diff --git a/Compono.slnx b/Compono.slnx
index e023dd0..5ec5559 100644
--- a/Compono.slnx
+++ b/Compono.slnx
@@ -3,6 +3,7 @@
     <Project Path="src/Compono/Compono.csproj" />
     <Project Path="src/Compono.Generators/Compono.Generators.csproj" />
     <Project Path="src/Compono.XunitV3/Compono.XunitV3.csproj" />
+    <Project Path="src/Compono.NSubstitute/Compono.NSubstitute.csproj" />
   </Folder>
   <Folder Name="/test/">
     <Project Path="test/Compono.Tests/Compono.Tests.csproj" />
diff --git a/docs/mvp.md b/docs/mvp.md
index 3cb86d4..86b4511 100644
--- a/docs/mvp.md
+++ b/docs/mvp.md
@@ -270,10 +270,12 @@ package: substitutable-shape rules including delegate types, `NSubstituteOptions
 diagnostics). **ADR-0024's core engine extension point is implemented
 (PLAN-0005 Phase 0)** — `builder.AddSemanticProvider(...)`/
 `builder.AddTestDoubleProvider(...)` are real, tested public API today.
-**`Compono.NSubstitute` itself (ADR-0025) is not yet implemented** — see
+**`Compono.NSubstitute` itself (ADR-0025) is implemented (PLAN-0005 Phase 1)**
+— `NSubstituteProvider`/`NSubstituteOptions`/`UseNSubstitute()` are real code,
+but not yet test-covered or end-to-end verified (PLAN-0005 Phase 2) — see
 [PLAN-0005](plans/0005-milestone-5-nsubstitute-integration.md) for the
 phase-by-phase tracker; this milestone's own exit criteria aren't met until
-that package ships.
+that verification lands.
 
 ### Scope
 
diff --git a/docs/plans/0005-milestone-5-nsubstitute-integration.md b/docs/plans/0005-milestone-5-nsubstitute-integration.md
index 3a12435..16784e0 100644
--- a/docs/plans/0005-milestone-5-nsubstitute-integration.md
+++ b/docs/plans/0005-milestone-5-nsubstitute-integration.md
@@ -123,15 +123,15 @@ Each phase ships as its own PR, per `design-decisions.md`'s phase rule.
 
 ### Phase 1: `Compono.NSubstitute` package (ADR-0025)
 
-**Status:** Not Started
+**Status:** Done
 
-- [ ] New `src/Compono.NSubstitute/Compono.NSubstitute.csproj` (matching
+- [x] New `src/Compono.NSubstitute/Compono.NSubstitute.csproj` (matching
       `Compono.XunitV3.csproj`'s TFM/packaging shape — `PackageReference` to
       `Compono` + `NSubstitute`, `PrivateAssets="none"` on the `Compono`
       reference per PLAN-0004 Phase 3's real packaging-bug lesson).
-- [ ] `NSubstituteOptions`: `SubstituteAbstractClasses` (`bool`, default
+- [x] `NSubstituteOptions`: `SubstituteAbstractClasses` (`bool`, default
       `true`).
-- [ ] `NSubstituteProvider : ICompositionValueProvider`: the
+- [x] `NSubstituteProvider : ICompositionValueProvider`: the
       `IsSubstitutable` static check (interface / delegate / conditionally
       unsealed-abstract-class), `Substitute.For(Type[], object[])` call for
       a match, `NotHandled` otherwise, no `context.Resolve<T>()` call
@@ -146,7 +146,7 @@ Each phase ships as its own PR, per `design-decisions.md`'s phase rule.
       **not** `typeof(Delegate).IsAssignableFrom(requestedType)` — the
       latter wrongly matches the non-substitutable `Delegate`/
       `MulticastDelegate` base types themselves.
-- [ ] `CompositionBuilderExtensions.UseNSubstitute()`/
+- [x] `CompositionBuilderExtensions.UseNSubstitute()`/
       `UseNSubstitute(Action<NSubstituteOptions>)` extension methods
       (C# 14 extension-block syntax, per `coding-standards.md`).
 
@@ -220,12 +220,22 @@ Each phase ships as its own PR, per `design-decisions.md`'s phase rule.
       versus `Compono.NSubstitute` itself not yet — done early, same PR
       #28 review round. Delegate substitution already in the scope list
       from the design-phase draft.
-- [ ] `docs/mvp.md`: mark this milestone's exit criteria met once
-      `Compono.NSubstitute` actually ships (Phase 1/2) — still open.
+- [x] `docs/mvp.md`: updated again in the Phase 1 PR (Codex, P2, same
+      pattern as the PR #28 round above) — `Compono.NSubstitute` (ADR-0025)
+      now says "implemented (PLAN-0005 Phase 1)," with test
+      coverage/end-to-end verification called out as still pending
+      (Phase 2), rather than leaving the whole package described as "not
+      yet implemented" once the Phase 1 PR actually shipped it. Milestone
+      exit criteria still correctly not marked met — that still needs
+      Phase 2.
 - [x] `docs/public-api.md`: Provider Extensibility section now says
       "Implemented" rather than "design target" — done early, same PR #28
       review round. NSubstitute Integration section correctly still says
       "not yet implemented."
+- [x] `docs/public-api.md`: NSubstitute Integration section updated again
+      in the Phase 1 PR (Codex, P2) — now says "Implemented (PLAN-0005
+      Phase 1)" rather than "not yet implemented," same reasoning as the
+      `docs/mvp.md` update above.
 - [x] `docs/public-api.md`: Naming Vocabulary gains `ICompositionValueProvider`
       — already added during the design phase.
 - [x] `docs/adr/README.md`/`docs/plans/README.md` index rows (already added
@@ -317,6 +327,39 @@ alpha must:
 
 ## Notes
 
+**Phase 1 (Done):**
+
+- Implemented exactly per ADR-0025's Amendment 1 corrected shapes — no
+  deviation from the ADR's own code sketches for `NSubstituteOptions`,
+  `NSubstituteProvider`, or `CompositionBuilderExtensions`.
+- `Compono.NSubstitute.csproj` mirrors `Compono.XunitV3.csproj`'s shape:
+  `net10.0;net11.0` TFMs, `ProjectReference` to `Compono` with
+  `PrivateAssets="none"` (PLAN-0004 Phase 3's packaging lesson, applied
+  proactively rather than rediscovered), `PackageReference` to
+  `NSubstitute` (version centrally managed, already present in
+  `Directory.Packages.props` from the test-project usage), and
+  `InternalsVisibleTo` for the not-yet-created `Compono.NSubstitute.Tests`
+  (Phase 2) so `IsSubstitutable` can stay `internal`.
+  `docs/architecture.md`'s package-dependency diagram already lists
+  `Compono.NSubstitute → Compono, NSubstitute` — matched exactly, no
+  reference to any other package.
+- Added to `Compono.slnx`. Whole-solution `dotnet build` green, no warnings.
+- No test project yet (Phase 2) — `IsSubstitutable`/`NSubstituteProvider`/
+  `UseNSubstitute` are implemented but only build-verified in this phase,
+  not test-verified; that's Phase 2's job per the plan's own phase split.
+- **PR review (Codex, one P2 finding) caught the same doc-staleness pattern
+  PR #28's second review round caught for Phase 0.** `docs/public-api.md`'s
+  NSubstitute Integration section and `docs/mvp.md`'s Milestone 5 section
+  both still said `Compono.NSubstitute` was "not yet implemented" after
+  this phase's package/provider/`UseNSubstitute()` code had already landed
+  in the same PR — pulled forward and fixed in this PR rather than
+  deferred to Phase 3, on the same reasoning as the earlier round: leaving
+  the docs contradict the code until a later phase lands is worse than a
+  small out-of-phase doc correction. Both now say "implemented (PLAN-0005
+  Phase 1)" and explicitly call out that test coverage/end-to-end
+  verification remain pending (Phase 2) — this milestone's exit criteria
+  still correctly not marked met.
+
 **Phase 0 (Done):**
 
 - Implemented in the same branch/PR as the design docs, per explicit user
diff --git a/docs/public-api.md b/docs/public-api.md
index 18e66c7..0a336be 100644
--- a/docs/public-api.md
+++ b/docs/public-api.md
@@ -422,9 +422,11 @@ same interface, registered via `AddSemanticProvider` instead of
 
 ## NSubstitute Integration
 
-Design target, resolved (not yet implemented) by
+Implemented (PLAN-0005 Phase 1) per
 [ADR-0025](adr/0025-compono-nsubstitute-package-design.md), built on the Provider
-Extensibility contract above.
+Extensibility contract above — `Compono.NSubstitute`'s `NSubstituteProvider`/
+`NSubstituteOptions`/`UseNSubstitute()` are real code today. Test coverage and
+end-to-end verification are still pending (PLAN-0005 Phase 2).
 
 Activation:
 
diff --git a/src/Compono.NSubstitute/Compono.NSubstitute.csproj b/src/Compono.NSubstitute/Compono.NSubstitute.csproj
new file mode 100644
index 0000000..f11047d
--- /dev/null
+++ b/src/Compono.NSubstitute/Compono.NSubstitute.csproj
@@ -0,0 +1,23 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+    <PropertyGroup>
+        <LangVersion>latest</LangVersion>
+        <ImplicitUsings>enable</ImplicitUsings>
+        <Nullable>enable</Nullable>
+        <TargetFrameworks>net10.0;net11.0</TargetFrameworks>
+        <Description>NSubstitute integration for Compono - a test-double provider composing interface, delegate, and (optionally) abstract-class parameters as real NSubstitute substitutes. See docs/adr/0024-public-provider-extensibility-model.md and docs/adr/0025-compono-nsubstitute-package-design.md.</Description>
+    </PropertyGroup>
+
+    <ItemGroup>
+        <!-- Compono packs its source generator as an analyzer asset. PrivateAssets="none" allows
+             that analyzer to flow through this integration package to consumers that reference
+             only Compono.NSubstitute. -->
+        <ProjectReference Include="..\Compono\Compono.csproj" PrivateAssets="none" />
+        <PackageReference Include="NSubstitute" />
+    </ItemGroup>
+
+    <ItemGroup>
+        <InternalsVisibleTo Include="Compono.NSubstitute.Tests" />
+    </ItemGroup>
+
+</Project>
diff --git a/src/Compono.NSubstitute/CompositionBuilderExtensions.cs b/src/Compono.NSubstitute/CompositionBuilderExtensions.cs
new file mode 100644
index 0000000..2b7c566
--- /dev/null
+++ b/src/Compono.NSubstitute/CompositionBuilderExtensions.cs
@@ -0,0 +1,28 @@
+namespace Compono;
+
+/// <summary>
+/// Activates <see cref="NSubstituteProvider"/> on a <see cref="CompositionBuilder"/>. See
+/// <c>docs/adr/0025-compono-nsubstitute-package-design.md</c>.
+/// </summary>
+public static class CompositionBuilderExtensions
+{
+    extension(CompositionBuilder builder)
+    {
+        /// <summary>
+        /// Registers an <see cref="NSubstituteProvider"/> with default <see cref="NSubstituteOptions"/>.
+        /// </summary>
+        public CompositionBuilder UseNSubstitute() => builder.UseNSubstitute(static _ => { });
+
+        /// <summary>
+        /// Registers an <see cref="NSubstituteProvider"/>, configured by <paramref name="configure"/>.
+        /// </summary>
+        /// <param name="configure">Sets the provider's <see cref="NSubstituteOptions"/>.</param>
+        public CompositionBuilder UseNSubstitute(Action<NSubstituteOptions> configure)
+        {
+            ArgumentNullException.ThrowIfNull(configure);
+            var options = new NSubstituteOptions();
+            configure(options);
+            return builder.AddTestDoubleProvider(new NSubstituteProvider(options));
+        }
+    }
+}
diff --git a/src/Compono.NSubstitute/NSubstituteOptions.cs b/src/Compono.NSubstitute/NSubstituteOptions.cs
new file mode 100644
index 0000000..11cde0d
--- /dev/null
+++ b/src/Compono.NSubstitute/NSubstituteOptions.cs
@@ -0,0 +1,15 @@
+namespace Compono;
+
+/// <summary>
+/// Configuration for <see cref="NSubstituteProvider"/>, set via
+/// <c>CompositionBuilderExtensions.UseNSubstitute(Action{NSubstituteOptions})</c>. See
+/// <c>docs/adr/0025-compono-nsubstitute-package-design.md</c>.
+/// </summary>
+public sealed class NSubstituteOptions
+{
+    /// <summary>
+    /// Whether an unsealed abstract class is substitutable, in addition to every interface and
+    /// delegate type. Defaults to <see langword="true"/>.
+    /// </summary>
+    public bool SubstituteAbstractClasses { get; set; } = true;
+}
diff --git a/src/Compono.NSubstitute/NSubstituteProvider.cs b/src/Compono.NSubstitute/NSubstituteProvider.cs
new file mode 100644
index 0000000..eaf4c63
--- /dev/null
+++ b/src/Compono.NSubstitute/NSubstituteProvider.cs
@@ -0,0 +1,46 @@
+using NSubstitute;
+
+namespace Compono;
+
+/// <summary>
+/// A stage-6 test-double provider that composes an interface, delegate, or (when
+/// <see cref="NSubstituteOptions.SubstituteAbstractClasses"/> allows it) unsealed abstract-class
+/// request as a real NSubstitute substitute, via <see cref="Substitute.For(Type[], object[])"/>.
+/// Registered via <c>CompositionBuilderExtensions.UseNSubstitute()</c>. See
+/// <c>docs/adr/0025-compono-nsubstitute-package-design.md</c> (Amendment 1 for the corrected shape
+/// below).
+/// </summary>
+public sealed class NSubstituteProvider : ICompositionValueProvider
+{
+    private readonly bool _substituteAbstractClasses;
+
+    /// <summary>
+    /// Creates an <see cref="NSubstituteProvider"/>, snapshotting <paramref name="options"/>'s
+    /// current values - the provider never retains the caller-owned <see cref="NSubstituteOptions"/>
+    /// instance itself, so a caller mutating it after construction can't change this
+    /// already-registered provider's behavior (ADR-0024's immutable-provider-registration guarantee).
+    /// </summary>
+    /// <param name="options">The options to snapshot.</param>
+    public NSubstituteProvider(NSubstituteOptions options)
+    {
+        ArgumentNullException.ThrowIfNull(options);
+        _substituteAbstractClasses = options.SubstituteAbstractClasses;
+    }
+
+    /// <inheritdoc />
+    public CompositionProviderResult TryProvide(in CompositionProviderRequest request, ICompositionContext context)
+    {
+        if (!IsSubstitutable(request.RequestedType, _substituteAbstractClasses))
+            return CompositionProviderResult.NotHandled;
+
+        return CompositionProviderResult.Handled(Substitute.For([request.RequestedType], []));
+    }
+
+    // ADR-0025 Amendment 1: IsSubclassOf(typeof(MulticastDelegate)), not
+    // typeof(Delegate).IsAssignableFrom(requestedType) - the latter wrongly matches the
+    // non-substitutable Delegate/MulticastDelegate framework base types themselves.
+    internal static bool IsSubstitutable(Type requestedType, bool substituteAbstractClasses) =>
+        requestedType.IsInterface
+        || requestedType.IsSubclassOf(typeof(MulticastDelegate))
+        || (substituteAbstractClasses && requestedType.IsAbstract && !requestedType.IsSealed && !requestedType.IsInterface);
+}
diff --git a/src/Compono.XunitV3/Compono.XunitV3.csproj b/src/Compono.XunitV3/Compono.XunitV3.csproj
index 699e03b..3fe7b91 100644
--- a/src/Compono.XunitV3/Compono.XunitV3.csproj
+++ b/src/Compono.XunitV3/Compono.XunitV3.csproj
@@ -9,17 +9,10 @@
     </PropertyGroup>
 
     <ItemGroup>
-        <!-- PrivateAssets="none" (PLAN-0004 Phase 3 real-packaged-consumer verification) - an ordinary
-             PackageReference/ProjectReference defaults PrivateAssets to "contentfiles;build;analyzers",
-             meaning those asset kinds stop at this package and never flow to a consumer of
-             Compono.XunitV3 - dotnet pack renders that default as exclude="Build,Analyzers" on the
-             generated <dependency> for Compono in this package's own nuspec. Left at its default, a
-             project that references only the Compono.XunitV3 package (never Compono directly - the
-             whole point of this package existing) never gets Compono.Generators transitively, and every
-             [Compose]-attributed type silently falls back to "no generated plan" - a real regression a
-             ProjectReference-based build can't surface. Caught by
-             test/Compono.XunitV3.SampleTests failing to compose a type reached only via a
-             [Compose]-attributed method until this was added. -->
+        <!-- Compono packs its source generator as an analyzer asset. PrivateAssets="none" allows
+             that analyzer to flow through this integration package to consumers that reference
+             only Compono.XunitV3 - caught by test/Compono.XunitV3.SampleTests failing to compose
+             a [Compose]-attributed type until this was added (PLAN-0004 Phase 3). -->
         <ProjectReference Include="..\Compono\Compono.csproj" PrivateAssets="none" />
         <!-- Ordinary reference, not PrivateAssets="all" - ComposeAttribute/ComposeAttribute{TProfile}
              derive from and return types this package owns publicly, so a consumer needs them