RFD 060: Config Explain

• Status: Discussion
• Category: Design
• Authors: Jean Mertz git@jeanmertz.com
• Date: 2026-07-21

Summary

Add a global --explain flag that prints the config resolution chain for all or a specific field, then exits without running the command. This gives users visibility into JP's layered configuration system — which files, environment variables, CLI flags, and conversation deltas contribute to the final resolved value.

Motivation

JP's configuration is loaded from up to 9 sources, merged in a specific order:

1. User global config ($XDG_CONFIG_HOME/jp/config.toml)
2. Workspace config (.jp/config.toml)
3. CWD config (.jp.toml, recursive upwards)
4. User workspace config ($XDG_DATA_HOME/jp/<id>/config.toml)
5. Environment variables (JP_CFG_*)
6. Conversation delta (stored in the active conversation's event stream)
7. CLI --cfg arguments
8. Command-specific CLI flags (--model, --reasoning, --tool, etc.)
9. Defaults (applied last via default_values)

When a config value isn't what the user expects, they have no way to determine which layer set it. The current debugging workflow is: check config.toml, check env vars, check conversation history, check CLI flags, give up and ask someone. This is particularly painful for:

• Model ID resolution: --model=opus resolves through the alias system to anthropic/claude-sonnet-4-5. Users can't see this transformation.
• Inheritance: A workspace config sets inherit = false, silently ignoring the global config. Users don't know why their global settings aren't applying.
• Conversation deltas: A previous jp config set or editor-provided config change is persisted in the conversation stream and silently overrides file config. Users don't know the conversation is carrying state.
• Conflicting layers: An env var sets one value, a --cfg flag sets another. Which wins?

--explain makes the resolution chain visible, turning "why is my config wrong" from a debugging session into a single command.

Design

User experience

Broad mode (no field specified)

$ jp query --model=opus --no-reasoning --explain

Prints all fields that differ from their defaults, grouped by the layer that set them:

Config resolution for `jp query --model=opus --no-reasoning`:

  User global config (~/.config/jp/config.toml):
    assistant.name = "JP"
    style.reasoning.display = "full"
    providers.llm.aliases.opus = "anthropic/claude-opus-4-6"

  Workspace config (/home/user/project/.jp/config.toml):
    conversation.tools.*.run = "ask"
    assistant.instructions.0 = { title = "Rust", ... }

  Environment:
    (none)

  Conversation delta (conversation jp-c1234):
    assistant.model.id = "anthropic/claude-haiku-4-5"

  CLI --cfg:
    (none)

  CLI flags:
    assistant.model.id = "anthropic/claude-opus-4-6"  (--model=opus, resolved alias)
    assistant.model.parameters.reasoning = "off"  (--no-reasoning)

  Final resolved config:
    assistant.model.id = "anthropic/claude-opus-4-6"
    assistant.model.parameters.reasoning = "off"
    assistant.name = "JP"
    conversation.tools.*.run = "ask"
    providers.llm.aliases.opus = "anthropic/claude-opus-4-6"
    style.reasoning.display = "full"
    ... (2 more fields)

The "Final resolved config" section shows the merged result — what the command would actually run with. Only non-default fields are shown.

Focused mode (specific field)

$ jp query --model=opus --explain=assistant.model.id

Traces a single field through every layer:

assistant.model.id = "anthropic/claude-opus-4-6"

  Resolution chain:
    1. User global     (~/.config/jp/config.toml)    (not set)
    2. Workspace       (.jp/config.toml)             (not set)
    3. CWD             (not found)
    4. User workspace  (not found)
    5. Environment     JP_CFG_ASSISTANT_MODEL_ID      (not set)
    6. Conversation    (conversation zy2a)            "anthropic/claude-haiku-4-5"
    7. CLI --cfg                                      (not set)
    8. CLI flag        --model=opus                   "opus" → alias → "anthropic/claude-opus-4-6"

  Documentation:
    The model to use for the assistant.
    Format: provider/model-name or an alias defined in providers.llm.aliases.

The focused mode shows every layer, including ones that didn't set the field ("not set") and config files that don't exist ("not found"). This makes the full resolution chain visible — users can see exactly where their value came from and what it overrode.

The "Documentation" section is pulled from the schema description (the /// doc comment on the corresponding AppConfig field using schematics schema introspection).

Dry-run semantics

--explain suppresses command execution. The command is parsed, config is fully resolved, and the provenance report is printed to stdout. The command itself (query, conversation ls, etc.) does not run.

This follows the precedent set by terraform plan, docker compose config, and systemd-analyze. The user re-runs without --explain to execute.

JSON output

--explain respects the --format flag:

$ jp query --model=opus --explain --format=json

{
  "field": null,
  "layers": [
    {
      "type": "file",
      "name": "user_global",
      "path": "~/.config/jp/config.toml",
      "config": {
        "assistant": { "name": "JP" },
        "style": { "reasoning": { "display": "full" } }
      }
    },
    ...
  ],
  "resolved": {
    "assistant": { "model": { "id": "anthropic/claude-opus-4-6" } },
    ...
  }
}

This enables scripting: jp query --explain --format=json | jq '.resolved.assistant.model.id'.

Architecture

Snapshot-and-diff provenance

Rather than threading provenance metadata through the config merge pipeline, we reconstruct it on demand. When --explain is active, load_partial_config() takes a JSON snapshot of the PartialAppConfig after each layer, alongside metadata about that layer (name, file path).

struct ConfigSnapshot {
    /// Human-readable layer name.
    name: &'static str,

    /// File path, if the layer comes from a file.
    path: Option<Utf8PathBuf>,

    /// The cumulative PartialAppConfig state after this layer was merged.
    state: PartialAppConfig,
}

After all layers are processed, the provenance for any field is determined by diffing adjacent snapshots using PartialAppConfig::delta() — the same method already used for conversation config deltas and config persistence:

fn layer_contributions(
    prev: &ConfigSnapshot,
    next: &ConfigSnapshot
) -> PartialAppConfig {
    prev.state.delta(next.state.clone())
}

A non-empty field in the delta means that layer changed the value. delta() already handles nested structs, IndexMap-based tool configs, and all the custom merge strategies in the config system.

Layer definitions

The layers correspond to the steps in load_partial_config():

Layer	Name	Source
1	user_global	$XDG_CONFIG_HOME/jp/config.toml +
		extends
2	workspace	.jp/config.toml + extends
3	cwd	.jp.toml (recursive upwards)
4	user_workspace	$XDG_DATA_HOME/jp/<id>/config.toml +
		extends
5	environment	JP_CFG_* variables
6	conversation	Active conversation's config delta
7	cli_cfg	--cfg KEY=VALUE arguments
8	cli_flags	Command-specific flags (--model, etc.)
9	defaults	PartialAppConfig::default_values()

Extends sub-layers

Config files can use extends to include other files. For example, a workspace config might have:

extends = ["config.d/**/*"]

This causes .jp/config.d/tools.toml, .jp/config.d/style.toml, etc. to be merged into the workspace layer. --explain decomposes these into sub-layers so the user can see exactly which file set a value:

  Workspace config (.jp/config.toml):
    assistant.model.id = "anthropic/claude-sonnet-4-5"

    extends: .jp/config.d/tools.toml:
      conversation.tools.*.run = "ask"

    extends: .jp/config.d/style.toml:
      style.reasoning.display = "full"

The implementation uses load_config_file_with_extends() which already processes extends files individually. Each call to loader.file() for an extended file gets its own snapshot. The snapshot metadata records both the parent file and the extends path:

struct ConfigSnapshot {
    name: &'static str,
    path: Option<Utf8PathBuf>,
    /// If this is an extends sub-layer, the parent file that declared it.
    extends_parent: Option<Utf8PathBuf>,
    state: PartialAppConfig,
}

The output groups sub-layers under their parent, indented to show the relationship. In focused mode, the trace shows the specific extends file:

    2a. Workspace       (.jp/config.toml)              (not set)
    2b. Workspace ext   (.jp/config.d/tools.toml)      "ask"
    2c. Workspace ext   (.jp/config.d/style.toml)      (not set)

The before and after ordering of extends paths (controlled by ExtendingRelativePath::is_before) is preserved in the snapshot sequence — before extensions appear as sub-layers before the parent file, after extensions appear after it.

Layers 1-4 are merged by load_partials_with_inheritance(). When inherit = false is set, earlier layers are skipped — the snapshot still records them, but marks them as skipped: true so the output can show the user that inheritance was disabled.

Layer 5 (environment variables) has higher precedence than file layers (1-4), per the documented ordering in configuration.md.

Layer 9 (defaults) is applied by build() via default_values().merge(partial). This layer fills in any fields not set by the earlier layers.

CLI flag reverse-mapping

Layer 8 (CLI flags) needs special handling. The apply_cli_config() method on each command converts typed CLI flags (like --model=opus) into PartialAppConfig mutations. But the snapshot only sees the resulting partial — it doesn't know that assistant.model.id = "anthropic/claude-sonnet-4-5" came from --model=opus with alias resolution.

The approach is to record provenance at the point where the mapping happens — inside apply_cli_config() itself. The code there already knows both sides: it reads the CLI flag value and writes the config field. We add an optional recorder parameter that captures this relationship:

trait IntoPartialAppConfig {
    fn apply_cli_config(
        &self,
        workspace: Option<&Workspace>,
        partial: PartialAppConfig,
        merged_config: Option<&PartialAppConfig>,
        recorder: Option<&mut CliRecorder>,  // new
    ) -> Result<PartialAppConfig, BoxedError>;
}

struct CliRecord {
    /// The config field path. Compile-time constant.
    field: &'static str,
    /// The clap argument ID that set it (e.g. `model`, `no_reasoning`).
    /// Compile-time constant.
    arg_id: &'static str,
    /// Optional note (e.g., "resolved via alias").
    note: Option<String>,
}

struct CliRecorder(Vec<CliRecord>);

CliRecord stores only the config field path, the clap argument ID, and an optional transformation note. It does not store the raw value or the display flag name (--model, -m). Both can be looked up at render time:

• Raw value: from ArgMatches::get_raw(arg_id), which is available in run_inner() via Clap's Cli::command().get_matches().
• Display flag name: from the Command definition via Command::find_subcommand() and Arg::get_long() / Arg::get_short().

This avoids cloning raw values in every apply_* helper and keeps the recorder lightweight — just two &'static str pointers and an occasional note.

The helper functions that bridge CLI flags to config fields record their assignments:

fn apply_model(
    partial: &mut PartialAppConfig,
    model: Option<&str>,
    recorder: Option<&mut CliRecorder>,
) {
    let Some(id) = model else { return };
    partial.assistant.model.id = id.into();

    if let Some(rec) = recorder {
        rec.record("assistant.model.id", "model", None);
    }
}

The &'static str for field and arg_id means these are compile-time constants, not runtime strings constructed elsewhere. The mapping lives next to the code that performs the mapping — the only place that can keep it accurate.

During normal execution (no --explain), the recorder is None and no allocation occurs. When --explain is active, run_inner() passes Some(&mut recorder) and the records are used to annotate the CLI flags layer in the output.

The recorder does not replace the snapshot diff — it augments it. If a flag sets a field but forgets to record it, the diff still shows the field changed at the CLI layer. The output just lacks the friendly flag name. The failure mode is "degraded display" not "wrong data."

Drift prevention

A test validates that all recorded field paths are real AppConfig fields:

#[test]
fn cli_recorder_field_paths_are_valid() {
    let fields: HashSet<_> = AppConfig::fields().into_iter().collect();
    let mut recorder = CliRecorder::default();
    // Run apply_cli_config with a populated Query struct
    // ...
    for record in &recorder.0 {
        assert!(
            fields.contains(record.field),
            "CLI recorder references unknown field: {}",
            record.field,
        );
    }
}

This follows the same pattern as the existing test_ensure_no_missing_assignments test — it catches field path drift at test time.

Schema documentation

Focused mode shows documentation for the explained field. This comes from the schematic::Schema description, which is populated from the /// doc comments on AppConfig fields:

fn field_description(field_path: &str) -> Option<String> {
    use schematic::{SchemaBuilder, SchemaType, Schematic as _};

    let builder = SchemaBuilder::default();
    let mut stack = vec![(AppConfig::build_schema(builder), "")];

    // Walk the schema tree to find the field and return its description.
    // Same traversal pattern as AppConfig::fields().
}

This reuses the same schema walking logic already in AppConfig::fields().

Integration point

--explain is a global flag on Globals:

#[derive(Debug, Default, clap::Args)]
struct Globals {
    // ... existing fields ...

    /// Explain config resolution and exit without running the command.
    ///
    /// Without a value, shows all non-default fields grouped by source.
    /// With a field path, traces that field through the resolution chain.
    #[arg(long = "explain", global = true, value_name = "FIELD")]
    explain: Option<Option<String>>,
}

Option<Option<String>>:

• None — flag not provided, normal execution.
• Some(None) — bare --explain, broad mode.
• Some(Some("assistant.model.id")) — focused mode for a specific field.

In run_inner(), after load_partial_config() completes (with snapshots collected), the explain check runs:

if let Some(field) = &cli.globals.explain {
    let report = build_explain_report(&snapshots, field.as_deref());
    printer.println(format_explain_report(&report, format));
    return Ok(());
}

This runs after config loading but before Ctx::new(), workspace stream initialization, and Commands::run(). The command is never executed.

Field validation

In focused mode, the provided field path is validated against AppConfig::fields(). If the field doesn't exist, an error is printed with suggestions (using the same fuzzy matching that missing_key() in assignment.rs uses):

Error: Unknown config field 'assistant.model'

Did you mean one of:
  assistant.model.id
  assistant.model.parameters.max_tokens
  assistant.model.parameters.reasoning
  assistant.model.parameters.temperature

Drawbacks

• Snapshot cloning cost: Cloning PartialAppConfig at each of the ~9 layers adds a small overhead to every --explain invocation. In practice this is sub-millisecond and only runs when --explain is present.

• CLI recorder is opt-in per call site: Each apply_* helper that bridges a CLI flag to a config field needs a recorder.record(...) call. Forgetting to add one when a new flag is introduced means the diff still shows the field changed, but without the flag annotation. A test validates that recorded arg_id values match real clap argument IDs and that field paths are valid, but cannot detect missing recordings — that requires code review discipline.

• Inheritable config complexity: When inherit = false is set, the explain output needs to show which layers were skipped and why. This adds conditional logic to the output formatting.

Alternatives

Eager provenance tracking

Thread a ProvenanceMap<String, (Value, LayerName)> through the entire merge pipeline. Every load_partial() and assign() call records where each field was set.

Rejected because: it adds complexity to every config load (even when --explain is not used), requires modifying the PartialAppConfig merge infrastructure, and the snapshot approach achieves the same result with less invasive changes.

jp config explain subcommand

A dedicated subcommand under jp config instead of a global flag.

Rejected because: it can't show CLI flag resolution. jp config explain assistant.model.id doesn't know that --model=opus would resolve to anthropic/claude-sonnet-4-5, because no command context is provided. The global flag captures the full resolution chain including command-specific transformations.

Verbose logging (-vvv)

Users can already get config resolution details via jp query -vvv, which outputs trace logs that include the merged config JSON. This shows the final state but not the per-layer provenance.

Not a replacement: trace logs are noisy, unstructured, and not designed for end-user consumption. --explain provides a curated, human-readable view of exactly what the user needs.

Non-Goals

• Config editing: --explain is read-only. It doesn't modify config files or suggest corrections. Config editing is the domain of jp config set and a future interactive --cfg feature.

• Runtime config changes: --explain shows the static config resolution at startup. It doesn't show config changes that happen during a turn (e.g., editor-provided config in the query editor).

• Dependency tracking: --explain doesn't show why a config value matters (e.g., "reasoning is off because your model doesn't support it"). It shows where the value came from, not what it does.

Risks and Open Questions

• Tool config fields: The conversation.tools.* fields use dynamic keys (tool names). The snapshot diff will show these correctly, but the field validation in focused mode needs to handle wildcard patterns (conversation.tools.fs_read_file.run) that aren't in the static AppConfig::fields() list.

• Conversation delta readability: The conversation delta is a PartialAppConfig stored in the event stream. For --explain, we show its fields as flat key-value pairs. But the delta might contain complex nested structures (tool configs, system prompt sections). The formatting needs to handle these gracefully — likely by truncating long values and showing a use --explain=<field> for details hint.

Implementation Plan

Phase 1: Snapshot infrastructure

1. Define ConfigSnapshot and ExplainReport types in a new jp_cli::explain module.
2. Add snapshot collection points to load_partial_config(), gated behind a boolean flag (only collect when --explain is active). This includes sub-layer snapshots for each extends file.
3. Refactor load_config_file_with_extends() to accept an optional snapshot collector, taking a snapshot after each extends file is loaded.
4. Implement the diff logic: given a sequence of snapshots (including sub-layers), determine which fields changed at each layer.
5. Unit test the diff logic with known partial configs, including extends scenarios.

Phase 2: Output formatting

1. Implement broad mode: group changed fields by layer, print to stdout.
2. Implement focused mode: trace a single field through all layers.
3. Add schema documentation lookup for focused mode.
4. Add JSON output format.
5. Add field validation with suggestions for unknown field paths.

Phase 3: CLI integration

1. Add --explain to Globals.
2. Wire the explain check into run_inner(), after config loading.
3. Add recorder: Option<&mut CliRecorder> parameter to IntoPartialAppConfig::apply_cli_config and thread it through the apply_* helper functions in Query and other command structs.
4. Add cli_recorder_field_paths_are_valid test.
5. Test with representative scenarios: alias resolution, inheritance cutoff, conversation deltas, env var overrides.

Phases 1 and 2 can be developed and tested independently. Phase 3 integrates them into the CLI and can be merged as a single PR.

References

• RFD 059: Shell completions and man pages
• Future RFD: Interactive config (bare --cfg flag)
• load_partial_config() in jp_cli/src/lib.rs — the 9-layer merge pipeline
• load_config_file_with_extends() in jp_config/src/util.rs — extends resolution
• AppConfig::fields() in jp_config/src/lib.rs — schema-driven field enumeration
• PartialAppConfig::delta() in jp_config/src/delta.rs — field-level diffing
• terraform plan — precedent for dry-run config explanation
• git config --show-origin --list — precedent for config provenance display