diff --git a/.github/workflows/_ci-clients.yml b/.github/workflows/_ci-clients.yml
index 310ab676b7..465c4b93ef 100644
--- a/.github/workflows/_ci-clients.yml
+++ b/.github/workflows/_ci-clients.yml
@@ -121,11 +121,16 @@ jobs:
 
   mobile-swift:
     name: Mobile Swift
-    runs-on: macos-latest
+    runs-on: xcode-27
     timeout-minutes: 30
     if: inputs.lane == 'mobile-swift' && inputs.mobile
     steps:
       - uses: actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10 # v6.0.3
+      - name: Select pinned Xcode
+        run: |
+          export DEVELOPER_DIR="/Applications/Xcode_$(cat .xcode-version).app/Contents/Developer"
+          scripts/check-xcode.sh
+          echo "DEVELOPER_DIR=$DEVELOPER_DIR" >> "$GITHUB_ENV"
       - uses: cashapp/activate-hermit@cea9af7913204a965fd488637a8d1811bba2e616 # v1
       - name: Install Flutter dependencies
         run: cd mobile && flutter pub get
diff --git a/.github/workflows/_ci-desktop-macos.yml b/.github/workflows/_ci-desktop-macos.yml
index d19ffa72eb..fc1038dfc2 100644
--- a/.github/workflows/_ci-desktop-macos.yml
+++ b/.github/workflows/_ci-desktop-macos.yml
@@ -23,18 +23,24 @@ env:
 jobs:
   desktop-build-macos:
     name: Desktop Build (macOS)
-    runs-on: macos-latest
+    runs-on: xcode-27
     timeout-minutes: 45
     if: github.event_name == 'push' || inputs.desktop || inputs.desktop_rust || inputs.rust
     permissions:
       contents: read
     steps:
       - uses: actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10 # v6.0.3
+      - name: Select pinned Xcode
+        run: |
+          export DEVELOPER_DIR="/Applications/Xcode_$(cat .xcode-version).app/Contents/Developer"
+          scripts/check-xcode.sh
+          echo "DEVELOPER_DIR=$DEVELOPER_DIR" >> "$GITHUB_ENV"
       - uses: cashapp/activate-hermit@cea9af7913204a965fd488637a8d1811bba2e616 # v1
       - name: Verify macOS icon representations
         run: scripts/macos-icon.sh --check
       - uses: Swatinem/rust-cache@e18b497796c12c097a38f9edb9d0641fb99eee32
         with:
+          key: xcode-${{ hashFiles('.xcode-version') }}
           workspaces: desktop/src-tauri
           save-if: ${{ github.event_name != 'pull_request' }}
       - name: Install desktop dependencies
@@ -64,7 +70,7 @@ jobs:
         uses: actions/cache/restore@caa296126883cff596d87d8935842f9db880ef25 # v5
         with:
           path: ${{ github.workspace }}/.cache/mesh-llama
-          key: mesh-llama-${{ runner.os }}-metal-${{ steps.mesh_rev.outputs.rev }}
+          key: mesh-llama-${{ runner.os }}-metal-${{ hashFiles('.xcode-version') }}-${{ steps.mesh_rev.outputs.rev }}
       - name: Build mesh llama native libraries
         if: steps.llama_cache.outputs.cache-hit != 'true'
         env:
@@ -88,7 +94,7 @@ jobs:
         uses: actions/cache/save@caa296126883cff596d87d8935842f9db880ef25 # v5
         with:
           path: ${{ github.workspace }}/.cache/mesh-llama
-          key: mesh-llama-${{ runner.os }}-metal-${{ steps.mesh_rev.outputs.rev }}
+          key: mesh-llama-${{ runner.os }}-metal-${{ hashFiles('.xcode-version') }}-${{ steps.mesh_rev.outputs.rev }}
       - name: Build Tauri app
         run: cd desktop && pnpm tauri build
         env:
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index a1ff1c47fe..2f6f4293de 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -71,6 +71,8 @@ jobs:
               - '.github/workflows/_ci-desktop.yml'
               - 'scripts/ci-apt-retry.sh'
               - 'Justfile'
+              - '.xcode-version'
+              - 'scripts/check-xcode.sh'
               - 'scripts/generate-macos-icon.py'
               - 'scripts/macos-icon.sh'
               - 'bin/uv'
@@ -86,6 +88,8 @@ jobs:
               - 'pnpm-lock.yaml'
             mobile:
               - 'mobile/**'
+              - '.xcode-version'
+              - 'scripts/check-xcode.sh'
               - 'scripts/mobile-release.sh'
               - 'scripts/mobile-worktree-overrides.sh'
               - 'scripts/mobile-worktree-clean.sh'
@@ -108,7 +112,9 @@ jobs:
           scripts/test-postgres-test-discovery.sh
           scripts/test-postgres-test-wrapper.sh
       - name: Release workflow source contract
-        run: scripts/test-release-ref-contract.sh
+        run: |
+          scripts/test-release-ref-contract.sh
+          scripts/test-xcode-version.sh
       - name: Relay image eligibility contract
         run: scripts/test-relay-image-eligibility-workflow.sh
       - name: Desktop release candidate contract
diff --git a/.github/workflows/desktop-release-cache-proof.yml b/.github/workflows/desktop-release-cache-proof.yml
index 461e25a60f..aaaac46472 100644
--- a/.github/workflows/desktop-release-cache-proof.yml
+++ b/.github/workflows/desktop-release-cache-proof.yml
@@ -12,7 +12,7 @@ jobs:
   macos:
     name: Prove macOS ${{ matrix.target }} cache visibility
     if: github.repository == 'block/buzz'
-    runs-on: macos-latest
+    runs-on: xcode-27
     timeout-minutes: 15
     strategy:
       fail-fast: false
@@ -28,6 +28,11 @@ jobs:
       - uses: actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10 # v6.0.3
         with:
           persist-credentials: false
+      - name: Select pinned Xcode
+        run: |
+          export DEVELOPER_DIR="/Applications/Xcode_$(cat .xcode-version).app/Contents/Developer"
+          scripts/check-xcode.sh
+          echo "DEVELOPER_DIR=$DEVELOPER_DIR" >> "$GITHUB_ENV"
       - uses: cashapp/activate-hermit@cea9af7913204a965fd488637a8d1811bba2e616 # v1
       - name: Patch proof dependency graph
         run: |
diff --git a/.github/workflows/macos-intel-canary.yml b/.github/workflows/macos-intel-canary.yml
index 35b05313c9..cc4ba34af2 100644
--- a/.github/workflows/macos-intel-canary.yml
+++ b/.github/workflows/macos-intel-canary.yml
@@ -13,7 +13,7 @@ jobs:
   build:
     name: Build macOS Intel canary
     if: github.repository == 'block/buzz'
-    runs-on: macos-latest
+    runs-on: xcode-27
     timeout-minutes: 60
     env:
       TARGET: x86_64-apple-darwin
@@ -31,6 +31,11 @@ jobs:
         with:
           persist-credentials: false
 
+      - name: Select pinned Xcode
+        run: |
+          export DEVELOPER_DIR="/Applications/Xcode_$(cat .xcode-version).app/Contents/Developer"
+          scripts/check-xcode.sh
+          echo "DEVELOPER_DIR=$DEVELOPER_DIR" >> "$GITHUB_ENV"
       - uses: cashapp/activate-hermit@cea9af7913204a965fd488637a8d1811bba2e616 # v1
 
       - name: Add Rust target
diff --git a/.github/workflows/release.yml b/.github/workflows/release.yml
index 040817361a..330508662c 100644
--- a/.github/workflows/release.yml
+++ b/.github/workflows/release.yml
@@ -52,7 +52,7 @@ jobs:
   release:
     name: Release
     if: github.repository == 'block/buzz'
-    runs-on: macos-latest
+    runs-on: xcode-27
     needs: setup
     timeout-minutes: 60
     permissions:
@@ -70,6 +70,11 @@ jobs:
           fetch-depth: 0
           persist-credentials: false
 
+      - name: Select pinned Xcode
+        run: |
+          export DEVELOPER_DIR="/Applications/Xcode_$(cat .xcode-version).app/Contents/Developer"
+          scripts/check-xcode.sh
+          echo "DEVELOPER_DIR=$DEVELOPER_DIR" >> "$GITHUB_ENV"
       - name: Verify tag-bound release source
         run: scripts/verify-release-ref.sh desktop-v "$VERSION"
 
@@ -108,7 +113,7 @@ jobs:
         uses: actions/cache/restore@caa296126883cff596d87d8935842f9db880ef25 # v5
         with:
           path: ${{ github.workspace }}/.cache/mesh-llama
-          key: mesh-llama-${{ runner.os }}-metal-${{ steps.mesh_rev.outputs.rev }}
+          key: mesh-llama-${{ runner.os }}-metal-${{ hashFiles('.xcode-version') }}-${{ steps.mesh_rev.outputs.rev }}
       - name: Build mesh llama native libraries
         if: steps.llama_cache.outputs.cache-hit != 'true'
         env:
@@ -132,7 +137,7 @@ jobs:
         uses: actions/cache/save@caa296126883cff596d87d8935842f9db880ef25 # v5
         with:
           path: ${{ github.workspace }}/.cache/mesh-llama
-          key: mesh-llama-${{ runner.os }}-metal-${{ steps.mesh_rev.outputs.rev }}
+          key: mesh-llama-${{ runner.os }}-metal-${{ hashFiles('.xcode-version') }}-${{ steps.mesh_rev.outputs.rev }}
 
       - name: Build unsigned Tauri app
         run: cd desktop && pnpm tauri build --verbose --no-sign --features mesh-llm --config src-tauri/tauri.release.conf.json
@@ -265,7 +270,7 @@ jobs:
   release-macos-x64:
     name: Release macOS (Intel)
     if: github.repository == 'block/buzz'
-    runs-on: macos-latest
+    runs-on: xcode-27
     needs: setup
     timeout-minutes: 60
     permissions:
@@ -284,6 +289,11 @@ jobs:
           fetch-depth: 0
           persist-credentials: false
 
+      - name: Select pinned Xcode
+        run: |
+          export DEVELOPER_DIR="/Applications/Xcode_$(cat .xcode-version).app/Contents/Developer"
+          scripts/check-xcode.sh
+          echo "DEVELOPER_DIR=$DEVELOPER_DIR" >> "$GITHUB_ENV"
       - name: Verify tag-bound release source
         run: scripts/verify-release-ref.sh desktop-v "$VERSION"
 
diff --git a/.github/workflows/signed-macos-canary.yml b/.github/workflows/signed-macos-canary.yml
index 5957f4785d..eaf091049f 100644
--- a/.github/workflows/signed-macos-canary.yml
+++ b/.github/workflows/signed-macos-canary.yml
@@ -13,7 +13,7 @@ jobs:
   build:
     name: Build signed macOS canary
     if: github.repository == 'block/buzz'
-    runs-on: macos-latest
+    runs-on: xcode-27
     timeout-minutes: 60
     permissions:
       contents: read
@@ -32,6 +32,11 @@ jobs:
         with:
           persist-credentials: false
 
+      - name: Select pinned Xcode
+        run: |
+          export DEVELOPER_DIR="/Applications/Xcode_$(cat .xcode-version).app/Contents/Developer"
+          scripts/check-xcode.sh
+          echo "DEVELOPER_DIR=$DEVELOPER_DIR" >> "$GITHUB_ENV"
       - uses: cashapp/activate-hermit@cea9af7913204a965fd488637a8d1811bba2e616 # v1
 
       - name: Get pnpm store directory
@@ -133,7 +138,7 @@ jobs:
         uses: actions/cache/restore@caa296126883cff596d87d8935842f9db880ef25 # v5
         with:
           path: ${{ github.workspace }}/.cache/mesh-llama
-          key: mesh-llama-${{ runner.os }}-metal-${{ steps.mesh_rev.outputs.rev }}
+          key: mesh-llama-${{ runner.os }}-metal-${{ hashFiles('.xcode-version') }}-${{ steps.mesh_rev.outputs.rev }}
 
       - name: Build mesh llama native libraries
         if: steps.llama_cache.outputs.cache-hit != 'true'
@@ -158,7 +163,7 @@ jobs:
         uses: actions/cache/save@caa296126883cff596d87d8935842f9db880ef25 # v5
         with:
           path: ${{ github.workspace }}/.cache/mesh-llama
-          key: mesh-llama-${{ runner.os }}-metal-${{ steps.mesh_rev.outputs.rev }}
+          key: mesh-llama-${{ runner.os }}-metal-${{ hashFiles('.xcode-version') }}-${{ steps.mesh_rev.outputs.rev }}
 
       - name: Build unsigned Tauri app
         run: cd desktop && pnpm tauri build --verbose --no-sign --features mesh-llm --config src-tauri/tauri.canary.conf.json
diff --git a/.xcode-version b/.xcode-version
new file mode 100644
index 0000000000..008c39a438
--- /dev/null
+++ b/.xcode-version
@@ -0,0 +1 @@
+27.0.0
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index dbe4ba5dd2..b20433b992 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -102,6 +102,29 @@ Hermit pins Rust, `just`, Node, pnpm, and other tools to the versions in
 upfront. If you don't use Hermit, ensure your toolchain meets the minimum
 versions in the table above.
 
+#### macOS: Xcode
+
+Apple builds use the exact version in [`.xcode-version`](.xcode-version),
+currently Xcode 27.0.0. Install that version and set `DEVELOPER_DIR` to its
+`Xcode.app/Contents/Developer` directory for your shell. For example:
+
+```bash
+export DEVELOPER_DIR=/Applications/Xcode_27.app/Contents/Developer
+scripts/check-xcode.sh
+```
+
+Local Tauri build/dev commands and native `just` recipes reject a different
+active version. Direct Cargo, Swift, Flutter, or Xcode invocations should run
+`scripts/check-xcode.sh` first in the same environment. Nothing installs Xcode
+or changes the system-wide `xcode-select` setting automatically. Xcode 27
+requires macOS 26.6 or later.
+
+GitHub Apple jobs use the `xcode-27` runner image and select the file's exact
+version before compiling. That runner image is currently a GitHub public
+preview, even though the selected Xcode 27.0 compiler is stable. When changing
+the pin, verify runner availability and coordinate the internal release
+pipeline's scheduling pin in `squareup/buzz-releases`.
+
 #### Linux: Tauri system libraries
 
 Hermit pins language toolchains, not system libraries. On Linux, the desktop
diff --git a/Justfile b/Justfile
index 04c6e46e70..69edf3e155 100644
--- a/Justfile
+++ b/Justfile
@@ -220,25 +220,30 @@ _ensure-migrations: _ensure-services
 # the default graph covers the `#[cfg(not(feature = "mesh-llm"))]` arms and
 # the feature-enabled graph covers the mesh code.
 desktop-tauri-clippy: _ensure-sidecar-stubs
+    ./scripts/check-xcode.sh
     cargo clippy --manifest-path {{desktop_tauri_manifest}} --workspace --all-targets -- -D warnings
     cargo clippy --manifest-path {{desktop_tauri_manifest}} --workspace --all-targets --features mesh-llm -- -D warnings
 
 # Check the desktop Tauri Rust crate compiles
 desktop-tauri-check: _ensure-sidecar-stubs
+    ./scripts/check-xcode.sh
     cargo check --manifest-path {{desktop_tauri_manifest}}
 
 # Run desktop Tauri Rust unit tests
 desktop-tauri-test: _ensure-sidecar-stubs
+    ./scripts/check-xcode.sh
     cd desktop/src-tauri && cargo test --workspace
 
 # Run the desktop Tauri lib tests with the mesh-llm feature graph
 desktop-tauri-test-mesh: _ensure-sidecar-stubs
+    ./scripts/check-xcode.sh
     cargo test --manifest-path {{desktop_tauri_manifest}} --features mesh-llm --lib
 
 # Run the native terminal latency gate explicitly on a known-idle host.
 # This is intentionally excluded from shared CI: scheduler contention makes a
 # wall-clock assertion flaky, and the release profile is the shipped shape.
 desktop-terminal-performance-test:
+    ./scripts/check-xcode.sh
     cargo test --manifest-path desktop/src-tauri/crates/buzz-terminal/Cargo.toml --release --test latency g3_renderer_acquire_stays_within_frame_budget -- --ignored --exact --nocapture
 
 # Verify compiled-flag behavior in each compile state.
@@ -255,6 +260,7 @@ desktop-terminal-performance-test:
 desktop-tauri-test-compiled-flags: _ensure-sidecar-stubs
     #!/usr/bin/env bash
     set -euo pipefail
+    ./scripts/check-xcode.sh
     cd desktop/src-tauri
     unset BUZZ_BUILD_AUTO_CONNECT_DEFAULT_RELAY BUZZ_BUILD_AGENT_ACCESS_OWNER_ONLY BUZZ_BUILD_DEMO_SLUG
     unset BUZZ_TEST_EXPECTED_AUTO_CONNECT_DEFAULT_RELAY BUZZ_TEST_EXPECTED_AGENT_ACCESS_OWNER_ONLY BUZZ_TEST_EXPECTED_DEMO_SLUG
@@ -314,6 +320,7 @@ desktop-tauri-test-compiled-flags: _ensure-sidecar-stubs
 desktop-release-build target="aarch64-apple-darwin":
     #!/usr/bin/env bash
     set -euo pipefail
+    ./scripts/check-xcode.sh
     TARGET={{target}}
     mkdir -p desktop/src-tauri/binaries
     touch "desktop/src-tauri/binaries/buzz-acp-$TARGET"
@@ -331,6 +338,7 @@ desktop-release-build target="aarch64-apple-darwin":
 desktop-demo-build demo_name target="aarch64-apple-darwin":
     #!/usr/bin/env bash
     set -euo pipefail
+    ./scripts/check-xcode.sh
     TARGET={{target}}
     [[ "$(uname -s)" == "Darwin" && "$TARGET" == *-apple-darwin ]] || { echo "Demo DMGs require a macOS Apple target" >&2; exit 2; }
     CONFIG_PATH="$(mktemp "${TMPDIR:-/tmp}/buzz-demo-config.XXXXXX")"
@@ -1015,6 +1023,7 @@ mobile-build-android:
 mobile-dev:
     #!/usr/bin/env bash
     set -euo pipefail
+    ./scripts/check-xcode.sh
     if ! pgrep -x Simulator &>/dev/null; then
         open -a Simulator
         sleep 3
diff --git a/desktop/scripts/tauri-command.mjs b/desktop/scripts/tauri-command.mjs
index bc53d607e7..49178359e9 100644
--- a/desktop/scripts/tauri-command.mjs
+++ b/desktop/scripts/tauri-command.mjs
@@ -31,6 +31,17 @@ function runTauri(args, options = {}) {
 }
 
 export function runTauriCommand(args) {
+  if (process.platform === "darwin" && ["build", "dev"].includes(args[0])) {
+    const check = spawnSync(
+      "bash",
+      [path.join(desktopRoot, "../scripts/check-xcode.sh")],
+      {
+        stdio: "inherit",
+      },
+    );
+    if (check.error) throw check.error;
+    if (check.status !== 0) return check.status ?? 1;
+  }
   if (args[0] !== "build") return runTauri(args);
 
   // Tauri runs beforeBuildCommand and then consumes frontendDist. Give the
diff --git a/scripts/check-xcode.sh b/scripts/check-xcode.sh
new file mode 100755
index 0000000000..50f69d8d85
--- /dev/null
+++ b/scripts/check-xcode.sh
@@ -0,0 +1,23 @@
+#!/usr/bin/env bash
+# Validate the active Apple toolchain without changing xcode-select globally.
+set -euo pipefail
+[[ "$(uname -s)" == Darwin ]] || exit 0
+root="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+expected="$(cat "$root/.xcode-version")"
+[[ "$expected" =~ ^[0-9]+\.[0-9]+\.[0-9]+$ ]] || {
+  echo "Invalid .xcode-version: expected a major.minor.patch version" >&2
+  exit 1
+}
+if ! version="$(xcodebuild -version)"; then
+  echo "Buzz requires Xcode $expected. Set DEVELOPER_DIR to its Xcode.app/Contents/Developer." >&2
+  exit 1
+fi
+actual="$(printf '%s\n' "$version" | sed -n 's/^Xcode //p')"
+[[ "$actual" == *.* ]] || actual="$actual.0"
+[[ "$actual" == *.*.* ]] || actual="$actual.0"
+if [[ "$actual" != "$expected" ]]; then
+  echo "Buzz requires Xcode $expected (.xcode-version); active toolchain reports: $version" >&2
+  echo "Set DEVELOPER_DIR to the matching Xcode.app/Contents/Developer and retry." >&2
+  exit 1
+fi
+printf '%s\n' "$version"
diff --git a/scripts/test-mobile-gateway-recipes.sh b/scripts/test-mobile-gateway-recipes.sh
index ae6741eb1a..111a75a23d 100644
--- a/scripts/test-mobile-gateway-recipes.sh
+++ b/scripts/test-mobile-gateway-recipes.sh
@@ -24,6 +24,7 @@ cat > "$fixture/scripts/mobile-worktree-overrides.sh" <<'STUB'
 exit 0
 STUB
 chmod +x "$fixture/bin/flutter" "$fixture/bin/pgrep" "$fixture/scripts/mobile-worktree-overrides.sh"
+cp "$fixture/scripts/mobile-worktree-overrides.sh" "$fixture/scripts/check-xcode.sh"
 export PATH="$fixture/bin:$PATH"
 export CALL_LOG="$fixture/arguments"
 unset BUZZ_PUSH_GATEWAY_URL
diff --git a/scripts/test-xcode-version.sh b/scripts/test-xcode-version.sh
new file mode 100755
index 0000000000..54e251f782
--- /dev/null
+++ b/scripts/test-xcode-version.sh
@@ -0,0 +1,33 @@
+#!/usr/bin/env bash
+# Exercise version enforcement without installing or switching a real Xcode.
+set -euo pipefail
+root="$(cd "$(dirname "$0")/.." && pwd)"
+fixture="$(mktemp -d)"
+trap 'rm -rf "$fixture"' EXIT
+mkdir -p "$fixture/scripts" "$fixture/bin"
+cp "$root/scripts/check-xcode.sh" "$fixture/scripts/"
+cp "$root/.xcode-version" "$fixture/"
+cat > "$fixture/bin/uname" <<'STUB'
+#!/usr/bin/env bash
+printf '%s\n' "${TEST_OS:-Darwin}"
+STUB
+cat > "$fixture/bin/xcodebuild" <<'STUB'
+#!/usr/bin/env bash
+[[ "${TEST_XCODE:-}" != broken ]] || exit 1
+printf 'Xcode %s\nBuild version fixture\n' "$TEST_XCODE"
+STUB
+chmod +x "$fixture/bin/"*
+export PATH="$fixture/bin:$PATH"
+check() { "$fixture/scripts/check-xcode.sh" >/dev/null 2>&1; }
+export TEST_XCODE=27.0
+check
+export TEST_XCODE=27
+check
+for TEST_XCODE in 26.6 27.1 '27.0 beta' broken ''; do
+  export TEST_XCODE
+  if check; then echo "Accepted wrong Xcode: $TEST_XCODE" >&2; exit 1; fi
+done
+printf 'latest\n' > "$fixture/.xcode-version"
+if check; then echo 'Accepted invalid pin' >&2; exit 1; fi
+TEST_OS=Linux check
+printf 'Xcode version enforcement passed\n'