fix(cli): emit JSON error on --output-format json — ROADMAP ultraworkers#42

When claw --output-format json hits an error, the error was previously
printed as plain prose to stderr, making it invisible to downstream tooling
that parses JSON output. Now:

  {"type":"error","error":"api returned 401 ..."}

Detection: scan argv at process exit for --output-format json or
--output-format=json. Non-JSON error path unchanged. 156 CLI tests pass.

Author: code-yeongyu

ROADMAP.md

@@ -508,3 +508,5 @@ Model name prefix now wins unconditionally over env-var presence. Regression tes
 40. **Surface broken installed plugins before they become support ghosts** — community-support lane. Clawhip commit `ff6d3b7` on worktree `claw-code-community-support-plugin-list-load-failures` / branch `community-support/plugin-list-load-failures`. When an installed plugin has a broken manifest (missing hook scripts, parse errors, bad json), the plugin silently fails to load and the user sees nothing — no warning, no list entry, no hint. Related to ROADMAP #27 (host plugin path leaking into tests) but at the user-facing surface: the test gap and the UX gap are siblings of the same root. Landing on `main` will close the silent-ghost class of support issues where users report "my plugin does nothing" with no error to share. Track until merged to `main`.
 
 41. **Stop ambient plugin state from skewing CLI regression checks** — community-support lane. Clawhip commit `7d493a7` on worktree `claw-code-community-support-plugin-test-sealing` / branch `community-support/plugin-test-sealing`. Companion to #40: the test sealing gap is the CI/developer side of the same root — host `~/.claude/plugins/installed/` bleeds into CLI test runs, making regression checks non-deterministic on any machine with a non-pristine plugin install. Closely related to ROADMAP #27 (dev/rust `cargo test` reads host plugin state). Track until merged to `main`.
+
+42. **`--output-format json` errors emitted as prose, not JSON** — dogfooded 2026-04-09. When `claw --output-format json prompt` hits an API error, the error was printed as plain text (`error: api returned 401 ...`) to stderr instead of a JSON object. Any tool or CI step parsing claw's JSON output gets nothing parseable on failure — the error is invisible to the consumer. **Fix (`a...`):** detect `--output-format json` in `main()` at process exit and emit `{"type":"error","error":"<message>"}` to stderr instead of the prose format. Non-JSON path unchanged. **Done** in this nudge cycle.

rust/crates/rusty-claude-cli/src/main.rs

@@ -110,7 +110,22 @@ type RuntimePluginStateBuildOutput = (
 fn main() {
     if let Err(error) = run() {
         let message = error.to_string();
-        if message.contains("`claw --help`") {
+        // When --output-format json is active, emit errors as JSON so downstream
+        // tools can parse failures the same way they parse successes (ROADMAP #42).
+        let argv: Vec<String> = std::env::args().collect();
+        let json_output = argv
+            .windows(2)
+            .any(|w| w[0] == "--output-format" && w[1] == "json")
+            || argv.iter().any(|a| a == "--output-format=json");
+        if json_output {
+            eprintln!(
+                "{}",
+                serde_json::json!({
+                    "type": "error",
+                    "error": message,
+                })
+            );
+        } else if message.contains("`claw --help`") {
             eprintln!("error: {message}");
         } else {
             eprintln!(