diff --git a/AGENTS.md b/AGENTS.md
index c0fea37b..a7dc8980 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -134,9 +134,9 @@ project:
 ## Markdown Guidance
 
 - Validate Markdown files using `make markdownlint`.
-- Run `make fmt` after any documentation changes to format all Markdown
-  files and fix table markup.
-- Validate Markdown Mermaid diagrams using by running `make nixie`.
+- Run `make fmt` after any documentation changes to format all Markdown files
+  and fix table markup.
+- Validate Markdown Mermaid diagrams by running `make nixie`.
 - Markdown paragraphs and bullet points must be wrapped at 80 columns.
 - Code blocks must be wrapped at 120 columns.
 - Tables and headings must not be wrapped.
diff --git a/docs/netsuke-design.md b/docs/netsuke-design.md
index bcbe3028..e9289375 100644
--- a/docs/netsuke-design.md
+++ b/docs/netsuke-design.md
@@ -62,10 +62,12 @@ before execution, a critical requirement for compatibility with Ninja.
 
    The AST is traversed to construct a canonical, fully resolved Intermediate
    Representation (IR) of the build. This IR represents the build as a static
-   dependency graph, with all file paths, commands, and dependencies explicitly
+   dependency graph with all file paths, commands, and dependencies explicitly
    defined. During this transformation, Netsuke performs critical validation
-   checks, such as verifying the existence of defined rules, detecting circular
-   dependencies, and ensuring all required inputs are accounted for.
+   checks. It verifies the existence of referenced rules, ensures each rule has
+   exactly one of `command` or `script`, and ensures every target specifies
+   exactly one of `rule`, `command`, or `script`. Circular dependencies and
+   missing inputs are also detected at this stage.
 
 5. Stage 5: Ninja Synthesis & Execution
 
@@ -162,13 +164,24 @@ top-level keys.
 
 ### 2.3 Defining `rules`
 
-Each entry in the `rules` list is a mapping that defines a command template.
+Each entry in the `rules` list is a mapping that defines a reusable action.
 
 - `name`: A unique string identifier for the rule.
 
-- `command`: The command string to be executed. This string uses placeholders
-  like `{ins}` and `{outs}` for input and output files. Netsuke's IR-to-Ninja
-  generator will translate these into Ninja's native `$in` and `$out` variables.
+- `command`: A single command string to be executed. This string uses
+  placeholders like `{ins}` and `{outs}` for input and output files. Netsuke's
+  IR-to-Ninja generator will translate these into Ninja's native `$in` and
+  `$out` variables.
+
+- `script`: A multi-line script declared with the YAML `|` block style. The
+  entire block is passed to an interpreter (currently `/bin/sh`).
+
+  Exactly one of `command` or `script` must be provided. The manifest parser
+  enforces this rule to prevent invalid states.
+
+  Internally, these options deserialise into a shared `Recipe` enum tagged with
+  a `kind` field. Serde aliases ensure manifests that omit the tag continue to
+  load correctly.
 
 - `description`: An optional, user-friendly string that is printed to the
   console when the rule is executed. This maps to Ninja's `description` field
@@ -190,6 +203,18 @@ Each entry in `targets` defines a build edge; placing a target in the optional
 - `rule`: The name of the rule (from the `rules` section) to use for building
   this target.
 
+- `command`: A single command string to run directly for this target.
+
+- `script`: A multi-line script passed to the interpreter. When present, it is
+  defined using the YAML `|` block style.
+
+  Only one of `rule`, `command`, or `script` may be specified. The parser
+  validates this exclusivity during deserialisation.
+
+  This union deserialises into the same `Recipe` enum used for rules. The parser
+  enforces that only one variant is present, maintaining backward compatibility
+  through serde aliases when `kind` is omitted.
+
 - `sources`: The input files required by the command. This can be a single
   string or a list of strings.
 
@@ -324,18 +349,29 @@ pub struct NetsukeManifest {
 #[serde(deny_unknown_fields)]
 pub struct Rule {
     pub name: String,
-    pub command: String,
+    pub recipe: Recipe,
     pub description: Option<String>,
     pub deps: Option<String>,
     // Additional fields like 'pool' or 'restat' can be added here
     // to map to more advanced Ninja features.
 }
 
+/// A union of execution styles for both rules and targets.
+#[serde(tag = "kind", rename_all = "lowercase")]
+pub enum Recipe {
+    #[serde(alias = "command")]
+    Command { command: String },
+    #[serde(alias = "script")]
+    Script { script: String },
+    #[serde(alias = "rule")]
+    Rule { rule: String },
+}
+
 /// Represents a single build target or edge in the dependency graph.
 #[serde(deny_unknown_fields)]
 pub struct Target {
     pub name: StringOrList,
-    pub rule: String,
+    pub recipe: Recipe,
 
     #[serde(default)]
     pub sources: StringOrList,
@@ -358,6 +394,7 @@ pub struct Target {
     pub always: bool,
 }
 
+
 /// An enum to handle fields that can be either a single string or a list of strings.
 #[serde(untagged)]
 pub enum StringOrList {
@@ -626,7 +663,7 @@ pub struct BuildGraph {
 
 /// Represents a reusable command, analogous to a Ninja 'rule'.
 pub struct Action {
-    pub command: String,
+    pub recipe: Recipe,
     pub description: Option<String>,
     pub depfile: Option<String>, // Template for the.d file path, e.g., "$out.d"
     pub deps_format: Option<String>, // "gcc" or "msvc"
@@ -705,6 +742,10 @@ structures to the Ninja file syntax.
    file. The command placeholders (`{ins}`, `{outs}`) are replaced with Ninja's
    variables (`$in`, `$out`).
 
+   When an action's `recipe` is a script, the generated rule wraps the script in
+   an invocation of `/bin/sh -e -c` so that multi-line scripts execute
+   consistently across platforms.
+
    Code snippet
 
    ````ninja