diff --git a/src/core/tools/tool-idempotent-replay.ts b/src/core/tools/tool-idempotent-replay.ts
index e03ed86c..7988f40d 100644
--- a/src/core/tools/tool-idempotent-replay.ts
+++ b/src/core/tools/tool-idempotent-replay.ts
@@ -37,6 +37,9 @@ export function findReplayableToolResultContent(
   sideEffectClass?: ToolSideEffectClass
 ): string | null {
   if (isToolAssumedMutating(toolName, sideEffectClass)) return null;
+  // Readonly describes side effects, not freshness. External observations and
+  // MCP search catalogs may change without a local write; ask the server again.
+  if (toolName.startsWith('mcp__')) return null;
 
   const want = stableSerializeToolInput(input);
   const wantedPathKey = toolPathKey(toolName, input);
diff --git a/test/mcp-read-freshness.spec.mjs b/test/mcp-read-freshness.spec.mjs
new file mode 100644
index 00000000..e67eb505
--- /dev/null
+++ b/test/mcp-read-freshness.spec.mjs
@@ -0,0 +1,70 @@
+import assert from 'node:assert/strict';
+import { createServer } from 'node:http';
+import test from 'node:test';
+import { MossAgent } from '../dist/core/agent/moss-agent.js';
+import { InMemorySessionStore } from '../dist/core/session/session.js';
+
+test('two Agent turns execute the same readonly MCP query and observe its changed backend state', async () => {
+  let requests = 0;
+  let completions = 0;
+  const observed = [];
+  const server = createServer((_req, response) => {
+    response.setHeader('content-type', 'application/json');
+    response.end(JSON.stringify({ status: ++requests === 1 ? 'running' : 'completed' }));
+  });
+  await new Promise((resolve) => server.listen(0, '127.0.0.1', resolve));
+  const url = `http://127.0.0.1:${server.address().port}/status`;
+  const name = 'mcp__fixture__training_status';
+  const agent = new MossAgent({
+    workspaceDir: process.cwd(),
+    llmProvider: {
+      id: 'freshness-fixture',
+      capabilities: { streaming: false },
+      async complete() {
+        return ++completions % 2
+          ? {
+              stopReason: 'tool_use',
+              content: [
+                {
+                  type: 'tool_use',
+                  id: `read-${completions}`,
+                  name,
+                  input: { runId: 'selected-run' },
+                },
+              ],
+            }
+          : { stopReason: 'end_turn', content: [{ type: 'text', text: 'checked' }] };
+      },
+    },
+    model: 'fixture',
+    sessionStore: new InMemorySessionStore(),
+    domainPrompt: false,
+    includeAgentBehaviorPrompt: false,
+    enableSteering: false,
+    enableCompaction: false,
+    enableFollowUpGuard: false,
+    maxAgentTurns: 4,
+    hooks: { onBeforeToolExec: async () => ({ approved: true }) },
+  });
+  agent.tools.register({
+    name,
+    description: 'Read current status from the owned fixture',
+    inputSchema: { type: 'object', properties: { runId: { type: 'string' } } },
+    metadata: { sideEffectClass: 'readonly' },
+    async execute() {
+      const result = await (await fetch(url)).json();
+      observed.push(result.status);
+      return JSON.stringify(result);
+    },
+  });
+  try {
+    await agent.chat('fixture-session', 'Check current status');
+    await agent.chat('fixture-session', 'Check the same status again');
+    assert.equal(requests, 2);
+    assert.deepEqual(observed, ['running', 'completed']);
+  } finally {
+    await agent.close();
+    server.closeAllConnections();
+    await new Promise((resolve) => server.close(resolve));
+  }
+});
diff --git a/test/tool-idempotent-replay.spec.mjs b/test/tool-idempotent-replay.spec.mjs
index 83d30f04..5952d8e5 100644
--- a/test/tool-idempotent-replay.spec.mjs
+++ b/test/tool-idempotent-replay.spec.mjs
@@ -22,6 +22,35 @@ function toolResult(id, content) {
   };
 }
 
+test('never replays external MCP observations, even when declared readonly', () => {
+  for (const name of [
+    'mcp__duck-lab__rdk_training_status',
+    'mcp__another-server__health',
+    'mcp__fixture__search',
+  ]) {
+    const input = { runId: 'same-run' };
+    const messages = [assistantToolUse('query-1', name, input), toolResult('query-1', 'running')];
+    assert.equal(findReplayableToolResultContent(messages, name, input, 32, 'readonly'), null);
+  }
+});
+
+test('retains stable local read reuse and never replays mutating tool results', () => {
+  const input = { path: 'example.txt' };
+  const messages = [
+    assistantToolUse('read-1', 'read_file', input),
+    toolResult('read-1', 'file contents'),
+  ];
+  assert.equal(
+    findReplayableToolResultContent(messages, 'read_file', input, 32, 'readonly'),
+    'file contents'
+  );
+  assert.equal(
+    findReplayableToolResultContent(messages, 'read_file', input, 32, 'local_write'),
+    null
+  );
+  assert.equal(findReplayableToolResultContent(messages, 'read_file', input), null);
+});
+
 test('does not replay a file read after the same path was mutated', () => {
   const messages = [
     assistantToolUse('read-1', 'read_file', { path: 'src/example.ts' }),