docs(tracker-client): define CLI I/O contract and ADRs

console/tracker-client/README.md

@@ -2,14 +2,19 @@
 
 A collection of console clients to make requests to BitTorrent trackers.
 
-> **Disclaimer**: This project is actively under development. We’re currently extracting and refining common functionality from the[Torrust Tracker](https://github.com/torrust/torrust-tracker) to make it available to the BitTorrent community in Rust. While these tools are functional, they are not yet ready for use in production or third-party projects.
+> **Disclaimer**: This project is actively under development. We’re currently extracting and refining common functionality from the [Torrust Tracker](https://github.com/torrust/torrust-tracker) to make it available to the BitTorrent community in Rust. While these tools are functional, they are not yet ready for use in production or third-party projects.
 
 There are currently three console clients available:
 
 - UDP Client
 - HTTP Client
 - Tracker Checker
 
+## Documentation
+
+- [Tracker CLI I/O Contract](docs/contracts/tracker-cli-io-contract.md)
+- [Tracker Client ADRs](docs/adrs/README.md)
+
 > **Notice**: [Console apps are planned to be merge into a single tracker client in the short-term](https://github.com/torrust/torrust-tracker/discussions/660).
 
 ## UDP Client
@@ -186,7 +191,7 @@ This program is free software: you can redistribute it and/or modify it under th
 
 This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the [GNU Lesser General Public License][LGPL_3_0] for more details.
 
-You should have received a copy of the *GNU Lesser General Public License* along with this program. If not, see <https://www.gnu.org/licenses/>.
+You should have received a copy of the _GNU Lesser General Public License_ along with this program. If not, see <https://www.gnu.org/licenses/>.
 
 Some files include explicit copyright notices and/or license notices.
 

console/tracker-client/docs/adrs/20260512080000_define_tracker_cli_io_contract_and_error_handling.md

@@ -0,0 +1,94 @@
+# ADR 20260512080000: Define Tracker CLI I/O Contract and Error Handling
+
+- Status: Accepted
+- Date: 2026-05-12
+- Scope: console/tracker-client
+
+## Context
+
+The tracker client is a growing CLI surface with multiple commands (UDP client, HTTP client,
+tracker checker, and monitor features under active development). The project intends to extract
+this application into an independent repository.
+
+Without an explicit contract, command outputs and error behavior can diverge, breaking user
+automation and increasing maintenance cost.
+
+At the same time, existing commands may not yet fully match the desired target behavior, so the
+team needs a migration policy, not a flag day rewrite.
+
+## Decision
+
+Define a global Tracker CLI I/O contract for console/tracker-client.
+
+### 1. Default output format
+
+- JSON is the default output format.
+
+### 2. Output channels
+
+- stdout: normal command results and machine-consumable output.
+- stderr: progress reporting, diagnostics, warnings, and error output.
+
+For monitor-style streaming behavior:
+
+- Progress/probe events may be emitted as one JSON object per line (NDJSON style).
+- If emitted as progress, they go to stderr.
+- Final command result summary goes to stdout as JSON.
+
+### 3. Exit-code semantics
+
+Exit codes represent tracker client app execution state, not tracker endpoint health status.
+
+- 0: command executed successfully, even if one or more trackers reported failures/timeouts.
+- 1: generic application/runtime failure (unexpected internal error).
+- 2: invalid tracker checker configuration/input errors.
+
+Tracker-specific failures (for example announce timeout, scrape timeout, non-200 HTTP from a
+tracker) are represented in JSON result payloads, not in non-zero exit codes.
+
+### 4. Progressive migration policy
+
+- New features and new subcommands must follow this contract.
+- Existing features that do not yet comply will be migrated progressively when touched by new
+  feature work or dedicated refactors.
+- No immediate broad rewrite is required.
+
+### 5. Scope location
+
+This policy is intentionally documented under console/tracker-client docs because the tracker
+client is expected to be extracted into its own repository.
+
+### 6. Auditability and testing strategy
+
+- Contracts should be auditable through stable structured payloads and explicit field definitions.
+- During the monorepo phase, conformance is enforced through issue specs and acceptance criteria.
+- After tracker-client extraction to its own repository, add dedicated E2E contract tests for
+  stdout/stderr behavior, exit codes, NDJSON events, and JSON schema conformance.
+
+## Consequences
+
+### Positive
+
+- Predictable behavior for shell pipelines and automation.
+- Clear separation between app-level failure and tracker-level status.
+- Lower migration risk through incremental adoption.
+- Documentation remains aligned with future repository extraction boundaries.
+- Auditable CLI behavior suitable for compliance and regression verification.
+
+### Negative
+
+- Transitional inconsistency until all legacy paths are migrated.
+- Additional implementation and review burden to keep channel/exit behavior consistent.
+- Full E2E contract coverage is deferred until extraction, so short-term assurance relies on
+  spec-driven validation.
+
+## Implementation Notes
+
+- Command specs should reference the tracker client I/O contract document.
+- New command acceptance criteria should include channel correctness and exit-code behavior.
+- Contract schema updates should be backward compatible or explicitly versioned.
+
+## References
+
+- [Tracker CLI I/O Contract](../contracts/tracker-cli-io-contract.md)
+- [console/tracker-client/README.md](../../README.md)

console/tracker-client/docs/adrs/README.md

@@ -0,0 +1,17 @@
+# Tracker Client ADRs
+
+Architecture Decision Records (ADRs) for the console tracker client live in this folder.
+
+These ADRs are scoped to the tracker client application and are intentionally separated from
+repository-level ADRs because the tracker client is expected to be extracted into its own
+repository in the future.
+
+## Goals
+
+- Capture durable decisions for tracker client behavior and architecture
+- Keep CLI/API contracts explicit and stable for automation users
+- Allow progressive migration of existing commands toward the target contract
+
+## Index
+
+See [ADR Index](index.md).

console/tracker-client/docs/adrs/index.md

@@ -0,0 +1,5 @@
+# ADR Index
+
+| ADR                                                                                   | Date       | Title                                              | Short Description                                                                                                                                                  |
+| ------------------------------------------------------------------------------------- | ---------- | -------------------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------ |
+| [20260512080000](20260512080000_define_tracker_cli_io_contract_and_error_handling.md) | 2026-05-12 | Define Tracker CLI I/O Contract and Error Handling | Standardize JSON-first output, stdout/stderr channel rules, and exit-code semantics for tracker checker commands with progressive migration for existing features. |

console/tracker-client/docs/contracts/tracker-cli-io-contract.md

@@ -0,0 +1,165 @@
+# Tracker CLI I/O Contract
+
+Status: Active
+
+Scope: console/tracker-client commands, with explicit emphasis on tracker checker behavior.
+
+## Purpose
+
+Define stable rules for:
+
+- output format
+- stdout/stderr channel usage
+- error payload structure
+- process exit codes
+
+This contract is designed for automation-first CLI usage and progressive adoption.
+
+## Core Rules
+
+### JSON-first output
+
+- JSON is the default output format for command results.
+- Result payloads on stdout are machine-consumable.
+
+### Channel usage
+
+- stdout:
+  - final command results
+  - structured status/results intended for downstream processing
+- stderr:
+  - progress reporting
+  - diagnostics and warnings
+  - application error output
+
+### Monitor/progress events
+
+For monitor commands (for example `tracker_checker monitor udp`):
+
+- Per-probe progress is emitted as NDJSON style: one JSON object per line.
+- Per-probe progress events go to stderr.
+- Final aggregate summary goes to stdout as JSON.
+
+## Error Payload Schema
+
+Application errors should use this envelope:
+
+```json
+{ "error": { "kind": "string", "source": "string", "message": "string" } }
+```
+
+Field meaning:
+
+- kind: machine-readable error category (for example `invalid_configuration`)
+- source: where the error originated (for example `TORRUST_CHECKER_CONFIG`, `config_path`, `runtime`)
+- message: human-readable detail
+
+## Exit Codes
+
+Exit codes represent CLI app execution status.
+
+- 0: command executed successfully (tracker failures can still be present in JSON results)
+- 1: generic application/runtime failure
+- 2: invalid tracker checker configuration/input
+
+Important:
+
+- Tracker endpoint failures do not map to non-zero process exit codes.
+- Tracker endpoint failures are part of result JSON payloads.
+
+## Distinguishing App Errors vs Tracker Failures
+
+- App errors:
+  - invalid CLI/config input
+  - internal command failures
+  - serialization/runtime failures
+  - reported via stderr error JSON and non-zero exit code
+- Tracker failures:
+  - timeout
+  - connection refused
+  - non-success status from tracker endpoint
+  - reported inside stdout result JSON, exit code remains 0
+
+## Stability and Migration
+
+- New features and subcommands must comply with this contract.
+- Legacy behavior is migrated progressively.
+- Contract changes should remain backward compatible; if a breaking change is required,
+  introduce a schema version and migration note.
+
+## Auditability Requirements
+
+This contract is intended to be auditable.
+
+- Prefer explicit structured payloads over ad-hoc text messages.
+- Keep field names stable once published.
+- If any required field changes, bump a schema version and document migration steps.
+
+Recommended metadata fields for auditable outputs:
+
+- `schema_version`
+- `command`
+- `timestamp`
+- `run_id`
+
+These fields can be added progressively as commands are migrated.
+
+## Verification Strategy
+
+### Current repository phase
+
+- Contract conformance is validated by documentation reviews and issue-level acceptance criteria.
+- New feature specs should include explicit checks for:
+  - stdout/stderr channel behavior
+  - JSON envelope conformance
+  - exit-code semantics
+
+### Post-extraction phase (target)
+
+When `console/tracker-client` is extracted to its own repository, add dedicated E2E conformance
+tests for this contract.
+
+Recommended E2E coverage:
+
+- golden stdout/stderr fixtures for representative command runs
+- exit-code assertions (`0`, `1`, `2`)
+- NDJSON per-line validation for monitor probe events
+- JSON schema validation for final summaries and error envelopes
+
+Until extraction, this remains a planned verification step.
+
+## Examples
+
+### Example 1: Successful run with tracker failures
+
+```text
+stdout:
+{"udp_trackers":[{"url":"udp://127.0.0.1:6969","status":{"code":"timeout","message":"announce timeout"}}]}
+
+stderr:
+{"event":"probe","url":"udp://127.0.0.1:6969","status":"timeout","elapsed_ms":null}
+
+exit code: 0
+```
+
+### Example 2: Invalid configuration
+
+```text
+stdout:
+
+stderr:
+{"error":{"kind":"invalid_configuration","source":"TORRUST_CHECKER_CONFIG","message":"JSON parse error: trailing comma at line 7 column 5"}}
+
+exit code: 2
+```
+
+### Example 3: Generic application failure
+
+```text
+stdout:
+
+stderr:
+{"error":{"kind":"runtime_failure","source":"runtime","message":"failed to initialize async runtime"}}
+
+exit code: 1
+```