diff --git a/src/tools/mod.rs b/src/tools/mod.rs
index 07028d5..0be616d 100644
--- a/src/tools/mod.rs
+++ b/src/tools/mod.rs
@@ -1,7 +1,9 @@
 pub mod calculator;
 pub mod registry;
+pub mod schema;
 pub mod traits;
 
 pub use calculator::CalculatorTool;
 pub use registry::ToolRegistry;
+pub use schema::{CleaningStrategy, SchemaCleanr};
 pub use traits::{Tool, ToolResult};
diff --git a/src/tools/schema.rs b/src/tools/schema.rs
new file mode 100644
index 0000000..91bca47
--- /dev/null
+++ b/src/tools/schema.rs
@@ -0,0 +1,721 @@
+//! JSON Schema cleaning and validation for LLM tool-calling compatibility.
+//!
+//! Different providers support different subsets of JSON Schema. This module
+//! normalizes tool schemas to improve cross-provider compatibility while
+//! preserving semantic intent.
+//!
+//! ## What this module does
+//!
+//! 1. Removes unsupported keywords per provider strategy
+//! 2. Resolves local `$ref` entries from `$defs` and `definitions`
+//! 3. Flattens literal `anyOf` / `oneOf` unions into `enum`
+//! 4. Strips nullable variants from unions and `type` arrays
+//! 5. Converts `const` to single-value `enum`
+//! 6. Detects circular references and stops recursion safely
+
+use serde_json::{Map, Value, json};
+use std::collections::{HashMap, HashSet};
+
+/// Keywords that Gemini rejects for tool schemas.
+pub const GEMINI_UNSUPPORTED_KEYWORDS: &[&str] = &[
+    // Schema composition
+    "$ref",
+    "$schema",
+    "$id",
+    "$defs",
+    "definitions",
+    // Property constraints
+    "additionalProperties",
+    "patternProperties",
+    // String constraints
+    "minLength",
+    "maxLength",
+    "pattern",
+    "format",
+    // Number constraints
+    "minimum",
+    "maximum",
+    "multipleOf",
+    // Array constraints
+    "minItems",
+    "maxItems",
+    "uniqueItems",
+    // Object constraints
+    "minProperties",
+    "maxProperties",
+    // Non-standard
+    "examples",
+];
+
+/// Keywords that should be preserved during cleaning (metadata).
+const SCHEMA_META_KEYWORDS: &[&str] = &["description", "title", "default"];
+
+/// Schema cleaning strategies for different LLM providers.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum CleaningStrategy {
+    /// Gemini (Google AI / Vertex AI) - Most restrictive
+    Gemini,
+    /// Anthropic Claude - Moderately permissive
+    Anthropic,
+    /// OpenAI GPT - Most permissive
+    OpenAI,
+    /// Conservative: Remove only universally unsupported keywords
+    Conservative,
+}
+
+impl CleaningStrategy {
+    /// Get the list of unsupported keywords for this strategy.
+    pub fn unsupported_keywords(self) -> &'static [&'static str] {
+        match self {
+            Self::Gemini => GEMINI_UNSUPPORTED_KEYWORDS,
+            Self::Anthropic => &["$ref", "$defs", "definitions"],
+            Self::OpenAI => &[],
+            Self::Conservative => &["$ref", "$defs", "definitions", "additionalProperties"],
+        }
+    }
+}
+
+/// JSON Schema cleaner optimized for LLM tool calling.
+pub struct SchemaCleanr;
+
+impl SchemaCleanr {
+    /// Clean schema for Gemini compatibility (strictest).
+    pub fn clean_for_gemini(schema: Value) -> Value {
+        Self::clean(schema, CleaningStrategy::Gemini)
+    }
+
+    /// Clean schema for Anthropic compatibility.
+    pub fn clean_for_anthropic(schema: Value) -> Value {
+        Self::clean(schema, CleaningStrategy::Anthropic)
+    }
+
+    /// Clean schema for OpenAI compatibility (most permissive).
+    pub fn clean_for_openai(schema: Value) -> Value {
+        Self::clean(schema, CleaningStrategy::OpenAI)
+    }
+
+    /// Clean schema with specified strategy.
+    pub fn clean(schema: Value, strategy: CleaningStrategy) -> Value {
+        let defs = if let Some(obj) = schema.as_object() {
+            Self::extract_defs(obj)
+        } else {
+            HashMap::new()
+        };
+        Self::clean_with_defs(schema, &defs, strategy, &mut HashSet::new())
+    }
+
+    /// Validate that a schema is suitable for LLM tool calling.
+    pub fn validate(schema: &Value) -> anyhow::Result<()> {
+        let obj = schema
+            .as_object()
+            .ok_or_else(|| anyhow::anyhow!("Schema must be an object"))?;
+
+        if !obj.contains_key("type") {
+            anyhow::bail!("Schema missing required 'type' field");
+        }
+
+        if let Some(Value::String(t)) = obj.get("type") {
+            if t == "object" && !obj.contains_key("properties") {
+                tracing::warn!("Object schema without 'properties' field may cause issues");
+            }
+        }
+
+        Ok(())
+    }
+
+    /// Extract $defs and definitions into a flat map for reference resolution.
+    fn extract_defs(obj: &Map<String, Value>) -> HashMap<String, Value> {
+        let mut defs = HashMap::new();
+
+        if let Some(Value::Object(defs_obj)) = obj.get("$defs") {
+            for (key, value) in defs_obj {
+                defs.insert(key.clone(), value.clone());
+            }
+        }
+
+        if let Some(Value::Object(defs_obj)) = obj.get("definitions") {
+            for (key, value) in defs_obj {
+                defs.insert(key.clone(), value.clone());
+            }
+        }
+
+        defs
+    }
+
+    /// Recursively clean a schema value.
+    fn clean_with_defs(
+        schema: Value,
+        defs: &HashMap<String, Value>,
+        strategy: CleaningStrategy,
+        ref_stack: &mut HashSet<String>,
+    ) -> Value {
+        match schema {
+            Value::Object(obj) => Self::clean_object(obj, defs, strategy, ref_stack),
+            Value::Array(arr) => Value::Array(
+                arr.into_iter()
+                    .map(|v| Self::clean_with_defs(v, defs, strategy, ref_stack))
+                    .collect(),
+            ),
+            other => other,
+        }
+    }
+
+    /// Clean an object schema.
+    fn clean_object(
+        obj: Map<String, Value>,
+        defs: &HashMap<String, Value>,
+        strategy: CleaningStrategy,
+        ref_stack: &mut HashSet<String>,
+    ) -> Value {
+        // Handle $ref resolution
+        if let Some(Value::String(ref_value)) = obj.get("$ref") {
+            return Self::resolve_ref(ref_value, &obj, defs, strategy, ref_stack);
+        }
+
+        // Handle anyOf/oneOf simplification
+        if obj.contains_key("anyOf") || obj.contains_key("oneOf") {
+            if let Some(simplified) = Self::try_simplify_union(&obj, defs, strategy, ref_stack) {
+                return simplified;
+            }
+        }
+
+        // Build cleaned object
+        let mut cleaned = Map::new();
+        let unsupported: HashSet<&str> = strategy.unsupported_keywords().iter().copied().collect();
+        let has_union = obj.contains_key("anyOf") || obj.contains_key("oneOf");
+
+        for (key, value) in obj {
+            // Skip unsupported keywords
+            if unsupported.contains(key.as_str()) {
+                continue;
+            }
+
+            match key.as_str() {
+                // Convert const to enum
+                "const" => {
+                    cleaned.insert("enum".to_string(), json!([value]));
+                }
+                // Skip type if we have anyOf/oneOf
+                "type" if has_union => {}
+                // Handle type arrays (remove null)
+                "type" if matches!(value, Value::Array(_)) => {
+                    let cleaned_value = Self::clean_type_array(value);
+                    cleaned.insert(key, cleaned_value);
+                }
+                // Recursively clean nested schemas
+                "properties" => {
+                    let cleaned_value = Self::clean_properties(value, defs, strategy, ref_stack);
+                    cleaned.insert(key, cleaned_value);
+                }
+                "items" => {
+                    let cleaned_value = Self::clean_with_defs(value, defs, strategy, ref_stack);
+                    cleaned.insert(key, cleaned_value);
+                }
+                "anyOf" | "oneOf" | "allOf" => {
+                    let cleaned_value = Self::clean_union(value, defs, strategy, ref_stack);
+                    cleaned.insert(key, cleaned_value);
+                }
+                _ => {
+                    let cleaned_value = match value {
+                        Value::Object(_) | Value::Array(_) => {
+                            Self::clean_with_defs(value, defs, strategy, ref_stack)
+                        }
+                        other => other,
+                    };
+                    cleaned.insert(key, cleaned_value);
+                }
+            }
+        }
+
+        Value::Object(cleaned)
+    }
+
+    /// Resolve a $ref to its definition.
+    fn resolve_ref(
+        ref_value: &str,
+        obj: &Map<String, Value>,
+        defs: &HashMap<String, Value>,
+        strategy: CleaningStrategy,
+        ref_stack: &mut HashSet<String>,
+    ) -> Value {
+        // Prevent circular references
+        if ref_stack.contains(ref_value) {
+            tracing::warn!("Circular $ref detected: {}", ref_value);
+            return Self::preserve_meta(obj, Value::Object(Map::new()));
+        }
+
+        if let Some(def_name) = Self::parse_local_ref(ref_value) {
+            if let Some(definition) = defs.get(def_name.as_str()) {
+                ref_stack.insert(ref_value.to_string());
+                let cleaned = Self::clean_with_defs(definition.clone(), defs, strategy, ref_stack);
+                ref_stack.remove(ref_value);
+                return Self::preserve_meta(obj, cleaned);
+            }
+        }
+
+        tracing::warn!("Cannot resolve $ref: {}", ref_value);
+        Self::preserve_meta(obj, Value::Object(Map::new()))
+    }
+
+    /// Parse a local JSON Pointer ref (#/$defs/Name or #/definitions/Name).
+    fn parse_local_ref(ref_value: &str) -> Option<String> {
+        ref_value
+            .strip_prefix("#/$defs/")
+            .or_else(|| ref_value.strip_prefix("#/definitions/"))
+            .map(Self::decode_json_pointer)
+    }
+
+    /// Decode JSON Pointer escaping (`~0` = `~`, `~1` = `/`).
+    fn decode_json_pointer(segment: &str) -> String {
+        if !segment.contains('~') {
+            return segment.to_string();
+        }
+
+        let mut decoded = String::with_capacity(segment.len());
+        let mut chars = segment.chars().peekable();
+
+        while let Some(ch) = chars.next() {
+            if ch == '~' {
+                match chars.peek().copied() {
+                    Some('0') => {
+                        chars.next();
+                        decoded.push('~');
+                    }
+                    Some('1') => {
+                        chars.next();
+                        decoded.push('/');
+                    }
+                    _ => decoded.push('~'),
+                }
+            } else {
+                decoded.push(ch);
+            }
+        }
+
+        decoded
+    }
+
+    /// Try to simplify anyOf/oneOf to a simpler form.
+    fn try_simplify_union(
+        obj: &Map<String, Value>,
+        defs: &HashMap<String, Value>,
+        strategy: CleaningStrategy,
+        ref_stack: &mut HashSet<String>,
+    ) -> Option<Value> {
+        let union_key = if obj.contains_key("anyOf") {
+            "anyOf"
+        } else if obj.contains_key("oneOf") {
+            "oneOf"
+        } else {
+            return None;
+        };
+
+        let variants = obj.get(union_key)?.as_array()?;
+
+        let cleaned_variants: Vec<Value> = variants
+            .iter()
+            .map(|v| Self::clean_with_defs(v.clone(), defs, strategy, ref_stack))
+            .collect();
+
+        // Strip null variants
+        let non_null: Vec<Value> = cleaned_variants
+            .into_iter()
+            .filter(|v| !Self::is_null_schema(v))
+            .collect();
+
+        // If only one variant remains after stripping nulls, return it
+        if non_null.len() == 1 {
+            return Some(Self::preserve_meta(obj, non_null[0].clone()));
+        }
+
+        // Try to flatten to enum if all variants are literals
+        if let Some(enum_value) = Self::try_flatten_literal_union(&non_null) {
+            return Some(Self::preserve_meta(obj, enum_value));
+        }
+
+        None
+    }
+
+    /// Check if a schema represents null type.
+    fn is_null_schema(value: &Value) -> bool {
+        if let Some(obj) = value.as_object() {
+            if let Some(Value::Null) = obj.get("const") {
+                return true;
+            }
+            if let Some(Value::Array(arr)) = obj.get("enum") {
+                if arr.len() == 1 && matches!(arr[0], Value::Null) {
+                    return true;
+                }
+            }
+            if let Some(Value::String(t)) = obj.get("type") {
+                if t == "null" {
+                    return true;
+                }
+            }
+        }
+        false
+    }
+
+    /// Try to flatten anyOf/oneOf with only literal values to enum.
+    fn try_flatten_literal_union(variants: &[Value]) -> Option<Value> {
+        if variants.is_empty() {
+            return None;
+        }
+
+        let mut all_values = Vec::new();
+        let mut common_type: Option<String> = None;
+
+        for variant in variants {
+            let obj = variant.as_object()?;
+
+            let literal_value = if let Some(const_val) = obj.get("const") {
+                const_val.clone()
+            } else if let Some(Value::Array(arr)) = obj.get("enum") {
+                if arr.len() == 1 {
+                    arr[0].clone()
+                } else {
+                    return None;
+                }
+            } else {
+                return None;
+            };
+
+            let variant_type = obj.get("type")?.as_str()?;
+            match &common_type {
+                None => common_type = Some(variant_type.to_string()),
+                Some(t) if t != variant_type => return None,
+                _ => {}
+            }
+
+            all_values.push(literal_value);
+        }
+
+        common_type.map(|t| {
+            json!({
+                "type": t,
+                "enum": all_values
+            })
+        })
+    }
+
+    /// Clean type array, removing null.
+    fn clean_type_array(value: Value) -> Value {
+        if let Value::Array(types) = value {
+            let non_null: Vec<Value> = types
+                .into_iter()
+                .filter(|v| v.as_str() != Some("null"))
+                .collect();
+
+            match non_null.len() {
+                0 => Value::String("null".to_string()),
+                1 => non_null.into_iter().next().unwrap_or(Value::String("null".to_string())),
+                _ => Value::Array(non_null),
+            }
+        } else {
+            value
+        }
+    }
+
+    /// Clean properties object.
+    fn clean_properties(
+        value: Value,
+        defs: &HashMap<String, Value>,
+        strategy: CleaningStrategy,
+        ref_stack: &mut HashSet<String>,
+    ) -> Value {
+        if let Value::Object(props) = value {
+            let cleaned: Map<String, Value> = props
+                .into_iter()
+                .map(|(k, v)| (k, Self::clean_with_defs(v, defs, strategy, ref_stack)))
+                .collect();
+            Value::Object(cleaned)
+        } else {
+            value
+        }
+    }
+
+    /// Clean union (anyOf/oneOf/allOf).
+    fn clean_union(
+        value: Value,
+        defs: &HashMap<String, Value>,
+        strategy: CleaningStrategy,
+        ref_stack: &mut HashSet<String>,
+    ) -> Value {
+        if let Value::Array(variants) = value {
+            let cleaned: Vec<Value> = variants
+                .into_iter()
+                .map(|v| Self::clean_with_defs(v, defs, strategy, ref_stack))
+                .collect();
+            Value::Array(cleaned)
+        } else {
+            value
+        }
+    }
+
+    /// Preserve metadata (description, title, default) from source to target.
+    fn preserve_meta(source: &Map<String, Value>, mut target: Value) -> Value {
+        if let Value::Object(target_obj) = &mut target {
+            for &key in SCHEMA_META_KEYWORDS {
+                if let Some(value) = source.get(key) {
+                    target_obj.insert(key.to_string(), value.clone());
+                }
+            }
+        }
+        target
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_remove_unsupported_keywords() {
+        let schema = json!({
+            "type": "string",
+            "minLength": 1,
+            "maxLength": 100,
+            "pattern": "^[a-z]+$",
+            "description": "A lowercase string"
+        });
+
+        let cleaned = SchemaCleanr::clean_for_gemini(schema);
+
+        assert_eq!(cleaned["type"], "string");
+        assert_eq!(cleaned["description"], "A lowercase string");
+        assert!(cleaned.get("minLength").is_none());
+        assert!(cleaned.get("maxLength").is_none());
+        assert!(cleaned.get("pattern").is_none());
+    }
+
+    #[test]
+    fn test_resolve_ref() {
+        let schema = json!({
+            "type": "object",
+            "properties": {
+                "age": {
+                    "$ref": "#/$defs/Age"
+                }
+            },
+            "$defs": {
+                "Age": {
+                    "type": "integer",
+                    "minimum": 0
+                }
+            }
+        });
+
+        let cleaned = SchemaCleanr::clean_for_gemini(schema);
+
+        assert_eq!(cleaned["properties"]["age"]["type"], "integer");
+        assert!(cleaned["properties"]["age"].get("minimum").is_none());
+        assert!(cleaned.get("$defs").is_none());
+    }
+
+    #[test]
+    fn test_flatten_literal_union() {
+        let schema = json!({
+            "anyOf": [
+                { "const": "admin", "type": "string" },
+                { "const": "user", "type": "string" },
+                { "const": "guest", "type": "string" }
+            ]
+        });
+
+        let cleaned = SchemaCleanr::clean_for_gemini(schema);
+
+        assert_eq!(cleaned["type"], "string");
+        assert!(cleaned["enum"].is_array());
+        let enum_values = cleaned["enum"].as_array().unwrap();
+        assert_eq!(enum_values.len(), 3);
+        assert!(enum_values.contains(&json!("admin")));
+        assert!(enum_values.contains(&json!("user")));
+        assert!(enum_values.contains(&json!("guest")));
+    }
+
+    #[test]
+    fn test_strip_null_from_union() {
+        let schema = json!({
+            "oneOf": [
+                { "type": "string" },
+                { "type": "null" }
+            ]
+        });
+
+        let cleaned = SchemaCleanr::clean_for_gemini(schema);
+
+        assert_eq!(cleaned["type"], "string");
+        assert!(cleaned.get("oneOf").is_none());
+    }
+
+    #[test]
+    fn test_const_to_enum() {
+        let schema = json!({
+            "const": "fixed_value",
+            "description": "A constant"
+        });
+
+        let cleaned = SchemaCleanr::clean_for_gemini(schema);
+
+        assert_eq!(cleaned["enum"], json!(["fixed_value"]));
+        assert_eq!(cleaned["description"], "A constant");
+        assert!(cleaned.get("const").is_none());
+    }
+
+    #[test]
+    fn test_preserve_metadata() {
+        let schema = json!({
+            "$ref": "#/$defs/Name",
+            "description": "User's name",
+            "title": "Name Field",
+            "default": "Anonymous",
+            "$defs": {
+                "Name": {
+                    "type": "string"
+                }
+            }
+        });
+
+        let cleaned = SchemaCleanr::clean_for_gemini(schema);
+
+        assert_eq!(cleaned["type"], "string");
+        assert_eq!(cleaned["description"], "User's name");
+        assert_eq!(cleaned["title"], "Name Field");
+        assert_eq!(cleaned["default"], "Anonymous");
+    }
+
+    #[test]
+    fn test_circular_ref_prevention() {
+        let schema = json!({
+            "type": "object",
+            "properties": {
+                "parent": {
+                    "$ref": "#/$defs/Node"
+                }
+            },
+            "$defs": {
+                "Node": {
+                    "type": "object",
+                    "properties": {
+                        "child": {
+                            "$ref": "#/$defs/Node"
+                        }
+                    }
+                }
+            }
+        });
+
+        let cleaned = SchemaCleanr::clean_for_gemini(schema);
+
+        assert_eq!(cleaned["properties"]["parent"]["type"], "object");
+    }
+
+    #[test]
+    fn test_validate_schema() {
+        let valid = json!({
+            "type": "object",
+            "properties": {
+                "name": { "type": "string" }
+            }
+        });
+
+        assert!(SchemaCleanr::validate(&valid).is_ok());
+
+        let invalid = json!({
+            "properties": {
+                "name": { "type": "string" }
+            }
+        });
+
+        assert!(SchemaCleanr::validate(&invalid).is_err());
+    }
+
+    #[test]
+    fn test_strategy_differences() {
+        let schema = json!({
+            "type": "string",
+            "minLength": 1,
+            "description": "A string field"
+        });
+
+        // Gemini: Most restrictive (removes minLength)
+        let gemini = SchemaCleanr::clean_for_gemini(schema.clone());
+        assert!(gemini.get("minLength").is_none());
+        assert_eq!(gemini["type"], "string");
+        assert_eq!(gemini["description"], "A string field");
+
+        // OpenAI: Most permissive (keeps minLength)
+        let openai = SchemaCleanr::clean_for_openai(schema.clone());
+        assert_eq!(openai["minLength"], 1);
+        assert_eq!(openai["type"], "string");
+    }
+
+    #[test]
+    fn test_nested_properties() {
+        let schema = json!({
+            "type": "object",
+            "properties": {
+                "user": {
+                    "type": "object",
+                    "properties": {
+                        "name": {
+                            "type": "string",
+                            "minLength": 1
+                        }
+                    },
+                    "additionalProperties": false
+                }
+            }
+        });
+
+        let cleaned = SchemaCleanr::clean_for_gemini(schema);
+
+        assert!(
+            cleaned["properties"]["user"]["properties"]["name"]
+                .get("minLength")
+                .is_none()
+        );
+        assert!(
+            cleaned["properties"]["user"]
+                .get("additionalProperties")
+                .is_none()
+        );
+    }
+
+    #[test]
+    fn test_type_array_null_removal() {
+        let schema = json!({
+            "type": ["string", "null"]
+        });
+
+        let cleaned = SchemaCleanr::clean_for_gemini(schema);
+
+        assert_eq!(cleaned["type"], "string");
+    }
+
+    #[test]
+    fn test_skip_type_when_non_simplifiable_union_exists() {
+        let schema = json!({
+            "type": "object",
+            "oneOf": [
+                {
+                    "type": "object",
+                    "properties": {
+                        "a": { "type": "string" }
+                    }
+                },
+                {
+                    "type": "object",
+                    "properties": {
+                        "b": { "type": "number" }
+                    }
+                }
+            ]
+        });
+
+        let cleaned = SchemaCleanr::clean_for_gemini(schema);
+
+        assert!(cleaned.get("type").is_none());
+        assert!(cleaned.get("oneOf").is_some());
+    }
+}
diff --git a/src/tools/traits.rs b/src/tools/traits.rs
index 10f0140..f3ffdc4 100644
--- a/src/tools/traits.rs
+++ b/src/tools/traits.rs
@@ -13,4 +13,19 @@ pub trait Tool: Send + Sync + 'static {
     fn description(&self) -> &str;
     fn parameters_schema(&self) -> serde_json::Value;
     async fn execute(&self, args: serde_json::Value) -> anyhow::Result<ToolResult>;
+
+    /// Whether this tool is side-effect free and safe to parallelize.
+    fn read_only(&self) -> bool {
+        false
+    }
+
+    /// Whether this tool can run alongside other concurrency-safe tools.
+    fn concurrency_safe(&self) -> bool {
+        self.read_only() && !self.exclusive()
+    }
+
+    /// Whether this tool should run alone even if concurrency is enabled.
+    fn exclusive(&self) -> bool {
+        false
+    }
 }