msd-core/docs/adr/0009-shell-command-projection-module.md

Shell Command Projection Module owns runtime-aware OS command rendering

• Status: Accepted
• Supersedes: ADR-0010 (File Operation Engine Module) — absorbed into this seam's Phases 3–4 (#3467–#3468), 2026-05-13
• Date: 2026-05-12

We propose introducing a Shell Command Projection Module that owns projection from typed command intent to concrete shell/runtime-specific command text. MSD currently hand-builds hook commands, PATH repair commands, shim scripts, and other serialized OS-facing command strings across installer call sites. That drift has repeatedly produced cross-shell regressions (#2376, #2979, #3002, #3011, #3181, #3393, #3413). The proposed seam concentrates quoting, path-style, and runtime-wrapper policy in one module while keeping real subprocess execution on array-arg/non-shell paths.

Decision

• Add a Shell Command Projection Module under msd-core/bin/lib/ as the single owner for runtime-aware command-text rendering.
• Feed the module typed inputs (platform, shell, runtime, executable token, args, path policy) instead of prebuilt shell strings.
• Keep callers as thin Adapters that request projected text for:
  • managed hook commands in settings.json
  • managed hook commands in runtime config TOML/JSON surfaces
  • user-facing PATH repair / setup instructions
  • generated shim / wrapper script text written to disk
• Keep internal subprocess execution (spawnSync, execFileSync, SDK query dispatch) outside this seam. The module does not become a generic command runner.
• Make runtime-specific wrappers explicit policy at the seam (for example, emit PowerShell call-operator prefixes only for shells/runtimes that require them).
• Make path-style projection explicit policy at the seam (native Windows, POSIX slash, $HOME-relative, project-dir-relative, etc.).
• Prefer typed IR outputs that tests can assert against directly, then render text at the final Adapter.

Initial Scope

First migration slice should cover installer/runtime surfaces already proving this bug class:

1. managed JS and .sh hook command construction
2. managed hook rewrite / normalization on reinstall
3. Codex hook block command rendering
4. PATH diagnostic action-line rendering
5. Windows shim / wrapper script text builders
6. local-install hook command rendering ($CLAUDE_PROJECT_DIR vs cwd-relative runtime paths)

It should not in the first pass expand into workflow markdown ```bash blocks or replace safe internal subprocess APIs with shell-string execution.

Migration Inventory

bin/install.js

These call sites should migrate behind the Shell Command Projection Module:

• formatHookCommandForShell() — bin/install.js:605-608
• formatManagedHookScriptToken() — bin/install.js:610-614
• rewriteLegacyManagedNodeHookCommands() projection path — bin/install.js:639-718
• buildCodexHookBlock() — bin/install.js:752-768
• rewriteLegacyCodexHookBlock() — bin/install.js:784-812
• buildHookCommand() — bin/install.js:827-857
• localCmd() builder — bin/install.js:8855-8857
• per-hook command constructors — bin/install.js:8858-8875, 9065-9067, 9090-9092, 9119-9121, 9143-9145, 9173-9176
• legacy PATH hint strings — bin/install.js:9764-9768
• formatSdkPathDiagnostic() render path — bin/install.js:10075-10084, builder at 10532-10582
• buildWindowsShimTriple() — bin/install.js:10487-10510

Tests expected to move with the seam

• tests/install.test.cjs (folds former bug-2979-hook-absolute-node, consolidation epic #1969)
• tests/sh-hook-paths.test.cjs
• tests/bug-3011-sdk-path-diagnostic.test.cjs
• tests/codex-config.test.cjs (folds former bug-3017-codex-hook-absolute-node, consolidation epic #1969)
• tests/path-replacement.test.cjs (folds former bug-2376-opencode-windows-home-path, consolidation epic #1969)
• tests/bug-3020-install-shell-path-probe.test.cjs
• tests/bug-3359-stale-msd-sdk-path-version.test.cjs

Interface sketch

The module should accept typed command intent rather than concatenated shell fragments. Example shape:

projectShellCommand({
  platform: 'linux' | 'darwin' | 'win32',
  shell: 'bash' | 'zsh' | 'cmd' | 'pwsh',
  runtime: 'claude' | 'gemini' | 'codex' | 'opencode' | 'copilot' | 'antigravity' | 'generic',
  executable: { kind: 'node' | 'bash' | 'pwsh' | 'literal', token: '...' },
  args: ['...'],
  pathStyle: 'native' | 'posix' | 'home-relative' | 'project-relative',
})

For user-facing multi-line guidance, the seam should return typed action IR first, then let the installer print it:

projectShellActions({ intent: 'prepend-path', platform, targetDir, runtime })

For generated shim/wrapper files, the seam should own script text rendering too:

projectShellScript({ shell: 'cmd' | 'pwsh' | 'sh', executable, argsTemplate })

Consequences

• Quoting, slash-direction, wrapper-prefix, and variable-expansion policy become local to one module.
• Installer/runtime call sites become thinner and stop inventing sibling string builders.
• Windows runtime-specific regressions become easier to classify as seam bugs instead of one-off installer bugs.
• Tests can assert against typed projection IR instead of source-grepping ad-hoc string concatenation sites.
• The first implementation will move a broad installer surface, so scope discipline matters: start with installer/runtime projection only, not every shell string in the repo.
• If accepted, CONTEXT.md should gain a canonical Shell Command Projection Module entry and future architecture reviews should treat out-of-seam command rendering as drift.

Open questions

• Whether hooks.shell_preference from #3082 should become an input policy consumed by this module or remain a higher-level runtime config concern.
• Whether Windows Git Bash should be modeled as explicit shell: 'bash' + platform: 'win32' or as a distinct shell target.
• Whether existing shim/script builders should migrate in the first pass or follow immediately after the hook/diagnostic path is stable.
• Whether the seam should live entirely in installer land or later become shared with other runtime-output surfaces outside bin/install.js.

References

• Feature issue: #3439
• Related bug history: #2376, #2979, #3002, #3011, #3017, #3020, #3082, #3181, #3393, #3413
• See 0005-sdk-architecture-seam-map.md
• See 0008-installer-migration-module.md

Update — 2026-05-13 (Phases 1–4 expansion, #3465–#3468)

The seam grew beyond the original "rendering only" scope. The "does not become a generic command runner" and "does not replace safe internal subprocess APIs" constraints (Decision §17, Initial Scope §33) were intentionally superseded.

Scope now owned by shell-command-projection.cjs:

• runtime-aware command-text rendering (original ADR scope)
• subprocess dispatch — execGit, execNpm, execTool, probeTty (Phase 2, #3466)
• platform file I/O — platformWriteSync, platformReadSync, platformEnsureDir, normalizeContent (Phase 3, #3467)
• legacy wrappers atomicWriteFileSync / safeReadFile / normalizeMd removed from core.cjs (Phase 4, #3468)

Result-shape invariant: all exec* return { exitCode, stdout, stderr } and never throw on non-zero exit. Platform-conditional logic (shell: process.platform === 'win32', probeTty Windows null return, .md-aware normalization) lives only at the seam.

Open question resolutions:

• Q4 (installer-only vs shared seam): resolved — shared. The seam lives in msd-core/bin/lib/, consumed by installer, planning workflow, and every fs/subprocess call site across the tool.
• Q1, Q2, Q3 (hooks.shell_preference, Windows Git Bash modeling, shim/script builder migration timing): unresolved, carried forward as projection-design concerns independent of the I/O expansion.

See CONTEXT.md "Shell Command Projection Module" entry for the canonical current-state description.