diff --git a/src/ClaudeDo.Worker/Lifecycle/ClaudeCliPreflight.cs b/src/ClaudeDo.Worker/Lifecycle/ClaudeCliPreflight.cs
new file mode 100644
index 0000000..1839e19
--- /dev/null
+++ b/src/ClaudeDo.Worker/Lifecycle/ClaudeCliPreflight.cs
@@ -0,0 +1,38 @@
+using System.Diagnostics;
+
+namespace ClaudeDo.Worker.Lifecycle;
+
+public static class ClaudeCliPreflight
+{
+    public sealed record Result(bool Ok, string Version, string Error, int ExitCode);
+
+    public static async Task<Result> CheckAsync(string claudeBin, CancellationToken ct = default)
+    {
+        try
+        {
+            var psi = new ProcessStartInfo
+            {
+                FileName = claudeBin,
+                Arguments = "--version",
+                UseShellExecute = false,
+                RedirectStandardOutput = true,
+                RedirectStandardError = true,
+                CreateNoWindow = true,
+            };
+            using var proc = Process.Start(psi);
+            if (proc is null) return new Result(false, "", "Process.Start returned null", -1);
+
+            var stdoutTask = proc.StandardOutput.ReadToEndAsync(ct);
+            var stderrTask = proc.StandardError.ReadToEndAsync(ct);
+            await proc.WaitForExitAsync(ct);
+
+            var stdout = (await stdoutTask).Trim();
+            var stderr = (await stderrTask).Trim();
+            return new Result(proc.ExitCode == 0, stdout, stderr, proc.ExitCode);
+        }
+        catch (Exception ex)
+        {
+            return new Result(false, "", ex.Message, -1);
+        }
+    }
+}
diff --git a/src/ClaudeDo.Worker/Program.cs b/src/ClaudeDo.Worker/Program.cs
index 54a9a0e..d89ab79 100644
--- a/src/ClaudeDo.Worker/Program.cs
+++ b/src/ClaudeDo.Worker/Program.cs
@@ -141,6 +141,22 @@ app.UseMiddleware<PlanningTokenAuthMiddleware>();
 app.MapHub<WorkerHub>("/hub");
 app.MapMcp("/mcp");
 
+// Claude CLI preflight: fail fast if the configured binary is unreachable or non-zero.
+// Skippable via CLAUDEDO_SKIP_CLI_PREFLIGHT=1 for environments without the CLI (e.g. tests).
+if (Environment.GetEnvironmentVariable("CLAUDEDO_SKIP_CLI_PREFLIGHT") != "1")
+{
+    var preflight = await ClaudeCliPreflight.CheckAsync(cfg.ClaudeBin);
+    if (!preflight.Ok)
+    {
+        app.Logger.LogCritical(
+            "Claude CLI preflight failed (bin: '{Bin}', exit: {Exit}): {Error}. " +
+            "Fix `claude_bin` in worker.config.json or set CLAUDEDO_SKIP_CLI_PREFLIGHT=1 to bypass.",
+            cfg.ClaudeBin, preflight.ExitCode, preflight.Error);
+        Environment.Exit(1);
+    }
+    app.Logger.LogInformation("Claude CLI preflight OK: {Version}", preflight.Version);
+}
+
 app.Logger.LogInformation("ClaudeDo.Worker listening on http://127.0.0.1:{Port} (db: {Db})",
     cfg.SignalRPort, cfg.DbPath);
 
diff --git a/tests/ClaudeDo.Worker.Tests/Lifecycle/ClaudeCliPreflightTests.cs b/tests/ClaudeDo.Worker.Tests/Lifecycle/ClaudeCliPreflightTests.cs
new file mode 100644
index 0000000..2334853
--- /dev/null
+++ b/tests/ClaudeDo.Worker.Tests/Lifecycle/ClaudeCliPreflightTests.cs
@@ -0,0 +1,29 @@
+using ClaudeDo.Worker.Lifecycle;
+using Xunit;
+
+namespace ClaudeDo.Worker.Tests.Lifecycle;
+
+public sealed class ClaudeCliPreflightTests
+{
+    [Fact]
+    public async Task NonExistentBinary_ReturnsNotOk_WithError()
+    {
+        var result = await ClaudeCliPreflight.CheckAsync("definitely-not-a-real-bin-xyz123");
+
+        Assert.False(result.Ok);
+        Assert.NotEqual(0, result.ExitCode);
+        Assert.NotEmpty(result.Error);
+    }
+
+    [Fact]
+    public async Task BinaryAcceptingVersionFlag_ReturnsOk_WithVersionOutput()
+    {
+        // `dotnet --version` is always available in this build environment, exits 0,
+        // and prints the SDK version on stdout — same shape as `claude --version`.
+        var result = await ClaudeCliPreflight.CheckAsync("dotnet");
+
+        Assert.True(result.Ok, $"expected ok, got exit={result.ExitCode} err='{result.Error}'");
+        Assert.Equal(0, result.ExitCode);
+        Assert.NotEmpty(result.Version);
+    }
+}