unknown;
+    checkpointPipelineState: unknown;
+  }>;
+}
+
 // ============================
 // /recall
 // ============================