Aura/docs/plans/0086-cost-flag-cli-ergonomics.md

# Cost-flag CLI ergonomics — Implementation Plan

> **Parent spec:** `docs/specs/0086-cost-flag-cli-ergonomics.md`
>
> **For agentic workers:** REQUIRED SUB-SKILL: use the `implement` skill to run
> this plan. Steps use `- [ ]` checkboxes for tracking.

**Goal:** Make the `aura run` cost-flag surface honest — units discoverable, a
named non-negativity diagnostic, and a refuse-don't-guess rejection of cost flags
on a non-R harness — with one C10 ledger note, changing no cost-model/R math.

**Architecture:** All code lands in the pure `parse_run_args` grammar (and two
print sites) in `crates/aura-cli/src/main.rs`; every new rejection is a pure
`Result::Err` printed by `main`'s existing `eprintln!("aura: {msg}")` + `exit(2)`
path (main.rs:3273-3279). One realization note is added to contract C10 in the
ledger. The accepted stage1-r cost path and its eight `net_expectancy_r`/bleed
goldens stay byte-identical.

**Tech Stack:** Rust; `crates/aura-cli/src/main.rs` (the `aura` binary + its
`mod tests`), `crates/aura-cli/tests/cli_run.rs` (integration), `docs/design/INDEX.md`.

---

**Files this plan creates or modifies:**

- Modify: `crates/aura-cli/src/main.rs` — new private `parse_nonneg_rate` fn + new
  `const COST_FLAGS_NOTE`; rewire the three cost-flag arms (:3185-3211); insert the
  non-R-harness guard (between :3215 and :3224); append the note to the `usage`
  closure (:3137-3140) and the `--help` print (:3265).
- Test: `crates/aura-cli/src/main.rs` `mod tests` (:3338) — pure `parse_run_args`
  units (negative-rate, non-R-harness, accepted-path, malformed-regression).
- Modify: `crates/aura-cli/tests/cli_run.rs` — update the perturbed negative-carry
  test (:1825-1841); add non-R-harness, negative-cost, and `--help`-note integration
  tests beside the cost block (:1631-1888) and the help block (:595-663).
- Modify: `docs/design/INDEX.md` — one C10 realization note at the cost-section tail
  (after `sweep-path cost. Decision log: #148.`, :831).

---

### Task 1: Named non-negativity diagnostic (spec item 2)

**Files:**
- Modify: `crates/aura-cli/src/main.rs` (new `parse_nonneg_rate`; arms :3185-3211)
- Test: `crates/aura-cli/src/main.rs` `mod tests` (:3338)
- Test: `crates/aura-cli/tests/cli_run.rs` (update :1825-1841; add one negative-cost test)

- [ ] **Step 1: Write the failing unit tests**

Add to `mod tests` in `crates/aura-cli/src/main.rs`, beside the existing
`parse_run_args_*` units (near :4480):

```rust
#[test]
fn parse_run_args_negative_cost_rates_are_named_not_usage() {
    for flag in ["--cost-per-trade", "--slip-vol-mult", "--carry-per-cycle"] {
        let err = parse_run_args(&["--harness", "stage1-r", flag, "-0.5"])
            .expect_err("a negative cost rate must be refused");
        assert!(err.contains("must be non-negative"), "{flag}: {err}");
        assert!(err.contains(flag), "message must name the offending flag: {err}");
        assert!(err.contains("-0.5"), "message must carry the value: {err}");
    }
}

#[test]
fn parse_run_args_malformed_cost_rate_stays_grammar_usage() {
    let err = parse_run_args(&["--harness", "stage1-r", "--cost-per-trade", "x"])
        .expect_err("a malformed rate is a grammar error");
    assert!(err.contains("usage"), "a malformed value stays the usage path: {err}");
}
```

- [ ] **Step 2: Run the unit tests to verify they fail**

Run: `cargo test -p aura-cli parse_run_args_negative_cost_rates_are_named_not_usage`
Expected: FAIL — the current arms `return Err(usage())` on a negative value, so the
message is the usage string and `err.contains("must be non-negative")` is false.

- [ ] **Step 3: Add the `parse_nonneg_rate` helper**

Insert immediately above `fn parse_run_args` (currently `crates/aura-cli/src/main.rs:3136`):

```rust
/// Parse a cost-rate flag value and enforce non-negativity, naming the offending
/// flag so a sign typo is distinguishable from a mistyped flag name. A *malformed*
/// value stays a grammar usage error (unchanged); only a parsed-but-negative value
/// yields the named message. Pure — no I/O, no exit.
fn parse_nonneg_rate(flag: &str, value: &str, usage: &impl Fn() -> String) -> Result<f64, String> {
    let v: f64 = value.parse().map_err(|_| usage())?;
    if v < 0.0 {
        return Err(format!("{flag} must be non-negative, got {v}"));
    }
    Ok(v)
}
```

- [ ] **Step 4: Rewire the three cost-flag arms to call the helper**

In `parse_run_args`, replace the `--cost-per-trade` arm (`main.rs:3185-3193`):

```rust
            "--cost-per-trade" if cost.is_none() => {
                let (value, t) = t.split_first().ok_or_else(usage)?;
                cost = Some(parse_nonneg_rate("--cost-per-trade", value, &usage)?);
                tail = t;
            }
```

the `--slip-vol-mult` arm (`main.rs:3194-3202`):

```rust
            "--slip-vol-mult" if slip_vol_mult.is_none() => {
                let (value, t) = t.split_first().ok_or_else(usage)?;
                slip_vol_mult = Some(parse_nonneg_rate("--slip-vol-mult", value, &usage)?);
                tail = t;
            }
```

and the `--carry-per-cycle` arm (`main.rs:3203-3211`):

```rust
            "--carry-per-cycle" if carry_per_cycle.is_none() => {
                let (value, t) = t.split_first().ok_or_else(usage)?;
                carry_per_cycle = Some(parse_nonneg_rate("--carry-per-cycle", value, &usage)?);
                tail = t;
            }
```

- [ ] **Step 5: Run the unit tests to verify they pass**

Run: `cargo test -p aura-cli parse_run_args_negative_cost_rates_are_named_not_usage`
Expected: PASS.
Run: `cargo test -p aura-cli parse_run_args_malformed_cost_rate_stays_grammar_usage`
Expected: PASS (a malformed value still routes through `usage()`).

- [ ] **Step 6: Update the now-perturbed integration test**

The existing `stage1_r_negative_carry_per_cycle_refused_with_usage_exit_2`
(`crates/aura-cli/tests/cli_run.rs:1825-1841`) asserts `stderr.contains("usage")`,
which the new named message no longer satisfies. Update its assertion and name.

Replace the assertion line (`cli_run.rs:1838`):

```rust
    assert!(stderr.contains("must be non-negative"), "negative carry stderr must name the cause: {stderr:?}");
```

Rename the function (`cli_run.rs:1826`) and adjust the doc line that says
"usage on stderr":

```rust
/// Property (cycle 0085 + #153, cost-flag validation symmetry): a NEGATIVE
/// `--carry-per-cycle` is refused at the binary boundary — exit 2, a named
/// non-negativity diagnostic on stderr, nothing on stdout, no `runs/` write — the
/// same shape the sibling `--cost-per-trade`/`--slip-vol-mult` flags enforce. The
/// `v < 0.0` guard is load-bearing: without it a negative carry reaches
/// `CarryCost::new` and PANICs, and a negative price-unit carry would CREDIT R —
/// inverting the accrual sign. Deterministic; the refusal precedes any data access.
#[test]
fn stage1_r_negative_carry_per_cycle_refused_non_negative_exit_2() {
```

- [ ] **Step 7: Add a sibling negative-cost integration test**

Add beside it in `crates/aura-cli/tests/cli_run.rs` (after the carry test, ~:1842):

```rust
/// Property (#153): a negative `--cost-per-trade` is refused at the binary
/// boundary — exit 2, a named non-negativity diagnostic on stderr, no `runs/`
/// write — the sibling of the carry guard, exercising the per-trade arm.
#[test]
fn run_negative_cost_per_trade_refused_non_negative_exit_2() {
    let dir = temp_cwd("cost-per-trade-neg");
    let out = Command::new(BIN)
        .current_dir(&dir)
        .args(["run", "--harness", "stage1-r", "--cost-per-trade", "-0.5"])
        .output()
        .expect("spawn aura run --cost-per-trade -0.5");
    assert_eq!(out.status.code(), Some(2), "negative cost must exit 2; stderr: {}",
        String::from_utf8_lossy(&out.stderr));
    assert!(out.stdout.is_empty(), "a refused run must not emit a report: {:?}", out.stdout);
    let stderr = String::from_utf8_lossy(&out.stderr);
    assert!(stderr.contains("must be non-negative"), "stderr must name the cause: {stderr:?}");
    assert!(!dir.join("runs").exists(), "a refused run must not write the registry");
    let _ = std::fs::remove_dir_all(&dir);
}
```

- [ ] **Step 8: Run the integration tests to verify they pass**

Run: `cargo test -p aura-cli --test cli_run stage1_r_negative_carry_per_cycle_refused_non_negative_exit_2`
Expected: PASS.
Run: `cargo test -p aura-cli --test cli_run run_negative_cost_per_trade_refused_non_negative_exit_2`
Expected: PASS.

---

### Task 2: Non-R-harness guard (spec item 3 — the spec_gap)

**Files:**
- Modify: `crates/aura-cli/src/main.rs` (insert guard after :3215)
- Test: `crates/aura-cli/src/main.rs` `mod tests` (:3338)
- Test: `crates/aura-cli/tests/cli_run.rs` (new integration test beside :1574-1589)

- [ ] **Step 1: Write the failing unit tests**

Add to `mod tests` in `crates/aura-cli/src/main.rs`, beside the Task-1 units:

```rust
#[test]
fn parse_run_args_cost_flags_require_r_harness() {
    let cases: [(&[&str], &str); 3] = [
        (&["--harness", "sma"], "--cost-per-trade"),
        (&["--harness", "macd"], "--slip-vol-mult"),
        (&["--harness", "sma"], "--carry-per-cycle"),
    ];
    for (harness, flag) in cases {
        let mut argv: Vec<&str> = harness.to_vec();
        argv.extend_from_slice(&[flag, "0.001"]);
        let err = parse_run_args(&argv).expect_err("cost flag on a non-R harness must be refused");
        assert!(err.contains("R-evaluator harness"), "{harness:?}+{flag}: {err}");
    }
}

#[test]
fn parse_run_args_cost_flag_without_harness_defaults_sma_and_is_refused() {
    let err = parse_run_args(&["--carry-per-cycle", "0.5"])
        .expect_err("the default harness is sma → cost flag refused");
    assert!(err.contains("R-evaluator harness"), "{err}");
}

#[test]
fn parse_run_args_cost_flags_accepted_on_stage1_r() {
    let a = parse_run_args(&["--harness", "stage1-r", "--cost-per-trade", "0.001"])
        .expect("cost flags are accepted on the R harness");
    assert!(matches!(a.harness, HarnessKind::Stage1R));
    assert_eq!(a.cost, Some(0.001));
}
```

- [ ] **Step 2: Run the unit tests to verify they fail**

Run: `cargo test -p aura-cli parse_run_args_cost_flags_require_r_harness`
Expected: FAIL — today a cost flag on `--harness sma` is accepted (silently
ignored downstream), so `parse_run_args` returns `Ok` and `expect_err` panics.

- [ ] **Step 3: Insert the non-R-harness guard**

In `parse_run_args`, immediately after `let harness = harness.unwrap_or(HarnessKind::Sma);`
(`main.rs:3215`) and before the `// --from/--to require --real.` comment:

```rust
    let harness = harness.unwrap_or(HarnessKind::Sma);
    // Cost flags are defined only against the R chain (gross-R -> net-R); a non-R
    // harness produces no R to charge against, so a cost flag there is a usage error,
    // not a silent no-op (refuse-don't-guess, C10).
    if !matches!(harness, HarnessKind::Stage1R)
        && (cost.is_some() || slip_vol_mult.is_some() || carry_per_cycle.is_some())
    {
        return Err(
            "cost flags require an R-evaluator harness (stage1-r); \
             --harness sma/macd produces no R to charge against"
                .to_string(),
        );
    }
```

- [ ] **Step 4: Run the unit tests to verify they pass**

Run: `cargo test -p aura-cli parse_run_args_cost_flags_require_r_harness`
Expected: PASS.
Run: `cargo test -p aura-cli parse_run_args_cost_flags_accepted_on_stage1_r`
Expected: PASS (the accepted path is unchanged).

- [ ] **Step 5: Write the failing integration test**

Add to `crates/aura-cli/tests/cli_run.rs`, beside `run_harness_sma_is_pip_only_no_r_block`
(~:1589):

```rust
/// Property (#153): a cost flag on a non-R harness (sma/macd, which produce no R)
/// is a usage error — exit 2 with a named diagnostic, no silent no-op, no `runs/`
/// write. The pre-#153 behaviour silently ignored the flag and exited 0.
#[test]
fn run_cost_flag_on_non_r_harness_is_refused_exit_2() {
    let dir = temp_cwd("sma-cost-refused");
    let out = Command::new(BIN)
        .current_dir(&dir)
        .args(["run", "--harness", "sma", "--cost-per-trade", "0.001"])
        .output()
        .expect("spawn aura run --harness sma --cost-per-trade");
    assert_eq!(out.status.code(), Some(2), "cost on a non-R harness must exit 2; stderr: {}",
        String::from_utf8_lossy(&out.stderr));
    assert!(out.stdout.is_empty(), "a refused run must not emit a report: {:?}", out.stdout);
    let stderr = String::from_utf8_lossy(&out.stderr);
    assert!(stderr.contains("R-evaluator harness"), "stderr must name the cause: {stderr:?}");
    assert!(!dir.join("runs").exists(), "a refused run must not write the registry");
    let _ = std::fs::remove_dir_all(&dir);
}
```

- [ ] **Step 6: Run the integration test to verify it passes**

Run: `cargo test -p aura-cli --test cli_run run_cost_flag_on_non_r_harness_is_refused_exit_2`
Expected: PASS.

---

### Task 3: Cost-flag units note (spec item 1)

**Files:**
- Modify: `crates/aura-cli/src/main.rs` (new `const COST_FLAGS_NOTE`; usage closure :3137; `--help` print :3265)
- Test: `crates/aura-cli/tests/cli_run.rs` (new `--help` note test beside :595-663)

- [ ] **Step 1: Write the failing integration test**

Add to `crates/aura-cli/tests/cli_run.rs`, beside the help tests (~:663):

```rust
/// Property (#153): the cost-flag units/constraints are discoverable from
/// `aura run --help` — the appended note states the in-R charge, the per-ENGINE-
/// cycle carry semantics, the `>= 0` constraint, and that cost flags are stage1-r
/// only. Pre-#153 the usage listed the flags with no unit or scale guidance.
#[test]
fn run_help_surfaces_cost_flag_units_note() {
    let out = Command::new(BIN).args(["run", "--help"]).output().expect("spawn aura run --help");
    assert_eq!(out.status.code(), Some(0), "run --help exits 0: {:?}", out.status);
    let stdout = String::from_utf8(out.stdout).expect("utf-8 stdout");
    assert!(stdout.contains("per ENGINE cycle"), "help must explain carry units: {stdout}");
    assert!(stdout.contains("stage1-r only"), "help must state the R-harness requirement: {stdout}");
}
```

- [ ] **Step 2: Run the integration test to verify it fails**

Run: `cargo test -p aura-cli --test cli_run run_help_surfaces_cost_flag_units_note`
Expected: FAIL — the current `--help` output (`USAGE`) carries no cost-flag note,
so `stdout.contains("per ENGINE cycle")` is false.

- [ ] **Step 3: Add the `COST_FLAGS_NOTE` const**

Insert immediately above `const USAGE: &str` (`main.rs:3244`):

```rust
/// One-line note appended to the `aura run` usage surfaces (the grammar-error
/// usage closure and `--help`) so the cost-flag units, scale, per-engine-cycle
/// semantics, non-negativity, and R-harness requirement are discoverable at the
/// point of use (#153). A leading newline puts it on its own line under the usage.
const COST_FLAGS_NOTE: &str = "\ncost flags (stage1-r only; all >= 0, charged in R as cost/|entry−stop|): \
--cost-per-trade = per-trade cost in price units; --slip-vol-mult = slippage multiplier on realized vol; \
--carry-per-cycle = carry in price units per ENGINE cycle (not per day, not an overnight swap)";
```

- [ ] **Step 4: Append the note to the `parse_run_args` usage closure**

Replace the `usage` closure (`main.rs:3137-3140`):

```rust
    let usage = || {
        format!(
            "usage: aura run [--harness <sma|macd|stage1-r>] [--real <SYMBOL> [--from <ms>] [--to <ms>]] [--trace <name>] [--cost-per-trade <f64>] [--slip-vol-mult <f64>] [--carry-per-cycle <f64>]{COST_FLAGS_NOTE}"
        )
    };
```

- [ ] **Step 5: Append the note to the `--help` print**

Replace the `--help` short-circuit print (`main.rs:3265`):

```rust
    if args.iter().any(|a| a == "--help" || a == "-h") {
        println!("{USAGE}{COST_FLAGS_NOTE}");
        return;
    }
```

- [ ] **Step 6: Run the integration test to verify it passes**

Run: `cargo test -p aura-cli --test cli_run run_help_surfaces_cost_flag_units_note`
Expected: PASS.

- [ ] **Step 7: Regression — the uniform help/usage pins still hold**

Run: `cargo test -p aura-cli --test cli_run per_subcommand_help_is_uniform_stdout_exit_zero`
Expected: PASS (the appended note keeps `contains("usage")` true and stderr empty).

---

### Task 4: C10 ledger note + full-suite gate

**Files:**
- Modify: `docs/design/INDEX.md` (C10 cost-section tail, after :831)

- [ ] **Step 1: Add the C10 realization note**

In `docs/design/INDEX.md`, replace the boundary between the cycle-0085 cost note
and the #117 HISTORY reframe (`:831-833`):

```markdown
sweep-path cost. Decision log: #148.

**Realization (cost-flag harness scoping, #153).** Cost flags are defined only
against an R-evaluator harness (the gross-R → net-R chain). On a non-R harness
(`--harness sma`/`macd`, which produce no R) a cost flag is a **usage error
(exit 2)**, not a silent no-op — refuse-don't-guess. Negative cost rates are
likewise rejected (exit 2) with a named diagnostic that identifies the offending
flag. CLI ergonomics only; the cost-model graph and the R math are untouched.

**Reframe (2026-06-23, #117 — exposure → bias, R as the signal-quality unit).**
```

- [ ] **Step 2: Full aura-cli suite gate — new tests pass, goldens byte-green**

Run: `cargo test -p aura-cli`
Expected: PASS — the whole `aura-cli` suite is green, including the eight accepted-path
cost goldens (`stage1_r_flat_cost_net_expectancy_r_golden` et al., cli_run.rs
:1686/1714/1736/1757/...) byte-unchanged, since only the grammar/usage strings and the
previously-unguarded sma-cost path changed.

- [ ] **Step 3: Lint gate**

Run: `cargo clippy -p aura-cli --all-targets -- -D warnings`
Expected: PASS — no warnings (the new helper, const, and guard are idiomatic).