diff --git a/apps/api/src/routes/chat.ts b/apps/api/src/routes/chat.ts
index 64392fdf9..7d16424e5 100644
--- a/apps/api/src/routes/chat.ts
+++ b/apps/api/src/routes/chat.ts
@@ -7,7 +7,7 @@
  * See: specs/018-project-first-architecture/tasks.md (T027)
  */
 import type { ChatSessionTaskEmbed } from '@simple-agent-manager/shared';
-import { isTaskExecutionStep, isTaskMode } from '@simple-agent-manager/shared';
+import { DEFAULT_SAM_HISTORY_LOAD_LIMIT, isTaskExecutionStep, isTaskMode } from '@simple-agent-manager/shared';
 import { and, eq, inArray } from 'drizzle-orm';
 import { drizzle } from 'drizzle-orm/d1';
 import { Hono } from 'hono';
@@ -28,6 +28,16 @@ const chatRoutes = new Hono<{ Bindings: Env }>();
 
 chatRoutes.use('/*', requireAuth(), requireApproved());
 
+function getSessionMessageLimit(env: Env, requestedLimit?: string): number {
+  const configuredLimit = parseInt(env.SAM_HISTORY_LOAD_LIMIT || '', 10);
+  const maxLimit = Number.isFinite(configuredLimit) && configuredLimit > 0
+    ? configuredLimit
+    : DEFAULT_SAM_HISTORY_LOAD_LIMIT;
+  const parsedLimit = parseInt(requestedLimit || '', 10);
+  const limit = Number.isFinite(parsedLimit) && parsedLimit > 0 ? parsedLimit : maxLimit;
+  return Math.min(limit, maxLimit);
+}
+
 /**
  * GET /api/projects/:projectId/sessions
  * List chat sessions for a project.
@@ -108,7 +118,7 @@ chatRoutes.get('/:sessionId', async (c) => {
     throw errors.notFound('Chat session');
   }
 
-  const limit = Math.min(parseInt(c.req.query('limit') || '1000', 10), 5000);
+  const limit = getSessionMessageLimit(c.env, c.req.query('limit'));
   const beforeParam = c.req.query('before');
   const before = beforeParam ? parseInt(beforeParam, 10) : null;
 
diff --git a/apps/api/tests/unit/routes/chat-session-agent-routing.test.ts b/apps/api/tests/unit/routes/chat-session-agent-routing.test.ts
index c947747db..3e6d36010 100644
--- a/apps/api/tests/unit/routes/chat-session-agent-routing.test.ts
+++ b/apps/api/tests/unit/routes/chat-session-agent-routing.test.ts
@@ -17,6 +17,7 @@ vi.mock('drizzle-orm/d1', () => ({
 }));
 
 vi.mock('@simple-agent-manager/shared', () => ({
+  DEFAULT_SAM_HISTORY_LOAD_LIMIT: 200,
   DEFAULT_WORKSPACE_PROFILE: 'full',
   isTaskExecutionStep: () => true,
   isTaskMode: (v: unknown) => v === 'task' || v === 'conversation',
@@ -187,6 +188,53 @@ describe('chatRoutes agent session routing', () => {
     expect(body.session.agentType).toBe('claude-code');
   });
 
+  it('caps session detail message loads to the configured history limit', async () => {
+    mocks.listAcpSessions.mockResolvedValue({
+      sessions: [],
+      total: 0,
+    });
+
+    const response = await app.request(
+      '/api/projects/proj-1/sessions/chat-1?limit=5000',
+      { method: 'GET' },
+      {
+        DATABASE: {} as D1Database,
+        SAM_HISTORY_LOAD_LIMIT: '25',
+      } as Env,
+    );
+
+    expect(response.status).toBe(200);
+    expect(mocks.getMessages).toHaveBeenCalledWith(
+      expect.anything(),
+      'proj-1',
+      'chat-1',
+      25,
+      null,
+    );
+  });
+
+  it('uses the default history limit when no session detail limit is requested', async () => {
+    mocks.listAcpSessions.mockResolvedValue({
+      sessions: [],
+      total: 0,
+    });
+
+    const response = await app.request(
+      '/api/projects/proj-1/sessions/chat-1',
+      { method: 'GET' },
+      { DATABASE: {} as D1Database } as Env,
+    );
+
+    expect(response.status).toBe(200);
+    expect(mocks.getMessages).toHaveBeenCalledWith(
+      expect.anything(),
+      'proj-1',
+      'chat-1',
+      200,
+      null,
+    );
+  });
+
   it('returns null agentType when ACP session has no agentType', async () => {
     mocks.listAcpSessions.mockResolvedValue({
       sessions: [
diff --git a/tasks/backlog/2026-05-01-tail-worker-log-ingest-auth.md b/tasks/backlog/2026-05-01-tail-worker-log-ingest-auth.md
new file mode 100644
index 000000000..6d7172e26
--- /dev/null
+++ b/tasks/backlog/2026-05-01-tail-worker-log-ingest-auth.md
@@ -0,0 +1,24 @@
+# Tail worker log ingest endpoint requires auth
+
+## Problem
+
+The production tail worker forwards Worker logs to `POST /api/admin/observability/logs/ingest`, but that internal service-binding request is being rejected with `Authentication required`.
+
+This generates thousands of `request_error` entries from `sam-api-prod` and pollutes Cloudflare Workers Observability, making real production errors harder to isolate.
+
+## Context
+
+Discovered while investigating production chat session load failures on 2026-05-01. Cloudflare Workers Observability showed repeated `request_error` events for:
+
+- `POST /api/admin/observability/logs/ingest`
+- error: `Authentication required`
+- source route: tail worker service binding request from `sam-tail-worker-prod`
+
+The chat UI failure had a separate root cause: oversized ProjectData DO RPC message responses.
+
+## Acceptance Criteria
+
+- [ ] Tail worker log ingestion succeeds without end-user authentication for internal service-binding calls.
+- [ ] The ingest route still rejects browser/user-originated unauthenticated requests.
+- [ ] A regression test covers the internal ingest path.
+- [ ] Production/staging observability no longer shows repeated `Authentication required` errors for `/api/admin/observability/logs/ingest`.