diff --git a/packages/adapters/claude-local/src/server/cli-capabilities.ts b/packages/adapters/claude-local/src/server/cli-capabilities.ts
index 92964823498..fb345b30103 100644
--- a/packages/adapters/claude-local/src/server/cli-capabilities.ts
+++ b/packages/adapters/claude-local/src/server/cli-capabilities.ts
@@ -78,9 +78,12 @@ export async function claudeCommandSupportsEffortFlag(input: {
   const cached = effortFlagSupportCache.get(key);
   if (cached) return cached;
 
-  const probe = probeClaudeCommandSupportsEffortFlag(input).catch((error) => {
+  // A thrown probe (e.g. sandbox connection error, ENOENT spawning the binary)
+  // must degrade to the conservative fallback rather than killing the run, so we
+  // resolve to null and drop the cache entry to retry on the next lease.
+  const probe = probeClaudeCommandSupportsEffortFlag(input).catch(() => {
     effortFlagSupportCache.delete(key);
-    throw error;
+    return null;
   });
   effortFlagSupportCache.set(key, probe);
   return probe;
diff --git a/packages/adapters/claude-local/src/server/index.ts b/packages/adapters/claude-local/src/server/index.ts
index 57a64c6bd65..31ba501f4fc 100644
--- a/packages/adapters/claude-local/src/server/index.ts
+++ b/packages/adapters/claude-local/src/server/index.ts
@@ -2,7 +2,10 @@ export { claudeSessionCwdMatchesExecutionTarget, execute, runClaudeLogin } from
 export { listClaudeSkills, syncClaudeSkills } from "./skills.js";
 export { listClaudeModels, refreshClaudeModels, resetClaudeModelsCacheForTests } from "./models.js";
 export { testEnvironment } from "./test.js";
-export { resetClaudeCliCapabilitiesCacheForTests } from "./cli-capabilities.js";
+export {
+  claudeCommandSupportsEffortFlag,
+  resetClaudeCliCapabilitiesCacheForTests,
+} from "./cli-capabilities.js";
 export {
   parseClaudeStreamJson,
   describeClaudeFailure,
diff --git a/server/src/__tests__/claude-local-execute.test.ts b/server/src/__tests__/claude-local-execute.test.ts
index 0ee4c86177b..281c8527221 100644
--- a/server/src/__tests__/claude-local-execute.test.ts
+++ b/server/src/__tests__/claude-local-execute.test.ts
@@ -4,6 +4,7 @@ import os from "node:os";
 import path from "node:path";
 import { runChildProcess } from "@paperclipai/adapter-utils/server-utils";
 import {
+  claudeCommandSupportsEffortFlag,
   claudeSessionCwdMatchesExecutionTarget,
   execute,
   resetClaudeCliCapabilitiesCacheForTests,
@@ -947,6 +948,48 @@ describe("claude execute", () => {
     }
   }, 10_000);
 
+  it("degrades to the conservative fallback (returns null) when the sandbox probe throws, and retries on the next lease", async () => {
+    let calls = 0;
+    const throwingRunner = {
+      execute: async () => {
+        calls += 1;
+        throw new Error("sandbox connection error");
+      },
+    };
+    const target = {
+      kind: "remote" as const,
+      transport: "sandbox" as const,
+      providerKey: "daytona",
+      environmentId: "env-1",
+      leaseId: "lease-1",
+      remoteCwd: "/remote/workspace",
+      timeoutMs: 30_000,
+      runner: throwingRunner,
+    };
+    const probeInput = {
+      runId: "run-probe-throws",
+      command: "/usr/local/bin/claude",
+      cwd: "/host/workspace",
+      env: {},
+      timeoutSec: 20,
+      graceSec: 5,
+    };
+
+    // A thrown probe must resolve to null (unknown) rather than reject and kill the run.
+    await expect(
+      claudeCommandSupportsEffortFlag({ ...probeInput, target }),
+    ).resolves.toBeNull();
+
+    // The failed result is not cached: a second lease re-probes instead of reusing the rejection.
+    await expect(
+      claudeCommandSupportsEffortFlag({
+        ...probeInput,
+        target: { ...target, leaseId: "lease-2" },
+      }),
+    ).resolves.toBeNull();
+    expect(calls).toBe(2);
+  });
+
   it("allows remote session resumes when saved cwd is the host workspace", () => {
     expect(claudeSessionCwdMatchesExecutionTarget({
       runtimeSessionCwd: "/host/workspace",