Bound MCP compile failure diagnostics without losing actionable errors - #67424

Closes #66823. Corroborating reports: #66455, #66792, #67119, #67350.

ADR: ADR-67424: Bound MCP Compile Diagnostics With a Streaming Head/Tail Collector. Completed for maintainer review; status remains Proposed until review and merge.

Changes

• Drain compile stderr through a compile-local streaming collector rather than buffering debug floods. Retain cause-first errors, multiline remediation, ANSI-normalized diagnostics, and explicit omission markers.
• Limit execution-failure messages to 4 KiB and serialized fallback JSON to 48 KiB, including JSON escaping. Preserve per-workflow invalid results when they fit; otherwise return an explicit invalid batch compile result with the affected workflow count.
• Preserve structured compiler/scanner JSON, even when it exceeds the fallback budget or the subprocess exits nonzero. Collect shellcheck findings separately so normal scanner diagnostics remain available. Strict validation, required shellcheck checks, and the deprecated/ignored max_tokens contract are unchanged.
• Match debug lines by the logger's namespace/message/elapsed format, without a fixed namespace allowlist. Preserve the active error block across interleaved debug lines. Shellcheck headers and finding lines cannot replace the cause or dependency warning.
• Add regressions using a real child process with DEBUG enabled and 12 MiB stderr; cover huge single lines, whitespace stdout, multiline/ANSI diagnostics, Unicode and byte boundaries, escape-heavy JSON, 334-workflow discovery, spawn failures, cancellation, concurrent request isolation, interleaved remediation, unlisted logger namespaces, and scanner/warning separation. No diagnostic-file persistence is needed.
• Document the fallback contract and add a patch changeset.

Validation

Validated the code committed as 63248a5ea648fa7ab7dac048f4052db0451e2fd0:

• npm_config_package=prettier@3.9.9 PATH="/opt/homebrew/bin:$(go env GOPATH)/bin:$PATH" make fmt — passed.
• npm_config_package=prettier@3.9.9 PATH="/opt/homebrew/bin:$(go env GOPATH)/bin:$PATH" make lint — passed, including full-repository Go lint, JavaScript lint, shellcheck and schema checks. Existing JavaScript warnings remain warnings; no rules were disabled.
• npm_config_package=prettier@3.9.9 PATH="/opt/homebrew/bin:$(go env GOPATH)/bin:$PATH" make agent-report-progress — passed, including build, changed-file standard/custom Go lint, impacted unit tests and workflow drift (all 334 workflow lock files in sync). Existing custom analyzer findings in unchanged files are handled by the repository's change-scoped gate.
• go test ./pkg/cli -run 'Test(CompileTool|MCPCompile|BuildCompileErrorResults|InjectShellcheckDiagnostics|InjectDockerUnavailableWarning|ExtractLastConsoleMessage|DevelopmentCompileFailsWithoutShellcheckOrDocker|MCPSubprocessGuardrail)' -count=1 — passed.
• go test -race ./pkg/cli -run 'Test(MCPCompile|CompileToolBounds|CompileToolPreservesStructuredFailureAfterDebugFlood|CompileToolDevelopmentPreservesBatchDiagnosticsOnFailure|CompileTool_UsesOnlyStdoutOnSuccess)' -count=1 — passed.
• git diff --check — passed.

The new review regressions reproduced the defects before the fixes and pass afterward. All five in-scope review threads were replied to and resolved.

Earlier publication encountered a shared golangci-lint lock and baseline custom-linter findings; the finishing pass's required local gates now pass. The default unpinned npx selected Prettier 4.0.0-alpha.13 and crashed on unrelated JSON; the declared dependencies and pinned Prettier 3.9.9 are used for validation without unrelated formatting changes.

CI hand-off: the two last-known CI checks were successful at triage. CI on the latest agent-pushed HEAD remains unverified until a maintainer re-triggers it. No Actions runs were triggered and the PR was not merged.