diff --git a/CHANGELOG.md b/CHANGELOG.md
index 0391521..95a805f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,11 @@
 # Changelog
 
+## Unreleased
+
+### Added
+
+- **`context-analyzer-tool config show`** (#4) — display effective configuration (TOML file path, merged values, optional highlight for non-default overrides). Supports `--format table|toml` and `--config`.
+
 ## 0.3.1 (2026-04-08)
 
 ### Bug Fixes
diff --git a/README.md b/README.md
index 207f669..ae04105 100644
--- a/README.md
+++ b/README.md
@@ -76,6 +76,7 @@ context-analyzer-tool status          View active sessions and recent tasks
 context-analyzer-tool anomalies       List recent anomalies with root causes
 context-analyzer-tool context-cost    Show context cost breakdown
 context-analyzer-tool health          Collector health check
+context-analyzer-tool config show     Display effective configuration (TOML + env)
 context-analyzer-tool rtk-status      Show RTK integration status and savings
 context-analyzer-tool prune           Clean up old data
 context-analyzer-tool clear           Clear all stored data and start fresh
diff --git a/src/context_analyzer_tool/cli.py b/src/context_analyzer_tool/cli.py
index 2aa6d7b..088b3e8 100644
--- a/src/context_analyzer_tool/cli.py
+++ b/src/context_analyzer_tool/cli.py
@@ -16,9 +16,12 @@
 from rich.table import Table
 
 from context_analyzer_tool.config import (
+    CATConfig,
+    config_to_toml,
     get_config_dir,
     get_config_path,
     load_config,
+    load_default_config,
     write_default_config,
 )
 
@@ -259,6 +262,116 @@ def _truncate_session_id(session_id: str, length: int = 8) -> str:
 # ---------------------------------------------------------------------------
 
 
+def _format_config_value(value: object) -> str:
+    """Format a config field value for CLI display."""
+    if isinstance(value, bool):
+        return "true" if value else "false"
+    if isinstance(value, list):
+        if not value:
+            return "[]"
+        return ", ".join(str(item) for item in value)
+    return str(value)
+
+
+def _iter_config_fields(cfg: CATConfig) -> list[tuple[str, str, object]]:
+    """Yield (section, field, value) tuples for every config field."""
+    rows: list[tuple[str, str, object]] = []
+    for section_name in (
+        "collector",
+        "anomaly",
+        "classifier",
+        "notifications",
+        "hooks",
+        "server",
+        "retention",
+        "dashboard",
+    ):
+        section = getattr(cfg, section_name)
+        for field_name in type(section).model_fields:
+            rows.append((section_name, field_name, getattr(section, field_name)))
+    return rows
+
+
+config_app = typer.Typer(
+    help="Inspect effective configuration (TOML file + CAT_* env overrides)",
+)
+
+
+@config_app.command("show")
+def config_show(
+    config_path: Path | None = typer.Option(
+        None, "--config", help="Config file path (default: ~/.context-analyzer-tool/config.toml)"
+    ),
+    output_format: str = typer.Option(
+        "table",
+        "--format",
+        "-f",
+        help="Output format: table or toml",
+    ),
+    highlight: bool = typer.Option(
+        True,
+        "--highlight/--no-highlight",
+        help="Highlight values that differ from built-in defaults",
+    ),
+) -> None:
+    """Show the effective configuration after merging TOML and env overrides."""
+    path = (config_path if config_path is not None else get_config_path()).expanduser()
+    try:
+        cfg = load_config(config_path)
+    except ValueError as exc:
+        console.print(f"[red]Configuration error:[/red] {exc}")
+        raise typer.Exit(1) from None
+
+    defaults = load_default_config()
+    default_lookup = {
+        (section, field): value
+        for section, field, value in _iter_config_fields(defaults)
+    }
+
+    exists = path.exists()
+    source_note = (
+        "[green]found[/green]" if exists else "[yellow]missing (using defaults)[/yellow]"
+    )
+    console.print(f"Config file: [bold]{path}[/bold] ({source_note})")
+    console.print(f"Config dir:  [bold]{get_config_dir()}[/bold]")
+    console.print()
+
+    if output_format == "toml":
+        console.print(config_to_toml(cfg), markup=False, end="")
+        return
+    if output_format != "table":
+        console.print(f"[red]Unknown format:[/red] {output_format!r} (use table or toml)")
+        raise typer.Exit(2)
+
+    table = Table(show_header=True, header_style="bold cyan", expand=True)
+    table.add_column("Section")
+    table.add_column("Key")
+    table.add_column("Value")
+    if highlight:
+        table.add_column("Note")
+
+    for section, field, value in _iter_config_fields(cfg):
+        default_value = default_lookup[(section, field)]
+        value_str = _format_config_value(value)
+        if highlight and value != default_value:
+            value_str = f"[yellow]{value_str}[/yellow]"
+            note = "[yellow]override[/yellow]"
+        elif highlight:
+            note = "[dim]default[/dim]"
+        else:
+            note = ""
+
+        if highlight:
+            table.add_row(section, field, value_str, note)
+        else:
+            table.add_row(section, field, value_str)
+
+    console.print(Panel(table, title="Effective Configuration", border_style="blue"))
+
+
+app.add_typer(config_app, name="config")
+
+
 @app.command()
 def serve(
     host: str | None = typer.Option(None, help="Bind host"),
diff --git a/src/context_analyzer_tool/config.py b/src/context_analyzer_tool/config.py
index 868538e..c76921b 100644
--- a/src/context_analyzer_tool/config.py
+++ b/src/context_analyzer_tool/config.py
@@ -245,6 +245,46 @@ def _apply_env_overrides(self) -> CATConfig:
 # ---------------------------------------------------------------------------
 
 
+def load_default_config() -> CATConfig:
+    """Return built-in defaults without ``CAT_*`` environment overrides."""
+    saved: dict[str, str | None] = {}
+    for key in list(os.environ):
+        if key.startswith(_ENV_PREFIX):
+            saved[key] = os.environ.pop(key)
+    try:
+        return CATConfig()
+    finally:
+        for key, value in saved.items():
+            if value is not None:
+                os.environ[key] = value
+
+
+def config_to_toml(cfg: CATConfig) -> str:
+    """Serialize *cfg* to a TOML-like string for display."""
+    lines: list[str] = []
+    for section_name, section in cfg.model_dump().items():
+        lines.append(f"[{section_name}]")
+        for key, value in section.items():
+            if isinstance(value, bool):
+                rendered = "true" if value else "false"
+            elif isinstance(value, str):
+                rendered = f'"{value}"'
+            elif isinstance(value, list):
+                if not value:
+                    rendered = "[]"
+                else:
+                    parts = [
+                        f'"{item}"' if isinstance(item, str) else str(item)
+                        for item in value
+                    ]
+                    rendered = f"[{', '.join(parts)}]"
+            else:
+                rendered = str(value)
+            lines.append(f"{key} = {rendered}")
+        lines.append("")
+    return "\n".join(lines).rstrip() + "\n"
+
+
 def load_config(config_path: Path | None = None) -> CATConfig:
     """Load config from a TOML file.
 
diff --git a/tests/test_cli_config.py b/tests/test_cli_config.py
new file mode 100644
index 0000000..59f69fe
--- /dev/null
+++ b/tests/test_cli_config.py
@@ -0,0 +1,81 @@
+"""Tests for the `config show` CLI command (#4)."""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+from typer.testing import CliRunner
+
+from context_analyzer_tool.cli import app
+from context_analyzer_tool.config import config_to_toml, load_config, load_default_config
+
+runner = CliRunner()
+
+
+def test_config_show_table_default(monkeypatch) -> None:
+    """`config show` prints the config path and effective values."""
+    with runner.isolated_filesystem():
+        cfg_dir = Path("cfg")
+        cfg_dir.mkdir()
+        monkeypatch.setenv("CAT_CONFIG_DIR", str(cfg_dir))
+
+        result = runner.invoke(app, ["config", "show"])
+
+    assert result.exit_code == 0, result.output
+    assert "Config file:" in result.output
+    assert "config.toml" in result.output
+    assert "Effective Configuration" in result.output
+    assert "7821" in result.output
+
+
+def test_config_show_toml_format(monkeypatch) -> None:
+    with runner.isolated_filesystem():
+        cfg_dir = Path("cfg")
+        cfg_dir.mkdir()
+        monkeypatch.setenv("CAT_CONFIG_DIR", str(cfg_dir))
+
+        result = runner.invoke(app, ["config", "show", "--format", "toml"])
+
+    assert result.exit_code == 0, result.output
+    assert "[collector]" in result.output
+    assert "port = 7821" in result.output
+
+
+def test_config_show_highlights_env_override(monkeypatch) -> None:
+    with runner.isolated_filesystem():
+        cfg_dir = Path("cfg")
+        cfg_dir.mkdir()
+        monkeypatch.setenv("CAT_CONFIG_DIR", str(cfg_dir))
+        monkeypatch.setenv("CAT_COLLECTOR_PORT", "9001")
+
+        result = runner.invoke(app, ["config", "show"])
+
+    assert result.exit_code == 0, result.output
+    assert "9001" in result.output
+    assert "override" in result.output
+
+
+def test_config_show_reads_custom_file(tmp_path: Path) -> None:
+    toml_file = tmp_path / "custom.toml"
+    toml_file.write_text(
+        "[collector]\nhost = \"10.0.0.1\"\nport = 8080\n",
+        encoding="utf-8",
+    )
+
+    result = runner.invoke(app, ["config", "show", "--config", str(toml_file)])
+
+    assert result.exit_code == 0, result.output
+    assert "custom.toml" in result.output
+    assert "10.0.0.1" in result.output
+    assert "8080" in result.output
+
+
+def test_config_to_toml_round_trip_defaults() -> None:
+    cfg = load_default_config()
+    rendered = config_to_toml(cfg)
+    assert "[collector]" in rendered
+    assert 'host = "127.0.0.1"' in rendered
+
+    reloaded = load_config(Path("/nonexistent/config.toml"))
+    # load_config on missing file returns defaults; compare key fields
+    assert reloaded.collector.host == cfg.collector.host