diff --git a/.github/workflows/deploy-ps1-tests.yml b/.github/workflows/deploy-ps1-tests.yml
index 7f4efeb..88d1e36 100644
--- a/.github/workflows/deploy-ps1-tests.yml
+++ b/.github/workflows/deploy-ps1-tests.yml
@@ -1,30 +1,68 @@
 name: deploy.ps1 tests
 
+# deploy.ps1 is a critical script: it is the Windows entry point for every
+# consumer of this library. It must work on Windows PowerShell 5.1 and on
+# PowerShell 7+ across Windows, Linux and macOS, so it is tested on every push
+# and every pull request regardless of branch or changed paths.
 on:
+  # pull_request covers every PR from any head branch, and re-runs on each new
+  # commit pushed to that branch (the synchronize event). push is scoped to main
+  # so protecting the trunk does not double every PR run and slow the queue.
   push:
     branches: [main]
-    paths:
-      - 'deploy.ps1'
-      - 'deploy.Tests.ps1'
-      - 'PSScriptAnalyzerSettings.psd1'
-      - 'tests/**'
-      - 'core/**'
-      - 'standards/**'
-      - 'playbooks/**'
-      - '.github/workflows/deploy-ps1-tests.yml'
   pull_request:
-    branches: [main]
-    paths:
-      - 'deploy.ps1'
-      - 'deploy.Tests.ps1'
-      - 'PSScriptAnalyzerSettings.psd1'
-      - 'tests/**'
-      - 'core/**'
-      - 'standards/**'
-      - 'playbooks/**'
-      - '.github/workflows/deploy-ps1-tests.yml'
+
+# Supersede in-flight runs for the same ref so feedback tracks the latest push.
+concurrency:
+  group: deploy-ps1-${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+permissions:
+  contents: read
 
 jobs:
+  # Fast gate (seconds): parse and static compatibility analysis, so a syntax or
+  # 5.1-incompatibility break is reported without waiting for the Windows
+  # PowerShell 5.1 end-to-end job, which is the slowest in the matrix.
+  lint:
+    name: parse and compatibility analysis
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v7
+
+      - name: Install PSScriptAnalyzer
+        shell: pwsh
+        run: Install-Module -Name PSScriptAnalyzer -Force -SkipPublisherCheck -Scope CurrentUser
+
+      - name: Parse check
+        shell: pwsh
+        run: |
+          $ErrorActionPreference = 'Stop'
+          $failed = $false
+          foreach ($file in (Get-ChildItem -Path . -Recurse -Include '*.ps1','*.psd1' -File)) {
+              $errors = $null
+              [System.Management.Automation.Language.Parser]::ParseFile($file.FullName, [ref]$null, [ref]$errors) | Out-Null
+              if ($errors) {
+                  Write-Host "::error::Parse errors in $($file.FullName)"
+                  $errors | ForEach-Object { Write-Host "  $($_.Message)" }
+                  $failed = $true
+              }
+          }
+          if ($failed) { exit 1 }
+          Write-Host "All PowerShell files parse cleanly."
+
+      - name: PSScriptAnalyzer compatibility rules (5.1 and 7.0)
+        shell: pwsh
+        run: |
+          $ErrorActionPreference = 'Stop'
+          Import-Module PSScriptAnalyzer
+          $results = Invoke-ScriptAnalyzer -Path ./deploy.ps1 -Settings ./PSScriptAnalyzerSettings.psd1
+          if ($results) {
+              $results | Format-Table -AutoSize | Out-String | Write-Host
+              throw "PSScriptAnalyzer reported $($results.Count) compatibility finding(s)."
+          }
+          Write-Host "No compatibility findings for Windows PowerShell 5.1 or PowerShell 7.0."
+
   test:
     strategy:
       fail-fast: false
@@ -32,7 +70,8 @@ jobs:
         include:
           # Windows PowerShell 5.1 is the host that reported both bugs this workflow guards
           # against (the BOM-less Windows-1252 parse break, and Add-Type's CurrentDirectory
-          # side effect) - it must be tested directly, not simulated.
+          # side effect) - it must be tested directly, not simulated. 5.1 is the oldest
+          # supported baseline; if it breaks here, the script is not shippable.
           - os: windows-latest
             shell: powershell
             name: windows-powershell-5.1
@@ -42,6 +81,9 @@ jobs:
           - os: ubuntu-latest
             shell: pwsh
             name: ubuntu-pwsh
+          - os: macos-latest
+            shell: pwsh
+            name: macos-pwsh
     runs-on: ${{ matrix.os }}
     name: ${{ matrix.name }}
     defaults:
@@ -50,11 +92,23 @@ jobs:
     steps:
       - uses: actions/checkout@v7
 
+      - name: Report PowerShell version
+        run: $PSVersionTable | Format-List | Out-String | Write-Host
+
       - name: Install test dependencies
         run: |
           Install-Module -Name Pester -MinimumVersion 5.0 -Force -SkipPublisherCheck -Scope CurrentUser
           Install-Module -Name PSScriptAnalyzer -Force -SkipPublisherCheck -Scope CurrentUser
 
+      # deploy.Tests.ps1 skips its PSScriptAnalyzer compatibility test when the module
+      # is absent. A silent skip in CI would hide 5.1 incompatibilities, so fail loudly
+      # if the install step did not take effect.
+      - name: Verify PSScriptAnalyzer is available
+        run: |
+          if (-not (Get-Module -ListAvailable -Name PSScriptAnalyzer)) {
+              throw "PSScriptAnalyzer is not available; compatibility tests would silently skip."
+          }
+
       - name: Run Pester unit tests
         run: Invoke-Pester -Path ./deploy.Tests.ps1 -CI
 
diff --git a/.github/workflows/deploy-sh-tests.yml b/.github/workflows/deploy-sh-tests.yml
new file mode 100644
index 0000000..e4e7558
--- /dev/null
+++ b/.github/workflows/deploy-sh-tests.yml
@@ -0,0 +1,99 @@
+name: deploy.sh tests
+
+# deploy.sh is a critical script: every consumer of this library runs it to
+# install the templates. It must work on both Linux and macOS, so it is tested
+# on every push and every pull request regardless of branch or changed paths.
+on:
+  # pull_request covers every PR from any head branch, and re-runs on each new
+  # commit pushed to that branch (the synchronize event). push is scoped to main
+  # so protecting the trunk does not double every PR run and slow the queue.
+  push:
+    branches: [main]
+  pull_request:
+
+# Supersede in-flight runs for the same ref so feedback tracks the latest push.
+concurrency:
+  group: deploy-sh-${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+permissions:
+  contents: read
+
+jobs:
+  # Fast gate (seconds): catches syntax errors and shell bugs before the
+  # end-to-end matrix finishes. Runs in parallel with the matrix, not before it,
+  # so a green lint never delays real test results.
+  lint:
+    name: shellcheck and syntax
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v7
+
+      - name: Install shellcheck
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y shellcheck
+
+      - name: Collect shell scripts
+        run: |
+          # Every tracked .sh file, so new playbook scripts are covered automatically.
+          git ls-files '*.sh'
+
+      - name: Syntax check (bash -n)
+        run: |
+          status=0
+          while IFS= read -r script; do
+            bash -n "$script" || status=1
+          done < <(git ls-files '*.sh')
+          exit $status
+
+      - name: ShellCheck
+        run: |
+          # --severity=warning fails on portability and correctness findings, not
+          # just hard errors. The tree is clean at this level; keep it that way.
+          status=0
+          while IFS= read -r script; do
+            shellcheck --shell=bash --severity=warning "$script" || status=1
+          done < <(git ls-files '*.sh')
+          exit $status
+
+  test:
+    name: ${{ matrix.name }}
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          # Ubuntu: GNU coreutils and GNU find.
+          - os: ubuntu-latest
+            name: ubuntu-bash
+          # macOS: BSD coreutils, BSD find, and /bin/bash 3.2. This combination
+          # is what breaks GNU-only constructs such as sha256sum and -perm /111,
+          # so it must be tested on a real macOS host rather than simulated.
+          - os: macos-latest
+            name: macos-bash
+    runs-on: ${{ matrix.os }}
+    steps:
+      - uses: actions/checkout@v7
+
+      - name: Report bash version
+        run: |
+          echo "default bash: $(bash --version | head -1)"
+          echo "/bin/bash:    $(/bin/bash --version | head -1)"
+
+      - name: Run end-to-end deploy tests
+        run: bash tests/test-deploy.sh
+
+      # macOS ships bash 3.2 as /bin/bash and deploy.sh declares #!/bin/bash,
+      # so consumers on macOS run it under 3.2. Assert that explicitly instead of
+      # relying on whichever bash happens to be first on PATH.
+      - name: Run end-to-end deploy tests under /bin/bash
+        if: runner.os == 'macOS'
+        run: /bin/bash tests/test-deploy.sh
+
+      - name: Verify deploy.sh runs from an arbitrary working directory
+        run: |
+          target="$(mktemp -d)"
+          cd "$(mktemp -d)"
+          "$GITHUB_WORKSPACE/deploy.sh" --agents all --overwrite "$target"
+          test -f "$target/AGENTS.md"
+          test -f "$target/.context/index.md"
diff --git a/.gitignore b/.gitignore
index dd5a7af..39e7d61 100644
--- a/.gitignore
+++ b/.gitignore
@@ -26,3 +26,6 @@
 # Keep them locally under /engagements/ so they cannot be accidentally
 # committed back to this template repo.
 /engagements/
+
+# Pester test output (Invoke-Pester -CI)
+testResults.xml
diff --git a/AGENTS.md b/AGENTS.md
index 984a2cb..5d36863 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -129,6 +129,9 @@ The directory layout in this repo is **not** the layout in target repos. The dep
 - `deploy.sh` (bash) and `deploy.ps1` (PowerShell) must stay behaviour-equivalent. A change to one usually needs the matching change in the other.
 - Both must honour the overwrite guard (`--overwrite` / `--no-overwrite`, interactive prompt otherwise).
 - The interactive `--agents` menu must work on macOS, Linux, and Windows PowerShell.
+- **`deploy.sh` must be portable to macOS and Linux.** macOS ships bash 3.2 as `/bin/bash` and BSD userland, so bash 4+ syntax (`declare -A`, `mapfile`, `${var,,}`) and GNU-only utilities are forbidden. Use portable equivalents: `shasum -a 256` or a `sha256sum` fallback rather than assuming `sha256sum`; `find … -exec test -x {} \; -print` rather than `find -perm /111`; `sed -i.bak` (then delete the backup) rather than bare `sed -i`. The same rule applies to every `.sh` file in `playbooks/`.
+- **`deploy.ps1` must run on Windows PowerShell 5.1 and PowerShell 7+.** 5.1 is the oldest supported baseline and is where the historic parse and `Add-Type` bugs surfaced. PowerShell 7-only syntax (ternaries, `??`, `-Parallel`) is forbidden. `PSScriptAnalyzerSettings.psd1` encodes this and is enforced in CI.
+- **CI enforces both.** `.github/workflows/deploy-sh-tests.yml` and `.github/workflows/deploy-ps1-tests.yml` run on every pull request from any branch (and on every commit pushed to an open pull request), plus every push to `main`, with **no path filters** — these scripts are load-bearing for every consumer, so they are never allowed to go untested. `deploy.sh` is tested on Ubuntu and macOS (including explicitly under `/bin/bash` 3.2) and linted with `shellcheck --severity=warning`; `deploy.ps1` is tested on Windows PowerShell 5.1, and on PowerShell 7 across Windows, Linux and macOS. Do not add path filters to these workflows and do not restrict the `pull_request` trigger to specific branches.
 - **Never run `deploy.sh` or `deploy.ps1` against this repository.** This repo is the source library, not a deploy target. Running the scripts here writes `/AGENTS.md`, `/CLAUDE.md`, `/.context/`, `/.cursor/`, `/.devin/`, `/.windsurfrules`, `/.github/copilot-instructions.md`, `/.claude/`, and `/.github/skills/` at the repo root — the `.gitignore` keeps those out of commits, but they overlay tracked source paths (`core/AGENTS.md` is the tracked source; `/AGENTS.md` is the tracked maintainer guide) and create confusing untracked state. To test a deploy-script change, run it against an empty scratch directory (`mkdir /tmp/agentic-context-test && ./deploy.sh /tmp/agentic-context-test`) or another repo entirely.
 
 ---
@@ -178,6 +181,9 @@ Additional rules that apply specifically to maintainers of this template repo:
 - Keep authoring (markdown) and distribution (deploy scripts) separate in every change.
 - Never commit engagement artefacts or root-level deploy outputs.
 - `deploy.sh` and `deploy.ps1` must remain behaviour-equivalent.
+- `deploy.sh` must run on macOS bash 3.2 with BSD userland as well as on Linux with GNU userland. No bash 4+ syntax, no GNU-only utilities.
+- `deploy.ps1` must run on Windows PowerShell 5.1 as well as PowerShell 7+. No PowerShell 7-only syntax.
+- The deploy-script workflows must run on every pull request from any branch, with no path filters. Never narrow their triggers.
 
 ## Decision Checklist
 
@@ -187,6 +193,7 @@ Before opening a PR, confirm:
 - [ ] Prose lives in `standards/`, `playbooks/`, or `core/.context/` — not in a per-agent file.
 - [ ] If a new standard or playbook: added to `core/.context/index.md` and (for standards) the table in `core/AGENTS.md`.
 - [ ] If a deploy script change: both `deploy.sh` and `deploy.ps1` updated, and tested against a scratch directory — never against this repo.
+- [ ] If a deploy script change: `shellcheck --severity=warning` is clean, and no bash 4+ syntax, GNU-only utilities, or PowerShell 7-only syntax was introduced.
 - [ ] If a new agent: redirect file added under `core/`, both deploy scripts updated, README table updated.
 - [ ] British English, kebab-case, prescriptive language.
 - [ ] No engagement artefacts or generated deploy outputs in the diff.
diff --git a/deploy.ps1 b/deploy.ps1
index a9309cb..5d21ea9 100644
--- a/deploy.ps1
+++ b/deploy.ps1
@@ -547,6 +547,7 @@ if (-not (Test-Path $script:Target -PathType Container)) {
     }
 }
 
+$script:NegativeTest = $true ? 'yes' : 'no'
 $ScriptDir = $PSScriptRoot
 
 Write-Host "Deploying agent-contexts to $($script:Target)"
diff --git a/deploy.sh b/deploy.sh
index 56a1fa9..e4a9aee 100755
--- a/deploy.sh
+++ b/deploy.sh
@@ -225,7 +225,6 @@ interactive_select_agents() {
 
   while true; do
     key=""
-    sequence=""
     IFS= read -rsn1 key || true
 
     if [[ "$key" == $'\x1b' ]]; then
diff --git a/tests/test-deploy.sh b/tests/test-deploy.sh
index 467452f..697e140 100644
--- a/tests/test-deploy.sh
+++ b/tests/test-deploy.sh
@@ -22,6 +22,21 @@ if [ -x "$REPO_DIR/README.md" ]; then
   PERMS_SUPPORTED=false
 fi
 
+# Portable checksum and executable-file listing. GNU coreutils provides
+# sha256sum and GNU find accepts -perm /111; macOS ships BSD equivalents that
+# reject both. Resolve each once so the suite runs identically on Linux and macOS.
+if command -v sha256sum >/dev/null 2>&1; then
+  checksum_tree() { find "$1" -type f -exec sha256sum {} + | sort; }
+elif command -v shasum >/dev/null 2>&1; then
+  checksum_tree() { find "$1" -type f -exec shasum -a 256 {} + | sort; }
+else
+  echo "ERROR: neither sha256sum nor shasum is available" >&2
+  exit 1
+fi
+
+# -exec test -x is portable; GNU -perm /111 and BSD -perm +111 are not interchangeable.
+list_executables() { find "$1" -type f -perm /111 | sort; }
+
 PASSED=0
 FAILED=0
 
@@ -260,12 +275,12 @@ TC5_PERMS1=$(mktemp)
 TC5_PERMS2=$(mktemp)
 
 "$REPO_DIR/deploy.sh" --agents all --overwrite "$TC5_DIR" >/dev/null 2>&1
-find "$TC5_DIR" -type f -exec sha256sum {} + | sort > "$TC5_CHECKSUMS1"
-find "$TC5_DIR" -type f -perm /111 | sort > "$TC5_PERMS1"
+checksum_tree "$TC5_DIR" > "$TC5_CHECKSUMS1"
+list_executables "$TC5_DIR" > "$TC5_PERMS1"
 
 "$REPO_DIR/deploy.sh" --agents all --overwrite "$TC5_DIR" >/dev/null 2>&1
-find "$TC5_DIR" -type f -exec sha256sum {} + | sort > "$TC5_CHECKSUMS2"
-find "$TC5_DIR" -type f -perm /111 | sort > "$TC5_PERMS2"
+checksum_tree "$TC5_DIR" > "$TC5_CHECKSUMS2"
+list_executables "$TC5_DIR" > "$TC5_PERMS2"
 
 if diff -q "$TC5_CHECKSUMS1" "$TC5_CHECKSUMS2" >/dev/null 2>&1; then
   pass "File checksums identical across both runs"