So I have discovered (after a few hours of debugging) that if configure a jvm tool

pants.ini

like:

[gen.wire]
wire_compiler: ':square-wire-compiler'

With a

BUILD.tools

like:

target(name='square-wire-compiler',
  dependencies=[
    '3rdparty:...',
    '3rdparty:...',
  ],
)

Pants will silently use the default jvm tool classpath for Wire, but will give the synthetic target the name

:square-wire-compiler

. (In wire_gen,

self.context.resolve(':square-wire-compiler')

returns a

JarLibrary(BuildFileAddress(FilesystemBuildFile(/Users/gmalmquist/Development/java/3rdparty/BUILD.gen), com.squareup.wire.wire-compiler))

). I eventually realized that the correct thing to do is specify

[gen.wire]
wire_compiler: //:square-wire-compiler

In which case

self.context.resolve(':square-wire-compiler')

correctly returns the

target()

I've defined in

BUILD.tools

.