diff --git a/src/Core/Core.fsproj b/src/Core/Core.fsproj
index 3fc6e80f..99c76640 100644
--- a/src/Core/Core.fsproj
+++ b/src/Core/Core.fsproj
@@ -42,6 +42,7 @@
     <Compile Include="RobustStats.fs" />
     <Compile Include="TemporalCoordinationDetection.fs" />
     <Compile Include="Veridicality.fs" />
+    <Compile Include="Graph.fs" />
     <Compile Include="Window.fs" />
     <Compile Include="Advanced.fs" />
     <Compile Include="Fusion.fs" />
diff --git a/src/Core/Graph.fs b/src/Core/Graph.fs
new file mode 100644
index 00000000..52a16f62
--- /dev/null
+++ b/src/Core/Graph.fs
@@ -0,0 +1,172 @@
+namespace Zeta.Core
+
+
+/// **Graph — ZSet-backed retraction-native graph substrate.**
+///
+/// A `Graph<'N>` is a structural wrapper around `ZSet<'N * 'N>`:
+/// every edge is an entry in the underlying ZSet with a signed
+/// `Weight`. Add-edge is a ZSet add; remove-edge is a ZSet sub.
+/// Net-zero entries compact by the existing ZSet consolidation
+/// pass (Spine-backed when persisted).
+///
+/// **Design contract:** `docs/DECISIONS/2026-04-24-graph-
+/// substrate-zset-backed-retraction-native.md` (Otto-123 ADR)
+/// codifies the 5 tightness properties: ZSet-backed, first-class
+/// event support, retractable, storage-format tight, operator-
+/// algebra composable.
+///
+/// **Attribution.**
+/// * Aaron — design bar ("tight in all aspects") Otto-121
+/// * Amara — formalization (11th + 12th + 13th + 14th ferries
+///   + validation-bar Otto-122 "can it detect a dumb cartel in
+///   a toy simulation?")
+/// * Otto — implementation (8th graduation under Otto-105
+///   cadence; first module that completes a cross-ferry arc
+///   from concept to running substrate)
+///
+/// **Scope of this first graduation.** Core type + minimal
+/// mutation operators + node/edge accessors + retraction-
+/// conservation property test. Detection primitives
+/// (`largestEigenvalue`, `modularityScore`) and toy cartel
+/// detector ship in follow-up PRs composing on this
+/// foundation. Splitting across multiple PRs per Otto-105
+/// small-graduation cadence; the ADR's single-PR preference
+/// defers to cadence discipline.
+///
+/// **Graph event semantics.** Directed edges by default. Multi-
+/// edges supported by ZSet weight (weight = count). Self-loops
+/// allowed (source = target is a legal edge). Nodes derived
+/// from edge-endpoint set — MVP; standalone-node tracking
+/// deferred to future graduation if needed.
+[<AutoOpen>]
+module Graph =
+
+    /// A directed-edge event emitted when a graph mutates.
+    /// Subscribers consume this via Zeta's existing Circuit /
+    /// Stream machinery — no graph-specific event plumbing.
+    type GraphEvent<'N> =
+        | EdgeAdded of source:'N * target:'N * weight:int64
+        | EdgeRemoved of source:'N * target:'N * weight:int64
+
+    /// `Graph<'N>` — a directed graph with signed-weight edges.
+    /// Internal representation is `ZSet<'N * 'N>` so that every
+    /// existing ZSet operator composes automatically.
+    type Graph<'N when 'N : comparison> =
+        internal
+            { Edges: ZSet<'N * 'N> }
+
+    /// Empty graph — no edges, no nodes.
+    [<GeneralizableValue>]
+    let empty<'N when 'N : comparison> : Graph<'N> =
+        { Edges = ZSet.empty<'N * 'N> }
+
+    /// `isEmpty g` — true when the graph has no edges with
+    /// non-zero weight.
+    let isEmpty (g: Graph<'N>) : bool = ZSet.isEmpty g.Edges
+
+    /// `edgeCount g` — number of distinct edges with non-zero
+    /// weight (multi-edges count as one; weight is not the
+    /// count here — `edgeWeight` exposes the multiplicity).
+    let edgeCount (g: Graph<'N>) : int = ZSet.count g.Edges
+
+    /// `edgeWeight g source target` — the signed multiplicity
+    /// of the edge `source → target`. Returns 0 when the edge
+    /// is absent or has been fully retracted.
+    let edgeWeight (source: 'N) (target: 'N) (g: Graph<'N>) : int64 =
+        ZSet.lookup (source, target) g.Edges
+
+    /// `addEdge g source target weight` — add `weight` to the
+    /// multiplicity of the edge `source → target`. Returns the
+    /// updated graph AND the emitted event. Weight of zero is
+    /// a no-op and emits no event.
+    let addEdge
+            (source: 'N)
+            (target: 'N)
+            (weight: int64)
+            (g: Graph<'N>)
+            : Graph<'N> * GraphEvent<'N> list =
+        if weight = 0L then (g, [])
+        else
+            let delta = ZSet.singleton (source, target) weight
+            let merged = ZSet.add g.Edges delta
+            ({ Edges = merged }, [ EdgeAdded(source, target, weight) ])
+
+    /// `removeEdge g source target weight` — subtract `weight`
+    /// from the multiplicity of the edge. NON-DESTRUCTIVE:
+    /// emits a negative-weight ZSet delta; if the result
+    /// net-zeros, ZSet consolidation drops the entry but the
+    /// Spine trace preserves the history. Weight of zero is
+    /// a no-op.
+    let removeEdge
+            (source: 'N)
+            (target: 'N)
+            (weight: int64)
+            (g: Graph<'N>)
+            : Graph<'N> * GraphEvent<'N> list =
+        if weight = 0L then (g, [])
+        else
+            let delta = ZSet.singleton (source, target) (-weight)
+            let merged = ZSet.add g.Edges delta
+            ({ Edges = merged }, [ EdgeRemoved(source, target, weight) ])
+
+    /// `fromEdgeSeq edges` — build a graph from an unordered
+    /// sequence of `(source, target, weight)` triples.
+    /// Duplicates sum via the underlying ZSet; zero-weight
+    /// triples are dropped.
+    let fromEdgeSeq (edges: ('N * 'N * int64) seq) : Graph<'N> =
+        let pairs = edges |> Seq.map (fun (s, t, w) -> (s, t), w)
+        { Edges = ZSet.ofSeq pairs }
+
+    /// `nodes g` — the set of nodes that appear as an endpoint
+    /// of any edge with non-zero weight. Derived from the edge
+    /// set; standalone-node tracking is a future graduation
+    /// candidate.
+    let nodes (g: Graph<'N>) : Set<'N> =
+        let mutable acc = Set.empty
+        let span = g.Edges.AsSpan()
+        for i in 0 .. span.Length - 1 do
+            let entry = span.[i]
+            let (s, t) = entry.Key
+            acc <- acc.Add s |> Set.add t
+        acc
+
+    /// `nodeCount g` — `|nodes g|`.
+    let nodeCount (g: Graph<'N>) : int = (nodes g).Count
+
+    /// `outNeighbors source g` — for each `(source, t, w)` in
+    /// the graph with non-zero `w`, return `(t, w)`. Does not
+    /// traverse; direct lookup against the edge set.
+    let outNeighbors (source: 'N) (g: Graph<'N>) : ('N * int64) list =
+        let span = g.Edges.AsSpan()
+        let mutable acc = []
+        for i in 0 .. span.Length - 1 do
+            let entry = span.[i]
+            let (s, t) = entry.Key
+            if s = source && entry.Weight <> 0L then
+                acc <- (t, entry.Weight) :: acc
+        List.rev acc
+
+    /// `inNeighbors target g` — dual of `outNeighbors`, edges
+    /// where `_ → target`.
+    let inNeighbors (target: 'N) (g: Graph<'N>) : ('N * int64) list =
+        let span = g.Edges.AsSpan()
+        let mutable acc = []
+        for i in 0 .. span.Length - 1 do
+            let entry = span.[i]
+            let (s, t) = entry.Key
+            if t = target && entry.Weight <> 0L then
+                acc <- (s, entry.Weight) :: acc
+        List.rev acc
+
+    /// `degree n g` — total in-degree + out-degree of `n`
+    /// (counting each edge-weight once). Self-loops count
+    /// twice (once as in-edge, once as out-edge).
+    let degree (n: 'N) (g: Graph<'N>) : int64 =
+        let span = g.Edges.AsSpan()
+        let mutable acc = 0L
+        for i in 0 .. span.Length - 1 do
+            let entry = span.[i]
+            let (s, t) = entry.Key
+            if s = n then acc <- acc + entry.Weight
+            if t = n then acc <- acc + entry.Weight
+        acc
diff --git a/tests/Tests.FSharp/Algebra/Graph.Tests.fs b/tests/Tests.FSharp/Algebra/Graph.Tests.fs
new file mode 100644
index 00000000..8168a23c
--- /dev/null
+++ b/tests/Tests.FSharp/Algebra/Graph.Tests.fs
@@ -0,0 +1,165 @@
+module Zeta.Tests.Algebra.GraphTests
+
+open FsUnit.Xunit
+open global.Xunit
+open Zeta.Core
+
+
+// ─── empty + basic accessors ─────────
+
+[<Fact>]
+let ``empty graph has zero edges and zero nodes`` () =
+    let g : Graph<int> = Graph.empty
+    Graph.isEmpty g |> should equal true
+    Graph.edgeCount g |> should equal 0
+    Graph.nodeCount g |> should equal 0
+
+[<Fact>]
+let ``edgeWeight returns 0 for absent edge`` () =
+    let g : Graph<int> = Graph.empty
+    Graph.edgeWeight 1 2 g |> should equal 0L
+
+
+// ─── addEdge ─────────
+
+[<Fact>]
+let ``addEdge sets edge weight and emits EdgeAdded event`` () =
+    let (g, events) = Graph.addEdge 1 2 5L Graph.empty
+    Graph.edgeWeight 1 2 g |> should equal 5L
+    events |> should equal [ EdgeAdded(1, 2, 5L) ]
+
+[<Fact>]
+let ``addEdge with zero weight is a no-op`` () =
+    let (g, events) = Graph.addEdge 1 2 0L Graph.empty
+    Graph.isEmpty g |> should equal true
+    events |> should equal ([]: GraphEvent<int> list)
+
+[<Fact>]
+let ``addEdge accumulates multi-edge weight`` () =
+    // Multi-edges are supported via ZSet signed-weight:
+    // two adds on the same edge sum to multiplicity 7.
+    let (g1, _) = Graph.addEdge 1 2 3L Graph.empty
+    let (g2, _) = Graph.addEdge 1 2 4L g1
+    Graph.edgeWeight 1 2 g2 |> should equal 7L
+    Graph.edgeCount g2 |> should equal 1
+
+
+// ─── removeEdge / retraction-native ─────────
+
+[<Fact>]
+let ``removeEdge subtracts weight and emits EdgeRemoved event`` () =
+    let (g1, _) = Graph.addEdge 1 2 5L Graph.empty
+    let (g2, events) = Graph.removeEdge 1 2 5L g1
+    Graph.edgeWeight 1 2 g2 |> should equal 0L
+    Graph.isEmpty g2 |> should equal true
+    events |> should equal [ EdgeRemoved(1, 2, 5L) ]
+
+[<Fact>]
+let ``removeEdge partial retraction leaves remainder`` () =
+    // Retraction-native: remove 3 from an edge of weight 5
+    // leaves weight 2, not "edge deleted".
+    let (g1, _) = Graph.addEdge 1 2 5L Graph.empty
+    let (g2, _) = Graph.removeEdge 1 2 3L g1
+    Graph.edgeWeight 1 2 g2 |> should equal 2L
+    Graph.edgeCount g2 |> should equal 1
+
+[<Fact>]
+let ``retraction-conservation: addEdge then removeEdge restores empty`` () =
+    // The load-bearing property from the ADR: apply(delta)
+    // followed by apply(-delta) restores prior state modulo
+    // compaction metadata.
+    let (g1, _) = Graph.addEdge 1 2 7L Graph.empty
+    let (g2, _) = Graph.removeEdge 1 2 7L g1
+    Graph.isEmpty g2 |> should equal true
+
+[<Fact>]
+let ``removeEdge on absent edge produces net-negative weight`` () =
+    // Remove-before-add is legal — ZSet signed-weight means
+    // the result is an anti-edge (negative multiplicity).
+    // Adding it later will cancel. This is what makes
+    // retraction-native counterfactuals O(|delta|).
+    let (g, _) = Graph.removeEdge 1 2 3L Graph.empty
+    Graph.edgeWeight 1 2 g |> should equal -3L
+
+
+// ─── nodes + neighbors ─────────
+
+[<Fact>]
+let ``nodes derives from edge endpoints`` () =
+    let g =
+        Graph.fromEdgeSeq [
+            (1, 2, 1L)
+            (2, 3, 1L)
+            (3, 1, 1L)
+        ]
+    Graph.nodes g |> should equal (Set.ofList [1; 2; 3])
+    Graph.nodeCount g |> should equal 3
+
+[<Fact>]
+let ``outNeighbors lists target nodes and weights`` () =
+    let g =
+        Graph.fromEdgeSeq [
+            (1, 2, 3L)
+            (1, 3, 5L)
+            (2, 3, 1L)
+        ]
+    let ns = Graph.outNeighbors 1 g
+    ns |> should equal [ (2, 3L); (3, 5L) ]
+
+[<Fact>]
+let ``inNeighbors is dual of outNeighbors`` () =
+    let g =
+        Graph.fromEdgeSeq [
+            (1, 3, 5L)
+            (2, 3, 1L)
+        ]
+    let ns = Graph.inNeighbors 3 g
+    ns |> List.sortBy fst |> should equal [ (1, 5L); (2, 1L) ]
+
+[<Fact>]
+let ``degree sums in+out edge weights`` () =
+    let g =
+        Graph.fromEdgeSeq [
+            (1, 2, 3L)    // out-edge for 1
+            (2, 1, 4L)    // in-edge for 1
+            (1, 3, 5L)    // out-edge for 1
+        ]
+    Graph.degree 1 g |> should equal (3L + 4L + 5L)
+
+
+// ─── self-loop support ─────────
+
+[<Fact>]
+let ``self-loop is a legal edge (source = target)`` () =
+    let (g, _) = Graph.addEdge 1 1 5L Graph.empty
+    Graph.edgeCount g |> should equal 1
+    Graph.edgeWeight 1 1 g |> should equal 5L
+
+[<Fact>]
+let ``self-loop counts twice in degree (once in, once out)`` () =
+    let (g, _) = Graph.addEdge 1 1 3L Graph.empty
+    Graph.degree 1 g |> should equal 6L  // 3 in + 3 out
+
+
+// ─── fromEdgeSeq ─────────
+
+[<Fact>]
+let ``fromEdgeSeq sums duplicate edges`` () =
+    let g =
+        Graph.fromEdgeSeq [
+            (1, 2, 3L)
+            (1, 2, 4L)
+        ]
+    Graph.edgeWeight 1 2 g |> should equal 7L
+    Graph.edgeCount g |> should equal 1
+
+[<Fact>]
+let ``fromEdgeSeq drops zero-weight triples`` () =
+    let g =
+        Graph.fromEdgeSeq [
+            (1, 2, 0L)
+            (2, 3, 1L)
+            (3, 4, 0L)
+        ]
+    Graph.edgeCount g |> should equal 1
+    Graph.edgeWeight 2 3 g |> should equal 1L
diff --git a/tests/Tests.FSharp/Tests.FSharp.fsproj b/tests/Tests.FSharp/Tests.FSharp.fsproj
index 9c6d4286..5799674c 100644
--- a/tests/Tests.FSharp/Tests.FSharp.fsproj
+++ b/tests/Tests.FSharp/Tests.FSharp.fsproj
@@ -20,6 +20,7 @@
     <Compile Include="Algebra/RobustStats.Tests.fs" />
     <Compile Include="Algebra/TemporalCoordinationDetection.Tests.fs" />
     <Compile Include="Algebra/Veridicality.Tests.fs" />
+    <Compile Include="Algebra/Graph.Tests.fs" />
 
     <!-- Circuit/ -->
     <Compile Include="Circuit/Circuit.Tests.fs" />