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diff --git a/src/SomhairlesDream.Cli/Arguments.fs b/src/SomhairlesDream.Cli/Arguments.fs
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+++ b/src/SomhairlesDream.Cli/Arguments.fs
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+namespace SomhairlesDream.Cli
+
+open System
+open SomhairlesDream.Shared
+
+[<CLIMutable>]
+type RunOptions =
+    { Ids: RunIds
+      OutputRoot: string
+      StageRoot: string
+      Python: string
+      Render: bool }
+
+type Command =
+    | Run of RunOptions
+    | Verify of string
+    | Help
+
+module Arguments =
+    let usage =
+        "usage: dotnet run --project src/SomhairlesDream.Cli -- run [options]\n"
+        + "       dotnet run --project src/SomhairlesDream.Cli -- verify <manifest.json>"
+
+    let private defaultPython () =
+        let configured = Environment.GetEnvironmentVariable("SOMHAIRLES_BPYTHON")
+        if String.IsNullOrWhiteSpace(configured) then "python3" else configured
+
+    let private defaults () =
+        { Ids =
+              { ProjectId = "heritage-001"
+                RunId = "run-001"
+                BaseId = "base-000"
+                TargetId = "target-003" }
+          OutputRoot = ".artifacts"
+          StageRoot = "stage"
+          Python = defaultPython ()
+          Render = false }
+
+    let private requiredValue (args: string array) (index: int) (optionName: string) =
+        if index + 1 >= args.Length || String.IsNullOrWhiteSpace(args[index + 1]) then
+            Error $"{optionName} requires a value"
+        else
+            Ok args[index + 1]
+
+    let private setValue optionName value (options: RunOptions) =
+        match optionName with
+        | "--project-id" -> Ok { options with Ids = { options.Ids with ProjectId = value } }
+        | "--run-id" -> Ok { options with Ids = { options.Ids with RunId = value } }
+        | "--base-id" -> Ok { options with Ids = { options.Ids with BaseId = value } }
+        | "--target-id" -> Ok { options with Ids = { options.Ids with TargetId = value } }
+        | "--output-root" -> Ok { options with OutputRoot = value }
+        | "--stage-root" -> Ok { options with StageRoot = value }
+        | "--python" -> Ok { options with Python = value }
+        | _ -> Error $"unknown option: {optionName}"
+
+    let private validateRunOptions (options: RunOptions) =
+        [| options.Ids.ProjectId; options.Ids.RunId; options.Ids.BaseId; options.Ids.TargetId |]
+        |> Array.tryFind (Ids.validateResourceId >> Result.isError)
+        |> function
+            | Some value -> Error $"invalid resource id: {value}"
+            | None -> Ok(Run options)
+
+    let private parseRun (args: string array) =
+        let rec loop (index: int) (options: RunOptions) =
+            if index >= args.Length then
+                validateRunOptions options
+            else
+                match args[index] with
+                | "--render" -> loop (index + 1) { options with Render = true }
+                | "--help" -> Ok Help
+                | optionName ->
+                    match requiredValue args index optionName with
+                    | Error message -> Error message
+                    | Ok value ->
+                        match setValue optionName value options with
+                        | Error message -> Error message
+                        | Ok updated -> loop (index + 2) updated
+
+        loop 0 (defaults ())
+
+    let parse (args: string array) =
+        match args with
+        | [||]
+        | [| "--help" |]
+        | [| "help" |] -> Ok Help
+        | [| "verify" |] -> Error "verify requires a manifest path"
+        | [| "verify"; path |] when not (String.IsNullOrWhiteSpace(path)) -> Ok(Verify path)
+        | [| "verify"; _ |] -> Error "verify requires a manifest path"
+        | values when values.Length > 2 && values[0] = "verify" -> Error "verify accepts exactly one manifest path"
+        | values when values.Length > 0 && values[0] = "run" -> parseRun (values |> Array.skip 1)
+        | _ -> Error usage