codex-tools: extract MCP schema adapters (#15928)

## Why

`codex-tools` already owns the shared tool input schema model and parser
from the first extraction step, but `core/src/tools/spec.rs` still owned
the MCP-specific adapter that normalizes `rmcp::model::Tool` schemas and
wraps `structuredContent` into the call result output schema.

Keeping that adapter in `codex-core` means the reusable MCP schema path
is still split across crates, and the unit tests for that logic stay
anchored in `codex-core` even though the runtime orchestration does not
need to move yet.

This change takes the next small step by moving the reusable MCP schema
adapter into `codex-tools` while leaving `ResponsesApiTool` assembly in
`codex-core`.

## What changed

- added `tools/src/mcp_tool.rs` and sibling
`tools/src/mcp_tool_tests.rs`
- introduced `ParsedMcpTool`, `parse_mcp_tool()`, and
`mcp_call_tool_result_output_schema()` in `codex-tools`
- updated `core/src/tools/spec.rs` to consume parsed MCP tool parts from
`codex-tools`
- removed the now-redundant MCP schema unit tests from
`core/src/tools/spec_tests.rs`
- expanded `codex-rs/tools/README.md` to describe this second migration
step

## Test plan

- `cargo test -p codex-tools`
- `cargo test -p codex-core --lib tools::spec::`

codex-rs/Cargo.lock

@@ -2633,6 +2633,7 @@ name = "codex-tools"
 version = "0.0.0"
 dependencies = [
  "pretty_assertions",
+ "rmcp",
  "serde",
  "serde_json",
 ]

codex-rs/core/src/tools/spec.rs

@@ -46,6 +46,7 @@ use codex_protocol::openai_models::WebSearchToolType;
 use codex_protocol::protocol::SandboxPolicy;
 use codex_protocol::protocol::SessionSource;
 use codex_protocol::protocol::SubAgentSource;
+use codex_tools::parse_mcp_tool;
 pub use codex_tools::parse_tool_input_schema;
 use codex_utils_absolute_path::AbsolutePathBuf;
 use serde::Deserialize;
@@ -58,6 +59,9 @@ use std::path::PathBuf;
 
 pub type JsonSchema = codex_tools::JsonSchema;
 
+#[cfg(test)]
+pub(crate) use codex_tools::mcp_call_tool_result_output_schema;
+
 const TOOL_SEARCH_DESCRIPTION_TEMPLATE: &str =
     include_str!("../../templates/search_tool/tool_description.md");
 const TOOL_SUGGEST_DESCRIPTION_TEMPLATE: &str =
@@ -2362,15 +2366,15 @@ pub(crate) fn mcp_tool_to_openai_tool(
     fully_qualified_name: String,
     tool: rmcp::model::Tool,
 ) -> Result<ResponsesApiTool, serde_json::Error> {
-    let (description, input_schema, output_schema) = mcp_tool_to_openai_tool_parts(tool)?;
+    let parsed_tool = parse_mcp_tool(&tool)?;
 
     Ok(ResponsesApiTool {
         name: fully_qualified_name,
-        description,
+        description: parsed_tool.description,
         strict: false,
         defer_loading: None,
-        parameters: input_schema,
+        parameters: parsed_tool.input_schema,
-        output_schema,
+        output_schema: Some(parsed_tool.output_schema),
     })
 }
 
@@ -2378,14 +2382,14 @@ pub(crate) fn mcp_tool_to_deferred_openai_tool(
     name: String,
     tool: rmcp::model::Tool,
 ) -> Result<ResponsesApiTool, serde_json::Error> {
-    let (description, input_schema, _) = mcp_tool_to_openai_tool_parts(tool)?;
+    let parsed_tool = parse_mcp_tool(&tool)?;
 
     Ok(ResponsesApiTool {
         name,
-        description,
+        description: parsed_tool.description,
         strict: false,
         defer_loading: Some(true),
-        parameters: input_schema,
+        parameters: parsed_tool.input_schema,
         output_schema: None,
     })
 }
@@ -2405,61 +2409,6 @@ fn dynamic_tool_to_openai_tool(
     })
 }
 
-fn mcp_tool_to_openai_tool_parts(
-    tool: rmcp::model::Tool,
-) -> Result<(String, JsonSchema, Option<JsonValue>), serde_json::Error> {
-    let rmcp::model::Tool {
-        description,
-        input_schema,
-        output_schema,
-        ..
-    } = tool;
-
-    let mut serialized_input_schema = serde_json::Value::Object(input_schema.as_ref().clone());
-
-    // OpenAI models mandate the "properties" field in the schema. Some MCP
-    // servers omit it (or set it to null), so we insert an empty object to
-    // match the behavior of the Agents SDK.
-    if let serde_json::Value::Object(obj) = &mut serialized_input_schema
-        && obj.get("properties").is_none_or(serde_json::Value::is_null)
-    {
-        obj.insert(
-            "properties".to_string(),
-            serde_json::Value::Object(serde_json::Map::new()),
-        );
-    }
-
-    let input_schema = parse_tool_input_schema(&serialized_input_schema)?;
-    let structured_content_schema = output_schema
-        .map(|output_schema| serde_json::Value::Object(output_schema.as_ref().clone()))
-        .unwrap_or_else(|| JsonValue::Object(serde_json::Map::new()));
-    let output_schema = Some(mcp_call_tool_result_output_schema(
-        structured_content_schema,
-    ));
-    let description = description.map(Into::into).unwrap_or_default();
-
-    Ok((description, input_schema, output_schema))
-}
-
-fn mcp_call_tool_result_output_schema(structured_content_schema: JsonValue) -> JsonValue {
-    json!({
-        "type": "object",
-        "properties": {
-            "content": {
-                "type": "array",
-                "items": {}
-            },
-            "structuredContent": structured_content_schema,
-            "isError": {
-                "type": "boolean"
-            },
-            "_meta": {}
-        },
-        "required": ["content"],
-        "additionalProperties": false
-    })
-}
-
 /// Builds the tool registry builder while collecting tool specs for later serialization.
 #[cfg(test)]
 pub(crate) fn build_specs(

codex-rs/core/src/tools/spec_tests.rs

@@ -63,139 +63,6 @@ fn search_capable_model_info() -> ModelInfo {
     model_info
 }
 
-#[test]
-fn mcp_tool_to_openai_tool_inserts_empty_properties() {
-    let mut schema = rmcp::model::JsonObject::new();
-    schema.insert("type".to_string(), serde_json::json!("object"));
-
-    let tool = rmcp::model::Tool {
-        name: "no_props".to_string().into(),
-        title: None,
-        description: Some("No properties".to_string().into()),
-        input_schema: std::sync::Arc::new(schema),
-        output_schema: None,
-        annotations: None,
-        execution: None,
-        icons: None,
-        meta: None,
-    };
-
-    let openai_tool =
-        mcp_tool_to_openai_tool("server/no_props".to_string(), tool).expect("convert tool");
-    let parameters = serde_json::to_value(openai_tool.parameters).expect("serialize schema");
-
-    assert_eq!(parameters.get("properties"), Some(&serde_json::json!({})));
-}
-
-#[test]
-fn mcp_tool_to_openai_tool_preserves_top_level_output_schema() {
-    let mut input_schema = rmcp::model::JsonObject::new();
-    input_schema.insert("type".to_string(), serde_json::json!("object"));
-
-    let mut output_schema = rmcp::model::JsonObject::new();
-    output_schema.insert(
-        "properties".to_string(),
-        serde_json::json!({
-            "result": {
-                "properties": {
-                    "nested": {}
-                }
-            }
-        }),
-    );
-    output_schema.insert("required".to_string(), serde_json::json!(["result"]));
-
-    let tool = rmcp::model::Tool {
-        name: "with_output".to_string().into(),
-        title: None,
-        description: Some("Has output schema".to_string().into()),
-        input_schema: std::sync::Arc::new(input_schema),
-        output_schema: Some(std::sync::Arc::new(output_schema)),
-        annotations: None,
-        execution: None,
-        icons: None,
-        meta: None,
-    };
-
-    let openai_tool = mcp_tool_to_openai_tool("mcp__server__with_output".to_string(), tool)
-        .expect("convert tool");
-
-    assert_eq!(
-        openai_tool.output_schema,
-        Some(serde_json::json!({
-            "type": "object",
-            "properties": {
-                "content": {
-                    "type": "array",
-                    "items": {}
-                },
-                "structuredContent": {
-                    "properties": {
-                        "result": {
-                            "properties": {
-                                "nested": {}
-                            }
-                        }
-                    },
-                    "required": ["result"]
-                },
-                "isError": {
-                    "type": "boolean"
-                },
-                "_meta": {}
-            },
-            "required": ["content"],
-            "additionalProperties": false
-        }))
-    );
-}
-
-#[test]
-fn mcp_tool_to_openai_tool_preserves_output_schema_without_inferred_type() {
-    let mut input_schema = rmcp::model::JsonObject::new();
-    input_schema.insert("type".to_string(), serde_json::json!("object"));
-
-    let mut output_schema = rmcp::model::JsonObject::new();
-    output_schema.insert("enum".to_string(), serde_json::json!(["ok", "error"]));
-
-    let tool = rmcp::model::Tool {
-        name: "with_enum_output".to_string().into(),
-        title: None,
-        description: Some("Has enum output schema".to_string().into()),
-        input_schema: std::sync::Arc::new(input_schema),
-        output_schema: Some(std::sync::Arc::new(output_schema)),
-        annotations: None,
-        execution: None,
-        icons: None,
-        meta: None,
-    };
-
-    let openai_tool = mcp_tool_to_openai_tool("mcp__server__with_enum_output".to_string(), tool)
-        .expect("convert tool");
-
-    assert_eq!(
-        openai_tool.output_schema,
-        Some(serde_json::json!({
-            "type": "object",
-            "properties": {
-                "content": {
-                    "type": "array",
-                    "items": {}
-                },
-                "structuredContent": {
-                    "enum": ["ok", "error"]
-                },
-                "isError": {
-                    "type": "boolean"
-                },
-                "_meta": {}
-            },
-            "required": ["content"],
-            "additionalProperties": false
-        }))
-    );
-}
-
 #[test]
 fn search_tool_deferred_tools_always_set_defer_loading_true() {
     let tool = mcp_tool(

codex-rs/tools/Cargo.toml

@@ -8,6 +8,12 @@ version.workspace = true
 workspace = true
 
 [dependencies]
+rmcp = { workspace = true, default-features = false, features = [
+    "base64",
+    "macros",
+    "schemars",
+    "server",
+] }
 serde = { workspace = true, features = ["derive"] }
 serde_json = { workspace = true }
 

codex-rs/tools/README.md

@@ -4,12 +4,15 @@
 shared across multiple crates and does not need to stay coupled to
 `codex-core`.
 
-Today this crate is intentionally small. It only owns the shared tool input
+Today this crate is intentionally small. It currently owns the shared tool
-schema model and parser that were previously defined in `core/src/tools/spec.rs`:
+schema primitives that no longer need to live in `core/src/tools/spec.rs`:
 
 - `JsonSchema`
 - `AdditionalProperties`
 - `parse_tool_input_schema()`
+- `ParsedMcpTool`
+- `parse_mcp_tool()`
+- `mcp_call_tool_result_output_schema()`
 
 That extraction is the first step in a longer migration. The goal is not to
 move all of `core/src/tools` into this crate in one shot. Instead, the plan is

codex-rs/tools/src/lib.rs

@@ -1,7 +1,11 @@
 //! Shared tool-schema parsing primitives that can live outside `codex-core`.
 
 mod json_schema;
+mod mcp_tool;
 
 pub use json_schema::AdditionalProperties;
 pub use json_schema::JsonSchema;
 pub use json_schema::parse_tool_input_schema;
+pub use mcp_tool::ParsedMcpTool;
+pub use mcp_tool::mcp_call_tool_result_output_schema;
+pub use mcp_tool::parse_mcp_tool;

codex-rs/tools/src/mcp_tool.rs

@@ -0,0 +1,65 @@
+use crate::JsonSchema;
+use crate::parse_tool_input_schema;
+use serde_json::Value as JsonValue;
+use serde_json::json;
+
+/// Parsed MCP tool metadata and schemas that can be adapted into a higher-level
+/// tool spec by downstream crates.
+#[derive(Debug, PartialEq)]
+pub struct ParsedMcpTool {
+    pub description: String,
+    pub input_schema: JsonSchema,
+    pub output_schema: JsonValue,
+}
+
+pub fn parse_mcp_tool(tool: &rmcp::model::Tool) -> Result<ParsedMcpTool, serde_json::Error> {
+    let mut serialized_input_schema = serde_json::Value::Object(tool.input_schema.as_ref().clone());
+
+    // OpenAI models mandate the "properties" field in the schema. Some MCP
+    // servers omit it (or set it to null), so we insert an empty object to
+    // match the behavior of the Agents SDK.
+    if let serde_json::Value::Object(obj) = &mut serialized_input_schema
+        && obj.get("properties").is_none_or(serde_json::Value::is_null)
+    {
+        obj.insert(
+            "properties".to_string(),
+            serde_json::Value::Object(serde_json::Map::new()),
+        );
+    }
+
+    let input_schema = parse_tool_input_schema(&serialized_input_schema)?;
+    let structured_content_schema = tool
+        .output_schema
+        .as_ref()
+        .map(|output_schema| serde_json::Value::Object(output_schema.as_ref().clone()))
+        .unwrap_or_else(|| JsonValue::Object(serde_json::Map::new()));
+
+    Ok(ParsedMcpTool {
+        description: tool.description.clone().map(Into::into).unwrap_or_default(),
+        input_schema,
+        output_schema: mcp_call_tool_result_output_schema(structured_content_schema),
+    })
+}
+
+pub fn mcp_call_tool_result_output_schema(structured_content_schema: JsonValue) -> JsonValue {
+    json!({
+        "type": "object",
+        "properties": {
+            "content": {
+                "type": "array",
+                "items": {}
+            },
+            "structuredContent": structured_content_schema,
+            "isError": {
+                "type": "boolean"
+            },
+            "_meta": {}
+        },
+        "required": ["content"],
+        "additionalProperties": false
+    })
+}
+
+#[cfg(test)]
+#[path = "mcp_tool_tests.rs"]
+mod tests;

codex-rs/tools/src/mcp_tool_tests.rs

@@ -0,0 +1,120 @@
+use super::ParsedMcpTool;
+use super::mcp_call_tool_result_output_schema;
+use super::parse_mcp_tool;
+use crate::JsonSchema;
+use pretty_assertions::assert_eq;
+use std::collections::BTreeMap;
+
+fn mcp_tool(name: &str, description: &str, input_schema: serde_json::Value) -> rmcp::model::Tool {
+    rmcp::model::Tool {
+        name: name.to_string().into(),
+        title: None,
+        description: Some(description.to_string().into()),
+        input_schema: std::sync::Arc::new(rmcp::model::object(input_schema)),
+        output_schema: None,
+        annotations: None,
+        execution: None,
+        icons: None,
+        meta: None,
+    }
+}
+
+#[test]
+fn parse_mcp_tool_inserts_empty_properties() {
+    let tool = mcp_tool(
+        "no_props",
+        "No properties",
+        serde_json::json!({
+            "type": "object"
+        }),
+    );
+
+    assert_eq!(
+        parse_mcp_tool(&tool).expect("parse MCP tool"),
+        ParsedMcpTool {
+            description: "No properties".to_string(),
+            input_schema: JsonSchema::Object {
+                properties: BTreeMap::new(),
+                required: None,
+                additional_properties: None,
+            },
+            output_schema: mcp_call_tool_result_output_schema(serde_json::json!({})),
+        }
+    );
+}
+
+#[test]
+fn parse_mcp_tool_preserves_top_level_output_schema() {
+    let mut tool = mcp_tool(
+        "with_output",
+        "Has output schema",
+        serde_json::json!({
+            "type": "object"
+        }),
+    );
+    tool.output_schema = Some(std::sync::Arc::new(rmcp::model::object(
+        serde_json::json!({
+            "properties": {
+                "result": {
+                    "properties": {
+                        "nested": {}
+                    }
+                }
+            },
+            "required": ["result"]
+        }),
+    )));
+
+    assert_eq!(
+        parse_mcp_tool(&tool).expect("parse MCP tool"),
+        ParsedMcpTool {
+            description: "Has output schema".to_string(),
+            input_schema: JsonSchema::Object {
+                properties: BTreeMap::new(),
+                required: None,
+                additional_properties: None,
+            },
+            output_schema: mcp_call_tool_result_output_schema(serde_json::json!({
+                "properties": {
+                    "result": {
+                        "properties": {
+                            "nested": {}
+                        }
+                    }
+                },
+                "required": ["result"]
+            })),
+        }
+    );
+}
+
+#[test]
+fn parse_mcp_tool_preserves_output_schema_without_inferred_type() {
+    let mut tool = mcp_tool(
+        "with_enum_output",
+        "Has enum output schema",
+        serde_json::json!({
+            "type": "object"
+        }),
+    );
+    tool.output_schema = Some(std::sync::Arc::new(rmcp::model::object(
+        serde_json::json!({
+            "enum": ["ok", "error"]
+        }),
+    )));
+
+    assert_eq!(
+        parse_mcp_tool(&tool).expect("parse MCP tool"),
+        ParsedMcpTool {
+            description: "Has enum output schema".to_string(),
+            input_schema: JsonSchema::Object {
+                properties: BTreeMap::new(),
+                required: None,
+                additional_properties: None,
+            },
+            output_schema: mcp_call_tool_result_output_schema(serde_json::json!({
+                "enum": ["ok", "error"]
+            })),
+        }
+    );
+}