Weather MCP server

An MCP server that exposes three weather tools to a Databricks Agent Bricks agent. It runs as a Databricks App at

https://weather-mcp-7474645136578041.aws.databricksapps.com

with the MCP endpoint at /mcp. That URL is what gets registered with the agent as an external MCP server.

The source lives at https://github.com/gisaf22/weather-mcp, which is where the commit history is — this zip carries none.

The app sits behind Databricks workspace authentication, so that URL is not reachable from outside the workspace: opening it returns a login page rather than the server. That is expected, not a broken deployment. The demonstration screenshots below are what show it running.

Data source and auth

Weather data comes from Open-Meteo. I picked it because it needs no API key and no signup — there are no secrets to manage at all.

That is a visible difference from the Alpaca paper-trading server this was patterned on. There, every call had to fetch credentials from a Databricks secret scope through a _secret() helper, and app.yaml needed an env block to point at the scope. Here app.yaml has no env block and weather_broker.py has no _secret() equivalent, because there is nothing to authenticate.

Locations are resolved through Open-Meteo's geocoding endpoint, so any place name the geocoder knows will work. There is no hardcoded list of supported cities.

Files

File	What it holds
weather_mcp_server.py	The three MCP tools and their docstrings. No requests calls live here.
weather_broker.py	Every HTTP call and all response parsing, plus the WMO weather-code table and the LocationNotFound exception.
app.yaml	Databricks App entrypoint. No env block — see above.
requirements.txt	fastmcp and requests.
agent/system_prompt.md	The system prompt configured on the agent, with notes on which rules came from observed failures.

The split matters: the tool functions compose broker calls and shape the result, and that is all they do. Swapping Open-Meteo for another provider means rewriting weather_broker.py and leaving the MCP surface alone.

Tools

get_current_weather(location: str) -> dict

Current conditions for a place name. Returns the resolved location alongside temperature_f, feels_like_f, humidity_pct, wind_mph, conditions, and observed_at.

get_forecast(location: str, days: int = 3) -> dict

Daily forecast, 1–7 days. Returns the resolved location plus one entry per day with date, high_f, low_f, precipitation_chance_pct, max_wind_mph, and conditions.

get_outdoor_recommendation(location: str, date: str | None = None) -> dict

Judges whether a day suits outdoor plans and says which weather factors drove the judgment. date is ISO YYYY-MM-DD and defaults to today at the location.

A factor fires when its condition holds:

Factor	Condition
rain likely	precipitation_chance_pct >= 50
possible rain	precipitation_chance_pct 30–49
heat	high_f >= 90
cold	low_f <= 45
wind	max_wind_mph >= 25
thunderstorm	WMO weather code 95–99

The verdict combines them:

Verdict	When
poor	a thunderstorm is forecast, or precipitation >= 50%, or high >= 95F
caution	none of those, but at least one other factor fired
good	no factor fired

Each factor carries the actual value that triggered it, not just the rule name:

{"rule": "possible rain", "value": 43, "note": "43% chance of precipitation"}

That is the point of the tool. It is the single factor that fired in demonstration (b) below, on a day with a high of 80.0F: the agent can answer "a 43% chance of precipitation" instead of only "caution", and the number it cites is the one the threshold actually tested.

All three tools return {"status": "error", "message": ...} on failure — an unrecognized place, a date outside the forecast range, or an API problem. A stack trace never reaches the agent; the traceback goes to the app logs.

Setup

git clone <this repo>
cd weather-mcp
python3 -m venv .venv
source .venv/bin/activate
pip install -r requirements.txt

Run it locally:

python weather_mcp_server.py     # serves on :8000, MCP at /mcp

weather_broker.py also runs standalone and exercises the API directly, which is the fastest way to check the data layer without the MCP wrapper:

python weather_broker.py

Deploy

Deploy as a Databricks App using this repo as the source, with app.yaml as the entrypoint. Then register https://<app-url>/mcp with the Agent Bricks agent as an external MCP server, and paste agent/system_prompt.md into the agent's instructions.

Redeploying: pushing to GitHub does not update the app. The Databricks Git folder has to pull the new commits first, and only then does a redeploy pick them up. Pushing and redeploying without the pull in between silently ships the old code — worth knowing before debugging a fix that appears not to have taken effect.

Demonstration

Four exchanges from the Agent Bricks playground, each showing the question, the tool call the agent made, and its reply.

(a) "What's the weather in Chicago right now?"

Routes to get_current_weather and reports the resolved location as Chicago, Illinois — so the user can tell it did not answer for Chicago, Jalisco.

(b) "Should I plan a picnic in Chicago tomorrow?"

Routes to get_outdoor_recommendation, which returns a caution verdict, and the agent cites the 43% precipitation figure carried in factors rather than repeating the verdict alone.

(c) "What's the forecast for Austin this weekend?"

Routes to get_forecast and reports each day by date — "August 9", "August 10", "August 11" — rather than computing weekday names.

(d) "What's the weather in Zzyzxville?"

Calls get_current_weather anyway rather than refusing on its own judgment, and relays the tool's own error message back to the user as a request to confirm the spelling.

The earlier failure this replaced — the agent skipping the tool entirely and declaring the place fictional — no longer reproduces with the system prompt in place, so there is no screenshot of it. It is described in Findings.

Findings

Three things I measured rather than assumed.

FastMCP shows the agent less of the docstring than I wrote

FastMCP builds a tool's description from only the first prose section of its docstring. Everything from the first section header onward is dropped. Args: survives, but as per-parameter descriptions inside the input schema, not as part of the description. Returns: is discarded entirely.

A bare Header: line followed by an indented block also parses as a section. That silently swallowed my verdict rules — the agent could see the factor thresholds but not how they combined into poor / caution / good. Renaming the header to Verdict rules (how the fired factors combine): was enough to stop the parser treating it as a section, because the parentheses break the pattern.

I found this by dumping the live tool descriptions over an MCP client connection and diffing them against the source, not by reading documentation. Before the fix, get_forecast was sending the agent 205 characters; after hoisting everything above Args:, 1,535. Everything the agent needs — resolved location semantics, field meanings, the thresholds, the error contract — now lives in that first prose section.

Two Open-Meteo fields answer different questions

weather_code and precipitation_probability_max are not two views of the same thing. The daily weather code is the most significant weather expected at any point in the day; the probability is the likelihood of measurable rain across the day. They routinely disagree.

Austin returned WMO code 82 — "violent rain showers" in the official table — next to a 3% precipitation chance. The mapping was correct; the pairing is just what a daily maximum looks like next to a daily likelihood. The agent reported it as a contradiction until the docstring explained the difference.

Two fixes. The docstrings now state what each field measures and that a low percentage beside a stormy label means brief and unlikely, not contradictory. And the shower labels were softened from the official slight/moderate/violent wording — "violent rain showers" reads as a severe-weather warning and badly oversells a 3% day, so codes 80/81/82 now render as "scattered showers", "rain showers", and "heavy showers possible".

The system prompt fixed the factual failures but not the stylistic one

Two failures went away once the prompt addressed them directly: inventing weekday names that did not match the dates, and skipping the tool call entirely to declare a place fictional. Both are now explicit rules, and both held.

The second one held in action but not in narration, which is the more interesting result. In demonstration (d) the agent calls the tool and relays its error message, exactly as instructed — but its reasoning line reads "I am going to use the get_current_weather tool... however I anticipate the tool will return an error because Zzyzxville does not appear to be a real location." The rule reliably stopped it from acting on its own judgment about whether a place exists. It did not stop it forming that judgment, or saying so out loud. An instruction can govern which tool call happens; it does not govern what the model believes on the way there.

One rule did not hold at all. The prompt says not to add generic advice the tools did not produce, naming hydration reminders and tents specifically. The agent still appends them: demonstration (c) closes with "Make sure to stay hydrated and plan for the heat", and (b) with "consider bringing umbrellas or a tent" — both visible in the screenshots above, neither traceable to anything a tool returned. I left it. The factual accuracy rules are what matter here, and chasing the padding with more prompt text was not worth the added instruction surface.

Known limitations

• The thresholds are chosen, not derived. 90F for heat, 25 mph for wind, 50% for rain likely — these are my judgment calls, not any published standard. They are stated numerically in the tool docstring so the agent can explain them, but a different set would be equally defensible.
• Ambiguous names resolve silently to the largest match. Open-Meteo orders geocoding results by population and the broker takes the first. "Chicago" gets Chicago, Illinois, not the real Chicago in Jalisco, Mexico. The resolved name, region, and country are returned so the agent can state which one it used, but nothing prompts the user to disambiguate before answering.
• Seven days is the ceiling. Requests beyond that are clamped, a limit of the free Open-Meteo forecast endpoint.