feat: validate endpoint integration, Expr quantity sizing, apply_func input field fix

- Add /api/v1/strategies/validate client to SwymClient; wire into agent loop
  before submission so all DSL errors are surfaced in one round-trip
- Update dsl-schema.json to upstream: quantity is now oneOf[DecimalString, Expr],
  ExprApplyFunc uses "input" field (renamed from "expr")
- Update prompts: document expression-based quantity sizing (fixed-fraction and
  ATR-based examples), fix apply_func to use "input" not "expr" throughout
- Remove unused ValidationError import

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/agent.rs

@@ -325,7 +325,7 @@ pub async fn run(cli: &Cli) -> Result<()> {
         let strat_path = cli.output_dir.join(format!("strategy_{iteration:03}.json"));
         std::fs::write(&strat_path, serde_json::to_string_pretty(&strategy)?)?;
 
-        // Hard validation errors: skip the expensive backtest and give immediate feedback.
+        // Hard client-side validation errors: skip without hitting the API.
         if !hard_errors.is_empty() {
             let record = IterationRecord {
                 iteration,
@@ -338,6 +338,40 @@ pub async fn run(cli: &Cli) -> Result<()> {
             continue;
         }
 
+        // Server-side validation: call /strategies/validate to get ALL DSL errors
+        // at once before submitting a backtest. This avoids burning a full backtest
+        // round-trip on a structurally invalid strategy and gives the model a
+        // complete list of errors to fix in one shot.
+        match swym.validate_strategy(&strategy).await {
+            Ok(api_errors) if !api_errors.is_empty() => {
+                for e in &api_errors {
+                    warn!("  DSL error at {}: {}", e.path, e.message);
+                }
+                let error_notes: Vec<String> = api_errors
+                    .iter()
+                    .map(|e| format!("DSL error at {}: {}", e.path, e.message))
+                    .collect();
+                validation_notes.extend(error_notes);
+                let record = IterationRecord {
+                    iteration,
+                    strategy: strategy.clone(),
+                    results: vec![],
+                    validation_notes,
+                };
+                info!("{}", record.summary());
+                history.push(record);
+                continue;
+            }
+            Ok(_) => {
+                // Valid — proceed to backtest
+            }
+            Err(e) => {
+                // Network/parse failure from the validate endpoint — log and proceed
+                // anyway so a transient API issue doesn't stall the run.
+                warn!("  strategy validation request failed (proceeding): {e:#}");
+            }
+        }
+
         // Run backtests against all instruments (in-sample)
         let mut results: Vec<BacktestResult> = Vec::new();
 

src/dsl-schema.json

@@ -66,8 +66,11 @@
       "properties": {
         "side": { "type": "string", "enum": ["buy", "sell"] },
         "quantity": {
-          "$ref": "#/definitions/DecimalString",
+          "description": "Per-order size in base asset units. Either a fixed decimal string (e.g. \"0.001\") or a dynamic Expr evaluated at candle close. When an Expr returns None the order is skipped; negative values are clamped to zero.",
-          "description": "Per-order size in base asset units, e.g. \"0.001\" for BTC."
+          "oneOf": [
+            { "$ref": "#/definitions/DecimalString" },
+            { "$ref": "#/definitions/Expr" }
+          ]
         }
       }
     },
@@ -280,7 +283,12 @@
         { "$ref": "#/definitions/ExprBinOp" },
         { "$ref": "#/definitions/ExprApplyFunc" },
         { "$ref": "#/definitions/ExprUnaryOp" },
-        { "$ref": "#/definitions/ExprBarsSince" }
+        { "$ref": "#/definitions/ExprBarsSince" },
+        { "$ref": "#/definitions/ExprEntryPrice" },
+        { "$ref": "#/definitions/ExprPositionQuantity" },
+        { "$ref": "#/definitions/ExprUnrealisedPnl" },
+        { "$ref": "#/definitions/ExprBarsSinceEntry" },
+        { "$ref": "#/definitions/ExprBalance" }
       ]
     },
     "ExprLiteral": {
@@ -417,6 +425,55 @@
           "description": "Maximum bars to look back."
         }
       }
+    },
+    "ExprEntryPrice": {
+      "description": "Volume-weighted average entry price of the current open position. Returns None when flat.",
+      "type": "object",
+      "required": ["kind"],
+      "additionalProperties": false,
+      "properties": {
+        "kind": { "const": "entry_price" }
+      }
+    },
+    "ExprPositionQuantity": {
+      "description": "Absolute quantity of the current open position in base asset units. Returns None when flat.",
+      "type": "object",
+      "required": ["kind"],
+      "additionalProperties": false,
+      "properties": {
+        "kind": { "const": "position_quantity" }
+      }
+    },
+    "ExprUnrealisedPnl": {
+      "description": "Estimated unrealised PnL of the current open position in quote asset. Returns None when flat.",
+      "type": "object",
+      "required": ["kind"],
+      "additionalProperties": false,
+      "properties": {
+        "kind": { "const": "unrealised_pnl" }
+      }
+    },
+    "ExprBarsSinceEntry": {
+      "description": "Number of complete primary-interval bars elapsed since the current position was opened. Computed as floor((now - time_enter) / primary_interval_secs). Returns None when flat.",
+      "type": "object",
+      "required": ["kind"],
+      "additionalProperties": false,
+      "properties": {
+        "kind": { "const": "bars_since_entry" }
+      }
+    },
+    "ExprBalance": {
+      "description": "Free balance of the named asset (matched case-insensitively). Returns None when the asset is not found or balance data is unavailable.",
+      "type": "object",
+      "required": ["kind", "asset"],
+      "additionalProperties": false,
+      "properties": {
+        "kind": { "const": "balance" },
+        "asset": {
+          "type": "string",
+          "description": "Internal asset name, e.g. \"usdt\", \"btc\". Case-insensitive."
+        }
+      }
     }
   }
 }

src/prompts.rs

@@ -74,21 +74,36 @@ bars_since_entry — complete bars elapsed since position was opened
 balance — free balance of a named asset (e.g. "usdt", "usdc")
 
 ### Quantity
-Action quantity MUST be a fixed decimal string that parses as a floating-point number.
+Action quantity is either a fixed decimal string **or** an Expr that evaluates to a number
-NEVER use an expression object for quantity — only plain decimal strings are accepted.
+at candle close. If the Expr returns None the order is skipped; negative values are clamped
-NEVER use placeholder strings like `"ATR_SIZED"`, `"FULL_BALANCE"`, `"percent_of_balance"`,
+to zero.
-`"dynamic"`, `"all"`, or any non-numeric string — these will be rejected immediately.
-To exit a position, use the SAME decimal string as the entry rule — the backtester matches
-open quantity automatically. There is no "close all" concept; just repeat the entry quantity.
 
-Size your quantity to represent roughly 5–10% of the initial $10,000 USDC balance at
+**Preferred: expression-based sizing** — instrument-agnostic, scales automatically:
-approximate current prices. Reference values (adjust if market prices differ significantly):
+
-- BTC: `"0.01"` ≈ $800 (8% of $10k)
+Fixed fraction of quote balance (5% of USDC balance, converted to base units at current price):
-- ETH: `"3.0"` ≈ $600 (6% of $10k)
+```json
-- SOL: `"50.0"` ≈ $700 (7% of $10k)
+{{"kind":"bin_op","op":"div",
-Since strategies run across all instruments with the same quantity string, choose a value
+  "left":{{"kind":"bin_op","op":"mul",
-appropriate for the primary instrument you are designing for. Avoid `"0.001"` — it produces
+    "left":{{"kind":"balance","asset":"usdc"}},
-negligible exposure and makes results statistically meaningless.
+    "right":{{"kind":"literal","value":"0.05"}}}},
+  "right":{{"kind":"field","field":"close"}}}}
+```
+
+ATR-based risk sizing (risk $200 per trade, sized by volatility):
+```json
+{{"kind":"bin_op","op":"div",
+  "left":{{"kind":"literal","value":"200"}},
+  "right":{{"kind":"func","name":"atr","period":14}}}}
+```
+
+For exit rules, use `position_quantity` to close the exact open position:
+```json
+{{"kind":"position_quantity"}}
+```
+
+**Fixed strings are also valid** when you want a specific size, e.g. `"0.01"`.
+NEVER use placeholder strings like `"ATR_SIZED"`, `"FULL_BALANCE"`, `"all"`, `"dynamic"` —
+these are rejected immediately. Use an Expr or a plain decimal string.
 
 ### Multi-timeframe
 Any expression can reference a different timeframe via "timeframe" field.
@@ -166,7 +181,7 @@ Common mistakes to NEVER make:
   with `ApplyFuncName` values: `highest`, `lowest`, `sma`, `ema`, `wma`, `std_dev`, `sum`,
   `bollinger_upper`, `bollinger_lower`.
 - `volume` is a candle FIELD, not a func name. Access it as `{{"kind":"field","field":"volume"}}`.
-  To compute EMA of volume: `{{"kind":"apply_func","name":"ema","period":20,"expr":{{"kind":"field","field":"volume"}}}}`.
+  To compute EMA of volume: `{{"kind":"apply_func","name":"ema","period":20,"input":{{"kind":"field","field":"volume"}}}}`.
 - `bollinger_upper` and `bollinger_lower` are FUNC NAMES, not Expr kinds. To compare close to the upper band:
   `{{"kind":"compare","left":{{"kind":"field","field":"close"}},"op":">","right":{{"kind":"func","name":"bollinger_upper","period":20}}}}`
   NEVER write `{{"kind":"bollinger_upper",...}}` — `bollinger_upper` is not an Expr kind.
@@ -374,7 +389,7 @@ The MACD line is `EMA(12) - EMA(26)`; the signal line is `EMA(9)` of the MACD li
             }},
             "right": {{
               "kind": "apply_func", "name": "ema", "period": 9,
-              "expr": {{
+              "input": {{
                 "kind": "bin_op", "op": "sub",
                 "left":  {{"kind": "func", "name": "ema", "period": 12}},
                 "right": {{"kind": "func", "name": "ema", "period": 26}}
@@ -403,7 +418,7 @@ The MACD line is `EMA(12) - EMA(26)`; the signal line is `EMA(9)` of the MACD li
                 }},
                 "right": {{
                   "kind": "apply_func", "name": "ema", "period": 9,
-                  "expr": {{
+                  "input": {{
                     "kind": "bin_op", "op": "sub",
                     "left":  {{"kind": "func", "name": "ema", "period": 12}},
                     "right": {{"kind": "func", "name": "ema", "period": 26}}
@@ -434,9 +449,10 @@ The MACD line is `EMA(12) - EMA(26)`; the signal line is `EMA(9)` of the MACD li
 }}
 ```
 
-Key pattern: `apply_func` wraps any `Expr` tree, enabling EMA-of-expression (signal line),
+Key pattern: `apply_func` wraps any `Expr` tree using the `"input"` field (NOT `"expr"`).
-WMA-of-expression (Hull MA), or std_dev-of-returns. There is NO native `macd` func name —
+This enables EMA-of-expression (signal line), WMA-of-expression (Hull MA), or std_dev-of-returns.
-always compose it as `bin_op(sub, func(ema,12), func(ema,26))` as shown above.
+There is NO native `macd` func name — always compose it as `bin_op(sub, func(ema,12), func(ema,26))` as shown above.
+CRITICAL: `apply_func` uses `"input"`, not `"expr"`. Writing `"expr":` will be rejected by the API.
 
 ## Anti-patterns to avoid
 

src/swym.rs

@@ -4,6 +4,20 @@ use serde::{Deserialize, Serialize};
 use serde_json::Value;
 use uuid::Uuid;
 
+/// Response from `POST /api/v1/strategies/validate`.
+#[derive(Debug, Deserialize)]
+pub struct ValidationResponse {
+    pub valid: bool,
+    #[serde(default)]
+    pub errors: Vec<ValidationError>,
+}
+
+#[derive(Debug, Deserialize, Clone)]
+pub struct ValidationError {
+    pub path: String,
+    pub message: String,
+}
+
 /// Client for the swym backtesting API.
 pub struct SwymClient {
     client: Client,
@@ -254,6 +268,32 @@ impl SwymClient {
         resp.json().await.context("parse candle coverage")
     }
 
+    /// Validate a strategy against the swym DSL schema.
+    ///
+    /// Calls `POST /api/v1/strategies/validate` and returns a structured list
+    /// of all validation errors. Returns `Ok(vec![])` when the strategy is valid.
+    /// Returns `Err` only on network or parse failures, not on DSL errors.
+    pub async fn validate_strategy(&self, strategy: &Value) -> Result<Vec<ValidationError>> {
+        let url = format!("{}/strategies/validate", self.base_url);
+        let resp = self
+            .client
+            .post(&url)
+            .json(strategy)
+            .send()
+            .await
+            .context("validate strategy request")?;
+
+        if !resp.status().is_success() {
+            let status = resp.status();
+            let body = resp.text().await.unwrap_or_default();
+            anyhow::bail!("validate strategy {status}: {body}");
+        }
+
+        let parsed: ValidationResponse =
+            resp.json().await.context("parse validation response")?;
+        Ok(parsed.errors)
+    }
+
     /// Submit a backtest run.
     pub async fn submit_backtest(
         &self,