bazel_build
Run a Bazel build and return structured diagnostics.
options are passed as extra bazel flags (e.g. --config=ci, -c opt).
LABSCO SUMMARY
Bazel's graph and its failures in a shape a model can act on.
The tool that changes a workflow is find_affected_targets: it turns a diff into the set of targets that need rebuilding, which is the same question CI asks and the one an assistant usually guesses at. Queries are read-only but can be slow — a bare //... over a large workspace will make you wait.
What it is
An MCP server over the Bazel command line. It runs queries, builds and tests inside your workspace and returns the results as structure rather than scrollback — failure diagnostics, per-target pass and fail with log excerpts, and dependency graphs an assistant can reason over.
What you get
- bazel_build — a build with structured failure diagnostics; extra Bazel flags such as --config=ci or -c opt pass through
- bazel_test — tests with per-target pass and fail results and excerpts from the failure logs
- bazel_query — an arbitrary query expression, read-only but expensive on broad patterns like //...
- list_targets — every target matching a package pattern, defaulting to the whole workspace
- get_deps and get_rdeps — a target's dependencies to a given depth, and its reverse dependencies within a scope, both excluding the target itself
- show_target_info — the BUILD rule definition for a target, noting that macro-generated targets may differ from the on-disk file
- explain_build_file — the contents of a BUILD file for analysis
- find_affected_targets — every target affected by a set of changed files, which is the CI-narrowing question
Requirements
uv, and a Bazel workspace for the commands to run in. Clients launch it with uvx bazel-mcp-server over stdio; if your client does not start it from the workspace root, pass --workspace-root explicitly. --bazel-path selects bazelisk or another binary, and --timeout and --max-output-chars bound how long a command may run and how much output comes back.
Setup effort
One command — uvx bazel-mcp-server
USE CASES
01
Narrowing what CI needs to run
Turn the changed files into the affected target set instead of rebuilding everything.
02
Chasing a build failure
Build, read the structured diagnostics, and look at the rule definition behind the failing target.
03
Understanding why a target is heavy
Walk its dependencies to a chosen depth and see what pulls it in from the other side.
04
Finding your way around a new repo
List the targets in a package and query the graph for the ones you care about.
9 tools
Counted once with only startup setup. Other versions, modes or optional credentials may expose a different list.
bazel_query
Run an arbitrary Bazel query expression.
Query is read-only but can be expensive on broad patterns like //... output_format: label, build, xml, package, location, graph, etc.
bazel_test
Run Bazel tests and return structured pass/fail results with failure log excerpts.
explain_build_file
Return BUILD file contents for analysis by the host LLM.
path is relative to the workspace root (e.g. foo/bar/BUILD or foo/bar).
find_affected_targets
Find all Bazel targets affected by a set of changed files.
This is useful for CI optimization - only rebuild/test what's changed.
get_deps
Get dependencies of a target up to the given depth, excluding the target itself.
get_rdeps
Find reverse dependencies of a target within scope, excluding the target itself.
list_targets
List all targets matching a package pattern.
Default //... lists targets across the entire workspace (can be slow). For a single package use //pkg or pkg (normalized to //pkg:all). For a subtree use //pkg/...
show_target_info
Show the BUILD rule definition for a target (bazel query --output=build).
Macro-generated targets may differ from on-disk BUILD files.
BEFORE YOU INSTALL
What to know first
Tools you get
9 tools when we connected.
Requires
uvx on PATH
CONTEXT COST
Light Standing cost
Rated by the size of the 9 tool definitions that ride along on every turn, not by how many there are
INSTALLATION
Connect your client
Add the block to your client, or run the command directly.
Server setup
Configuration file
JSON
{
"mcpServers": {
"bazel-mcp-server": {
"command": "uvx",
"args": [
"bazel-mcp-server"
]
}
}
}uvx
TERMINAL
uvx bazel-mcp-server
