diff --git a/src/cmdforge/gui/dialogs/test_step_dialog.py b/src/cmdforge/gui/dialogs/test_step_dialog.py
new file mode 100644
index 0000000..cd34bf7
--- /dev/null
+++ b/src/cmdforge/gui/dialogs/test_step_dialog.py
@@ -0,0 +1,496 @@
+"""Test Step Dialog for interactive step testing."""
+
+import json
+import re
+import time
+from typing import Union
+
+from PySide6.QtWidgets import (
+    QDialog, QVBoxLayout, QHBoxLayout, QFormLayout, QGroupBox,
+    QLabel, QLineEdit, QPlainTextEdit, QPushButton, QComboBox,
+    QTextEdit, QTableWidget, QTableWidgetItem, QHeaderView,
+    QSplitter, QWidget, QMessageBox
+)
+from PySide6.QtCore import Qt, QThread, Signal
+from PySide6.QtGui import QColor
+
+from ...tool import PromptStep, CodeStep, ToolStep
+from ...runner import execute_prompt_step, execute_code_step, execute_tool_step
+from ...providers import load_providers
+
+
+class StepTestWorker(QThread):
+    """Background worker for executing step tests."""
+    finished = Signal(dict)  # Emits result dict
+
+    def __init__(self, step: Union[PromptStep, CodeStep, ToolStep], variables: dict, provider_override: str = None):
+        super().__init__()
+        self.step = step
+        self.variables = variables
+        self.provider_override = provider_override
+
+    def run(self):
+        result = {
+            "success": False,
+            "output": "",
+            "output_vars": {},
+            "error": None,
+            "elapsed_ms": 0
+        }
+
+        start_time = time.time()
+
+        try:
+            if isinstance(self.step, PromptStep):
+                output, success = execute_prompt_step(
+                    self.step, self.variables, self.provider_override
+                )
+                result["success"] = success
+                result["output"] = output
+                result["output_vars"] = {self.step.output_var: output}
+                if not success:
+                    result["error"] = "Provider call failed"
+
+            elif isinstance(self.step, CodeStep):
+                outputs, success = execute_code_step(
+                    self.step, self.variables, step_num=1
+                )
+                result["success"] = success
+                result["output_vars"] = outputs
+                result["output"] = "\n".join(f"{k} = {v}" for k, v in outputs.items())
+                if not success:
+                    result["error"] = "Code execution failed"
+
+            elif isinstance(self.step, ToolStep):
+                output, success = execute_tool_step(
+                    self.step, self.variables,
+                    depth=0,
+                    provider_override=self.provider_override,
+                    dry_run=False,
+                    verbose=False
+                )
+                result["success"] = success
+                result["output"] = output
+                result["output_vars"] = {self.step.output_var: output}
+                if not success:
+                    result["error"] = f"Tool '{self.step.tool}' execution failed"
+
+        except Exception as e:
+            result["success"] = False
+            result["error"] = str(e)
+
+        result["elapsed_ms"] = int((time.time() - start_time) * 1000)
+        self.finished.emit(result)
+
+
+class TestStepDialog(QDialog):
+    """Dialog for interactively testing a single step."""
+
+    # Assertion types available
+    ASSERTION_TYPES = [
+        ("not_empty", "Not Empty", "Output must not be empty"),
+        ("contains", "Contains", "Output must contain the specified text"),
+        ("not_contains", "Does Not Contain", "Output must NOT contain the specified text"),
+        ("equals", "Equals", "Output must exactly equal the expected value"),
+        ("valid_json", "Valid JSON", "Output must be valid JSON"),
+        ("valid_python", "Valid Python", "Output must be valid Python syntax"),
+        ("matches_regex", "Matches Regex", "Output must match the regular expression"),
+        ("min_length", "Min Length", "Output must be at least N characters"),
+        ("max_length", "Max Length", "Output must be at most N characters"),
+    ]
+
+    def __init__(self, parent, step: Union[PromptStep, CodeStep, ToolStep], available_vars: list = None):
+        super().__init__(parent)
+        self.step = step
+        self.available_vars = available_vars or ["input"]
+        self._worker = None
+
+        # Determine step type for title
+        if isinstance(step, PromptStep):
+            step_type = "Prompt"
+        elif isinstance(step, CodeStep):
+            step_type = "Code"
+        elif isinstance(step, ToolStep):
+            step_type = f"Tool ({step.tool})"
+        else:
+            step_type = "Unknown"
+
+        step_name = step.name if step.name else step_type
+        self.setWindowTitle(f"Test Step: {step_name}")
+        self.setMinimumSize(800, 700)
+
+        self._setup_ui()
+        self._detect_variables()
+
+    def _setup_ui(self):
+        """Set up the dialog UI."""
+        layout = QVBoxLayout(self)
+        layout.setSpacing(12)
+
+        # Main splitter: top (inputs) | bottom (output)
+        splitter = QSplitter(Qt.Vertical)
+
+        # Top section: Variables and Assertions
+        top_widget = QWidget()
+        top_layout = QHBoxLayout(top_widget)
+        top_layout.setContentsMargins(0, 0, 0, 0)
+
+        # Left: Variables input
+        vars_group = QGroupBox("Input Variables")
+        vars_layout = QVBoxLayout(vars_group)
+
+        vars_help = QLabel("Provide test values for variables used in this step:")
+        vars_help.setStyleSheet("color: #718096; font-size: 11px;")
+        vars_layout.addWidget(vars_help)
+
+        # Variables form
+        self.vars_form = QFormLayout()
+        self.vars_form.setSpacing(8)
+        self.var_inputs = {}  # variable name -> QLineEdit or QPlainTextEdit
+        vars_layout.addLayout(self.vars_form)
+        vars_layout.addStretch()
+
+        top_layout.addWidget(vars_group, 1)
+
+        # Right: Assertions
+        assert_group = QGroupBox("Assertions (Optional)")
+        assert_layout = QVBoxLayout(assert_group)
+
+        assert_help = QLabel("Define checks to validate the step output:")
+        assert_help.setStyleSheet("color: #718096; font-size: 11px;")
+        assert_layout.addWidget(assert_help)
+
+        # Assertions table
+        self.assertions_table = QTableWidget(0, 3)
+        self.assertions_table.setHorizontalHeaderLabels(["Type", "Value", ""])
+        self.assertions_table.horizontalHeader().setSectionResizeMode(0, QHeaderView.ResizeToContents)
+        self.assertions_table.horizontalHeader().setSectionResizeMode(1, QHeaderView.Stretch)
+        self.assertions_table.horizontalHeader().setSectionResizeMode(2, QHeaderView.ResizeToContents)
+        self.assertions_table.verticalHeader().setVisible(False)
+        self.assertions_table.setMaximumHeight(150)
+        assert_layout.addWidget(self.assertions_table)
+
+        # Add assertion button
+        btn_add_assertion = QPushButton("+ Add Assertion")
+        btn_add_assertion.clicked.connect(self._add_assertion_row)
+        assert_layout.addWidget(btn_add_assertion)
+
+        top_layout.addWidget(assert_group, 1)
+
+        splitter.addWidget(top_widget)
+
+        # Bottom section: Controls and Output
+        bottom_widget = QWidget()
+        bottom_layout = QVBoxLayout(bottom_widget)
+        bottom_layout.setContentsMargins(0, 0, 0, 0)
+
+        # Controls row
+        controls_layout = QHBoxLayout()
+
+        # Provider override (for prompt and tool steps)
+        if isinstance(self.step, (PromptStep, ToolStep)):
+            controls_layout.addWidget(QLabel("Provider:"))
+            self.provider_combo = QComboBox()
+            self.provider_combo.addItem("(use step's default)")
+            providers = load_providers()
+            for provider in sorted(providers, key=lambda p: p.name):
+                self.provider_combo.addItem(provider.name)
+            # Add common defaults
+            for default in ["mock"]:
+                if self.provider_combo.findText(default) < 0:
+                    self.provider_combo.addItem(default)
+            self.provider_combo.setMinimumWidth(150)
+            controls_layout.addWidget(self.provider_combo)
+        else:
+            self.provider_combo = None
+
+        controls_layout.addStretch()
+
+        # Run button
+        self.btn_run = QPushButton("Run Step")
+        self.btn_run.setMinimumHeight(36)
+        self.btn_run.setMinimumWidth(120)
+        self.btn_run.clicked.connect(self._run_test)
+        controls_layout.addWidget(self.btn_run)
+
+        bottom_layout.addLayout(controls_layout)
+
+        # Output section
+        output_group = QGroupBox("Output")
+        output_layout = QVBoxLayout(output_group)
+
+        # Status line
+        self.status_label = QLabel("Click 'Run Step' to test this step")
+        self.status_label.setStyleSheet("color: #718096;")
+        output_layout.addWidget(self.status_label)
+
+        # Output display
+        self.output_display = QTextEdit()
+        self.output_display.setReadOnly(True)
+        self.output_display.setPlaceholderText("Step output will appear here...")
+        font = self.output_display.font()
+        font.setFamily("Consolas, Monaco, monospace")
+        self.output_display.setFont(font)
+        output_layout.addWidget(self.output_display)
+
+        # Assertion results
+        self.assertion_results = QLabel("")
+        self.assertion_results.setWordWrap(True)
+        output_layout.addWidget(self.assertion_results)
+
+        bottom_layout.addWidget(output_group)
+
+        splitter.addWidget(bottom_widget)
+        splitter.setSizes([300, 400])
+
+        layout.addWidget(splitter)
+
+        # Dialog buttons
+        buttons_layout = QHBoxLayout()
+        buttons_layout.addStretch()
+
+        btn_close = QPushButton("Close")
+        btn_close.clicked.connect(self.accept)
+        buttons_layout.addWidget(btn_close)
+
+        layout.addLayout(buttons_layout)
+
+    def _detect_variables(self):
+        """Detect variables used in the step and create input fields."""
+        # Get template text based on step type
+        template = ""
+        if isinstance(self.step, PromptStep):
+            template = self.step.prompt
+        elif isinstance(self.step, CodeStep):
+            template = self.step.code
+        elif isinstance(self.step, ToolStep):
+            template = self.step.input_template
+            # Also add args values
+            for value in self.step.args.values():
+                template += " " + str(value)
+
+        # Find all {variable} references (excluding escaped {{ }})
+        # Simple regex: match {word} but not {{ or }}
+        var_pattern = r'\{([a-zA-Z_][a-zA-Z0-9_]*)\}'
+        found_vars = set(re.findall(var_pattern, template))
+
+        # Combine with available_vars (from previous steps)
+        all_vars = sorted(set(self.available_vars) | found_vars)
+
+        # Create input fields for each variable
+        for var_name in all_vars:
+            if var_name == "input":
+                # Use multiline for input
+                widget = QPlainTextEdit()
+                widget.setPlaceholderText("Enter test input text...")
+                widget.setMaximumHeight(80)
+            else:
+                widget = QLineEdit()
+                widget.setPlaceholderText(f"Value for {{{var_name}}}")
+
+            self.var_inputs[var_name] = widget
+            self.vars_form.addRow(f"{{{var_name}}}:", widget)
+
+    def _add_assertion_row(self):
+        """Add a new assertion row to the table."""
+        row = self.assertions_table.rowCount()
+        self.assertions_table.insertRow(row)
+
+        # Type dropdown
+        type_combo = QComboBox()
+        for type_id, display_name, tooltip in self.ASSERTION_TYPES:
+            type_combo.addItem(display_name, type_id)
+            idx = type_combo.count() - 1
+            type_combo.setItemData(idx, tooltip, Qt.ToolTipRole)
+        self.assertions_table.setCellWidget(row, 0, type_combo)
+
+        # Value input
+        value_edit = QLineEdit()
+        value_edit.setPlaceholderText("Expected value (if applicable)")
+        self.assertions_table.setCellWidget(row, 1, value_edit)
+
+        # Remove button
+        btn_remove = QPushButton("×")
+        btn_remove.setFixedWidth(30)
+        btn_remove.clicked.connect(lambda: self._remove_assertion_row(row))
+        self.assertions_table.setCellWidget(row, 2, btn_remove)
+
+    def _remove_assertion_row(self, row: int):
+        """Remove an assertion row."""
+        self.assertions_table.removeRow(row)
+        # Update remove button connections for remaining rows
+        for i in range(self.assertions_table.rowCount()):
+            btn = self.assertions_table.cellWidget(i, 2)
+            if btn:
+                btn.clicked.disconnect()
+                btn.clicked.connect(lambda checked=False, r=i: self._remove_assertion_row(r))
+
+    def _get_assertions(self) -> list:
+        """Get list of assertions from the table."""
+        assertions = []
+        for row in range(self.assertions_table.rowCount()):
+            type_combo = self.assertions_table.cellWidget(row, 0)
+            value_edit = self.assertions_table.cellWidget(row, 1)
+            if type_combo:
+                assertions.append({
+                    "type": type_combo.currentData(),
+                    "display": type_combo.currentText(),
+                    "value": value_edit.text() if value_edit else ""
+                })
+        return assertions
+
+    def _run_test(self):
+        """Run the step test."""
+        # Collect variable values
+        variables = {}
+        for var_name, widget in self.var_inputs.items():
+            if isinstance(widget, QPlainTextEdit):
+                variables[var_name] = widget.toPlainText()
+            else:
+                variables[var_name] = widget.text()
+
+        # Get provider override
+        provider_override = None
+        if self.provider_combo and self.provider_combo.currentIndex() > 0:
+            provider_override = self.provider_combo.currentText()
+
+        # Disable run button and show loading
+        self.btn_run.setEnabled(False)
+        self.btn_run.setText("Running...")
+        self.status_label.setText("Executing step...")
+        self.status_label.setStyleSheet("color: #718096;")
+        self.output_display.clear()
+        self.assertion_results.clear()
+
+        # Start worker thread
+        self._worker = StepTestWorker(self.step, variables, provider_override)
+        self._worker.finished.connect(self._on_test_finished)
+        self._worker.start()
+
+    def _on_test_finished(self, result: dict):
+        """Handle test completion."""
+        self.btn_run.setEnabled(True)
+        self.btn_run.setText("Run Step")
+
+        # Display result
+        if result["success"]:
+            self.status_label.setText(f"✓ Step completed in {result['elapsed_ms']}ms")
+            self.status_label.setStyleSheet("color: #38a169; font-weight: bold;")
+
+            # Show output
+            output_text = result["output"]
+            if result["output_vars"]:
+                output_text += "\n\n--- Output Variables ---\n"
+                for var, value in result["output_vars"].items():
+                    preview = value[:200] + "..." if len(value) > 200 else value
+                    output_text += f"{var} = {preview}\n"
+            self.output_display.setPlainText(output_text)
+        else:
+            self.status_label.setText(f"✗ Step failed ({result['elapsed_ms']}ms)")
+            self.status_label.setStyleSheet("color: #e53e3e; font-weight: bold;")
+
+            error_text = result.get("error", "Unknown error")
+            self.output_display.setHtml(f"<span style='color: #e53e3e;'><b>Error:</b> {error_text}</span>")
+
+        # Run assertions
+        assertions = self._get_assertions()
+        if assertions and result["success"]:
+            self._run_assertions(result["output"], assertions)
+
+    def _run_assertions(self, output: str, assertions: list):
+        """Run assertions against the output."""
+        results = []
+        all_passed = True
+
+        for assertion in assertions:
+            a_type = assertion["type"]
+            a_value = assertion["value"]
+            a_display = assertion["display"]
+            passed = False
+            message = ""
+
+            try:
+                if a_type == "not_empty":
+                    passed = bool(output.strip())
+                    message = "Output is not empty" if passed else "Output is empty"
+
+                elif a_type == "contains":
+                    passed = a_value in output
+                    message = f"Output contains '{a_value}'" if passed else f"Output does not contain '{a_value}'"
+
+                elif a_type == "not_contains":
+                    passed = a_value not in output
+                    message = f"Output does not contain '{a_value}'" if passed else f"Output contains '{a_value}'"
+
+                elif a_type == "equals":
+                    passed = output.strip() == a_value.strip()
+                    message = "Output equals expected" if passed else "Output does not equal expected"
+
+                elif a_type == "valid_json":
+                    try:
+                        json.loads(output)
+                        passed = True
+                        message = "Output is valid JSON"
+                    except json.JSONDecodeError as e:
+                        passed = False
+                        message = f"Invalid JSON: {e}"
+
+                elif a_type == "valid_python":
+                    try:
+                        import ast
+                        ast.parse(output)
+                        passed = True
+                        message = "Output is valid Python"
+                    except SyntaxError as e:
+                        passed = False
+                        message = f"Invalid Python: {e}"
+
+                elif a_type == "matches_regex":
+                    try:
+                        passed = bool(re.search(a_value, output))
+                        message = f"Output matches regex" if passed else f"Output does not match regex"
+                    except re.error as e:
+                        passed = False
+                        message = f"Invalid regex: {e}"
+
+                elif a_type == "min_length":
+                    try:
+                        min_len = int(a_value)
+                        passed = len(output) >= min_len
+                        message = f"Length {len(output)} >= {min_len}" if passed else f"Length {len(output)} < {min_len}"
+                    except ValueError:
+                        passed = False
+                        message = "Invalid minimum length value"
+
+                elif a_type == "max_length":
+                    try:
+                        max_len = int(a_value)
+                        passed = len(output) <= max_len
+                        message = f"Length {len(output)} <= {max_len}" if passed else f"Length {len(output)} > {max_len}"
+                    except ValueError:
+                        passed = False
+                        message = "Invalid maximum length value"
+
+            except Exception as e:
+                passed = False
+                message = f"Error: {e}"
+
+            if not passed:
+                all_passed = False
+
+            results.append((a_display, passed, message))
+
+        # Display results
+        result_html = "<b>Assertion Results:</b><br>"
+        for display, passed, message in results:
+            icon = "✓" if passed else "✗"
+            color = "#38a169" if passed else "#e53e3e"
+            result_html += f"<span style='color: {color};'>{icon} {display}: {message}</span><br>"
+
+        if all_passed:
+            result_html = f"<span style='color: #38a169; font-weight: bold;'>All {len(results)} assertion(s) passed!</span><br>" + result_html
+        else:
+            failed_count = sum(1 for _, p, _ in results if not p)
+            result_html = f"<span style='color: #e53e3e; font-weight: bold;'>{failed_count} of {len(results)} assertion(s) failed</span><br>" + result_html
+
+        self.assertion_results.setText(result_html)
diff --git a/src/cmdforge/gui/pages/tool_builder_page.py b/src/cmdforge/gui/pages/tool_builder_page.py
index f8ee2e8..145ec9c 100644
--- a/src/cmdforge/gui/pages/tool_builder_page.py
+++ b/src/cmdforge/gui/pages/tool_builder_page.py
@@ -233,6 +233,12 @@ class ToolBuilderPage(QWidget):
         self.btn_edit_step.clicked.connect(self._edit_step)
         steps_btns.addWidget(self.btn_edit_step)
 
+        self.btn_test_step = QPushButton("Test")
+        self.btn_test_step.setObjectName("secondary")
+        self.btn_test_step.setToolTip("Test the selected step with custom input")
+        self.btn_test_step.clicked.connect(self._test_step)
+        steps_btns.addWidget(self.btn_test_step)
+
         self.btn_del_step = QPushButton("Delete")
         self.btn_del_step.setObjectName("danger")
         self.btn_del_step.setToolTip("Delete the selected step")
@@ -691,6 +697,21 @@ class ToolBuilderPage(QWidget):
                 self._add_tool_dependency(new_step.tool)
             self._refresh_steps()
 
+    def _test_step(self):
+        """Test the selected step with custom input."""
+        items = self.steps_list.selectedItems()
+        if not items:
+            QMessageBox.information(self, "Test Step", "Please select a step to test")
+            return
+
+        step = items[0].data(Qt.UserRole)
+        idx = self.steps_list.row(items[0])
+        available_vars = self._get_available_vars(up_to_step=idx)
+
+        from ..dialogs.test_step_dialog import TestStepDialog
+        dialog = TestStepDialog(self, step, available_vars=available_vars)
+        dialog.exec()
+
     def _delete_step(self):
         """Delete selected step."""
         items = self.steps_list.selectedItems()