Smash Story: The Demo Script That Out-Debugged My Test Suite

This is a Smash Stories submission for the DEV Summer Bug Smash: a debugging story about the gap between "all tests pass" and "it actually works" — and the unlikely hero that closed it.

The project is a small MCP (Model Context Protocol) server that wraps Google's gemini-3.1-flash-lite-image

model. It exposes image generation and stateful image editing as four tools that any MCP-speaking agent can call — Claude Code, a Google ADK agent, and a Rust CLI all consume the same ~300-line Python server. (Full architecture write-up here.)

By every signal a developer normally trusts, it was healthy:

Then I wrote a demo script. It found a production bug in under a minute of runtime.

demo.sh

walks the stack live: discover the tools, generate an image, then do a stateful edit. To keep the demo cheap, step 2 requested the lowest quality tier the server documents:

cargo run --quiet -- generate "a tiny robot chef cooking ramen" 16:9 minimal

First run:

🔴 Image generation failed: Error code: 400 - {'error': {'message':
"'minimal' is not a supported thinking level for this model.
Allowed values are: low, high.", 'code': 'invalid_request'}}

Wait. The server's own validation had approved minimal

before sending it. Here's that validation:

SUPPORTED_THINKING_LEVELS = {"minimal", "low", "medium", "high"}

@mcp.tool()
def generate_image(
    prompt: str, aspect_ratio: str = "1:1", thinking_level: str = "medium"
) -> str:
    ...

Four allowed values. The live API accepts two: low

and high

. And look at the default — medium

. That's the real smash-worthy find:

Every live call that didn't explicitly overrideThe validation layer wasn't validating the API's contract — it was validating a stale memory of it.thinking_level

was a guaranteed HTTP 400.

The suite mocks the Gemini client, as unit tests should:

@patch("server._get_client")
def test_generate_image_success(self, mock_get_client):
    mock_client.interactions.create.return_value = mock_interaction
    result = generate_image(prompt="test", thinking_level="medium")
    self.assertIn("🟢 Image successfully saved!", result)

The mock returns success for any input — including inputs the real API rejects. The tests correctly proved "the server forwards medium

faithfully." Faithfully forwarding an invalid value is still a bug; it's just invisible from inside the mock boundary.

Two conditions had to align for this to ship:

SUPPORTED_THINKING_LEVELS

was a cached copy of a fact only the API owns. Cached copies drift.high

for quality — so the broken default and the two phantom values were never exercised. f(x)

being called a hundred times tells you nothing about f()

.Two lines of production code, plus the part that actually takes discipline — locking the discovery in so it can't regress:

-SUPPORTED_THINKING_LEVELS = {"minimal", "low", "medium", "high"}
+SUPPORTED_THINKING_LEVELS = {"low", "high"}

-    prompt: str, aspect_ratio: str = "1:1", thinking_level: str = "medium"
+    prompt: str, aspect_ratio: str = "1:1", thinking_level: str = "low"
result = generate_image(prompt="test", thinking_level="medium")
self.assertIn("Unsupported thinking level 'medium'", result)

Then the sweep (three tool signatures, docstrings, the server's self-describing get_help

, every doc that repeated the wrong values) and a rebuild + push of the published Docker image, which had been shipping the bug to anyone who pulled it.

Elapsed time from first failure to fixed-image-on-Docker-Hub: about ten minutes — because the failing tool call came back as readable text (Allowed values are: low, high

) instead of a stack trace. Error messages that name the fix are half the debugging.

DEMO_FAST=1 ./demo.sh

).The whole project is built on Google AI, end to end:

gemini-3.1-flash-lite-image

store=True

• previous_interaction_id

) are what make multi-turn image editing work: the demo's edit step adds a neon RAMEN sign to LlmAgent

on gemini-2.5-flash

MCPToolset

— Gemini calling Gemini, with the bug fix sitting in between.Allowed values are: low, high

) is what made this a ten-minute smash.xbill9/nb2lite-mcp