diff --git a/src/logging/diagnostic-stability.test.ts b/src/logging/diagnostic-stability.test.ts
index 6610157a390..ba2e4d22e8b 100644
--- a/src/logging/diagnostic-stability.test.ts
+++ b/src/logging/diagnostic-stability.test.ts
@@ -97,6 +97,45 @@ describe("diagnostic stability recorder", () => {
     expect(snapshot.events[1]).not.toHaveProperty("reason");
   });
 
+  it("summarizes assembled context diagnostics without prompt text", async () => {
+    startDiagnosticStabilityRecorder();
+
+    emitDiagnosticEvent({
+      type: "context.assembled",
+      runId: "run-secret",
+      sessionId: "session-secret",
+      provider: "openai",
+      model: "gpt-5.4",
+      channel: "telegram",
+      trigger: "user-message",
+      messageCount: 4,
+      historyTextChars: 1200,
+      historyImageBlocks: 1,
+      maxMessageTextChars: 800,
+      systemPromptChars: 300,
+      promptChars: 100,
+      promptImages: 1,
+      contextTokenBudget: 200_000,
+      reserveTokens: 20_000,
+    });
+    await new Promise<void>((resolve) => setImmediate(resolve));
+
+    const snapshot = getDiagnosticStabilitySnapshot({ limit: 10 });
+
+    expect(snapshot.events[0]).toMatchObject({
+      type: "context.assembled",
+      provider: "openai",
+      model: "gpt-5.4",
+      channel: "telegram",
+      count: 4,
+      context: { limit: 200_000 },
+    });
+    expect(snapshot.events[0]).not.toHaveProperty("runId");
+    expect(snapshot.events[0]).not.toHaveProperty("sessionId");
+    expect(snapshot.events[0]).not.toHaveProperty("promptChars");
+    expect(snapshot.events[0]).not.toHaveProperty("systemPromptChars");
+  });
+
   it("sanitizes tool and model diagnostic error categories", async () => {
     startDiagnosticStabilityRecorder();
 
diff --git a/src/logging/diagnostic-stability.ts b/src/logging/diagnostic-stability.ts
index ea209f62261..50b0357a3e9 100644
--- a/src/logging/diagnostic-stability.ts
+++ b/src/logging/diagnostic-stability.ts
@@ -246,6 +246,14 @@ function sanitizeDiagnosticEvent(event: DiagnosticEventPayload): DiagnosticStabi
     case "run.attempt":
       record.count = event.attempt;
       break;
+    case "context.assembled":
+      record.channel = event.channel;
+      record.provider = event.provider;
+      record.model = event.model;
+      record.count = event.messageCount;
+      record.context =
+        event.contextTokenBudget !== undefined ? { limit: event.contextTokenBudget } : undefined;
+      break;
     case "diagnostic.heartbeat":
       record.webhooks = { ...event.webhooks };
       record.active = event.active;