hooks/builtins: name the resolved shell in the env block

docs/configuration/agents/index.md

@@ -93,7 +93,7 @@ agents:
 | `toolsets`                  | array   | ✗        | List of tool configurations. See [Tool Config](../tools/index.md).                                                                                                        |
 | `fallback`                  | object  | ✗        | Automatic model failover configuration.                                                                                                                                       |
 | `add_date`                  | boolean | ✗        | When `true`, injects the current date into the agent's context.                                                                                                               |
-| `add_environment_info`      | boolean | ✗        | When `true`, injects working directory, OS, CPU architecture, and git info into context.                                                                                      |
+| `add_environment_info`      | boolean | ✗        | When `true`, injects working directory, OS, CPU architecture, git info, and the resolved shell into context.                                                                  |
 | `add_prompt_files`          | array   | ✗        | List of file paths whose contents are appended to the system prompt. Useful for including coding standards, guidelines, or additional context.                                |
 | `add_description_parameter` | boolean | ✗        | When `true`, adds agent descriptions as a parameter in tool schemas. Helps with tool selection in multi-agent scenarios.                                                      |
 | `redact_secrets`            | boolean | ✗        | When `true`, scrubs detected secrets (API keys, tokens, private keys, etc.) out of tool-call arguments, outgoing chat messages, and tool output before they reach a tool, the model, or downstream consumers. See [Redacting Secrets](#redacting-secrets) below.   |

docs/configuration/hooks/index.md

@@ -206,7 +206,7 @@ Built-ins are typically zero-config and faster than equivalent shell hooks becau
 | Builtin                 | Event                                                                                     | Args                  | What it does                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
 | ----------------------- | ----------------------------------------------------------------------------------------- | --------------------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
 | `add_date`              | `turn_start`                                                                              | _none_                | Prepends `Today's date: YYYY-MM-DD` so the model always knows the current date.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
-| `add_environment_info`  | `session_start`                                                                           | _none_                | Adds the working directory, git-repo status, OS, and CPU architecture.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
+| `add_environment_info`  | `session_start`                                                                           | _none_                | Adds the working directory, git-repo status, OS, CPU architecture, and the resolved shell.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
 | `add_prompt_files`      | `turn_start`                                                                              | `[file1, file2, ...]` | Reads each named file from the workdir hierarchy (walking up) and the home directory, and appends their contents.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
 | `add_git_status`        | `turn_start`                                                                              | _none_                | Adds the output of `git status --short --branch` (no-op outside a git repo or when git isn't installed).                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
 | `add_git_diff`          | `turn_start`                                                                              | _none_, or `["full"]` | Adds `git diff --stat` by default. Pass `args: ["full"]` to emit the full unified diff. Output is capped to 4 KB.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |

pkg/hooks/builtins/add_environment_info.go

@@ -6,34 +6,37 @@ import (
 	"runtime"
 
 	"github.com/docker/docker-agent/pkg/hooks"
+	"github.com/docker/docker-agent/pkg/shellpath"
 )
 
 // AddEnvironmentInfo is the registered name of the add_environment_info builtin.
 const AddEnvironmentInfo = "add_environment_info"
 
-// addEnvironmentInfo emits cwd / git / OS / arch info as session_start
-// additional context. No-op when Cwd is empty.
+// addEnvironmentInfo emits cwd/git/OS/arch/shell as session_start context.
+// No-op when Cwd is empty.
 func addEnvironmentInfo(_ context.Context, in *hooks.Input, _ []string) (*hooks.Output, error) {
 	if in == nil || in.Cwd == "" {
 		return nil, nil
 	}
 	return hooks.NewAdditionalContextOutput(hooks.EventSessionStart, environmentInfo(in.Cwd)), nil
 }
 
-// environmentInfo formats the env block injected at session_start:
-// working directory, git-repo status, and human-readable OS / arch.
+// environmentInfo builds the <env> block. Long-form dialect rules live in
+// shellSyntaxHint (tool description) so this stays terse.
 func environmentInfo(workingDir string) string {
 	gitRepo := "No"
 	if isGitRepo(workingDir) {
 		gitRepo = "Yes"
 	}
+	shellPath, _ := shellpath.DetectShell() // second value is argsPrefix, unused here
 	return fmt.Sprintf(`Here is useful information about the environment you are running in:
 	<env>
 	Working directory: %s
 	Is directory a git repo: %s
 	Operating System: %s
 	CPU Architecture: %s
-	</env>`, workingDir, gitRepo, displayOS(), displayArch())
+	Shell: %s (%s)
+	</env>`, workingDir, gitRepo, displayOS(), displayArch(), shellpath.ShellBaseName(shellPath), shellPath)
 }
 
 // displayOS returns a friendlier label for the common values of

pkg/hooks/builtins/add_environment_info_test.go

@@ -7,6 +7,8 @@ import (
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
+
+	"github.com/docker/docker-agent/pkg/shellpath"
 )
 
 func TestEnvironmentInfo(t *testing.T) {
@@ -49,12 +51,14 @@ func TestEnvironmentInfo(t *testing.T) {
 			if tt.expectGit {
 				gitStatus = "Yes"
 			}
+			shellPath, _ := shellpath.DetectShell()
 			expected := `Here is useful information about the environment you are running in:
 	<env>
 	Working directory: ` + dir + `
 	Is directory a git repo: ` + gitStatus + `
 	Operating System: ` + displayOS() + `
 	CPU Architecture: ` + displayArch() + `
+	Shell: ` + shellpath.ShellBaseName(shellPath) + ` (` + shellPath + `)
 	</env>`
 
 			assert.Equal(t, expected, environmentInfo(dir))

pkg/shellpath/shellpath.go

@@ -7,8 +7,19 @@ import (
 	"os/exec"
 	"path/filepath"
 	"runtime"
+	"strings"
 )
 
+// ShellBaseName returns the lowercase shell name without extension. Splits on
+// both separators so results are stable when the path came from another host OS.
+func ShellBaseName(shellPath string) string {
+	base := shellPath
+	if i := strings.LastIndexAny(base, `/\`); i >= 0 {
+		base = base[i+1:]
+	}
+	return strings.ToLower(strings.TrimSuffix(base, filepath.Ext(base)))
+}
+
 // WindowsCmdExe returns the absolute path to cmd.exe on Windows using the
 // SystemRoot environment variable (e.g. C:\Windows\System32\cmd.exe).
 // This avoids resolving cmd.exe through PATH, which would be vulnerable

pkg/shellpath/shellpath_test.go

@@ -7,6 +7,32 @@ import (
 	"testing"
 )
 
+func TestShellBaseName(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		path     string
+		expected string
+	}{
+		{path: "/bin/zsh", expected: "zsh"},
+		{path: "/usr/local/bin/fish", expected: "fish"},
+		{path: "/bin/sh", expected: "sh"},
+		{path: `C:\Windows\System32\WindowsPowerShell\v1.0\powershell.exe`, expected: "powershell"},
+		{path: `C:\Program Files\PowerShell\7\pwsh.exe`, expected: "pwsh"},
+		{path: `C:\Windows\System32\cmd.exe`, expected: "cmd"},
+		{path: "", expected: ""},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.path, func(t *testing.T) {
+			t.Parallel()
+			if got := ShellBaseName(tt.path); got != tt.expected {
+				t.Errorf("ShellBaseName(%q) = %q, want %q", tt.path, got, tt.expected)
+			}
+		})
+	}
+}
+
 func TestWindowsCmdExe_ComSpec(t *testing.T) {
 	t.Setenv("ComSpec", `C:\Custom\cmd.exe`)
 	got := WindowsCmdExe()

pkg/tools/builtin/shell/shell.go

@@ -368,7 +368,7 @@ func (t *ToolSet) Instructions() string {
 - Use "cwd" parameter instead of cd within commands
 - Combine operations with pipes, redirections, and heredocs
 - Non-zero exit codes return error info with output; timed-out commands are terminated`,
-		shellBaseName(t.handler.shell), displayOS())
+		shellpath.ShellBaseName(t.handler.shell), displayOS())
 }
 
 func (t *ToolSet) Tools(context.Context) ([]tools.Tool, error) {
@@ -409,7 +409,7 @@ func (t *ToolSet) Stop(context.Context) error {
 // resolved shell is PowerShell or cmd.exe (e.g. "pwd && ls -la" is a
 // parse error under Windows PowerShell 5.1).
 func shellToolDescription(shellPath string) string {
-	name := shellBaseName(shellPath)
+	name := shellpath.ShellBaseName(shellPath)
 	desc := fmt.Sprintf("Executes the given shell command with %s on %s.", name, displayOS())
 	if hint := shellSyntaxHint(name); hint != "" {
 		desc += " " + hint
@@ -435,18 +435,6 @@ func shellSyntaxHint(name string) string {
 	}
 }
 
-// shellBaseName reduces a resolved shell path to a lowercase name the
-// model can recognize (C:\...\powershell.exe -> powershell, /bin/zsh -> zsh).
-// Splits on both separators instead of filepath.Base so the result is
-// deterministic regardless of the host OS the path came from.
-func shellBaseName(shellPath string) string {
-	base := shellPath
-	if i := strings.LastIndexAny(base, `/\`); i >= 0 {
-		base = base[i+1:]
-	}
-	return strings.ToLower(strings.TrimSuffix(base, filepath.Ext(base)))
-}
-
 // displayOS returns a friendlier label for the common values of
 // runtime.GOOS, falling back to GOOS itself for anything exotic.
 func displayOS() string {

pkg/tools/builtin/shell/shell_test.go

@@ -15,6 +15,7 @@ import (
 	"github.com/stretchr/testify/require"
 
 	"github.com/docker/docker-agent/pkg/config"
+	"github.com/docker/docker-agent/pkg/shellpath"
 	"github.com/docker/docker-agent/pkg/tools"
 )
 
@@ -200,7 +201,7 @@ func TestShellTool_Instructions(t *testing.T) {
 	instructions := tool.Instructions()
 
 	assert.Contains(t, instructions, "Shell Tools")
-	assert.Contains(t, instructions, shellBaseName(tool.handler.shell),
+	assert.Contains(t, instructions, shellpath.ShellBaseName(tool.handler.shell),
 		"instructions must name the resolved shell so the model uses its syntax")
 	assert.Contains(t, instructions, displayOS())
 	assert.NotContains(t, instructions, "run_background_job")
@@ -219,32 +220,10 @@ func TestShellTool_DescriptionNamesInterpreter(t *testing.T) {
 	require.Len(t, allTools, 1)
 
 	description := allTools[0].Description
-	assert.Contains(t, description, shellBaseName(tool.handler.shell))
+	assert.Contains(t, description, shellpath.ShellBaseName(tool.handler.shell))
 	assert.Contains(t, description, displayOS())
 }
 
-func TestShellBaseName(t *testing.T) {
-	t.Parallel()
-
-	tests := []struct {
-		path     string
-		expected string
-	}{
-		{path: "/bin/zsh", expected: "zsh"},
-		{path: "/usr/local/bin/fish", expected: "fish"},
-		{path: `C:\Windows\System32\WindowsPowerShell\v1.0\powershell.exe`, expected: "powershell"},
-		{path: `C:\Program Files\PowerShell\7\pwsh.exe`, expected: "pwsh"},
-		{path: `C:\Windows\System32\cmd.exe`, expected: "cmd"},
-	}
-
-	for _, tt := range tests {
-		t.Run(tt.path, func(t *testing.T) {
-			t.Parallel()
-			assert.Equal(t, tt.expected, shellBaseName(tt.path))
-		})
-	}
-}
-
 func TestResolveWorkDir(t *testing.T) {
 	t.Parallel()