diff --git a/packages/core/src/git.ts b/packages/core/src/git.ts
index 0041c3353f43..e15a9e07edc9 100644
--- a/packages/core/src/git.ts
+++ b/packages/core/src/git.ts
@@ -165,11 +165,13 @@ export const layer = Layer.effect(
     const fetch = Effect.fn("Git.fetch")((directory: string) => execute(directory, proc)(["fetch", "--all", "--prune"]))
 
     const fetchBranch = Effect.fn("Git.fetchBranch")((directory: string, branch: string) =>
-      execute(directory, proc)(["fetch", "origin", `+refs/heads/${branch}:refs/remotes/origin/${branch}`]),
+      execute(directory, proc)(["fetch", "origin", fetchRefSpec(branch)]),
     )
 
     const checkout = Effect.fn("Git.checkout")((directory: string, branch: string) =>
-      execute(directory, proc)(["checkout", "-B", branch, `origin/${branch}`]),
+      branch.startsWith("refs/")
+        ? execute(directory, proc)(["checkout", "--detach", trackingRef(branch)])
+        : execute(directory, proc)(["checkout", "-B", branch, trackingRef(branch)]),
     )
 
     const reset = Effect.fn("Git.reset")((directory: string, target: string) =>
@@ -443,3 +445,15 @@ function resolvePath(cwd: string, value: string) {
   if (path.isAbsolute(normalized)) return path.normalize(normalized)
   return path.resolve(cwd, normalized)
 }
+
+function fetchRefSpec(ref: string) {
+  if (ref.startsWith("refs/")) return `+${ref}:${trackingRef(ref)}`
+  return `+refs/heads/${ref}:${trackingRef(ref)}`
+}
+
+function trackingRef(ref: string) {
+  if (ref.startsWith("refs/heads/")) return `refs/remotes/origin/${ref.slice("refs/heads/".length)}`
+  if (ref.startsWith("refs/tags/")) return `refs/remotes/origin/tags/${ref.slice("refs/tags/".length)}`
+  if (ref.startsWith("refs/")) return `refs/remotes/origin/${ref.slice("refs/".length)}`
+  return `refs/remotes/origin/${ref}`
+}
diff --git a/packages/core/src/repository-cache.ts b/packages/core/src/repository-cache.ts
index 894dc38faa62..d80c6d05e0bd 100644
--- a/packages/core/src/repository-cache.ts
+++ b/packages/core/src/repository-cache.ts
@@ -159,7 +159,11 @@ export const layer: Layer.Layer<Service, never, FSUtil.Service | Git.Service | E
 
                 if (status === "cloned") {
                   const result = yield* git
-                    .clone({ remote: input.reference.remote, target: localPath, branch: input.branch })
+                    .clone({
+                      remote: input.reference.remote,
+                      target: localPath,
+                      branch: input.branch?.startsWith("refs/") ? undefined : input.branch,
+                    })
                     .pipe(
                       Effect.mapError((error) => new CloneFailedError({ repository, message: errorMessage(error) })),
                     )
@@ -169,6 +173,39 @@ export const layer: Layer.Layer<Service, never, FSUtil.Service | Git.Service | E
                       message: resultMessage(result, `Failed to clone ${repository}`),
                     })
                   }
+
+                  if (input.branch?.startsWith("refs/")) {
+                    const requestedBranch = input.branch
+                    const fetchBranch = yield* git
+                      .fetchBranch(localPath, requestedBranch)
+                      .pipe(
+                        Effect.mapError((error) => new FetchFailedError({ repository, message: errorMessage(error) })),
+                      )
+                    if (fetchBranch.exitCode !== 0) {
+                      return yield* new FetchFailedError({
+                        repository,
+                        message: resultMessage(fetchBranch, `Failed to fetch ${requestedBranch}`),
+                      })
+                    }
+
+                    const checkout = yield* git.checkout(localPath, requestedBranch).pipe(
+                      Effect.mapError(
+                        (error) =>
+                          new CheckoutFailedError({
+                            repository,
+                            branch: requestedBranch,
+                            message: errorMessage(error),
+                          }),
+                      ),
+                    )
+                    if (checkout.exitCode !== 0) {
+                      return yield* new CheckoutFailedError({
+                        repository,
+                        branch: requestedBranch,
+                        message: resultMessage(checkout, `Failed to checkout ${requestedBranch}`),
+                      })
+                    }
+                  }
                 }
 
                 if (status === "refreshed") {
@@ -276,7 +313,7 @@ function cacheOperation<A, E, R>(effect: Effect.Effect<A, E, R>, operation: stri
 }
 
 const resetTarget = Effect.fnUntraced(function* (git: Git.Interface, cwd: string, requestedBranch?: string) {
-  if (requestedBranch) return `origin/${requestedBranch}`
+  if (requestedBranch) return trackingRef(requestedBranch)
   const remoteHead = yield* git.remoteHead(cwd)
   if (remoteHead) return remoteHead
   const currentBranch = yield* git.branch(cwd)
@@ -284,6 +321,13 @@ const resetTarget = Effect.fnUntraced(function* (git: Git.Interface, cwd: string
   return "HEAD"
 })
 
+function trackingRef(ref: string) {
+  if (ref.startsWith("refs/heads/")) return `refs/remotes/origin/${ref.slice("refs/heads/".length)}`
+  if (ref.startsWith("refs/tags/")) return `refs/remotes/origin/tags/${ref.slice("refs/tags/".length)}`
+  if (ref.startsWith("refs/")) return `refs/remotes/origin/${ref.slice("refs/".length)}`
+  return `refs/remotes/origin/${ref}`
+}
+
 function resultMessage(result: Git.Result, fallback: string) {
   return result.stderr.trim() || result.text.trim() || fallback
 }
diff --git a/packages/core/src/repository.ts b/packages/core/src/repository.ts
index dbc6a8fbcae6..9942574d7829 100644
--- a/packages/core/src/repository.ts
+++ b/packages/core/src/repository.ts
@@ -103,10 +103,19 @@ export function parseRemote(input: string): RemoteReference {
 }
 
 export function validateBranch(branch: string): void {
-  if (/^[A-Za-z0-9/_.-]+$/.test(branch) && !branch.startsWith("-") && !branch.includes("..")) return
+  if (
+    branch &&
+    !branch.startsWith("-") &&
+    !branch.includes("..") &&
+    !branch.includes("@{") &&
+    !branch.endsWith(".") &&
+    !branch.endsWith(".lock") &&
+    !/[\s~^:?*[\\\x00-\x1f\x7f]/.test(branch)
+  )
+    return
   throw new InvalidBranchError({
     branch,
-    message: "Branch must contain only alphanumeric characters, /, _, ., and -, and cannot start with - or contain ..",
+    message: "Branch must be a valid git ref name and cannot start with -, contain .., or contain @{",
   })
 }
 
diff --git a/packages/core/test/fixture/git.ts b/packages/core/test/fixture/git.ts
index f02da400af6a..ae019926821b 100644
--- a/packages/core/test/fixture/git.ts
+++ b/packages/core/test/fixture/git.ts
@@ -44,6 +44,14 @@ export async function branch(source: string, name: string, content: string) {
   await git(source, "push", "-u", "origin", name)
 }
 
+export async function tag(source: string, name: string, content: string) {
+  await fs.writeFile(path.join(source, "README.md"), content)
+  await git(source, "add", "README.md")
+  await git(source, "commit", "-m", name)
+  await git(source, "tag", name)
+  await git(source, "push", "origin", name)
+}
+
 export async function git(cwd: string, ...args: string[]) {
   await exec("git", args, { cwd })
 }
diff --git a/packages/core/test/git.test.ts b/packages/core/test/git.test.ts
index e09e3e1a5d4e..ba9dddaf5fae 100644
--- a/packages/core/test/git.test.ts
+++ b/packages/core/test/git.test.ts
@@ -5,7 +5,7 @@ import path from "path"
 import { Effect } from "effect"
 import { Git } from "@opencode-ai/core/git"
 import { AbsolutePath } from "@opencode-ai/core/schema"
-import { branch, commit, gitRemote } from "./fixture/git"
+import { branch, commit, gitRemote, tag } from "./fixture/git"
 import { tmpdir } from "./fixture/tmpdir"
 import { testEffect } from "./lib/effect"
 
@@ -50,6 +50,22 @@ describe("Git", () => {
       }),
     ),
   )
+
+  it.live("fetches and checks out full tag refs", () =>
+    withRemote((fixture) =>
+      Effect.gen(function* () {
+        const git = yield* Git.Service
+        const target = path.join(fixture.root, "checkout")
+        yield* git.clone({ remote: fixture.remote, target })
+
+        yield* Effect.promise(() => tag(fixture.source, "effect@4.0.0-beta.65", "tagged\n"))
+        expect((yield* git.fetchBranch(target, "refs/tags/effect@4.0.0-beta.65")).exitCode).toBe(0)
+        expect((yield* git.checkout(target, "refs/tags/effect@4.0.0-beta.65")).exitCode).toBe(0)
+        expect(yield* git.branch(target)).toBeUndefined()
+        expect(yield* read(path.join(target, "README.md"))).toBe("tagged\n")
+      }),
+    ),
+  )
 })
 
 function withRemote<A, E, R>(body: (fixture: Awaited<ReturnType<typeof gitRemote>>) => Effect.Effect<A, E, R>) {
diff --git a/packages/core/test/repository-cache.test.ts b/packages/core/test/repository-cache.test.ts
index a99daea8e290..9b2010ee8fcf 100644
--- a/packages/core/test/repository-cache.test.ts
+++ b/packages/core/test/repository-cache.test.ts
@@ -9,7 +9,7 @@ import { Global } from "@opencode-ai/core/global"
 import { Repository } from "@opencode-ai/core/repository"
 import { RepositoryCache } from "@opencode-ai/core/repository-cache"
 import { EffectFlock } from "@opencode-ai/core/util/effect-flock"
-import { git, gitRemote } from "./fixture/git"
+import { git, gitRemote, tag } from "./fixture/git"
 import { tmpdir } from "./fixture/tmpdir"
 import { testEffect } from "./lib/effect"
 
@@ -67,6 +67,24 @@ describe("RepositoryCache", () => {
     ),
   )
 
+  it.live("materializes full tag refs", () =>
+    withRemote((fixture) =>
+      Effect.gen(function* () {
+        const cache = yield* RepositoryCache.Service
+        yield* Effect.promise(() => tag(fixture.source, "effect@4.0.0-beta.65", "tagged\n"))
+
+        const result = yield* cache.ensure({
+          reference: fixture.reference,
+          branch: "refs/tags/effect@4.0.0-beta.65",
+        })
+
+        expect(result.status).toBe("cloned")
+        expect(result.branch).toBeUndefined()
+        expect(yield* read(path.join(result.localPath, "README.md"))).toBe("tagged\n")
+      }).pipe(Effect.provide(cacheLayer(fixture.root))),
+    ),
+  )
+
   it.live("returns typed validation and clone failures", () =>
     withRemote((fixture) =>
       Effect.gen(function* () {
diff --git a/packages/core/test/repository.test.ts b/packages/core/test/repository.test.ts
index 5b18f8b1d693..22fdf7c8ddcd 100644
--- a/packages/core/test/repository.test.ts
+++ b/packages/core/test/repository.test.ts
@@ -51,9 +51,13 @@ describe("Repository", () => {
     expect(() => Repository.parseRemote("not-a-repo")).toThrow(Repository.InvalidReferenceError)
     expect(() => Repository.parseRemote("git@github.com:../../../etc/passwd")).toThrow(Repository.InvalidReferenceError)
     expect(() => Repository.validateBranch("feature/docs.v1")).not.toThrow()
+    expect(() => Repository.validateBranch("refs/heads/feature/docs.v1")).not.toThrow()
+    expect(() => Repository.validateBranch("refs/tags/effect@4.0.0-beta.65")).not.toThrow()
     expect(() => Repository.validateBranch("-bad")).toThrow(Repository.InvalidBranchError)
     expect(() => Repository.validateBranch("bad..branch")).toThrow(Repository.InvalidBranchError)
     expect(() => Repository.validateBranch("bad branch")).toThrow(Repository.InvalidBranchError)
+    expect(() => Repository.validateBranch("bad@{branch")).toThrow(Repository.InvalidBranchError)
+    expect(() => Repository.validateBranch("bad:branch")).toThrow(Repository.InvalidBranchError)
   })
 
   test("compares cache identity independent of input spelling", () => {