diff --git a/server/drivers/claude.test.ts b/server/drivers/claude.test.ts
index 6b4a6f7bc2..ba9b10dc9e 100644
--- a/server/drivers/claude.test.ts
+++ b/server/drivers/claude.test.ts
@@ -1434,6 +1434,19 @@ describe("ClaudeDriver turns (fake CLI)", () => {
     });
   });
 
+  it("withholds --autocompact from a CLI above the floor whose --help does not list it", async () => {
+    // 2.1.129 clears the 2.1.122 floor yet rejects the flag ("unknown option")
+    const dump = join(scratch, "no-autocompact-cli.json");
+    await create(undefined, { FAKE_CLAUDE_DUMP: dump, FAKE_CLAUDE_VERSION: "2.1.129", FAKE_CLAUDE_AUTOCOMPACT: "0" });
+    await instance.snapshot();
+    await instance.adapter.sendTurn({ threadId: "t-no-autocompact", text: "hi" });
+    await recorder.until((e) => e.type === "turn.completed");
+
+    const seen = JSON.parse(readFileSync(dump, "utf8"));
+    expect(seen.argv).not.toContain("--autocompact");
+    expect(seen.argv).toContain("--strict-mcp-config");
+  });
+
   it("keeps only the isolation flag a very old CLI accepts", async () => {
     // 1.0.100: --strict-mcp-config exists (1.0.60), --setting-sources does
     // not yet (1.0.122)
diff --git a/server/drivers/claude.ts b/server/drivers/claude.ts
index ec9feeceb8..b2cb3311ef 100644
--- a/server/drivers/claude.ts
+++ b/server/drivers/claude.ts
@@ -1182,6 +1182,12 @@ export const ClaudeDriver: ProviderDriver<ClaudeConfig> = {
     // other context controls and an unknown flag would reject that request.
     let cliVersion: ClaudeCliVersion | null = null;
     let cliVersionChecked = false;
+    // Whether `claude --help` lists --autocompact, read once per CLI version
+    // by snapshot(). The flag is not in every build above its version floor
+    // (2.1.129 rejects it), so the listing wins over the floor; null until
+    // probed, or when the probe fails.
+    let cliHasAutocompact: boolean | null = null;
+    let cliHelpVersion: string | null = null;
     const readCliVersion = (env: NodeJS.ProcessEnv): Promise<string | null> =>
       new Promise((resolve) => {
         execCli(config.cli, ["--version"], { timeout: 8000, env }, (err, stdout) =>
@@ -1439,7 +1445,7 @@ export const ClaudeDriver: ProviderDriver<ClaudeConfig> = {
         if (claudeCliSupports(cliVersion, "--setting-sources")) args.push("--setting-sources", "project");
       }
       const compactWindow = autoCompactWindow(turnEnvironment);
-      if (compactWindow && claudeCliSupports(cliVersion, "--autocompact")) {
+      if (compactWindow && (cliHasAutocompact ?? claudeCliSupports(cliVersion, "--autocompact"))) {
         args.push("--autocompact", compactWindow);
       }
       // An old pair conversation can still carry its first assignment in
@@ -2431,6 +2437,13 @@ export const ClaudeDriver: ProviderDriver<ClaudeConfig> = {
       if (!version) return { state: "unavailable", reason: `\`${config.cli}\` CLI not found` };
       cliVersion = parseClaudeCliVersion(version);
       cliVersionChecked = true;
+      if (version !== cliHelpVersion) {
+        const help = await new Promise<string | null>((resolve) => {
+          execCli(config.cli, ["--help"], { timeout: 8000, env }, (err, stdout) => resolve(err ? null : stdout));
+        });
+        cliHasAutocompact = help === null ? null : /^\s*--autocompact\b/m.test(help);
+        cliHelpVersion = version;
+      }
       const update = claudeCliUpdate(version, config.cli);
       const warning = claudeInheritWarning(env);
       if (config.requireApiKey) {
diff --git a/server/testing/fake-claude-cli.ts b/server/testing/fake-claude-cli.ts
index 0008bee95a..6864125bf3 100755
--- a/server/testing/fake-claude-cli.ts
+++ b/server/testing/fake-claude-cli.ts
@@ -237,6 +237,18 @@ if (argv[0] === "--version") {
   process.exit(0);
 }
 
+if (argv[0] === "--help") {
+  // Lists --autocompact in the option column like the real CLI, unless the
+  // fake stands in for a build without it: FAKE_CLAUDE_AUTOCOMPACT=0, or a
+  // version below the 2.1.122 floor.
+  const [maj = 0, min = 0, pat = 0] = (process.env.FAKE_CLAUDE_VERSION ?? "2.1.232").split(".").map(Number);
+  const has = process.env.FAKE_CLAUDE_AUTOCOMPACT !== "0" && (maj > 2 || (maj === 2 && (min > 1 || (min === 1 && pat >= 122))));
+  process.stdout.write(
+    `Usage: claude [options]\n\nOptions:\n  --model <model>  Model\n${has ? "  --autocompact <tokens>  Compaction window\n" : ""}  -h, --help  Display help\n`,
+  );
+  process.exit(0);
+}
+
 if (argv[0] === "update") {
   if (process.env.FAKE_CLAUDE_UPDATE === "fail") {
     process.stderr.write("fake-claude: simulated update failure\n");