feat(plugin): bound vcs adapter diffs

# Conflicts:
#	packages/core/src/config.ts
#	packages/core/src/plugin/internal.ts
#	packages/core/src/project.ts
#	packages/core/test/project.test.ts
#	packages/plugin/src/v2/effect/context.ts
#	packages/plugin/src/v2/effect/index.ts

packages/client/src/promise/generated/types.ts

@@ -2461,7 +2461,7 @@ export type CredentialRemoveOutput = void
 export type ProjectListOutput = ReadonlyArray<{
   readonly id: string
   readonly worktree: string
-  readonly vcs?: "git" | "hg"
+  readonly vcs?: string
   readonly name?: string
   readonly icon?: { readonly url?: string; readonly override?: string; readonly color?: string }
   readonly commands?: { readonly start?: string }

packages/core/src/config.ts

@@ -24,6 +24,7 @@ import { ConfigProvider } from "./config/provider"
 import { ConfigReference } from "./config/reference"
 import { ConfigToolOutput } from "./config/tool-output"
 import { ConfigVariable } from "./config/variable"
+import { ConfigVcs } from "./config/vcs"
 import { ConfigWatcher } from "./config/watcher"
 import { ConfigV1 } from "./v1/config/config"
 import { ConfigMigrateV1 } from "./v1/config/migrate"
@@ -104,6 +105,10 @@ export class Info extends Schema.Class<Info>("Config.Info")({
   plugins: ConfigPlugin.Plugins.pipe(Schema.optional).annotate({
     description: "Ordered plugin enablement directives and external package declarations",
   }),
+  vcs: ConfigVcs.Info.pipe(Schema.optional).annotate({
+    description:
+      "Plugin-provided VCS backends keyed by type; detection markers are only honored in global configuration",
+  }),
   providers: Schema.Record(Schema.String, ConfigProvider.Info).pipe(Schema.optional),
 }) {}
 

packages/core/src/config/vcs.ts

@@ -0,0 +1,57 @@
+export * as ConfigVcs from "./vcs"
+
+import path from "path"
+import { Effect, Option, Schema } from "effect"
+import { parse, type ParseError } from "jsonc-parser"
+import { FSUtil } from "../fs-util"
+
+const RESERVED = new Set(["git", "hg"])
+
+export const Type = Schema.String.check(Schema.isPattern(/^[a-z][a-z0-9-]{0,31}$/)).check(
+  Schema.makeFilter<string>((value) =>
+    RESERVED.has(value) ? `'${value}' has built-in detection and cannot be redeclared` : undefined,
+  ),
+)
+
+const Marker = Schema.String.check(
+  Schema.makeFilter<string>((value) => {
+    if (!value || value === "." || value === ".." || /[\\/]/.test(value)) {
+      return `marker must be a single path segment such as ".jj"`
+    }
+    return undefined
+  }),
+)
+
+export class Backend extends Schema.Class<Backend>("ConfigV2.Vcs.Backend")({
+  marker: Marker.annotate({
+    description: 'Directory name that marks a repository root for this backend, such as ".jj"',
+  }),
+}) {}
+
+export const Info = Schema.Record(Type, Backend).check(
+  Schema.makeFilter<Readonly<Record<string, Backend>>>((value) => {
+    const markers = Object.values(value).map((backend) => backend.marker)
+    return new Set(markers).size === markers.length ? undefined : "vcs backends must declare distinct markers"
+  }),
+)
+export type Info = typeof Info.Type
+
+const decode = Schema.decodeUnknownOption(Schema.Struct({ vcs: Info.pipe(Schema.optional) }), {
+  onExcessProperty: "ignore",
+})
+
+export const readGlobal = Effect.fnUntraced(function* (fs: FSUtil.Interface, configDirectory: string) {
+  const backends = new Map<string, Backend>()
+  for (const name of ["opencode.json", "opencode.jsonc"]) {
+    const text = yield* fs
+      .readFileStringSafe(path.join(configDirectory, name))
+      .pipe(Effect.catch(() => Effect.succeed(undefined)))
+    if (!text) continue
+    const errors: ParseError[] = []
+    const input: unknown = parse(text, errors, { allowTrailingComma: true })
+    if (errors.length) continue
+    const info = Option.getOrUndefined(decode(input))?.vcs
+    for (const [type, backend] of Object.entries(info ?? {})) backends.set(type, backend)
+  }
+  return backends
+})

packages/core/src/location-services.ts

@@ -54,6 +54,7 @@ import { ToolRegistry } from "./tool/registry"
 import { WebSearchTool } from "./tool/websearch"
 import { ToolOutputStore } from "./tool-output-store"
 import { Vcs } from "./vcs"
+import { VcsBackends } from "./vcs/backends"
 
 export { LocationServiceMap } from "./location-service-map"
 
@@ -90,6 +91,7 @@ const pluginSupervisorNode = makeLocationNode({
     Shell.node,
     SkillV2.node,
     ToolRegistry.toolsNode,
+    VcsBackends.node,
     WebSearchTool.configNode,
   ],
 })
@@ -137,6 +139,7 @@ const locationServiceNodes = [
   SessionTitle.node,
   Snapshot.node,
   SessionRunnerLLM.node,
+  VcsBackends.node,
   Vcs.node,
   // Start repository watches only after boot-critical filesystem and Git work.
   LocationWatcher.node,

packages/core/src/location.ts

@@ -10,6 +10,7 @@ export { Info, Ref, response }
 
 export interface Interface extends Info {
   readonly vcs?: Project.Vcs
+  readonly vcsBackend?: Project.Vcs
 }
 
 export class Service extends Context.Service<Service, Interface>()("@opencode/Location") {}
@@ -27,6 +28,7 @@ const layer = (ref: Ref) =>
         workspaceID: ref.workspaceID,
         project: { id: resolved.id, directory: resolved.directory },
         vcs: resolved.vcs,
+        vcsBackend: resolved.vcsBackend,
       })
     }),
   )

packages/core/src/plugin.ts

@@ -18,6 +18,7 @@ import { SkillV2 } from "./skill"
 import { State } from "./state"
 import { ToolRegistry } from "./tool/registry"
 import { ToolHooks } from "./tool/hooks"
+import { VcsBackends } from "./vcs/backends"
 
 export interface Interface {
   readonly activate: (plugins: readonly { readonly plugin: Plugin; readonly version?: string }[]) => Effect.Effect<void>
@@ -131,6 +132,7 @@ export const node = makeLocationNode({
     SkillV2.node,
     ToolRegistry.toolsNode,
     ToolHooks.node,
+    VcsBackends.node,
     PluginRuntime.node,
   ],
 })

packages/core/src/plugin/host.ts

@@ -20,6 +20,7 @@ import { AbsolutePath, type DeepMutable } from "../schema"
 import { SkillV2 } from "../skill"
 import { Tools } from "../tool/tools"
 import { ToolHooks } from "../tool/hooks"
+import { VcsBackends } from "../vcs/backends"
 import { WorkspaceV2 } from "../workspace"
 
 const mutable = <T>(value: T) => value as DeepMutable<T>
@@ -35,6 +36,7 @@ export const make = Effect.fn("PluginHost.make")(function* (plugin: PluginV2.Int
   const skill = yield* SkillV2.Service
   const tools = yield* Tools.Service
   const toolHooks = yield* ToolHooks.Service
+  const vcsBackends = yield* VcsBackends.Service
   const runtime = yield* PluginRuntime.Service
   const locationInfo = () =>
     new Location.Info({
@@ -341,6 +343,9 @@ export const make = Effect.fn("PluginHost.make")(function* (plugin: PluginV2.Int
           }),
       },
     },
+    vcs: {
+      register: (backend) => vcsBackends.register(backend),
+    },
     session: {
       create: (input) =>
         runtime.session.create({

packages/core/src/plugin/internal.ts

@@ -46,6 +46,7 @@ import { Tools } from "../tool/tools"
 import { WebFetchTool } from "../tool/webfetch"
 import { WebSearchTool } from "../tool/websearch"
 import { WriteTool } from "../tool/write"
+import { VcsBackends } from "../vcs/backends"
 import { AgentPlugin } from "./agent"
 import { CommandPlugin } from "./command"
 import { ModelsDevPlugin } from "./models-dev"
@@ -83,6 +84,7 @@ const services = Effect.fn("PluginInternal.services")(function* () {
   const skill = yield* SkillV2.Service
   const tools = yield* Tools.Service
   const websearch = yield* WebSearchTool.ConfigService
+  const vcs = yield* VcsBackends.Service
   return Context.mergeAll(
     Context.make(AgentV2.Service, agent),
     Context.make(Catalog.Service, catalog),
@@ -112,6 +114,7 @@ const services = Effect.fn("PluginInternal.services")(function* () {
     Context.make(SkillV2.Service, skill),
     Context.make(Tools.Service, tools),
     Context.make(WebSearchTool.ConfigService, websearch),
+    Context.make(VcsBackends.Service, vcs),
   )
 })
 

packages/core/src/project.ts

@@ -6,9 +6,11 @@ import { ChildProcess } from "effect/unstable/process"
 import { asc, desc } from "drizzle-orm"
 import path from "path"
 import { AbsolutePath } from "./schema"
+import { ConfigVcs } from "./config/vcs"
 import { Database } from "./database/database"
 import { FSUtil } from "./fs-util"
 import { Git } from "./git"
+import { Global } from "./global"
 import { AppProcess } from "./process"
 import { makeGlobalNode } from "./effect/app-node"
 import { Hash } from "./util/hash"
@@ -42,8 +44,14 @@ export interface Resolved {
   readonly id: ID
   readonly directory: AbsolutePath
   readonly vcs?: Vcs
+  readonly vcsBackend?: Vcs
 }
 
+type Discovery =
+  | { readonly type: "git"; readonly directory: AbsolutePath; readonly vcs: Vcs; readonly repo: Git.Repository }
+  | { readonly type: "hg"; readonly directory: AbsolutePath; readonly vcs: Vcs }
+  | { readonly type: "plugin"; readonly directory: AbsolutePath; readonly vcs: Vcs }
+
 // Keep this filesystem-only; permission checks use it and should not execute VCS commands.
 export const root = Effect.fn("Project.root")(function* (
   fs: FSUtil.Interface,
@@ -103,6 +111,7 @@ const layer = Layer.effect(
   Effect.gen(function* () {
     const fs = yield* FSUtil.Service
     const git = yield* Git.Service
+    const global = yield* Global.Service
     const proc = yield* AppProcess.Service
     const db = (yield* Database.Service).db
     const projectDirectories = yield* ProjectDirectories.Service
@@ -195,34 +204,85 @@ const layer = Layer.effect(
         Effect.catch(() => Effect.succeed(undefined)),
       )
       if (!dotHg) return undefined
-      const worktree = AbsolutePath.make(path.dirname(dotHg))
-      const store = AbsolutePath.make(dotHg)
-      const previous = yield* cached(store)
-      const id = previous ?? (yield* hgRoot(worktree))
       return {
-        previous,
-        id: id ?? ID.global,
-        directory: worktree,
-        vcs: { type: "hg" as const, store },
-      }
+        type: "hg" as const,
+        directory: AbsolutePath.make(path.dirname(dotHg)),
+        vcs: { type: "hg", store: AbsolutePath.make(dotHg) },
+      } satisfies Discovery
+    })
+
+    const markerDiscover = Effect.fnUntraced(function* (input: AbsolutePath) {
+      const backends = yield* ConfigVcs.readGlobal(fs, global.config)
+      if (backends.size === 0) return undefined
+      const types = new Map([...backends].map(([type, backend]) => [backend.marker, type] as const))
+      const match = yield* fs.up({ targets: [...types.keys()], start: input }).pipe(
+        Effect.map((matches) => matches[0]),
+        Effect.catch(() => Effect.succeed(undefined)),
+      )
+      if (!match) return undefined
+      const type = types.get(path.basename(match))
+      if (!type) return undefined
+      return {
+        type: "plugin" as const,
+        directory: AbsolutePath.make(path.dirname(match)),
+        vcs: { type, store: AbsolutePath.make(match) },
+      } satisfies Discovery
     })
 
     const resolve = Effect.fn("Project.resolve")(function* (input: AbsolutePath) {
       const repo = yield* git.repo.discover(input)
-      if (repo) {
-        const previous = yield* cached(repo.commonDirectory)
-        const id = (yield* remote(repo)) ?? previous ?? (yield* root(repo))
+      const [hg, marker] = yield* Effect.all([hgDiscover(input), markerDiscover(input)])
+      const discoveries: Discovery[] = [
+        ...(repo
+          ? [
+              {
+                type: "git" as const,
+                directory: repo.worktree,
+                vcs: { type: "git", store: repo.commonDirectory },
+                repo,
+              } satisfies Discovery,
+            ]
+          : []),
+        ...(hg ? [hg] : []),
+        ...(marker ? [marker] : []),
+      ]
+      const distance = (directory: AbsolutePath) =>
+        path.relative(directory, input).split(path.sep).filter(Boolean).length
+      const selected = discoveries.toSorted((a, b) => distance(a.directory) - distance(b.directory))[0]
+      if (!selected) return { id: ID.global, directory: AbsolutePath.make(path.parse(input).root), vcs: undefined }
+
+      const vcsBackend = marker?.directory === selected.directory ? marker.vcs : selected.vcs
+      if (selected.type === "git") {
+        const previous = yield* cached(selected.repo.commonDirectory)
+        const id = (yield* remote(selected.repo)) ?? previous ?? (yield* root(selected.repo))
         return {
           previous,
           id: id ?? ID.global,
-          directory: repo.worktree,
-          vcs: { type: "git" as const, store: repo.commonDirectory },
+          directory: selected.directory,
+          vcs: selected.vcs,
+          vcsBackend,
         }
       }
 
-      const hg = yield* hgDiscover(input)
-      if (hg) return hg
-      return { id: ID.global, directory: AbsolutePath.make(path.parse(input).root), vcs: undefined }
+      const previous = yield* cached(selected.vcs.store)
+      if (selected.type === "hg") {
+        const id = previous ?? (yield* hgRoot(selected.directory))
+        return {
+          previous,
+          id: id ?? ID.global,
+          directory: selected.directory,
+          vcs: selected.vcs,
+          vcsBackend,
+        }
+      }
+
+      return {
+        previous,
+        id: previous ?? ID.make(Hash.fast(`vcs-store:${selected.vcs.store}`)),
+        directory: selected.directory,
+        vcs: selected.vcs,
+        vcsBackend,
+      }
     })
 
     const commit = Effect.fn("Project.commit")(function* (input: { store: AbsolutePath; id: ID }) {
@@ -236,5 +296,5 @@ const layer = Layer.effect(
 export const node = makeGlobalNode({
   service: Service,
   layer: layer,
-  deps: [Database.node, FSUtil.node, Git.node, AppProcess.node, ProjectDirectories.node],
+  deps: [Database.node, FSUtil.node, Git.node, Global.node, AppProcess.node, ProjectDirectories.node],
 })

packages/core/src/project/schema.ts

@@ -22,14 +22,8 @@ export type DirectoriesInput = typeof DirectoriesInput.Type
 export const Directories = Project.Directories
 export type Directories = typeof Directories.Type
 
-export const Vcs = Schema.Union([
-  Schema.Struct({
-    type: Schema.Literal("git"),
-    store: AbsolutePath,
-  }),
-  Schema.Struct({
-    type: Schema.Literal("hg"),
-    store: AbsolutePath,
-  }),
-])
+export const Vcs = Schema.Struct({
+  type: Schema.String,
+  store: AbsolutePath,
+})
 export type Vcs = typeof Vcs.Type

packages/core/src/project/sql.ts

@@ -6,7 +6,7 @@ import { ProjectSchema } from "./schema"
 export const ProjectTable = sqliteTable("project", {
   id: text().$type<ProjectSchema.ID>().primaryKey(),
   worktree: absoluteColumn().notNull(),
-  vcs: text().$type<"git" | "hg">(),
+  vcs: text(),
   name: text(),
   icon_url: text(),
   icon_url_override: text(),

packages/core/src/vcs.ts

@@ -7,8 +7,10 @@ import { makeLocationNode } from "./effect/app-node"
 import { FSUtil } from "./fs-util"
 import { Location } from "./location"
 import { AppProcess } from "./process"
+import { VcsBackends } from "./vcs/backends"
 import { VcsGit } from "./vcs/git"
 import { VcsHg } from "./vcs/hg"
+import { MAX_TOTAL_PATCH_BYTES, PATCH_CONTEXT_LINES } from "./vcs/patch"
 
 export { FileStatus, Mode }
 
@@ -17,19 +19,16 @@ export interface DiffOptions {
 }
 
 export interface Interface {
-  readonly status: () => Effect.Effect<FileStatus[]>
-  readonly diff: (mode: Mode, options?: DiffOptions) => Effect.Effect<FileDiff.Info[]>
+  readonly status: () => Effect.Effect<readonly FileStatus[]>
+  readonly diff: (mode: Mode, options?: DiffOptions) => Effect.Effect<readonly FileDiff.Info[]>
 }
 
 export class Service extends Context.Service<Service, Interface>()("@opencode/v2/Vcs") {}
 
-// Adapter seam: one working-copy implementation per VCS type, selected by the
-// resolved location. Locations without a supported VCS degrade to empty
-// results so callers never need to special-case.
-const adapter = (proc: AppProcess.Interface, fs: FSUtil.Interface, location: Location.Interface) => {
+const builtIn = (proc: AppProcess.Interface, fs: FSUtil.Interface, location: Location.Interface) => {
   const scope = { directory: location.directory, worktree: location.project.directory }
-  if (location.vcs?.type === "git") return VcsGit.make(proc, scope)
-  if (location.vcs?.type === "hg") return VcsHg.make(proc, fs, scope)
+  if (location.vcsBackend?.type === "git") return VcsGit.make(proc, scope)
+  if (location.vcsBackend?.type === "hg") return VcsHg.make(proc, fs, scope)
 }
 
 const layer = Layer.effect(
@@ -38,15 +37,34 @@ const layer = Layer.effect(
     const proc = yield* AppProcess.Service
     const fs = yield* FSUtil.Service
     const location = yield* Location.Service
-    const impl = adapter(proc, fs, location)
+    const backends = yield* VcsBackends.Service
+    const native = builtIn(proc, fs, location)
+    let warned = false
+
+    const adapter = Effect.fnUntraced(function* () {
+      if (native) return native
+      if (!location.vcsBackend) return undefined
+      const plugin = backends.get(location.vcsBackend.type)
+      if (!plugin && !warned) {
+        warned = true
+        yield* Effect.logWarning("vcs backend declared but not registered", { type: location.vcsBackend.type })
+      }
+      return plugin
+    })
+
     return Service.of({
       status: Effect.fn("Vcs.status")(function* () {
+        const impl = yield* adapter()
         if (!impl) return []
         return yield* impl.status()
       }),
       diff: Effect.fn("Vcs.diff")(function* (mode: Mode, options?: DiffOptions) {
+        const impl = yield* adapter()
         if (!impl) return []
-        return yield* impl.diff(mode, options)
+        return yield* impl.diff(mode, {
+          context: options?.context ?? PATCH_CONTEXT_LINES,
+          maxOutputBytes: MAX_TOTAL_PATCH_BYTES,
+        })
       }),
     })
   }),
@@ -55,5 +73,5 @@ const layer = Layer.effect(
 export const node = makeLocationNode({
   service: Service,
   layer: layer,
-  deps: [AppProcess.node, FSUtil.node, Location.node],
+  deps: [AppProcess.node, FSUtil.node, Location.node, VcsBackends.node],
 })

packages/core/src/vcs/backends.ts

@@ -0,0 +1,120 @@
+export * as VcsBackends from "./backends"
+
+import { Vcs } from "@opencode-ai/plugin/v2/effect"
+import { FileDiff } from "@opencode-ai/schema/file-diff"
+import { FileStatus } from "@opencode-ai/schema/vcs"
+import { Cause, Context, Effect, Exit, Layer, Option, Schema } from "effect"
+import type { Scope } from "effect"
+import { ConfigVcs } from "../config/vcs"
+import { makeLocationNode } from "../effect/app-node"
+import { Location } from "../location"
+
+export interface Interface {
+  readonly register: (backend: Vcs.Backend) => Effect.Effect<void, Vcs.RegistrationError, Scope.Scope>
+  readonly get: (type: string) => Vcs.Adapter | undefined
+}
+
+export class Service extends Context.Service<Service, Interface>()("@opencode/v2/VcsBackends") {}
+
+const decodeType = Schema.decodeUnknownOption(ConfigVcs.Type)
+const decodeStatus = Schema.decodeUnknownOption(Schema.Array(FileStatus))
+const decodeDiff = Schema.decodeUnknownOption(Schema.Array(FileDiff.Info))
+
+interface Entry {
+  readonly backend: Vcs.Backend
+  adapter?: Vcs.Adapter
+}
+
+const layer = Layer.effect(
+  Service,
+  Effect.gen(function* () {
+    const location = yield* Location.Service
+    const registry = new Map<string, Entry>()
+
+    return Service.of({
+      register: (backend) =>
+        Effect.gen(function* () {
+          if (Option.isNone(decodeType(backend.type))) {
+            return yield* new Vcs.RegistrationError({
+              type: backend.type,
+              message: `Invalid vcs backend type '${backend.type}'`,
+            })
+          }
+          if (registry.has(backend.type)) {
+            return yield* new Vcs.RegistrationError({
+              type: backend.type,
+              message: `Vcs backend '${backend.type}' is already registered`,
+            })
+          }
+          yield* Effect.uninterruptible(
+            Effect.gen(function* () {
+              registry.set(backend.type, { backend })
+              yield* Effect.addFinalizer(() => Effect.sync(() => registry.delete(backend.type)))
+            }),
+          )
+        }),
+      get: (type) => {
+        const vcs = location.vcsBackend
+        const entry = registry.get(type)
+        if (!entry || vcs?.type !== type) return undefined
+        entry.adapter ??= guard(type, () =>
+          entry.backend.make({
+            directory: location.directory,
+            worktree: location.project.directory,
+            store: vcs.store,
+          }),
+        )
+        return entry.adapter
+      },
+    })
+  }),
+)
+
+export const node = makeLocationNode({ service: Service, layer: layer, deps: [Location.node] })
+
+function guard(type: string, make: () => Vcs.Adapter): Vcs.Adapter {
+  let underlying: Vcs.Adapter | undefined
+  const adapter = Effect.sync(() => (underlying ??= make()))
+  return {
+    status: () => adapter.pipe(Effect.flatMap((impl) => impl.status()), sanitize(type, "status", decodeStatus)),
+    diff: (mode, options) =>
+      adapter.pipe(
+        Effect.flatMap((impl) => impl.diff(mode, options)),
+        sanitize(type, "diff", decodeDiff),
+        Effect.map((rows) => boundDiff(rows, options.maxOutputBytes)),
+      ),
+  }
+}
+
+function boundDiff(rows: readonly FileDiff.Info[], maxOutputBytes: number) {
+  let total = 0
+  return rows.map((row) => {
+    if (row.patch === undefined) return row
+    const bytes = Buffer.byteLength(row.patch)
+    if (total + bytes > maxOutputBytes) return { ...row, patch: undefined }
+    total += bytes
+    return row
+  })
+}
+
+function sanitize<A>(type: string, operation: string, decode: (input: unknown) => Option.Option<readonly A[]>) {
+  return <E, R>(effect: Effect.Effect<readonly A[], E, R>) =>
+    effect.pipe(
+      Effect.exit,
+      Effect.flatMap((exit) => {
+        if (Exit.isFailure(exit)) {
+          if (Cause.hasInterrupts(exit.cause)) return Effect.failCause(exit.cause)
+          return Effect.logWarning("vcs backend failed", { type, operation, cause: exit.cause }).pipe(
+            Effect.as([] as readonly A[]),
+          )
+        }
+        return Option.match(decode(exit.value), {
+          onNone: () =>
+            Effect.logWarning("vcs backend returned invalid data", { type, operation }).pipe(
+              Effect.as([] as readonly A[]),
+            ),
+          onSome: (value) => Effect.succeed(value),
+        })
+      }),
+    )
+}

packages/core/test/config-vcs.test.ts

@@ -0,0 +1,33 @@
+import { describe, expect, test } from "bun:test"
+import { Option, Schema } from "effect"
+import { ConfigVcs } from "@opencode-ai/core/config/vcs"
+
+const decode = Schema.decodeUnknownOption(ConfigVcs.Info)
+
+describe("ConfigVcs", () => {
+  test("accepts backend declarations keyed by type", () => {
+    const result = Option.getOrUndefined(decode({ jj: { marker: ".jj" } }))
+    expect(result?.["jj"]?.marker).toBe(".jj")
+  })
+
+  test("rejects reserved built-in types", () => {
+    expect(Option.isNone(decode({ git: { marker: ".mygit" } }))).toBe(true)
+    expect(Option.isNone(decode({ hg: { marker: ".myhg" } }))).toBe(true)
+  })
+
+  test("rejects invalid type slugs", () => {
+    expect(Option.isNone(decode({ "Not A Slug": { marker: ".x" } }))).toBe(true)
+    expect(Option.isNone(decode({ "9starts-with-digit": { marker: ".x" } }))).toBe(true)
+  })
+
+  test("rejects markers that are not a single path segment", () => {
+    expect(Option.isNone(decode({ jj: { marker: "" } }))).toBe(true)
+    expect(Option.isNone(decode({ jj: { marker: ".." } }))).toBe(true)
+    expect(Option.isNone(decode({ jj: { marker: "a/b" } }))).toBe(true)
+  })
+
+  test("rejects duplicate markers across types", () => {
+    expect(Option.isNone(decode({ jj: { marker: ".x" }, piper: { marker: ".x" } }))).toBe(true)
+    expect(Option.isSome(decode({ jj: { marker: ".jj" }, piper: { marker: ".piper" } }))).toBe(true)
+  })
+})

packages/core/test/fixture/location.ts

@@ -4,12 +4,16 @@ import { AbsolutePath } from "@opencode-ai/core/schema"
 import { Effect, Layer } from "effect"
 import { tmpdir } from "./tmpdir"
 
-export function location(ref: Location.Ref, input: { projectDirectory?: AbsolutePath; vcs?: Project.Vcs } = {}) {
+export function location(
+  ref: Location.Ref,
+  input: { projectDirectory?: AbsolutePath; vcs?: Project.Vcs; vcsBackend?: Project.Vcs } = {},
+) {
   return {
     directory: ref.directory,
     workspaceID: ref.workspaceID,
     project: { id: Project.ID.global, directory: input.projectDirectory ?? ref.directory },
     vcs: input.vcs,
+    vcsBackend: input.vcsBackend ?? input.vcs,
   } satisfies Location.Interface
 }
 

packages/core/test/plugin/fixture.ts

@@ -18,6 +18,7 @@ import { Reference } from "@opencode-ai/core/reference"
 import { SkillV2 } from "@opencode-ai/core/skill"
 import { ToolHooks } from "@opencode-ai/core/tool/hooks"
 import { ToolRegistry } from "@opencode-ai/core/tool/registry"
+import { VcsBackends } from "@opencode-ai/core/vcs/backends"
 import { Effect, Layer } from "effect"
 import { tempLocationLayer } from "../fixture/location"
 
@@ -50,6 +51,7 @@ export const PluginTestLayer = AppNodeBuilder.build(
     SkillV2.node,
     ToolHooks.node,
     ToolRegistry.toolsNode,
+    VcsBackends.node,
   ]),
   [
     [Location.node, tempLocationLayer],

packages/core/test/plugin/host.ts

@@ -84,6 +84,9 @@ export function host(overrides: Overrides = {}): PluginContext {
       command: () => Effect.die("unused session.command"),
       interrupt: () => Effect.die("unused session.interrupt"),
     },
+    vcs: overrides.vcs ?? {
+      register: () => Effect.die("unused vcs.register"),
+    },
   }
 }
 

packages/core/test/project.test.ts

@@ -1,10 +1,11 @@
-import { describe, expect } from "bun:test"
+import { afterAll, describe, expect } from "bun:test"
 import { $ } from "bun"
 import fs from "fs/promises"
 import path from "path"
 import { Effect, Layer, Schema } from "effect"
 import { AppNodeBuilder } from "@opencode-ai/core/effect/app-node-builder"
 import { Database } from "@opencode-ai/core/database/database"
+import { Global } from "@opencode-ai/core/global"
 import { ProjectV2 } from "@opencode-ai/core/project"
 import { ProjectTable } from "@opencode-ai/core/project/sql"
 import { AbsolutePath } from "@opencode-ai/core/schema"
@@ -68,6 +69,12 @@ describe("ProjectV2.list", () => {
   )
 })
 
+const globalConfig = await tmpdir()
+afterAll(() => globalConfig[Symbol.asyncDispose]())
+const itMarker = testEffect(
+  AppNodeBuilder.build(ProjectV2.node, [[Global.node, Global.layerWith({ config: globalConfig.path })]]),
+)
+
 function remoteID(remote: string) {
   return ProjectV2.ID.make(Hash.fast(`git-remote:${remote}`))
 }
@@ -271,6 +278,7 @@ describe("ProjectV2.resolve", () => {
       const result = yield* project.resolve(abs(path.join(tmp.path, "a", "b")))
 
       expect(result.vcs?.type).toBe("hg")
+      expect(result.vcsBackend?.type).toBe("hg")
       expect(result.directory).toBe(abs(tmp.path))
       expect(result.id).not.toBe(ProjectV2.ID.make("global"))
       expect(result.previous).toBeUndefined()
@@ -290,6 +298,83 @@ describe("ProjectV2.resolve", () => {
       const result = yield* project.resolve(abs(tmp.path))
 
       expect(result.vcs?.type).toBe("git")
+      expect(result.vcsBackend?.type).toBe("git")
+    }),
+  )
+
+  itMarker.live("uses a plugin backend without replacing colocated git repository identity", () =>
+    Effect.gen(function* () {
+      const tmp = yield* Effect.acquireRelease(
+        Effect.promise(() => tmpdir()),
+        (tmp) => Effect.promise(() => tmp[Symbol.asyncDispose]()),
+      )
+      yield* Effect.promise(async () => {
+        await Bun.write(
+          path.join(globalConfig.path, "opencode.json"),
+          JSON.stringify({ vcs: { jj: { marker: ".jj" } } }),
+        )
+        await initRepo(tmp.path, { commit: true })
+        await fs.mkdir(path.join(tmp.path, ".jj"))
+      })
+      const project = yield* ProjectV2.Service
+
+      const result = yield* project.resolve(abs(tmp.path))
+
+      expect(result.vcs?.type).toBe("git")
+      expect(result.vcsBackend?.type).toBe("jj")
+      expect(result.directory).toBe(yield* real(tmp.path))
+    }),
+  )
+
+  itMarker.live("prefers a nested plugin repository over an ancestor git repository", () =>
+    Effect.gen(function* () {
+      const tmp = yield* Effect.acquireRelease(
+        Effect.promise(() => tmpdir()),
+        (tmp) => Effect.promise(() => tmp[Symbol.asyncDispose]()),
+      )
+      const nested = path.join(tmp.path, "nested")
+      yield* Effect.promise(async () => {
+        await Bun.write(
+          path.join(globalConfig.path, "opencode.json"),
+          JSON.stringify({ vcs: { jj: { marker: ".jj" } } }),
+        )
+        await initRepo(tmp.path, { commit: true })
+        await fs.mkdir(path.join(nested, ".jj"), { recursive: true })
+        await fs.mkdir(path.join(nested, "src"))
+      })
+      const project = yield* ProjectV2.Service
+
+      const result = yield* project.resolve(abs(path.join(nested, "src")))
+
+      expect(result.vcs?.type).toBe("jj")
+      expect(result.vcsBackend?.type).toBe("jj")
+      expect(result.directory).toBe(abs(nested))
+    }),
+  )
+
+  itMarker.live("detects plugin backends from global config markers", () =>
+    Effect.gen(function* () {
+      const tmp = yield* Effect.acquireRelease(
+        Effect.promise(() => tmpdir()),
+        (tmp) => Effect.promise(() => tmp[Symbol.asyncDispose]()),
+      )
+      yield* Effect.promise(async () => {
+        await Bun.write(
+          path.join(globalConfig.path, "opencode.json"),
+          JSON.stringify({ vcs: { jj: { marker: ".jj" } } }),
+        )
+        await fs.mkdir(path.join(tmp.path, ".jj"))
+        await fs.mkdir(path.join(tmp.path, "a", "b"), { recursive: true })
+      })
+      const project = yield* ProjectV2.Service
+
+      const result = yield* project.resolve(abs(path.join(tmp.path, "a", "b")))
+
+      expect(result.vcs?.type).toBe("jj")
+      expect(result.vcsBackend?.type).toBe("jj")
+      expect(result.vcs?.store).toBe(abs(path.join(tmp.path, ".jj")))
+      expect(result.directory).toBe(abs(tmp.path))
+      expect(result.id).toBe(ProjectV2.ID.make(Hash.fast(`vcs-store:${path.join(tmp.path, ".jj")}`)))
     }),
   )
 

packages/core/test/vcs-backends.test.ts

@@ -0,0 +1,158 @@
+import { describe, expect } from "bun:test"
+import { Cause, Effect, Exit, Fiber, Layer, Scope } from "effect"
+import { Vcs as PluginVcs } from "@opencode-ai/plugin/v2/effect"
+import { LayerNode } from "@opencode-ai/core/effect/layer-node"
+import { Location } from "@opencode-ai/core/location"
+import { AbsolutePath } from "@opencode-ai/core/schema"
+import { Vcs } from "@opencode-ai/core/vcs"
+import { VcsBackends } from "@opencode-ai/core/vcs/backends"
+import { location } from "./fixture/location"
+import { it } from "./lib/effect"
+
+const directory = AbsolutePath.make("/repo")
+
+const provide = Effect.provide(
+  LayerNode.compile(LayerNode.group([Vcs.node, VcsBackends.node]), [
+    [
+      Location.node,
+      Layer.succeed(
+        Location.Service,
+        Location.Service.of(
+          location({ directory }, { vcs: { type: "fake", store: AbsolutePath.make("/repo/.fake") } }),
+        ),
+      ),
+    ],
+  ]),
+)
+
+const status = [{ file: "a.txt", additions: 1, deletions: 0, status: "added" as const }]
+const diff = [{ file: "a.txt", patch: "+hello", additions: 1, deletions: 0, status: "added" as const }]
+
+const backend = (overrides: Partial<PluginVcs.Adapter> = {}): PluginVcs.Backend => ({
+  type: "fake",
+  make: () => ({
+    status: () => Effect.succeed(status),
+    diff: () => Effect.succeed(diff),
+    ...overrides,
+  }),
+})
+
+const register = (input: PluginVcs.Backend) =>
+  Effect.gen(function* () {
+    const backends = yield* VcsBackends.Service
+    return yield* backends.register(input)
+  })
+
+describe("VcsBackends", () => {
+  it.live("serves status and diff through a registered backend", () =>
+    Effect.gen(function* () {
+      yield* register(backend())
+      const vcs = yield* Vcs.Service
+      expect(yield* vcs.status()).toEqual(status)
+      expect(yield* vcs.diff("working")).toEqual(diff)
+    }).pipe(Effect.scoped, provide),
+  )
+
+  it.live("passes normalized options and bounds plugin diffs", () =>
+    Effect.gen(function* () {
+      let received: Parameters<PluginVcs.Adapter["diff"]>[1] | undefined
+      yield* register(
+        backend({
+          diff: (_mode, options) => {
+            received = options
+            return Effect.succeed([{ ...diff[0], patch: "x".repeat(options.maxOutputBytes + 1) }])
+          },
+        }),
+      )
+      const vcs = yield* Vcs.Service
+      expect(yield* vcs.diff("working", { context: 3 })).toEqual([{ ...diff[0], patch: undefined }])
+      expect(received).toEqual({ context: 3, maxOutputBytes: 10_000_000 })
+    }).pipe(Effect.scoped, provide),
+  )
+
+  it.live("passes the location scope to the adapter factory", () =>
+    Effect.gen(function* () {
+      let scope: PluginVcs.AdapterScope | undefined
+      yield* register({
+        type: "fake",
+        make: (input) => {
+          scope = input
+          return backend().make(input)
+        },
+      })
+      yield* (yield* Vcs.Service).status()
+      expect(scope).toEqual({ directory: "/repo", worktree: "/repo", store: "/repo/.fake" })
+    }).pipe(Effect.scoped, provide),
+  )
+
+  it.live("returns empty results when no backend is registered", () =>
+    Effect.gen(function* () {
+      const vcs = yield* Vcs.Service
+      expect(yield* vcs.status()).toEqual([])
+      expect(yield* vcs.diff("working")).toEqual([])
+    }).pipe(Effect.scoped, provide),
+  )
+
+  it.live("frees the type when the registration scope closes", () =>
+    Effect.gen(function* () {
+      const backends = yield* VcsBackends.Service
+      const scope = yield* Scope.make()
+      yield* backends.register(backend()).pipe(Scope.provide(scope))
+      expect((yield* (yield* Vcs.Service).status()).length).toBe(1)
+      yield* Scope.close(scope, Exit.void)
+      expect(yield* (yield* Vcs.Service).status()).toEqual([])
+      yield* register(backend())
+    }).pipe(Effect.scoped, provide),
+  )
+
+  it.live("rejects duplicate and reserved types", () =>
+    Effect.gen(function* () {
+      yield* register(backend())
+      const duplicate = yield* register(backend()).pipe(Effect.exit)
+      expect(Exit.isFailure(duplicate)).toBe(true)
+      const reserved = yield* register({ ...backend(), type: "git" }).pipe(Effect.exit)
+      expect(Exit.isFailure(reserved)).toBe(true)
+      const invalid = yield* register({ ...backend(), type: "Not A Slug" }).pipe(Effect.exit)
+      expect(Exit.isFailure(invalid)).toBe(true)
+    }).pipe(Effect.scoped, provide),
+  )
+
+  it.live("degrades failing adapters to empty results", () =>
+    Effect.gen(function* () {
+      yield* register(
+        backend({
+          status: () => Effect.die(new Error("backend exploded")),
+          diff: () => Effect.sync(() => {
+            throw new Error("sync explosion")
+          }),
+        }),
+      )
+      const vcs = yield* Vcs.Service
+      expect(yield* vcs.status()).toEqual([])
+      expect(yield* vcs.diff("working")).toEqual([])
+    }).pipe(Effect.scoped, provide),
+  )
+
+  it.live("preserves adapter interruption", () =>
+    Effect.gen(function* () {
+      yield* register(backend({ status: () => Effect.never }))
+      const vcs = yield* Vcs.Service
+      const fiber = yield* Effect.forkChild(vcs.status())
+      yield* Fiber.interrupt(fiber)
+      const exit = yield* Fiber.await(fiber)
+      expect(Exit.isFailure(exit)).toBe(true)
+      if (Exit.isFailure(exit)) expect(Cause.hasInterrupts(exit.cause)).toBe(true)
+    }).pipe(Effect.scoped, provide),
+  )
+
+  it.live("drops rows that fail schema validation", () =>
+    Effect.gen(function* () {
+      yield* register(
+        backend({
+          status: () => Effect.succeed([{ file: "a.txt", additions: -1, deletions: 0, status: "added" }]),
+        }),
+      )
+      expect(yield* (yield* Vcs.Service).status()).toEqual([])
+    }).pipe(Effect.scoped, provide),
+  )
+})

packages/plugin/src/v2/effect/context.ts

@@ -10,6 +10,7 @@ import type { ReferenceHooks } from "./reference.js"
 import type { SkillHooks } from "./skill.js"
 import type { ToolDomain } from "./tool.js"
 import type { SessionHooks } from "./runtime.js"
+import type { VcsDomain } from "./vcs.js"
 
 export interface PluginContext {
   readonly options: PluginOptions
@@ -24,4 +25,5 @@ export interface PluginContext {
   readonly skill: SkillHooks
   readonly tool: ToolDomain
   readonly session: SessionHooks
+  readonly vcs: VcsDomain
 }

packages/plugin/src/v2/effect/index.ts

@@ -12,3 +12,5 @@ export type { SkillDraft, SkillHooks } from "./skill.js"
 export * as Tool from "./tool.js"
 export type { ToolDomain, ToolExecuteBeforeEvent, ToolExecuteAfterEvent } from "./tool.js"
 export type { SessionHooks } from "./runtime.js"
+export { Vcs } from "./vcs.js"
+export type { VcsDomain } from "./vcs.js"

packages/plugin/src/v2/effect/vcs.ts

@@ -0,0 +1,35 @@
+export * as Vcs from "./vcs.js"
+
+import { FileDiff } from "@opencode-ai/schema/file-diff"
+import { FileStatus, Mode } from "@opencode-ai/schema/vcs"
+import { Schema, type Effect, type Scope } from "effect"
+
+export class RegistrationError extends Schema.TaggedErrorClass<RegistrationError>()("Vcs.RegistrationError", {
+  type: Schema.String,
+  message: Schema.String,
+}) {}
+
+export interface AdapterScope {
+  readonly directory: string
+  readonly worktree: string
+  readonly store: string
+}
+
+export interface DiffOptions {
+  readonly context: number
+  readonly maxOutputBytes: number
+}
+
+export interface Adapter {
+  readonly status: () => Effect.Effect<readonly FileStatus[]>
+  readonly diff: (mode: Mode, options: DiffOptions) => Effect.Effect<readonly FileDiff.Info[]>
+}
+
+export interface Backend {
+  readonly type: string
+  readonly make: (scope: AdapterScope) => Adapter
+}
+
+export interface VcsDomain {
+  readonly register: (backend: Backend) => Effect.Effect<void, RegistrationError, Scope.Scope>
+}

packages/schema/src/project.ts

@@ -8,7 +8,7 @@ import { ProjectID } from "./project-id.js"
 export const ID = ProjectID
 export type ID = typeof ID.Type
 
-export const Vcs = Schema.Literals(["git", "hg"]).annotate({ identifier: "Project.Vcs" })
+export const Vcs = Schema.String.annotate({ identifier: "Project.Vcs" })
 export const Current = Schema.Struct({
   id: ID,
   directory: AbsolutePath,

packages/sdk/js/src/gen/types.gen.ts

@@ -744,7 +744,7 @@ export type Project = {
   id: string
   worktree: string
   vcsDir?: string
-  vcs?: "git" | "hg"
+  vcs?: string
   time: {
     created: number
     initialized?: number

packages/sdk/js/src/v2/gen/sdk.gen.ts

@@ -334,6 +334,8 @@ import type {
   V2ProjectCurrentResponses,
   V2ProjectDirectoriesErrors,
   V2ProjectDirectoriesResponses,
+  V2ProjectListErrors,
+  V2ProjectListResponses,
   V2ProviderGetErrors,
   V2ProviderGetResponses,
   V2ProviderListErrors,
@@ -7032,6 +7034,18 @@ export class Credential extends HeyApiClient {
 }
 
 export class Project2 extends HeyApiClient {
+  /**
+   * List projects
+   *
+   * List known projects.
+   */
+  public list<ThrowOnError extends boolean = false>(options?: Options<never, ThrowOnError>) {
+    return (options?.client ?? this.client).get<V2ProjectListResponses, V2ProjectListErrors, ThrowOnError>({
+      url: "/api/project",
+      ...options,
+    })
+  }
+
   /**
    * Get current project
    *

packages/sdk/js/src/v2/gen/types.gen.ts

@@ -3492,7 +3492,7 @@ export type FormAnswer = {
   [key: string]: FormValue
 }
 
-export type ProjectVcs = "git" | "hg"
+export type ProjectVcs = string
 
 export type ProjectIcon = {
   url?: string
@@ -9329,6 +9329,38 @@ export type McpServer2 = {
   integrationID?: string
 }
 
+export type ProjectVcs2 = string
+
+export type ProjectIcon2 = {
+  url?: string
+  override?: string
+  color?: string
+}
+
+export type ProjectCommands2 = {
+  /**
+   * Startup script to run when creating a new workspace (worktree)
+   */
+  start?: string
+}
+
+export type ProjectTime2 = {
+  created: number
+  updated: number
+  initialized?: number
+}
+
+export type ProjectV2 = {
+  id: string
+  worktree: string
+  vcs?: ProjectVcs2
+  name?: string
+  icon?: ProjectIcon2
+  commands?: ProjectCommands2
+  time: ProjectTime2
+  sandboxes: Array<string>
+}
+
 export type ProjectCurrent2 = {
   id: string
   directory: string
@@ -17353,6 +17385,35 @@ export type V2CredentialUpdateResponses = {
 
 export type V2CredentialUpdateResponse = V2CredentialUpdateResponses[keyof V2CredentialUpdateResponses]
 
+export type V2ProjectListData = {
+  body?: never
+  path?: never
+  query?: never
+  url: "/api/project"
+}
+
+export type V2ProjectListErrors = {
+  /**
+   * InvalidRequestError
+   */
+  400: InvalidRequestErrorV2
+  /**
+   * UnauthorizedError
+   */
+  401: UnauthorizedErrorV2
+}
+
+export type V2ProjectListError = V2ProjectListErrors[keyof V2ProjectListErrors]
+
+export type V2ProjectListResponses = {
+  /**
+   * Success
+   */
+  200: Array<ProjectV2>
+}
+
+export type V2ProjectListResponse = V2ProjectListResponses[keyof V2ProjectListResponses]
+
 export type V2ProjectCurrentData = {
   body?: never
   path?: never