Nanaimo data projectHow long are the Bowen Park trails? An agent that must read every page
The City of Nanaimo publishes its trails as open data, split into short segments with a name, a surface type and a length in metres. Our agent answers one question by paging through them. The code, the stand-in plans and every figure are ours, run on the City's layer as downloaded on 23 October 2026.
The City's trail segments at a glance (our tally of 2,190 rows; lengths in metres per the layer's own unit)| What we counted | Result |
|---|
| Segments in the layer | 2,190, of which 147 have no trail name |
| Surface types | Wilderness 725, Hard 637, Soft 564, Stairs 141, Boardwalk 84, Bridge 39 |
| Segments named Bowen Park | 219, more than any other name |
| Segments named Westwood Lake Park | 158 |
| Sum of the Bowen Park segment lengths (our calculation) | 7,535 m, about 7.53 km |
The design: the agent's only tool hands back at most 50 segment lengths at a time, plus the number of segments the trail has. With 219 Bowen Park segments, a correct answer needs five calls. The loop has two guards. A step budget of eight stops a plan that never finishes, and an answer is accepted only if the agent has read every segment and its total matches the sum of what its own tool calls returned.
The full program, with three plans written by us in place of a model:# Nanaimo trail agent with a step budget, and a check that redoes the sum (our teaching example)
import json
SEGMENTS = []
for page in ("trails-p0.json", "trails-p2000.json"):
SEGMENTS += [f["attributes"] for f in json.load(open(page, encoding="utf-8"))["features"]]
PAGE = 50
def get_segments(name, offset):
"""Tool: up to 50 segment lengths (metres) of one named trail, from offset, plus the trail's row count."""
rows = [s for s in SEGMENTS if s["NAME"] == name]
return {"lengths_m": [s["Shape__Length"] for s in rows[offset:offset + PAGE]], "total_rows": len(rows)}
def run(plan, max_steps=8):
"""Execute tool calls until the plan answers; stop when the step budget is spent."""
seen, total = [], None
for step, call in enumerate(plan, start=1):
if step > max_steps:
return "STOPPED: budget of %d steps spent, no answer" % max_steps
if call["type"] == "answer":
recomputed = round(sum(seen) / 1000, 2)
if len(seen) != total:
return "REJECTED: read %d of %d segments" % (len(seen), total)
if call["km"] != recomputed:
return "REJECTED: claimed %s km, tool results add to %s km" % (call["km"], recomputed)
return "OK: %s km from %d segments" % (call["km"], total)
result = get_segments(call["name"], call["offset"])
seen += result["lengths_m"]
total = result["total_rows"]
return "plan ended without an answer"
# Three stand-in plans in place of a model, so the loop and its checks run with no AI service.
name = "Bowen Park"
rows = get_segments(name, 0)["total_rows"]
calls = [{"type": "tool", "name": name, "offset": o} for o in range(0, rows, PAGE)]
honest = calls + [{"type": "answer", "km": 7.53}]
stuck = [calls[0]] * 20 # asks for the first page again and again
early = calls[:2] + [{"type": "answer", "km": 3.26}] # answers after two pages of five
for label, plan in (("honest", honest), ("stuck", stuck), ("early", early)):
print(label, "->", run(plan))
What it printed:honest -> OK: 7.53 km from 219 segments
stuck -> STOPPED: budget of 8 steps spent, no answer
early -> REJECTED: read 100 of 219 segments
The stuck plan
It asks for the first page twenty times. Without a budget the loop would run until someone noticed; with one, it stops after eight steps and says plainly that it has no answer. An honest stop is a result.
The early plan
It reads two pages of five and answers 3.26 km. That number is the true sum of what it read, so a check that only re-adds its tool results would pass it. The completeness rule catches it: 100 of 219 segments is not the trail.
The honest plan
Five calls, every segment read, a total that matches the tool results: 7.53 km. The figure is our sum of the City's segment lengths, not a published trail length, and segments without a name are not included.
What the file does not settle
Bowen Park segments include 13 of stairs and 2 of bridge, so the 7.53 km mixes surfaces a walker and a cyclist would treat very differently. The layer has no date column, and a trail split across two names would need a person to decide whether to merge them. We report the sum for one name and stop there.
Used under the Open Government Licence - Nanaimo, which grants use including for commercial purposes (Open Data Catalogue Licence). Contains information licenced under the Open Government Licence - Nanaimo. No endorsement by the City of Nanaimo is implied.
