feat: explain missing POSIX sh on Windows workspaces (#27048)

When a workspace has no POSIX sh on PATH (typical for fresh Windows workspaces), the execute tool fails with a raw `exec: "sh": executable file not found in %PATH%` error the model cannot act on. 

This change:
- Enriches the above error in chattool with remediation steps and a docs link.
- Documents the requirement in the Coder Agents architecture page.

> This PR was generated by Coder Agents on behalf of @johnstcn

coderd/x/chatd/chattool/execute.go

@@ -49,6 +49,31 @@ var fileDumpPatterns = []*regexp.Regexp{
 	regexp.MustCompile(`^(rg|grep)\s+-l\s+`),
 }
 
+const (
+	// shNotFoundFragment omits the trailing path variable
+	// (%PATH% vs $PATH) for OS portability. Only transport
+	// errors from StartProcess contain it, never command output.
+	shNotFoundFragment = `exec: "sh": executable file not found`
+
+	// shNotFoundGuidance is model-facing remediation text, relayed
+	// to the user. Keep the docs anchor in sync with
+	// docs/ai-coder/agents/architecture.md.
+	shNotFoundGuidance = "The workspace has no POSIX shell (sh) on its PATH. " +
+		"Coder Agents run commands with \"sh -c\". On Windows, install sh " +
+		"via Git Bash, MSYS2, or WSL, then restart the workspace to pick " +
+		"up the updated PATH. See " +
+		"https://coder.com/docs/ai-coder/agents/architecture#windows-workspace-shell-requirement"
+)
+
+// enrichStartError appends actionable guidance when a StartProcess
+// error indicates the workspace has no sh binary.
+func enrichStartError(msg string) string {
+	if strings.Contains(msg, shNotFoundFragment) {
+		return msg + "\n\n" + shNotFoundGuidance
+	}
+	return msg
+}
+
 // ExecuteResult is the structured response from the execute
 // tool.
 type ExecuteResult struct {
@@ -162,7 +187,7 @@ func executeBackground(
 		Background: true,
 	})
 	if err != nil {
-		return errorResult(fmt.Sprintf("start background process: %v", err))
+		return errorResult(enrichStartError(fmt.Sprintf("start background process: %v", err)))
 	}
 
 	result := ExecuteResult{
@@ -212,7 +237,7 @@ func executeForeground(
 		Background: false,
 	})
 	if err != nil {
-		return errorResult(fmt.Sprintf("start process: %v", err))
+		return errorResult(enrichStartError(fmt.Sprintf("start process: %v", err)))
 	}
 
 	result := waitForProcess(cmdCtx, ctx, conn, resp.ID, timeout)

coderd/x/chatd/chattool/execute_test.go

@@ -408,6 +408,72 @@ func TestExecuteTool(t *testing.T) {
 		require.NoError(t, json.Unmarshal([]byte(resp.Content), &result))
 		assert.False(t, result.Success)
 		assert.Contains(t, result.Error, "connection lost")
+		// Unrelated errors must not trigger the missing-shell
+		// guidance.
+		assert.NotContains(t, result.Error, "Git Bash")
+		assert.NotContains(t, result.Error, "coder.com/docs")
+	})
+
+	t.Run("MissingShellError", func(t *testing.T) {
+		t.Parallel()
+
+		// OS rendering differs (%PATH% vs $PATH); the fragment
+		// omits the suffix to match both.
+		tests := []struct {
+			name       string
+			input      string
+			agentErr   string
+			wantPrefix string
+		}{
+			{
+				name:       "ForegroundWindows",
+				input:      `{"command":"echo hi"}`,
+				agentErr:   "unexpected status code 500: Failed to start process.\n\tError: start process: exec: \"sh\": executable file not found in %PATH%",
+				wantPrefix: "start process:",
+			},
+			{
+				name:       "BackgroundWindows",
+				input:      `{"command":"echo hi","run_in_background":true}`,
+				agentErr:   "unexpected status code 500: Failed to start process.\n\tError: start process: exec: \"sh\": executable file not found in %PATH%",
+				wantPrefix: "start background process:",
+			},
+			{
+				name:       "ForegroundPOSIX",
+				input:      `{"command":"echo hi"}`,
+				agentErr:   "unexpected status code 500: Failed to start process.\n\tError: start process: exec: \"sh\": executable file not found in $PATH",
+				wantPrefix: "start process:",
+			},
+		}
+		for _, tt := range tests {
+			t.Run(tt.name, func(t *testing.T) {
+				t.Parallel()
+				ctrl := gomock.NewController(t)
+				mockConn := agentconnmock.NewMockAgentConn(ctrl)
+
+				mockConn.EXPECT().
+					StartProcess(gomock.Any(), gomock.Any()).
+					Return(workspacesdk.StartProcessResponse{}, xerrors.New(tt.agentErr))
+
+				tool := newExecuteTool(t, mockConn)
+				ctx := testutil.Context(t, testutil.WaitMedium)
+				resp, err := tool.Run(ctx, fantasy.ToolCall{
+					ID:    "call-1",
+					Name:  "execute",
+					Input: tt.input,
+				})
+				require.NoError(t, err)
+				assert.False(t, resp.IsError)
+
+				var result chattool.ExecuteResult
+				require.NoError(t, json.Unmarshal([]byte(resp.Content), &result))
+				assert.False(t, result.Success)
+				// The result keeps the original error for debugging.
+				assert.Contains(t, result.Error, tt.wantPrefix)
+				assert.Contains(t, result.Error, `exec: "sh": executable file not found`)
+				assert.Contains(t, result.Error, "Git Bash")
+				assert.Contains(t, result.Error, "https://coder.com/docs/ai-coder/agents/architecture#windows-workspace-shell-requirement")
+			})
+		}
 	})
 
 	t.Run("ProcessOutputError", func(t *testing.T) {