# Build Failure RCA - Python Version Mismatch

**Date**: 2025-01-27
**Incident ID**: BUILD-FAIL-001
**Severity**: L1
**Status**: Root Cause Identified

## Executive Summary

FastMCP Cloud build failed due to Python version incompatibility. Build environment uses Python 3.12.12, but project requires Python >=3.13.

## Failure Point

**Location**: Docker build step 4/5 - Dependency installation
**Error**: `uv pip install --system .` failed with unsatisfiable requirements

```
× No solution found when resolving dependencies:
  ╰─▶ Because the current Python version (3.12.12) does not satisfy
      Python>=3.13 and sef-agents==0.1.0 depends on Python>=3.13
```

## Root Cause Analysis

### Primary Cause
**Python version mismatch between build environment and project requirements**

1. **Project Configuration**:
   - `pyproject.toml` line 6: `requires-python = ">=3.13"`
   - `.python-version`: `3.13`
   - All CI workflows use Python 3.13

2. **Build Environment**:
   - Base image: `342547628772.dkr.ecr.us-east-1.amazonaws.com/fastmcp-prd-base-images:mcp-base-python3.12`
   - Actual Python: 3.12.12
   - Build config specifies: "Python version: 3.12"

3. **Dependency Resolution**:
   - `uv pip install --system .` reads `pyproject.toml`
   - Detects `requires-python = ">=3.13"`
   - Fails because runtime Python is 3.12.12

### Contributing Factors
- FastMCP Cloud build system auto-detects Python version from base image
- No explicit Python version override in build configuration
- Base image tag (`mcp-base-python3.12`) hardcodes Python 3.12

## Evidence

### Build Log Excerpt
```
Build configuration:
  • Python version: 3.12
  • Entrypoint: src/sef_agents/server.py
  • Git SHA: df19ea69

#5 [1/5] FROM ...mcp-base-python3.12...
#9 [4/5] RUN ...uv pip install --system .
0.336   × No solution found when resolving dependencies:
0.337   ╰─▶ Because the current Python version (3.12.12) does not satisfy
0.337       Python>=3.13
```

### Project Files
- `pyproject.toml`: `requires-python = ">=3.13"`
- `.python-version`: `3.13`
- `.github/workflows/security-scan.yml`: `uv python install 3.13`

## Impact

- **Build Status**: Failed
- **Deployment**: Blocked
- **User Impact**: None (pre-deployment failure)

## Resolution

### Immediate Fix Required
1. **Update FastMCP Cloud build configuration** to use Python 3.13 base image
2. **Verify base image availability**: `mcp-base-python3.13` or equivalent
3. **Override Python version** in build config if base image selection is not possible

### Verification Steps
1. Confirm FastMCP Cloud supports Python 3.13 base images
2. Update build configuration to specify Python 3.13
3. Re-run build and verify dependency installation succeeds

## Prevention

- Add build-time validation: Check Python version matches `requires-python` before install
- Document Python version requirements in deployment docs
- Add pre-build check in CI to validate Python version compatibility

## Related Artifacts

- Build log: FastMCP Cloud deployment logs
- Git SHA: df19ea69
- Repository: https://github.com/Mishtert/sef-agents
