No implicit event-file name: intercept names it, semantic reads stdin

Context

Through Bear 4.1.x, bear intercept wrote its event stream to events.json by default and bear semantic read events.json by default. The paired defaults made the split two-step workflow flagless, at the price of a magic rendezvous filename in the current working directory: the two subcommands coordinated through a name neither the user chose nor any external tool consumes. That carried a real hazard - a stale events.json from an earlier run could be silently reprocessed by a bare bear semantic - and a hidden coupling: nothing in either invocation said the two commands were connected.

Shipping bear parse-sh in 4.2.0 changed the balance. The pipe bear parse-sh | bear semantic becomes a primary way to feed semantic, and the events.json default fought it: forgetting --input - died with a baffling "Event file not found: events.json". Viewed as a UNIX tool, semantic is a pure filter - no build, stdout free - and filters read standard input when no file is named. intercept is the opposite case: it can never use standard output (the build owns that stream), so no natural default exists for it at all.

The two file names in play are not equivalent. compile_commands.json is an ecosystem contract - Clang tooling finds the database by that exact name - so defaulting to it carries meaning. events.json was a name Bear invented; no third-party contract depends on it.

Alternatives considered:

• Keep the paired defaults and only improve the missing-file error with a "reading from a pipe? pass --input -" hint. Rejected: it treats the symptom and keeps both the magic-filename coupling and the stale-file hazard.
• Default semantic --input to - only when stdin is not a TTY. Rejected: isatty-dependent semantics make the identical command line behave differently in a terminal and in CI, and CI stdin is often an empty pipe even when the user meant a file - a silent empty-database path.
• Give intercept a stream default by redirecting the build's own stdout to stderr, freeing standard output for the event stream. Rejected: it silently changes observable build behavior - tools that detect a tty or write results to stdout would break - for a pipeline no producer needs.
• Mandate --input on semantic too, symmetric with intercept. Rejected: a pure filter refusing to read stdin by default fights the convention this redesign invokes, permanently bakes --input - into the flagship parse-sh pipeline, and diverges from parse-sh, whose input already defaults to -. The asymmetry with intercept is principled: require the flag where no natural default exists; use the conventional default where one does.
• A positional file operand (bear semantic [FILE], stdin when omitted), the purest filter spelling. Not taken: -i/--input defaulting to - is equally conventional and keeps semantic consistent with parse-sh's existing flags.

Decision

bear intercept requires an explicit --output; there is no default event-file name. bear semantic --input defaults to -, reading the event stream from standard input like any filter. The compile_commands.json output default stays everywhere. Two guards soften the filter footguns: semantic prints a stderr notice when it is about to read events from a terminal, and warns when the event stream turned out empty.

Update (pre-4.2.0 release): parse-sh later stopped emitting the event stream and produces the database directly (parse-sh-database-product), so the parse-sh pipeline this entry cites no longer exists. The decision stands on its remaining grounds: the stale-rendezvous hazard, and semantic as the filter fed by bear intercept event files and third-party producers.

Consequences

• Piping a producer into bear semantic is flagless. The confusing missing-file failure dissolves instead of being papered over with a hint. (At the time of the decision the flagship pipe was bear parse-sh | bear semantic; see the update note above.)
• The stale-events.json hazard is gone; an event file is only ever read because the user named it.
• The old two-step habit breaks loudly and early: bear intercept without --output is a usage error before the build runs, the best available failure mode.
• Scripts written for 4.1.x split workflows must add --output / --input. This is a deliberate breaking change shipped in 4.2.0, before parse-sh examples ossify; it only gets more expensive later.
• A bare bear semantic on a terminal blocks reading stdin like sort or jq would; the TTY notice names the way out.

References

• Requirement: interception-events-format (the seam whose endpoints this names).
• Related rationale: parse-sh-producer (the producer that made the pipe a first-class feed).
• The footgun this supersedes: a bare bear semantic fed from a pipe failed with "Event file not found: events.json" instead of reading standard input.