@@ -43,6 +43,18 @@ automatically validate a definition passed to `execute()`. Guard invalid
 configurations in `execute()` too, returning a failed result rather than a
 successful default or an unhandled exception.
 
+The built-in shell step preserves raw `run` interpolation by default.
+Per-step `strict: true` rejects expression markers in `run` during validation
+and execution. Its optional `env` mapping evaluates string templates separately,
+requires NUL-free string results encodable for the process environment on
+POSIX, and overrides only the child environment.
+`SPECKIT_WORKFLOW_DIR` remains engine-owned. This prevents workflow expression
+values from becoming shell source in strict mode; it is not a sandbox or a
+guarantee that an authored script handles environment data safely.
+Environment bindings and inherited settings can intentionally affect executable
+lookup, module loading, and runtime startup. Strict mode does not filter them;
+workflow source review must cover both `run` and `env` and referenced code.
+
 New runs persist an execution tree. Resume replays completed occurrences into
 their expression contexts without calling their implementations, then continues
 at the unfinished occurrence. Selected branches, custom expansions, loop
 
@@ -335,6 +335,11 @@ non-default source.
 
 > **Vet both the bundle source and its component catalogs.** A project can supply its own bundle catalog and `install-allowed` component catalogs (extension, preset, workflow, and step) under `.specify/`; that project configuration existing is not evidence anything in it was reviewed. Before installing a bundle from an unfamiliar project, run `specify bundle catalog list` — and the equivalent `extension`/`preset`/`workflow catalog list` and `specify workflow step catalog list` commands for the components it pulls in — and treat any source you didn't add yourself as unvetted until you've reviewed it.
 
+For workflow components, inspect shell `run` and `env` fields and their
+referenced code, including changes introduced by active overlays. This applies
+even to `strict: true` steps: environment bindings can change executable lookup
+or runtime loading. See [Reviewing shell execution](workflows.md#reviewing-shell-execution).
+
 ### List the Catalog Stack
 
 ```bash
 
@@ -453,7 +453,7 @@ referents, so a pre-existing cache-file symlink can be replaced without changing
 its target. If a cache cannot be safely written, a warning is logged and
 discovery continues using the fetched catalog.
 
-> **A project's `.specify/workflow-catalogs.yml` can point `add` and `search` at a catalog you didn't choose.** Before running a workflow from an unfamiliar project, run `specify workflow catalog list` (and `specify workflow step catalog list` for the step catalogs its steps can pull in) — a project supplying that config is not evidence its workflows or steps were vetted. Maintainers do not audit `run` fields; read a workflow's shell steps yourself before running it (see [Who maintains workflows?](#who-maintains-workflows)).
+> **A project's `.specify/workflow-catalogs.yml` can point `add` and `search` at a catalog you didn't choose.** Before running a workflow from an unfamiliar project, run `specify workflow catalog list` (and `specify workflow step catalog list` for the step catalogs its steps can pull in) — a project supplying that config is not evidence its workflows or steps were vetted. Maintainers do not audit workflow code; review both `run` and `env` in its shell steps yourself, including strict steps (see [Reviewing shell execution](#reviewing-shell-execution) and [Who maintains workflows?](#who-maintains-workflows)).
 
 ### List Catalogs
 
@@ -782,6 +782,32 @@ qualified `step_id`. A step emits completion after its checkpoint; containers
 emit before their children, while replayed completed occurrences emit no step
 events.
 
+### Shell step
+
+`type: shell` executes `run` through the platform shell, retaining support for
+pipes, redirects, and multiple commands. Set `strict: true` to reject `{{`
+expression markers anywhere in `run`, including inside quotes. This is checked
+both during validation and immediately before execution. `strict` must be a
+boolean; omitted or `false` preserves existing raw interpolation behavior.
+
+Use an `env` mapping to bind dynamic values without inserting them into shell
+source. For example, on POSIX:
+
+```yaml
+- id: display-result
+  type: shell
+  strict: true
+  env:
+    RESULT: "{{ steps.plan.output.stdout }}"
+  run: 'printf "%s\n" "$RESULT"'
+```
+
+For cross-platform scripts, read the environment in the program itself, such
+as `os.environ["RESULT"]` in Python. See
+[Shell Step Environment Variables](#shell-step-environment-variables) for the
+binding contract and [Interpolation and shell safety](#interpolation-and-shell-safety)
+for the remaining trust boundary.
+
 > **Security note:** a `shell` step runs a local command with **your** privileges. There is no capability sandbox — `requires` is an advisory pre-condition block (spec-kit version, integrations), not a runtime gate, so it does **not** restrict what a step can do. In particular there is no `requires.permissions` capability gate: it is rejected by validation precisely because it would imply a sandbox that does not exist. Review any catalog or downloaded workflow before running it, and use a `gate` step to require explicit approval before sensitive or destructive shell commands.
 
 ### GitHub step
@@ -1237,9 +1263,11 @@ If an interpolated value can contain characters like `;`, `|`, `&`, `$( )`, back
 - **Workflow `inputs.*`** — supplied by whoever runs the workflow.
 - **A prior step's output**, e.g. `{{ steps.plan.output.stdout }}` — for a `command` or `prompt` step with [`capture: true`](#capturing-command-and-prompt-output) this is **text produced by the AI agent**, which can in turn be influenced by files, tickets, or web content the agent read. Treat agent output as untrusted when it flows into a `shell` step.
 
-There is **no shell-escaping filter** in the expression language and **no sandbox** around a `shell` step, so none of the practices below can be treated as a guarantee that a hostile value is neutralised. The only reliable control is to constrain what an interpolated value *can* be, and to keep values you cannot constrain out of `run` fields entirely. Scrutinise every `run` field that interpolates a value you do not control, and at minimum:
+There is **no shell-escaping filter** in the expression language and **no sandbox** around a `shell` step. Prefer [`strict: true`](#shell-step) with `env` bindings to keep dynamic values out of shell source. Strict mode does not sanitise data, inspect commands, or prevent an authored script from interpreting data as code. Do not use `eval`, build another `sh -c` command from a variable, or pass untrusted values through shell startup variables such as `BASH_ENV`. On Windows, `cmd.exe` expansion of `%VALUE%` can introduce shell syntax; read untrusted values inside a program instead. On POSIX, quote variable expansions and use the target program's argument validation or `--` where appropriate to prevent option injection.
 
-- **Constrain the value at the source with `enum`/an allowlist.** When `inputs.*` feeds a `run` field, restrict it to a fixed set of known-safe values so a caller cannot supply arbitrary shell text at all. This is the strongest control the engine offers — prefer it over any downstream mitigation.
+For compatibility, raw interpolation remains enabled when `strict` is omitted or `false`. Its injection risk remains unchanged. Keep values you cannot constrain out of `run` fields entirely. Scrutinise every non-strict `run` field that interpolates a value you do not control, and at minimum:
+
+- **Constrain the value at the source with `enum`/an allowlist.** When `inputs.*` feeds a non-strict `run` field, restrict it to a fixed set of known-safe values so a caller cannot supply arbitrary shell text at all.
 
   ```yaml
   inputs:
@@ -1262,6 +1290,60 @@ Shell steps automatically receive the following environment variables:
 | -------- | ----------- |
 | `SPECKIT_WORKFLOW_DIR` | Resolved absolute path to the workflow source directory (same value as `{{ context.workflow_dir }}`). Not set when the workflow has no source path. |
 
+An optional per-step `env` mapping is available in both strict and non-strict
+mode. Names must match `[A-Za-z_][A-Za-z0-9_]*`; names are literal, not
+expressions. Values must be strings and may contain workflow expressions.
+Each value is evaluated once and must resolve to a string without NUL
+characters. On POSIX, the string must also be encodable for the process
+environment using the filesystem encoding and its error handler.
+Invalid bindings fail the step before launching a process.
+
+Single-expression bindings preserve the expression's result type:
+`COUNT: "{{ inputs.count }}"` rejects a number, boolean, null, list, or mapping
+instead of converting it to text. Mixed templates use standard expression
+interpolation: `COUNT: "n={{ inputs.count }}"` produces `n=42` for integer
+`42`; embedded null becomes empty text, and other values use their string
+representation. To serialize structured data explicitly, use `to_json`.
+
+Bindings override inherited environment values for that subprocess only;
+they do not modify the parent process or subsequent steps. An empty string is
+valid. `SPECKIT_WORKFLOW_DIR` is reserved (case-insensitively) and cannot be
+overridden. Keep environment names and executable-loading settings under the
+workflow author's control: environment binding is a data channel, not a sandbox.
+
+### Reviewing shell execution
+
+Before installing or running an unfamiliar workflow, audit each shell step's
+`run` **and** `env`, including nested branches and fan-out templates. Review
+referenced scripts and resources, called workflows, and active overlay YAML
+that can replace or insert steps. Apply the same review to workflow components
+of bundles, and repeat it when definitions, overlays, or referenced code change.
+Custom step packages require their own source review because loading them
+executes Python code.
+
+An environment value can control execution rather than merely supply data.
+For example, `PATH` selects executables, `PYTHONPATH` affects Python imports,
+`BASH_ENV` can select Bash startup code, and `NODE_OPTIONS` can change Node.js
+startup behavior. Native loaders and other programs have their own settings;
+these examples are not an exhaustive list. A fixed `run` can therefore execute
+different code even with `strict: true` and no `eval`. Strict mode neither
+filters execution-controlling bindings nor cleans the inherited environment.
+Keep those settings literal and author-vetted, or constrain dynamic values to
+an approved set of trusted paths/options. Use unconstrained input or agent
+output only for application-data variables whose consumers handle it as data.
+
+When a shell step invokes Specify itself, catalog overrides such as
+`SPECKIT_WORKFLOW_CATALOG_URL` can also change where child commands discover
+or install components. Vet those bindings alongside the
+[active catalog sources](catalogs.md#discovery-and-trust).
+
+Workflow validation checks configuration, not whether environment settings or
+commands are safe. `specify workflow info` shows metadata and step identities;
+`specify workflow resolve` shows layer and step attribution. Neither displays
+the complete `run`/`env` configuration or replaces reading the contributing
+source files. Catalog install permission, bundle installation, and strict mode
+are not security endorsements.
+
 ## Input Types
 
 | Type      | Coercion                                          |
 
@@ -7,6 +7,7 @@
 import locale
 import math
 import os
+import re
 import subprocess
 from typing import Any
 
@@ -23,6 +24,13 @@ class ShellStep(StepBase):
     type_key = "shell"
 
     def execute(self, config: dict[str, Any], context: StepContext) -> StepResult:
+        binding_errors = self._binding_errors(config)
+        if binding_errors:
+            return StepResult(
+                status=StepStatus.FAILED,
+                error="; ".join(binding_errors),
+                output={"exit_code": -1, "stdout": "", "stderr": "invalid shell bindings"},
+            )
         run_cmd = config.get("run", "")
         if isinstance(run_cmd, str) and "{{" in run_cmd:
             run_cmd = evaluate_expression(run_cmd, context)
@@ -45,15 +53,33 @@ def execute(self, config: dict[str, Any], context: StepContext) -> StepResult:
             )
 
         env = {**os.environ}
+        for name, template in config.get("env", {}).items():
+            try:
+                value = evaluate_expression(template, context)
+            except ValueError as exc:
+                return StepResult(
+                    status=StepStatus.FAILED,
+                    error=f"Shell step {config.get('id', '?')!r}: env {name!r}: {exc}",
+                    output={"exit_code": -1, "stdout": "", "stderr": "invalid environment expression"},
+                )
+            value_error = self._environment_value_error(name, value)
+            if value_error is not None:
+                return StepResult(
+                    status=StepStatus.FAILED,
+                    error=f"Shell step {config.get('id', '?')!r}: {value_error}",
+                    output={"exit_code": -1, "stdout": "", "stderr": "invalid environment value"},
+                )
+            env[name.upper() if os.name == "nt" else name] = value
         if context.workflow_dir:
             env["SPECKIT_WORKFLOW_DIR"] = context.workflow_dir
         else:
             env.pop("SPECKIT_WORKFLOW_DIR", None)
 
         # NOTE: shell=True is required to support pipes, redirects, and
         # multi-command expressions in workflow YAML.  Workflow authors
-        # control commands; catalog-installed workflows should be reviewed
-        # before use (see PUBLISHING.md for security guidance).
+        # control commands. strict=True rejects expression interpolation in
+        # run; dynamic data can instead be bound through env without becoming
+        # shell source. Neither mode sandboxes authored shell commands.
         try:
             proc = subprocess.run(  # noqa: S602 -- intentional shell=True (see NOTE above)
                 run_cmd,
@@ -121,6 +147,51 @@ def execute(self, config: dict[str, Any], context: StepContext) -> StepResult:
                 output={"exit_code": -1, "stdout": "", "stderr": str(exc)},
             )
 
+    @staticmethod
+    def _environment_value_error(name: str, value: Any) -> str | None:
+        if not isinstance(value, str):
+            return f"env {name!r} must be a string, got {type(value).__name__}."
+        if "\x00" in value:
+            return f"env {name!r} must not contain NUL characters."
+        if os.name == "posix":
+            try:
+                os.fsencode(value)
+            except UnicodeEncodeError:
+                return f"env {name!r} cannot be encoded for the process environment."
+        return None
+
+    @classmethod
+    def _binding_errors(cls, config: dict[str, Any]) -> list[str]:
+        errors: list[str] = []
+        prefix = f"Shell step {config.get('id', '?')!r}: "
+        strict = config.get("strict", False)
+        if not isinstance(strict, bool):
+            errors.append(prefix + "'strict' must be a boolean.")
+        elif strict:
+            run = config.get("run")
+            if not isinstance(run, str):
+                errors.append(prefix + "'run' must be a string in strict mode.")
+            elif "{{" in run:
+                errors.append(
+                    prefix + "strict mode forbids expressions in 'run'; "
+                    "bind dynamic values through 'env' instead."
+                )
+
+        bindings = config.get("env", {})
+        if not isinstance(bindings, dict):
+            errors.append(prefix + "'env' must be a mapping.")
+            return errors
+        for name, template in bindings.items():
+            if not isinstance(name, str) or re.fullmatch(r"[A-Za-z_][A-Za-z0-9_]*", name) is None:
+                errors.append(prefix + f"invalid environment variable name {name!r}.")
+                continue
+            if name.upper() == "SPECKIT_WORKFLOW_DIR":
+                errors.append(prefix + f"environment variable {name!r} is reserved.")
+            value_error = cls._environment_value_error(name, template)
+            if value_error is not None:
+                errors.append(prefix + value_error)
+        return errors
+
     @staticmethod
     def _timeout_error(config: dict[str, Any]) -> str | None:
         """Return an error message if ``config['timeout']`` is invalid, else None.
@@ -157,20 +228,20 @@ def _timeout_error(config: dict[str, Any]) -> str | None:
 
     def validate(self, config: dict[str, Any]) -> list[str]:
         errors = super().validate(config)
-        if "run" not in config:
-            errors.append(
-                f"Shell step {config.get('id', '?')!r} is missing 'run' field."
-            )
-        elif not isinstance(config["run"], str):
-            # execute() str()-coerces run and invokes it under shell=True, so a
-            # null or list 'run' would run the Python repr ('None', "['echo']")
-            # as a command. Reject non-strings at validation, mirroring the
-            # command-step input/options and gate options type checks. An
-            # expression like "{{ ... }}" is still a str, so it stays valid.
-            errors.append(
-                f"Shell step {config.get('id', '?')!r}: 'run' must be a string, "
-                f"got {type(config['run']).__name__}."
-            )
+        errors.extend(self._binding_errors(config))
+        # Strict run errors are already reported by _binding_errors().
+        if config.get("strict") is not True:
+            if "run" not in config:
+                errors.append(
+                    f"Shell step {config.get('id', '?')!r} is missing 'run' field."
+                )
+            elif not isinstance(config["run"], str):
+                # Legacy execution coerces run to shell source; validation
+                # must still reject non-strings without changing that path.
+                errors.append(
+                    f"Shell step {config.get('id', '?')!r}: 'run' must be a string, "
+                    f"got {type(config['run']).__name__}."
+                )
         output_format = config.get("output_format")
         if output_format is not None and output_format != "json":
             errors.append(