diff --git a/cstat-agent-use.md b/cstat-agent-use.md
index f753593..3880374 100644
--- a/cstat-agent-use.md
+++ b/cstat-agent-use.md
@@ -35,7 +35,11 @@ remain available under `cstat advanced ...`.
 
 Each accepts `--json` for structured output.
 
-- `cstat loc --json --path .` — line counts and directory breakdown.
+- `cstat loc --explain --json` — machine-readable `loc` contract: modes,
+  `code_lines` rules, project JSON fields, and selected-file JSON fields.
+- `cstat loc --json --path .` — project size-shape data.
+- `cstat loc --json --path src/lib.rs` — selected-file projected static line
+  reachability.
 - `cstat symbols --json --path .` — symbol totals by kind and per file.
 - `cstat deps --json --path .` — dependency edges, coupling, fan-in/fan-out,
   and cohesion.
@@ -82,5 +86,8 @@ Use these only when the focused report points to a question they answer:
   test/benchmark roots can reach a function or edge. They are not runtime hit-count profiling.
 - Dependency edges come from source-level `use`/`mod` relationships. Generated
   code and macro expansion can hide edges.
+- For exact `loc` `code_lines` rules and JSON fields, run
+  `cstat loc --explain` or `cstat loc --explain --json`; that command is the
+  canonical contract.
 - Prefer targeted reductions: remove dead code, split large files, move symbols
   across modules, then reduce per-function complexity.
diff --git a/src/guide.rs b/src/guide.rs
index 91cbdc9..b6fc0a8 100644
--- a/src/guide.rs
+++ b/src/guide.rs
@@ -64,30 +64,31 @@ when learning what the metrics mean.",
 fn topic_size() -> TopicContent {
     TopicContent {
         description: "\
-Size metrics measure the volume of code at file and function granularity. \
-Large files and functions are not inherently problematic, but they correlate \
-with higher defect density, harder navigation, and merge conflicts. The \
-distribution shape matters more than any single value.",
+Size metrics measure the volume and distribution of Rust source. `cstat loc` owns the line-count contract; run `cstat loc --explain` for modes, code_lines rules, and JSON fields.",
         metrics: &[
-            ("file LoC", "Total lines of code per file. High values may indicate a module doing too much."),
-            ("function LoC", "Lines per function body. Long functions tend to have higher cognitive load."),
-            ("mean / median", "Central tendency of the distribution. A large gap between mean and median indicates skew from outliers."),
-            ("std_dev", "Spread of the distribution. High values mean uneven file sizes."),
-            ("max", "The largest single file or function. Often the first place to investigate."),
+            (
+                "file size shape",
+                "Project-level file-size distribution from `cstat loc --path .`.",
+            ),
+            (
+                "selected-file line reachability",
+                "Projected static reachable/unreachable production spans from `cstat loc --path <file.rs>`.",
+            ),
+            (
+                "distribution statistics",
+                "Mean, median, std_dev, min, and max summarize project file sizes.",
+            ),
         ],
         commands: &[
-            "cstat loc               — per-file LoC with bar charts and directory breakdown",
-            "cstat loc --json        — structured file-level size data",
+            "cstat loc --explain       — loc modes, code_lines rules, and JSON field contract",
+            "cstat loc --path .        — project size-shape report",
+            "cstat loc --path src/lib.rs — selected-file projected line reachability",
             "cstat dist --metric loc — histogram and outlier analysis of LoC distribution",
         ],
         patterns: &[
-            "A right-skewed LoC distribution (long tail) usually means a few files have \
-grown disproportionately. Check whether those files contain multiple concerns.",
-            "Files above 500 lines often contain function clusters that could be separate \
-modules. Cross-reference with `cstat deps` to see if the file's functions form \
-distinct groups with few cross-calls.",
-            "Uniform file sizes are not a goal — some modules are naturally larger. The \
-signal is when a file is large AND has low cohesion or high internal complexity.",
+            "Use `cstat loc --explain` as the canonical reference before consuming loc output.",
+            "A right-skewed project-mode code_lines distribution means a few files dominate size.",
+            "Selected-file loc is a static projection from function spans and test/benchmark reachability; do not read it as runtime coverage.",
         ],
     }
 }
diff --git a/src/loc.rs b/src/loc.rs
index 2d37ab2..cb28c55 100644
--- a/src/loc.rs
+++ b/src/loc.rs
@@ -377,11 +377,13 @@ pub fn render_loc(files: &[PathBuf], project_path: &Path, top_n: Option<usize>,
 
     if verbose {
         render::verbose_block(&[
-            "Bar chart: files ranked by code lines (excluding blank lines and comments).",
-            "Bar length is proportional to LoC relative to the largest file.",
+            "Bar chart: files ranked by code_lines.",
+            "code_lines excludes blank lines, // comment-only lines, and block-comment-only regions.",
+            "Line classification is static text scanning, not semantic Rust parsing.",
+            "Bar length is proportional to code_lines relative to the largest file.",
             "Color gradient: red = top of the ranking (most lines), green = bottom.",
+            "Full loc reference: cstat loc --explain.",
         ]);
-        render::guide_ref("size");
     }
 
     println!(
@@ -477,6 +479,71 @@ pub fn render_loc_file_json(rs_files: &[PathBuf], project_path: &Path, file: &Pa
     }
 }
 
+pub fn render_loc_explain(json: bool) {
+    if json {
+        render_loc_explain_json();
+    } else {
+        render_loc_explain_human();
+    }
+}
+
+fn render_loc_explain_human() {
+    render::section_header("loc reference");
+    println!("  Project mode: cstat loc --path .");
+    println!("  Project JSON: cstat loc --path . --json");
+    println!("  Selected-file mode: cstat loc --path src/lib.rs");
+    println!("  Selected-file JSON: cstat loc --path src/lib.rs --json");
+    println!("  Machine contract: cstat loc --explain --json");
+    println!();
+    println!("  code_lines: static textual classification");
+    println!("    excludes blank lines, // comment-only lines, and block-comment-only regions");
+    println!("    not semantic Rust parsing");
+    println!();
+    println!("  Project JSON fields: cstat_version, files, aggregate, directory_breakdown");
+    println!("  Selected-file JSON fields: file, total_lines, code_lines, projected_reachable_lines, projected_unreachable_lines, reachable_spans, unreachable_spans");
+}
+
+fn render_loc_explain_json() {
+    let output = serde_json::json!({
+        "cstat_version": env!("CARGO_PKG_VERSION"),
+        "probe": "loc",
+        "commands": {
+            "project_human": "cstat loc --path .",
+            "project_json": "cstat loc --path . --json",
+            "selected_file_human": "cstat loc --path src/lib.rs",
+            "selected_file_json": "cstat loc --path src/lib.rs --json",
+            "explain_human": "cstat loc --explain",
+            "explain_json": "cstat loc --explain --json",
+        },
+        "code_lines": {
+            "kind": "static textual classification",
+            "excludes": [
+                "blank lines",
+                "// comment-only lines",
+                "block-comment-only regions via simple /* ... */ state tracking",
+            ],
+            "not": "semantic Rust parsing",
+        },
+        "project_json_fields": {
+            "cstat_version": "cstat version string",
+            "files": ["path", "total_lines", "code_lines"],
+            "aggregate": ["total_files", "total_loc", "mean", "std_dev", "median", "min", "max"],
+            "directory_breakdown": ["directory", "code_lines"],
+        },
+        "selected_file_json_fields": {
+            "file": "selected file path relative to the crate root",
+            "total_lines": "physical lines in the selected file",
+            "code_lines": "production function-span code lines after exclusions",
+            "projected_reachable_lines": "production code lines in statically reachable function spans",
+            "projected_unreachable_lines": "production code lines not in statically reachable function spans",
+            "reachable_spans": ["function", "line_start", "line_end"],
+            "unreachable_spans": ["function", "line_start", "line_end"],
+        },
+    });
+
+    println!("{}", serde_json::to_string(&output).unwrap());
+}
+
 pub fn analyze_file_projected_line_reachability_from_files(
     rs_files: &[PathBuf],
     project_path: &Path,
@@ -511,8 +578,10 @@ pub fn render_file_projected_line_reachability_report(
     if verbose {
         render::verbose_block(&[
             "Physical lines count every source line in the selected file.",
-            "Code lines exclude blank/comment-only lines and test/support/wrapper spans for selected-file production accounting.",
+            "code_lines is production function-span code after excluding tests, benches, test-support helpers, and selected binary wrapper main().",
+            "Line classification excludes blank lines, // comment-only lines, and block-comment-only regions with simple text scanning.",
             "Projected reachable lines are production code lines inside functions statically reached from project tests/benches.",
+            "Full loc reference: cstat loc --explain.",
         ]);
     }
 
diff --git a/src/main.rs b/src/main.rs
index 63b6733..822ab98 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -33,7 +33,7 @@ struct Cli {
     #[command(subcommand)]
     command: Option<Commands>,
 
-    /// Path to the Rust project directory (defaults to current directory)
+    /// Rust project directory or Rust source file to analyze (defaults to current directory)
     #[arg(long, default_value = ".", global = true)]
     path: PathBuf,
 
@@ -56,11 +56,40 @@ enum Commands {
     Summary,
     /// Alias for the focused module metrics report
     Report,
-    /// Lines-of-code analysis with bar charts
+    #[command(
+        about = "Lines-of-code size-shape analysis",
+        long_about = r#"Lines-of-code size-shape analysis for Rust source.
+
+Project mode:
+  cstat loc --path .
+  cstat loc --path . --json
+
+Selected-file mode:
+  cstat loc --path src/lib.rs
+  cstat loc --path src/lib.rs --json
+
+Explain mode:
+  cstat loc --explain
+  cstat loc --explain --json
+
+Project mode reports total physical lines, code_lines, aggregate stats, per-file ranking, and directory breakdown for discovered Rust files. Selected-file mode reports projected static reachable/unreachable production line spans for one Rust source file.
+
+code_lines is a static textual classification: blank lines are excluded, // comment-only lines are excluded, and block-comment-only regions are excluded using simple /* ... */ state tracking. It is not semantic Rust parsing.
+
+Project JSON fields: cstat_version, files[{path,total_lines,code_lines}], aggregate{total_files,total_loc,mean,std_dev,median,min,max}, directory_breakdown[{directory,code_lines}].
+Selected-file JSON fields: file, total_lines, code_lines, projected_reachable_lines, projected_unreachable_lines, reachable_spans[{function,line_start,line_end}], unreachable_spans[{function,line_start,line_end}].
+
+Use --explain to print this usage and JSON field contract without running analysis. --path, --top, and -v are ignored by --explain.
+
+--top only limits the project-mode per-file ranking (ignored for selected-file and --explain modes)."#
+    )]
     Loc {
-        /// Show only the top N files
+        /// Show only the top N files in project mode (ignored for selected-file and --explain modes)
         #[arg(long)]
         top: Option<usize>,
+        /// Print the loc usage and JSON field contract without running analysis
+        #[arg(long)]
+        explain: bool,
     },
     /// Symbol counts by kind and file
     Symbols,
@@ -162,6 +191,12 @@ fn main() {
     }
 
     let verbose = cli.verbose;
+    let command = cli.command.unwrap_or(Commands::Summary);
+
+    if let Commands::Loc { explain: true, .. } = &command {
+        loc::render_loc_explain(json);
+        return;
+    }
     let target = match discovery::resolve_target(&cli.path) {
         Ok(target) => target,
         Err(e) => {
@@ -177,8 +212,6 @@ fn main() {
     let project_path = &target.project_path;
     let rs_files = discovery::files_for_project(&target);
 
-    let command = cli.command.unwrap_or(Commands::Summary);
-
     if let Some(file) = discovery::selected_file(&target) {
         let project_rs_files = discovery::files_for_project(&target);
         let target_rs_files = discovery::files_for_target(&target);
@@ -307,7 +340,7 @@ fn main() {
                 report::render_report(&rs_files, &project_path, verbose);
             }
         }
-        Commands::Loc { top } => {
+        Commands::Loc { top, .. } => {
             if json {
                 loc::render_loc_json(&rs_files, &project_path);
             } else {
diff --git a/tests/loc_cli.rs b/tests/loc_cli.rs
new file mode 100644
index 0000000..b20b1df
--- /dev/null
+++ b/tests/loc_cli.rs
@@ -0,0 +1,308 @@
+use serde_json::Value;
+use std::fs;
+use std::path::{Path, PathBuf};
+use std::process::{Command, Output};
+use std::time::{SystemTime, UNIX_EPOCH};
+
+fn temp_project(name: &str) -> PathBuf {
+    let unique = SystemTime::now()
+        .duration_since(UNIX_EPOCH)
+        .unwrap()
+        .as_nanos();
+    let root = std::env::temp_dir().join(format!("cstat-loc-cli-{name}-{unique}"));
+    fs::create_dir_all(root.join("src")).unwrap();
+    fs::write(
+        root.join("Cargo.toml"),
+        "[package]\nname = \"fixture\"\nversion = \"0.1.0\"\nedition = \"2021\"\n",
+    )
+    .unwrap();
+    fs::write(
+        root.join("src/lib.rs"),
+        r#"
+pub fn live() {
+    helper();
+}
+
+/*
+block comment only
+*/
+fn helper() {}
+
+/* single-line block comment only */
+fn orphan() {
+    // comment-only
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn covers_live() {
+        live();
+    }
+}
+"#,
+    )
+    .unwrap();
+    root
+}
+
+fn run_cstat_raw(args: &[&str]) -> Output {
+    let bin = env!("CARGO_BIN_EXE_cstat");
+    let mut command = Command::new(bin);
+    command.args(args);
+    command.output().expect("invoke cstat binary")
+}
+
+fn run_cstat(path: &Path, args: &[&str]) -> Output {
+    let bin = env!("CARGO_BIN_EXE_cstat");
+    let mut command = Command::new(bin);
+    command.args(["--no-color", "--path"]);
+    command.arg(path);
+    command.args(args);
+    command.output().expect("invoke cstat binary")
+}
+
+fn assert_success(output: &Output) {
+    assert!(
+        output.status.success(),
+        "cstat failed: status={:?}\nstderr={}\nstdout={}",
+        output.status,
+        String::from_utf8_lossy(&output.stderr),
+        String::from_utf8_lossy(&output.stdout),
+    );
+}
+
+fn span_array_contains_function(array: &Value, expected: &str) -> bool {
+    array
+        .as_array()
+        .expect("span array")
+        .iter()
+        .any(|item| item.get("function").and_then(Value::as_str) == Some(expected))
+}
+
+#[test]
+fn loc_help_exposes_project_and_selected_file_modes() {
+    let output = run_cstat_raw(&["loc", "--help"]);
+    assert_success(&output);
+
+    let stdout = String::from_utf8_lossy(&output.stdout);
+    for expected in [
+        "Rust project directory or Rust source file",
+        "Project mode:",
+        "Selected-file mode:",
+        "cstat loc --path .",
+        "cstat loc --path src/lib.rs",
+        "cstat loc --explain",
+        "cstat loc --explain --json",
+        "code_lines is a static textual classification",
+        "blank lines are excluded",
+        "// comment-only lines are excluded",
+        "block-comment-only regions are excluded",
+        "Project JSON fields:",
+        "Selected-file JSON fields:",
+        "--top only limits the project-mode",
+        "ignored for selected-file and --explain modes",
+    ] {
+        assert!(stdout.contains(expected), "missing {expected}: {stdout}");
+    }
+}
+
+#[test]
+fn loc_explain_human_is_canonical_reference() {
+    let output = run_cstat_raw(&[
+        "--path",
+        "/definitely/missing/cstat/path",
+        "loc",
+        "--explain",
+    ]);
+    assert_success(&output);
+
+    let stdout = String::from_utf8_lossy(&output.stdout);
+    for expected in [
+        "loc reference",
+        "Project mode: cstat loc --path .",
+        "Machine contract: cstat loc --explain --json",
+        "code_lines: static textual classification",
+        "not semantic Rust parsing",
+        "Project JSON fields:",
+        "Selected-file JSON fields:",
+    ] {
+        assert!(stdout.contains(expected), "missing {expected}: {stdout}");
+    }
+
+    let stderr = String::from_utf8_lossy(&output.stderr);
+    assert!(
+        !stderr.contains("Error discovering files"),
+        "unexpected discovery error: {stderr}",
+    );
+}
+
+#[test]
+fn loc_explain_json_exposes_machine_contract() {
+    let output = run_cstat_raw(&["--json", "loc", "--explain"]);
+    assert_success(&output);
+
+    let stdout = String::from_utf8(output.stdout).unwrap();
+    let value: Value = serde_json::from_str(&stdout).expect("parse loc explain JSON");
+    assert_eq!(value["probe"], "loc");
+    assert_eq!(
+        value["commands"]["explain_json"],
+        "cstat loc --explain --json",
+    );
+    assert_eq!(value["code_lines"]["kind"], "static textual classification",);
+    assert!(
+        value["project_json_fields"]["files"]
+            .as_array()
+            .expect("project fields")
+            .iter()
+            .any(|field| field.as_str() == Some("code_lines")),
+        "json={stdout}",
+    );
+    assert!(
+        value["selected_file_json_fields"]["reachable_spans"]
+            .as_array()
+            .expect("selected fields")
+            .iter()
+            .any(|field| field.as_str() == Some("function")),
+        "json={stdout}",
+    );
+}
+
+#[test]
+fn loc_verbose_points_to_loc_explain_not_guide() {
+    let root = temp_project("verbose-loc-reference");
+    let output = run_cstat(&root, &["-v", "loc"]);
+    assert_success(&output);
+
+    let stdout = String::from_utf8_lossy(&output.stdout);
+    assert!(
+        stdout.contains("Full loc reference: cstat loc --explain"),
+        "stdout={stdout}",
+    );
+    assert!(
+        !stdout.contains("cstat guide size"),
+        "stdout unexpectedly referenced guide: {stdout}",
+    );
+
+    fs::remove_dir_all(root).unwrap();
+}
+
+#[test]
+fn loc_project_json_exposes_shape_contract() {
+    let root = temp_project("project-json");
+    let output = run_cstat(&root, &["--json", "loc"]);
+    assert_success(&output);
+
+    let stdout = String::from_utf8(output.stdout).unwrap();
+    let value: Value = serde_json::from_str(&stdout).expect("parse loc JSON");
+    assert_eq!(value["cstat_version"], env!("CARGO_PKG_VERSION"));
+
+    let files = value["files"].as_array().expect("files array");
+    assert!(!files.is_empty(), "json={stdout}");
+    let lib = files
+        .iter()
+        .find(|file| file["path"] == "src/lib.rs")
+        .expect("src/lib.rs row");
+    let total_lines = lib["total_lines"].as_u64().expect("total_lines");
+    let code_lines = lib["code_lines"].as_u64().expect("code_lines");
+    assert!(
+        total_lines >= code_lines && code_lines > 0,
+        "lib row={lib:?}",
+    );
+
+    let aggregate = &value["aggregate"];
+    for field in [
+        "total_files",
+        "total_loc",
+        "mean",
+        "std_dev",
+        "median",
+        "min",
+        "max",
+    ] {
+        assert!(aggregate.get(field).is_some(), "missing {field}: {stdout}");
+    }
+
+    let directories = value["directory_breakdown"]
+        .as_array()
+        .expect("directory_breakdown array");
+    assert!(
+        directories.iter().any(|directory| {
+            directory["directory"] == "src"
+                && directory["code_lines"]
+                    .as_u64()
+                    .is_some_and(|code_lines| code_lines > 0)
+        }),
+        "directory_breakdown={directories:?}",
+    );
+
+    fs::remove_dir_all(root).unwrap();
+}
+
+#[test]
+fn loc_selected_file_human_is_usable() {
+    let root = temp_project("selected-human");
+    let file = root.join("src/lib.rs");
+    let output = run_cstat(&file, &["loc"]);
+    assert_success(&output);
+
+    let stdout = String::from_utf8_lossy(&output.stdout);
+    for expected in [
+        "Projected line reachability",
+        "file src/lib.rs",
+        "physical lines",
+        "production code lines",
+        "statically reachable lines",
+        "not statically reachable lines",
+        "reachable function spans:",
+        "unreachable function spans:",
+        "live",
+        "orphan",
+    ] {
+        assert!(stdout.contains(expected), "missing {expected}: {stdout}");
+    }
+
+    fs::remove_dir_all(root).unwrap();
+}
+
+#[test]
+fn loc_selected_file_json_exposes_projected_reachability_contract() {
+    let root = temp_project("selected-json");
+    let file = root.join("src/lib.rs");
+    let output = run_cstat(&file, &["--json", "loc"]);
+    assert_success(&output);
+
+    let stdout = String::from_utf8(output.stdout).unwrap();
+    let value: Value = serde_json::from_str(&stdout).expect("parse selected-file loc JSON");
+    assert_eq!(value["file"], "src/lib.rs");
+    let total_lines = value["total_lines"].as_u64().expect("total_lines");
+    let code_lines = value["code_lines"].as_u64().expect("code_lines");
+    let projected_reachable_lines = value["projected_reachable_lines"]
+        .as_u64()
+        .expect("projected_reachable_lines");
+    let projected_unreachable_lines = value["projected_unreachable_lines"]
+        .as_u64()
+        .expect("projected_unreachable_lines");
+    assert!(total_lines >= code_lines, "json={stdout}");
+    assert_eq!(
+        projected_reachable_lines + projected_unreachable_lines,
+        code_lines,
+        "json={stdout}",
+    );
+    assert!(
+        span_array_contains_function(&value["reachable_spans"], "live"),
+        "json={stdout}",
+    );
+    assert!(
+        span_array_contains_function(&value["reachable_spans"], "helper"),
+        "json={stdout}",
+    );
+    assert!(
+        span_array_contains_function(&value["unreachable_spans"], "orphan"),
+        "json={stdout}",
+    );
+
+    fs::remove_dir_all(root).unwrap();
+}