diff --git a/docs/config.md b/docs/config.md
index 80e7cef1..52a30324 100644
--- a/docs/config.md
+++ b/docs/config.md
@@ -91,6 +91,42 @@ append a pipeline's `steps` sequence. An overlay replaces sequences as defined
 by ConfigKit, so changing an individual ordered binding requires supplying the
 whole replacement sequence.
 
+## Sibling Inheritance
+
+ConfigKit v0.5.0 supports native `extends` composition in mappings. A directive
+names exactly one immediate sibling in the same mapping; it is not a path or a
+cross-scope lookup. Imports, the selected overlay, and explicit overrides are
+composed before inheritance, which runs before typed decoding and normal
+Notarius validation. Every definition, including a parent and definitions not
+selected for a run, remains subject to ordinary validation.
+
+Child mappings recursively overlay parent mappings. A child sequence, scalar,
+or null replaces the inherited value. This makes pipeline inheritance useful
+for reusing a complete pipeline while replacing whole ordered lists:
+
+~~~yaml
+pipelines:
+  production:
+    input: {module: seriatim}
+    llm_profile: dnd-extraction
+    # Other complete pipeline settings and ordered steps...
+  development:
+    extends: production
+    structured_output_repair_attempts: 0
+~~~
+
+Pipeline names are siblings within `pipelines`; nested mappings such as named
+artifact lanes also have their own sibling namespace. Step entries are a
+sequence, so a step cannot extend another step. Replacing `steps` or a validator
+chain supplies the complete replacement sequence; entries are not appended or
+merged by identifier.
+
+`extends` is now a reserved key in every input mapping, including dynamic
+module options and named maps. Existing ordinary data or entry names using that
+key must be renamed along with any consumer that reads them. Configuration
+overlays, pipeline inheritance, and PromptKit profile `base_profile`
+inheritance are separate mechanisms.
+
 ## PromptKit Profiles
 
 The optional **promptkit** object selects one source of profile definitions and
diff --git a/docs/roadmap/implementation.md b/docs/roadmap/implementation.md
index 3101541b..725c84da 100644
--- a/docs/roadmap/implementation.md
+++ b/docs/roadmap/implementation.md
@@ -108,6 +108,8 @@ adjustments. Leave new inheritance coverage to the following stages.
 
 ## Stage 2 — Prove Pipeline Composition And Runtime Conversion
 
+> Completion: complete.
+
 Goal: establish whole-pipeline reuse through the real configuration boundary.
 
 Inspect `load.go`, `document.go`, `loaded_config.go`, `pipeline_document.go`,
diff --git a/internal/core/config/inheritance_test.go b/internal/core/config/inheritance_test.go
new file mode 100644
index 00000000..55af0387
--- /dev/null
+++ b/internal/core/config/inheritance_test.go
@@ -0,0 +1,133 @@
+package config
+
+import (
+	"testing"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+)
+
+func TestPipelineInheritancePreservesCompositionAndReplacesSteps(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name        string
+		childSteps  string
+		wantStepIDs []string
+	}{
+		{
+			name:        "inherits ordered steps",
+			wantStepIDs: []string{"prepare", "summarize"},
+		},
+		{
+			name: "replaces ordered steps",
+			childSteps: `
+    steps:
+      - id: replacement
+        artifacts:
+          summary:
+            extract: {module: extract}`,
+			wantStepIDs: []string{"replacement"},
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			source := `version: 5
+pipelines:
+  parent:
+    input: {module: input}
+    structured_output_repair_attempts: 2
+    steps:
+      - id: prepare
+        artifacts:
+          score:
+            extract: {module: extract, retries: 2}
+      - id: summarize
+        artifacts:
+          notes:
+            extract: {module: extract}
+  child:
+    extends: parent
+    llm_profile: local
+    structured_output_repair_attempts: 0` + test.childSteps + "\n"
+			loaded, err := loadConfigSource(configkit.Bytes("inheritance.yml", []byte(source)), LoadOptions{})
+			if err != nil {
+				t.Fatalf("loadConfigSource() error = %v", err)
+			}
+
+			parent := loaded.Config.Pipelines["parent"]
+			child := loaded.Config.Pipelines["child"]
+			if parent.Input.Module != "input" || child.Input.Module != "input" {
+				t.Fatalf("parent/child inputs = %#v / %#v", parent.Input, child.Input)
+			}
+			if child.LLMProfile != "local" ||
+				child.StructuredOutputRepairAttempts == nil || *child.StructuredOutputRepairAttempts != 0 {
+				t.Fatalf("child overrides = %#v", child)
+			}
+			if parent.StructuredOutputRepairAttempts == nil || *parent.StructuredOutputRepairAttempts != 2 {
+				t.Fatalf("parent settings changed: %#v", parent)
+			}
+			if len(child.Steps) != len(test.wantStepIDs) {
+				t.Fatalf("child steps = %#v, want %v", child.Steps, test.wantStepIDs)
+			}
+			for i, want := range test.wantStepIDs {
+				if child.Steps[i].ID != want {
+					t.Fatalf("child step %d = %q, want %q", i, child.Steps[i].ID, want)
+				}
+			}
+			if test.childSteps == "" && child.Steps[0].Artifacts["score"].Extract.Module != "extract" {
+				t.Fatalf("inherited first artifact = %#v", child.Steps[0].Artifacts["score"])
+			}
+
+			for _, id := range []string{"parent", "child"} {
+				resolved, err := loaded.Config.Resolve(ResolveInput{PipelineID: id, Catalog: effectiveCatalog(t)})
+				if err != nil {
+					t.Fatalf("Resolve(%q) error = %v", id, err)
+				}
+				if resolved.ResolvedPipeline.ID != id {
+					t.Fatalf("resolved pipeline ID = %q, want %q", resolved.ResolvedPipeline.ID, id)
+				}
+				wantResolvedSteps := len(test.wantStepIDs)
+				if id == "parent" {
+					wantResolvedSteps = 2
+				}
+				if len(resolved.ResolvedPipeline.Steps) != wantResolvedSteps {
+					t.Fatalf("resolved %q steps = %#v", id, resolved.ResolvedPipeline.Steps)
+				}
+				if id == "child" && resolved.ResolvedPipeline.Steps[0].ArtifactLanes[0].Extract.LLMProfile != "local" {
+					t.Fatalf("resolved child profile = %q", resolved.ResolvedPipeline.Steps[0].ArtifactLanes[0].Extract.LLMProfile)
+				}
+			}
+		})
+	}
+}
+
+func TestArtifactLaneInheritanceUsesSiblingMappingComposition(t *testing.T) {
+	t.Parallel()
+
+	loaded, err := loadConfigSource(configkit.Bytes("lanes.yml", []byte(`version: 5
+pipelines:
+  session:
+    input: {module: input}
+    artifacts:
+      base:
+        extract: {module: extract, retries: 2}
+      derived:
+        extends: base
+        extract: {retries: 4}
+`)), LoadOptions{})
+	if err != nil {
+		t.Fatalf("loadConfigSource() error = %v", err)
+	}
+
+	lanes := loaded.Config.Pipelines["session"].Artifacts
+	if lanes["base"].Extract.Module != "extract" || lanes["base"].Extract.Retries != 2 {
+		t.Fatalf("base lane = %#v", lanes["base"])
+	}
+	if lanes["derived"].Extract.Module != "extract" || lanes["derived"].Extract.Retries != 4 {
+		t.Fatalf("derived lane = %#v", lanes["derived"])
+	}
+	if _, err := loaded.Config.Resolve(ResolveInput{PipelineID: "session", Catalog: effectiveCatalog(t)}); err != nil {
+		t.Fatalf("Resolve() error = %v", err)
+	}
+}