Common Mistakes by AI Coding Agents

Patterns observed across multiple AI agent interactions on the ProxySQL codebase, with root cause analysis and prevention strategies.

1. Wrong Branch Target

Symptom: PR targets v3.0 (main) instead of the feature branch.

Root cause: Agents prioritize technical content over administrative instructions. Even when branch info is present in the issue, agents often skim past it while focusing on code requirements, then use heuristics (e.g., most recent branch, default branch) to fill the gap they don't realize they have.

Prevention: Place git workflow instructions at the very top of the issue, before the technical description. Agents read top-down with decreasing attention — administrative details buried after exciting code specs will be skipped.

### FIRST: Git workflow (do this before reading anything else)
- Create branch `v3.0-XXXX` from `v3.0-5473`
- PR target: `v3.0-5473`

Detection: Check gh pr view <number> --json baseRefName after PR creation.

2. Reimplemented Functions in Test Files

Symptom: Test file contains copy-pasted reimplementations of the functions under test. Tests validate the copy, not the real production code.

Root cause: Agent doesn't know the build system links tests against libproxysql.a, so it creates standalone tests that don't depend on the library.

Prevention:

Explain that tests link against libproxysql.a (the real functions are available at link time)
Add to DO NOT list: "Do NOT reimplement extracted functions in the test file"
Provide the Makefile rule that shows the linking

Detection: grep -c "static.*calculate_eviction\|static.*evaluate_pool" test_file.cpp — if > 0, functions were reimplemented.

3. Test Files in Wrong Directory

Symptom: Test placed in test/tap/tests/ (E2E test directory) instead of test/tap/tests/unit/ (unit test directory).

Root cause: Agent sees existing test files in test/tap/tests/ and follows that pattern. Doesn't know about the unit/ subdirectory.

Prevention: Specify the exact file path including directory in the issue deliverables.

Detection: ls test/tap/tests/*unit* should return nothing — unit tests belong in test/tap/tests/unit/.

4. Manual TAP Symbol Stubs

Symptom: Test file manually defines noise_failures, noise_failure_mutex, stop_noise_tools(), get_noise_tools_count().

Root cause: Agent compiles tap.cpp which references these symbols. Without the harness, the agent must define them. This is a signal the agent isn't using the harness.

Prevention:

Explain that test_globals.cpp already provides all TAP stubs
Add to DO NOT list: "Do NOT define noise_failures or stop_noise_tools"

Detection: grep -c "noise_failures\|stop_noise_tools" test_file.cpp — if > 0, harness not used.

5. Merged Instead of Rebased

Symptom: PR diff includes dozens of unrelated files because the agent ran git merge <upstream> into its branch.

Root cause: Agent's default strategy for incorporating upstream changes is merge. This creates a merge commit that brings all upstream changes into the PR diff.

Prevention: Explicit instruction: "Use git rebase, NOT git merge."

Detection: git log --merges <branch> --not <base> — any merge commits indicate merging.

6. Circular Include Dependencies

Symptom: Production code compiles on the agent's machine (or doesn't get tested) but fails in CI or on other platforms with "unknown type name" errors.

Root cause: ProxySQL has circular include chains (proxysql.h → cpp.h → MySQL_HostGroups_Manager.h → Base_HostGroups_Manager.h → proxysql.h). Placing new declarations in these headers can result in the declarations being invisible depending on include order.

Prevention:

Create standalone headers with their own include guards (e.g., ConnectionPoolDecision.h)
Explicitly warn about the circular chain in the issue
Require make build_lib -j4 as a verification step

Detection: Compilation failure with "unknown type name" for a type that clearly exists in a header.

7. Modified Existing Test Files Instead of Creating New Ones

Symptom: Agent adds tests to an existing test file instead of creating a new one for the new feature.

Root cause: Agent sees a test file for a related component and assumes new tests belong there.

Prevention: Specify the exact test file name in the issue: "Create test/tap/tests/unit/my_feature_unit-t.cpp."

8. Didn't Verify Compilation

Symptom: PR contains code that doesn't compile. Agent submitted without building.

Root cause: Some agents don't have access to the build environment, or don't run the build as part of their workflow.

Prevention:

Add explicit verification step: "make build_lib -j4 must exit with code 0"
Add to acceptance criteria as a hard requirement

9. Overly Broad Changes

Symptom: Agent refactors callers, updates documentation, fixes unrelated bugs, or "improves" code outside the task scope.

Root cause: Agent optimizes for perceived quality/completeness and makes changes it considers beneficial.

Prevention:

Explicitly scope: "Only modify <list of files>"
Add: "Do not refactor code outside the scope of this task"
Add: "Do not fix pre-existing issues you notice — file separate issues for those"

Summary: Red Flags in Agent PRs

Quick checks to run on any agent-generated PR:

bash

# Wrong base branch?
gh pr view <PR> --json baseRefName -q '.baseRefName'

# Test in wrong directory?
gh pr diff <PR> | grep "^+++ b/test/tap/tests/[^u]"

# Reimplemented functions?
gh pr diff <PR> | grep "^+static.*calculate_\|^+static.*evaluate_\|^+static.*should_"

# Manual TAP stubs?
gh pr diff <PR> | grep "^+.*noise_failures\|^+.*stop_noise_tools"

# Merge commits?
gh pr view <PR> --json commits --jq '.commits[].messageHeadline' | grep -i merge

# Unrelated files changed?
gh pr diff <PR> | grep "^+++ b/" | grep -v "<expected_files_pattern>"