diff --git a/Dockerfile b/Dockerfile
index 19c00f6..8c99528 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -13,6 +13,7 @@ RUN pip install --upgrade pip
 
 # Copy the project and install editable with dev extras.
 COPY cli ./cli
+COPY obs ./obs
 COPY researchers ./researchers
 COPY orchestrator ./orchestrator
 COPY tests ./tests
diff --git a/cli/main.py b/cli/main.py
index aa135c6..0a0443e 100644
--- a/cli/main.py
+++ b/cli/main.py
@@ -7,7 +7,10 @@ ResearchResult contracts to the terminal.
 import asyncio
 import json
 import os
+import re
 import sys
+from collections import defaultdict
+from datetime import datetime, timedelta, timezone
 from pathlib import Path
 from typing import Optional
 
@@ -20,6 +23,7 @@ from rich.table import Table
 from rich.text import Text
 
 from obs import configure_logging, get_logger
+from obs.costs import DEFAULT_LEDGER_PATH
 from researchers.web.models import ResearchResult
 
 
@@ -298,5 +302,215 @@ def replay(trace_id: str, trace_dir: Optional[str]) -> None:
     render_trace(entries, trace_id, console)
 
 
+# ---------------------------------------------------------------------------
+# costs command
+# ---------------------------------------------------------------------------
+
+
+_RELATIVE_RE = re.compile(r"^(\d+)([dwhm])$")
+
+
+def _parse_when(value: str) -> datetime:
+    """Parse an ISO date or a relative shorthand like '7d', '24h'."""
+    m = _RELATIVE_RE.match(value)
+    if m:
+        n = int(m.group(1))
+        unit = m.group(2)
+        delta = {
+            "h": timedelta(hours=n),
+            "d": timedelta(days=n),
+            "w": timedelta(weeks=n),
+            "m": timedelta(days=30 * n),
+        }[unit]
+        return datetime.now(timezone.utc) - delta
+    # Otherwise treat as ISO date / datetime
+    dt = datetime.fromisoformat(value)
+    if dt.tzinfo is None:
+        dt = dt.replace(tzinfo=timezone.utc)
+    return dt
+
+
+def _load_ledger(path: Path) -> list[dict]:
+    if not path.exists():
+        return []
+    entries: list[dict] = []
+    with open(path, "r", encoding="utf-8") as f:
+        for lineno, line in enumerate(f, 1):
+            line = line.strip()
+            if not line:
+                continue
+            try:
+                entries.append(json.loads(line))
+            except json.JSONDecodeError:
+                # Skip a corrupt line rather than blow up the whole report
+                continue
+    return entries
+
+
+def _filter_entries(
+    entries: list[dict],
+    since: Optional[datetime],
+    until: Optional[datetime],
+    model: Optional[str],
+) -> list[dict]:
+    out = []
+    for e in entries:
+        ts_str = e.get("timestamp", "")
+        try:
+            ts = datetime.fromisoformat(ts_str.replace("Z", "+00:00"))
+        except ValueError:
+            continue
+        if since and ts < since:
+            continue
+        if until and ts > until:
+            continue
+        if model and e.get("model_id") != model:
+            continue
+        out.append(e)
+    return out
+
+
+def render_costs(entries: list[dict], console: Console) -> None:
+    """Render a cost summary from filtered ledger entries."""
+    if not entries:
+        console.print("[dim]No cost data yet.[/dim]")
+        return
+
+    total_calls = len(entries)
+    total_tokens = sum(e.get("tokens_used", 0) for e in entries)
+    total_input = sum(e.get("tokens_input") or 0 for e in entries)
+    total_output = sum(e.get("tokens_output") or 0 for e in entries)
+    total_tavily = sum(e.get("tavily_searches", 0) for e in entries)
+    total_spend = sum(
+        e.get("estimated_cost_usd") or 0.0 for e in entries
+    )
+    unknown_cost_calls = sum(
+        1 for e in entries if e.get("estimated_cost_usd") is None
+    )
+
+    # Summary panel
+    summary = Text()
+    summary.append(f"Calls: {total_calls}\n", style="bold")
+    summary.append(f"Total spend: ${total_spend:.4f}\n", style="bold green")
+    summary.append(f"Total tokens: {total_tokens:,} ")
+    summary.append(f"(in {total_input:,} / out {total_output:,})\n", style="dim")
+    summary.append(f"Tavily searches: {total_tavily}\n")
+    if unknown_cost_calls:
+        summary.append(
+            f"Calls with unknown model price: {unknown_cost_calls}\n",
+            style="yellow",
+        )
+    console.print(Panel(summary, title="Cost Summary", border_style="green"))
+
+    # Per-day breakdown
+    per_day: dict[str, dict] = defaultdict(lambda: {"calls": 0, "tokens": 0, "spend": 0.0})
+    for e in entries:
+        day = e.get("timestamp", "")[:10]
+        per_day[day]["calls"] += 1
+        per_day[day]["tokens"] += e.get("tokens_used", 0)
+        per_day[day]["spend"] += e.get("estimated_cost_usd") or 0.0
+    day_table = Table(title="Per Day", show_lines=False, expand=True)
+    day_table.add_column("Date", style="dim")
+    day_table.add_column("Calls", justify="right")
+    day_table.add_column("Tokens", justify="right")
+    day_table.add_column("Spend (USD)", justify="right", style="green")
+    for day in sorted(per_day.keys()):
+        d = per_day[day]
+        day_table.add_row(
+            day, str(d["calls"]), f"{d['tokens']:,}", f"${d['spend']:.4f}"
+        )
+    console.print(day_table)
+
+    # Per-model breakdown
+    per_model: dict[str, dict] = defaultdict(
+        lambda: {"calls": 0, "tokens": 0, "spend": 0.0}
+    )
+    for e in entries:
+        m = e.get("model_id", "(unknown)")
+        per_model[m]["calls"] += 1
+        per_model[m]["tokens"] += e.get("tokens_used", 0)
+        per_model[m]["spend"] += e.get("estimated_cost_usd") or 0.0
+    model_table = Table(title="Per Model", show_lines=False, expand=True)
+    model_table.add_column("Model")
+    model_table.add_column("Calls", justify="right")
+    model_table.add_column("Tokens", justify="right")
+    model_table.add_column("Spend (USD)", justify="right", style="green")
+    for m in sorted(per_model.keys()):
+        d = per_model[m]
+        model_table.add_row(
+            m, str(d["calls"]), f"{d['tokens']:,}", f"${d['spend']:.4f}"
+        )
+    console.print(model_table)
+
+    # Highest-cost call
+    costed = [e for e in entries if e.get("estimated_cost_usd") is not None]
+    if costed:
+        top = max(costed, key=lambda e: e["estimated_cost_usd"])
+        top_text = Text()
+        top_text.append(f"trace_id: {top.get('trace_id', '?')}\n")
+        top_text.append(f"question: {top.get('question', '')[:120]}\n")
+        top_text.append(f"model: {top.get('model_id', '?')}\n")
+        top_text.append(f"tokens: {top.get('tokens_used', 0):,}\n")
+        top_text.append(
+            f"spend: ${top.get('estimated_cost_usd', 0):.4f}\n",
+            style="bold green",
+        )
+        console.print(
+            Panel(top_text, title="Highest-Cost Call", border_style="yellow")
+        )
+
+
+@cli.command()
+@click.option(
+    "--since",
+    default=None,
+    help="Filter by start time. ISO date or relative (e.g. 7d, 24h, 2w).",
+)
+@click.option(
+    "--until",
+    default=None,
+    help="Filter by end time. ISO date or relative.",
+)
+@click.option(
+    "--model",
+    default=None,
+    help="Filter to a specific model_id.",
+)
+@click.option(
+    "--json",
+    "as_json",
+    is_flag=True,
+    default=False,
+    help="Emit raw filtered ledger entries as JSON instead of the table.",
+)
+@click.option(
+    "--ledger",
+    default=None,
+    help=f"Override ledger path (default: {DEFAULT_LEDGER_PATH}).",
+)
+def costs(
+    since: Optional[str],
+    until: Optional[str],
+    model: Optional[str],
+    as_json: bool,
+    ledger: Optional[str],
+) -> None:
+    """Show cost summary from the research ledger."""
+    console = Console()
+    path = Path(os.path.expanduser(ledger or DEFAULT_LEDGER_PATH))
+    entries = _load_ledger(path)
+
+    since_dt = _parse_when(since) if since else None
+    until_dt = _parse_when(until) if until else None
+    filtered = _filter_entries(entries, since_dt, until_dt, model)
+
+    if as_json:
+        for e in filtered:
+            click.echo(json.dumps(e))
+        return
+
+    render_costs(filtered, console)
+
+
 if __name__ == "__main__":
     cli()
diff --git a/tests/test_cli.py b/tests/test_cli.py
index 41c5b79..6ba2c6b 100644
--- a/tests/test_cli.py
+++ b/tests/test_cli.py
@@ -4,7 +4,7 @@ from unittest.mock import patch
 
 from click.testing import CliRunner
 
-from cli.main import cli, render_result, render_trace
+from cli.main import cli, render_costs, render_result, render_trace
 from researchers.web.models import (
     Citation,
     ConfidenceFactors,
@@ -186,3 +186,139 @@ class TestReplayCommand:
         out = console.export_text()
         assert "empty-trace" in out
         assert "empty" in out.lower()
+
+
+# ---------------------------------------------------------------------------
+# costs command
+# ---------------------------------------------------------------------------
+
+
+import json as _json
+
+
+def _write_ledger(path, entries):
+    path.write_text("\n".join(_json.dumps(e) for e in entries) + "\n")
+
+
+def _ledger_fixture(tmp_path):
+    path = tmp_path / "costs.jsonl"
+    entries = [
+        {
+            "timestamp": "2026-04-06T10:00:00Z",
+            "trace_id": "t1",
+            "question": "What is X?",
+            "model_id": "claude-sonnet-4-6",
+            "tokens_used": 1000,
+            "tokens_input": 800,
+            "tokens_output": 200,
+            "iterations_run": 1,
+            "wall_time_sec": 5.0,
+            "tavily_searches": 1,
+            "estimated_cost_usd": 0.005,
+            "budget_exhausted": False,
+            "confidence": 0.9,
+        },
+        {
+            "timestamp": "2026-04-07T11:00:00Z",
+            "trace_id": "t2",
+            "question": "Bigger query",
+            "model_id": "claude-opus-4-6",
+            "tokens_used": 50000,
+            "tokens_input": 40000,
+            "tokens_output": 10000,
+            "iterations_run": 5,
+            "wall_time_sec": 120.0,
+            "tavily_searches": 8,
+            "estimated_cost_usd": 1.25,
+            "budget_exhausted": True,
+            "confidence": 0.7,
+        },
+        {
+            "timestamp": "2026-04-08T12:00:00Z",
+            "trace_id": "t3",
+            "question": "Unknown model run",
+            "model_id": "future-model-7",
+            "tokens_used": 500,
+            "tokens_input": 400,
+            "tokens_output": 100,
+            "iterations_run": 1,
+            "wall_time_sec": 2.0,
+            "tavily_searches": 0,
+            "estimated_cost_usd": None,
+            "budget_exhausted": False,
+            "confidence": 0.5,
+        },
+    ]
+    _write_ledger(path, entries)
+    return path
+
+
+class TestCostsCommand:
+    def test_renders_summary(self, tmp_path):
+        path = _ledger_fixture(tmp_path)
+        runner = CliRunner()
+        result = runner.invoke(cli, ["costs", "--ledger", str(path)])
+        assert result.exit_code == 0, result.output
+        # Summary
+        assert "Calls: 3" in result.output
+        assert "$1.2550" in result.output
+        # Per-day rows
+        assert "2026-04-06" in result.output
+        assert "2026-04-07" in result.output
+        assert "2026-04-08" in result.output
+        # Per-model rows
+        assert "claude-sonnet-4-6" in result.output
+        assert "claude-opus-4-6" in result.output
+        # Highest-cost panel
+        assert "t2" in result.output
+        # Unknown model warning
+        assert "unknown model price" in result.output
+
+    def test_filter_by_model(self, tmp_path):
+        path = _ledger_fixture(tmp_path)
+        runner = CliRunner()
+        result = runner.invoke(
+            cli,
+            ["costs", "--ledger", str(path), "--model", "claude-opus-4-6"],
+        )
+        assert result.exit_code == 0
+        assert "Calls: 1" in result.output
+        assert "claude-sonnet-4-6" not in result.output
+
+    def test_filter_by_since_iso(self, tmp_path):
+        path = _ledger_fixture(tmp_path)
+        runner = CliRunner()
+        result = runner.invoke(
+            cli,
+            ["costs", "--ledger", str(path), "--since", "2026-04-08"],
+        )
+        assert result.exit_code == 0
+        assert "Calls: 1" in result.output
+        assert "future-model-7" in result.output
+        assert "claude-sonnet-4-6" not in result.output
+
+    def test_json_output(self, tmp_path):
+        path = _ledger_fixture(tmp_path)
+        runner = CliRunner()
+        result = runner.invoke(
+            cli,
+            ["costs", "--ledger", str(path), "--json"],
+        )
+        assert result.exit_code == 0
+        lines = [l for l in result.output.strip().splitlines() if l]
+        assert len(lines) == 3
+        first = _json.loads(lines[0])
+        assert first["trace_id"] == "t1"
+
+    def test_empty_ledger(self, tmp_path):
+        path = tmp_path / "missing.jsonl"
+        runner = CliRunner()
+        result = runner.invoke(cli, ["costs", "--ledger", str(path)])
+        assert result.exit_code == 0
+        assert "No cost data yet" in result.output
+
+    def test_render_costs_handles_empty(self):
+        console = Console(record=True, width=120)
+        render_costs([], console)
+        out = console.export_text()
+        assert "No cost data yet" in out