feat(unstable-v2): add session injection schema

agent-client-protocol-schema/Cargo.toml

@@ -33,6 +33,7 @@ unstable = [
     "unstable_nes",
     "unstable_plan_operations",
     "unstable_session_fork",
+    "unstable_session_inject",
     "unstable_session_compaction",
     "unstable_session_notices",
     "unstable_end_turn_token_usage",
@@ -42,6 +43,9 @@ unstable = [
 # It introduces a parallel `v2` module with a different wire version, so it
 # must be opted into explicitly.
 unstable_protocol_v2 = []
+# Session injection is a v2-only draft extension. Pair it with
+# `unstable_protocol_v2` to expose the v2 types.
+unstable_session_inject = []
 unstable_llm_providers = []
 unstable_mcp_over_acp = []
 unstable_nes = []

agent-client-protocol-schema/src/v2/agent.rs

@@ -20,6 +20,14 @@ use super::{
 };
 use crate::{IntoOption, ProtocolVersion, SkipListener};
 
+#[cfg(feature = "unstable_session_inject")]
+use super::session_inject::{
+    InjectSessionRequest, InjectSessionResponse, ReplaceInjectSessionRequest,
+    ReplaceInjectSessionResponse, RevokeInjectSessionRequest, RevokeInjectSessionResponse,
+    SESSION_INJECT_METHOD_NAME, SESSION_REPLACE_INJECT_METHOD_NAME,
+    SESSION_REVOKE_INJECT_METHOD_NAME, SessionInjectCapabilities,
+};
+
 #[cfg(feature = "unstable_mcp_over_acp")]
 use super::mcp::{
     MCP_MESSAGE_METHOD_NAME, MessageMcpNotification, MessageMcpRequest, MessageMcpResponse,
@@ -4162,6 +4170,19 @@ pub struct SessionCapabilities {
     ///
     /// This capability is not part of the spec yet, and may be removed or changed at any point.
     ///
+    /// Mid-turn user-message injection supported by the agent.
+    ///
+    /// Optional. Omitted or `null` means the agent does not support
+    /// `session/inject` or the mandatory `session/revoke_inject` companion method.
+    #[cfg(feature = "unstable_session_inject")]
+    #[serde_as(deserialize_as = "DefaultOnError")]
+    #[cfg_attr(feature = "schemars", schemars(extend("x-deserialize-default-on-error" = true)))]
+    #[serde(default)]
+    pub inject: Option<SessionInjectCapabilities>,
+    /// **UNSTABLE**
+    ///
+    /// This capability is not part of the spec yet, and may be removed or changed at any point.
+    ///
     /// Whether the agent supports `session/fork`.
     ///
     /// Optional. Omitted or `null` both mean the agent does not advertise support.
@@ -4238,6 +4259,17 @@ impl SessionCapabilities {
         self
     }
 
+    #[cfg(feature = "unstable_session_inject")]
+    /// Mid-turn user-message injection supported by the agent.
+    ///
+    /// Advertising this capability also requires support for
+    /// `session/revoke_inject`.
+    #[must_use]
+    pub fn inject(mut self, inject: impl IntoOption<SessionInjectCapabilities>) -> Self {
+        self.inject = inject.into_option();
+        self
+    }
+
     #[cfg(feature = "unstable_session_fork")]
     /// Whether the agent supports `session/fork`.
     ///
@@ -4926,6 +4958,15 @@ pub struct AgentMethodNames {
     pub session_set_config_option: &'static str,
     /// Method for sending a prompt to the agent.
     pub session_prompt: &'static str,
+    /// Method for injecting a message into a running session.
+    #[cfg(feature = "unstable_session_inject")]
+    pub session_inject: &'static str,
+    /// Method for revoking a pending injected message.
+    #[cfg(feature = "unstable_session_inject")]
+    pub session_revoke_inject: &'static str,
+    /// Method for replacing a pending injected message.
+    #[cfg(feature = "unstable_session_inject")]
+    pub session_replace_inject: &'static str,
     /// Notification for cancelling operations.
     pub session_cancel: &'static str,
     /// Method for exchanging MCP-over-ACP messages.
@@ -4989,6 +5030,12 @@ pub const AGENT_METHOD_NAMES: AgentMethodNames = AgentMethodNames {
     session_new: SESSION_NEW_METHOD_NAME,
     session_set_config_option: SESSION_SET_CONFIG_OPTION_METHOD_NAME,
     session_prompt: SESSION_PROMPT_METHOD_NAME,
+    #[cfg(feature = "unstable_session_inject")]
+    session_inject: SESSION_INJECT_METHOD_NAME,
+    #[cfg(feature = "unstable_session_inject")]
+    session_revoke_inject: SESSION_REVOKE_INJECT_METHOD_NAME,
+    #[cfg(feature = "unstable_session_inject")]
+    session_replace_inject: SESSION_REPLACE_INJECT_METHOD_NAME,
     session_cancel: SESSION_CANCEL_METHOD_NAME,
     #[cfg(feature = "unstable_mcp_over_acp")]
     mcp_message: MCP_MESSAGE_METHOD_NAME,
@@ -5184,6 +5231,27 @@ pub enum ClientRequest {
     ///
     /// See protocol docs: [Prompt Lifecycle](https://agentclientprotocol.com/protocol/prompt-lifecycle)
     PromptRequest(Box<PromptRequest>),
+    /// **UNSTABLE**
+    ///
+    /// This capability is not part of the spec yet, and may be removed or changed at any point.
+    ///
+    /// Injects a user message for pending delivery.
+    #[cfg(feature = "unstable_session_inject")]
+    InjectSessionRequest(Box<InjectSessionRequest>),
+    /// **UNSTABLE**
+    ///
+    /// This capability is not part of the spec yet, and may be removed or changed at any point.
+    ///
+    /// Revokes a pending injected message. Mandatory when injection is advertised.
+    #[cfg(feature = "unstable_session_inject")]
+    RevokeInjectSessionRequest(Box<RevokeInjectSessionRequest>),
+    /// **UNSTABLE**
+    ///
+    /// This capability is not part of the spec yet, and may be removed or changed at any point.
+    ///
+    /// Replaces pending injected content when `pending.replace` is advertised.
+    #[cfg(feature = "unstable_session_inject")]
+    ReplaceInjectSessionRequest(Box<ReplaceInjectSessionRequest>),
     #[cfg(feature = "unstable_nes")]
     /// **UNSTABLE**
     ///
@@ -5247,6 +5315,12 @@ impl ClientRequest {
             Self::CloseSessionRequest(_) => AGENT_METHOD_NAMES.session_close,
             Self::SetSessionConfigOptionRequest(_) => AGENT_METHOD_NAMES.session_set_config_option,
             Self::PromptRequest(_) => AGENT_METHOD_NAMES.session_prompt,
+            #[cfg(feature = "unstable_session_inject")]
+            Self::InjectSessionRequest(_) => AGENT_METHOD_NAMES.session_inject,
+            #[cfg(feature = "unstable_session_inject")]
+            Self::RevokeInjectSessionRequest(_) => AGENT_METHOD_NAMES.session_revoke_inject,
+            #[cfg(feature = "unstable_session_inject")]
+            Self::ReplaceInjectSessionRequest(_) => AGENT_METHOD_NAMES.session_replace_inject,
             #[cfg(feature = "unstable_nes")]
             Self::StartNesRequest(_) => AGENT_METHOD_NAMES.nes_start,
             #[cfg(feature = "unstable_nes")]
@@ -5304,6 +5378,15 @@ pub enum AgentResponse {
     SetSessionConfigOptionResponse(Box<SetSessionConfigOptionResponse>),
     /// Successful result returned for a `session/prompt` request.
     PromptResponse(Box<PromptResponse>),
+    /// Successful result returned for a `session/inject` request.
+    #[cfg(feature = "unstable_session_inject")]
+    InjectSessionResponse(Box<InjectSessionResponse>),
+    /// Successful result returned for a `session/revoke_inject` request.
+    #[cfg(feature = "unstable_session_inject")]
+    RevokeInjectSessionResponse(#[serde(default)] Box<RevokeInjectSessionResponse>),
+    /// Successful result returned for a `session/replace_inject` request.
+    #[cfg(feature = "unstable_session_inject")]
+    ReplaceInjectSessionResponse(Box<ReplaceInjectSessionResponse>),
     /// Successful result returned for a `nes/start` request.
     #[cfg(feature = "unstable_nes")]
     StartNesResponse(Box<StartNesResponse>),

agent-client-protocol-schema/src/v2/error.rs

@@ -127,6 +127,44 @@ impl Error {
         }
     }
 
+    /// A steer was rejected because the session has no running turn.
+    #[cfg(feature = "unstable_session_inject")]
+    #[must_use]
+    pub fn inject_no_running_turn() -> Self {
+        Self::from(ErrorCode::InjectPreconditionFailed)
+            .data(serde_json::json!({ "reason": "no_running_turn" }))
+    }
+
+    /// A pending injected message was delivered before it could be revoked or replaced.
+    #[cfg(feature = "unstable_session_inject")]
+    #[must_use]
+    pub fn inject_already_delivered(message_id: impl Into<super::MessageId>) -> Self {
+        Self::from(ErrorCode::InjectPreconditionFailed).data(serde_json::json!({
+            "reason": "already_delivered",
+            "messageId": message_id.into(),
+        }))
+    }
+
+    /// Pending injected-message replacement is not supported.
+    #[cfg(feature = "unstable_session_inject")]
+    #[must_use]
+    pub fn inject_replace_not_supported(message_id: impl Into<super::MessageId>) -> Self {
+        Self::from(ErrorCode::InjectPreconditionFailed).data(serde_json::json!({
+            "reason": "replace_not_supported",
+            "messageId": message_id.into(),
+        }))
+    }
+
+    /// The injected message ID is unknown for the requested session.
+    #[cfg(feature = "unstable_session_inject")]
+    #[must_use]
+    pub fn inject_unknown_message_id(message_id: impl Into<super::MessageId>) -> Self {
+        Self::from(ErrorCode::ResourceNotFound).data(serde_json::json!({
+            "reason": "unknown_message_id",
+            "messageId": message_id.into(),
+        }))
+    }
+
     /// Converts a standard error into an internal JSON-RPC error.
     ///
     /// The error's string representation is included as additional data.
@@ -185,6 +223,21 @@ pub enum ErrorCode {
     #[cfg_attr(feature = "schemars", schemars(transform = error_code_transform))]
     #[strum(to_string = "Resource not found")]
     ResourceNotFound, // -32002
+    /// **UNSTABLE**
+    ///
+    /// This error is not part of the spec yet, and may be removed or changed at any point.
+    ///
+    /// A session injection precondition failed.
+    ///
+    /// `error.data` uses a `reason` of `already_delivered`, `no_running_turn`,
+    /// or `replace_not_supported`. Pending-message failures also include the
+    /// `messageId`. Unknown message IDs instead use `-32002` with
+    /// `data: { reason: "unknown_message_id", messageId }`. The error data remains
+    /// open JSON, consistent with the protocol's shared [`Error::data`] field.
+    #[cfg(feature = "unstable_session_inject")]
+    #[cfg_attr(feature = "schemars", schemars(transform = error_code_transform))]
+    #[strum(to_string = "Inject precondition failed")]
+    InjectPreconditionFailed, // -32010
     /// Other undefined error code.
     #[cfg_attr(feature = "schemars", schemars(untagged))]
     #[strum(to_string = "Unknown error")]
@@ -202,6 +255,8 @@ impl From<i32> for ErrorCode {
             -32800 => ErrorCode::RequestCancelled,
             -32000 => ErrorCode::AuthRequired,
             -32002 => ErrorCode::ResourceNotFound,
+            #[cfg(feature = "unstable_session_inject")]
+            -32010 => ErrorCode::InjectPreconditionFailed,
             _ => ErrorCode::Other(value),
         }
     }
@@ -218,6 +273,8 @@ impl From<ErrorCode> for i32 {
             ErrorCode::RequestCancelled => -32800,
             ErrorCode::AuthRequired => -32000,
             ErrorCode::ResourceNotFound => -32002,
+            #[cfg(feature = "unstable_session_inject")]
+            ErrorCode::InjectPreconditionFailed => -32010,
             ErrorCode::Other(value) => value,
         }
     }
@@ -245,6 +302,8 @@ fn error_code_transform(schema: &mut Schema) {
         "RequestCancelled" => ErrorCode::RequestCancelled,
         "AuthRequired" => ErrorCode::AuthRequired,
         "ResourceNotFound" => ErrorCode::ResourceNotFound,
+        #[cfg(feature = "unstable_session_inject")]
+        "InjectPreconditionFailed" => ErrorCode::InjectPreconditionFailed,
         _ => panic!("Unexpected error code name {name}"),
     };
     let mut description = schema
@@ -340,4 +399,50 @@ mod tests {
             );
         }
     }
+
+    #[cfg(feature = "unstable_session_inject")]
+    #[test]
+    fn session_inject_errors_include_required_data() {
+        assert_eq!(
+            serde_json::to_value(Error::inject_no_running_turn()).unwrap(),
+            serde_json::json!({
+                "code": -32010,
+                "message": "Inject precondition failed",
+                "data": { "reason": "no_running_turn" },
+            })
+        );
+        assert_eq!(
+            serde_json::to_value(Error::inject_already_delivered("message-1")).unwrap(),
+            serde_json::json!({
+                "code": -32010,
+                "message": "Inject precondition failed",
+                "data": {
+                    "reason": "already_delivered",
+                    "messageId": "message-1",
+                },
+            })
+        );
+        assert_eq!(
+            serde_json::to_value(Error::inject_replace_not_supported("message-1")).unwrap(),
+            serde_json::json!({
+                "code": -32010,
+                "message": "Inject precondition failed",
+                "data": {
+                    "reason": "replace_not_supported",
+                    "messageId": "message-1",
+                },
+            })
+        );
+        assert_eq!(
+            serde_json::to_value(Error::inject_unknown_message_id("message-1")).unwrap(),
+            serde_json::json!({
+                "code": -32002,
+                "message": "Resource not found",
+                "data": {
+                    "reason": "unknown_message_id",
+                    "messageId": "message-1",
+                },
+            })
+        );
+    }
 }

agent-client-protocol-schema/src/v2/mod.rs

@@ -23,6 +23,8 @@ mod plan;
 mod protocol_level;
 #[cfg(feature = "schemars")]
 pub(crate) mod schema_util;
+#[cfg(feature = "unstable_session_inject")]
+mod session_inject;
 mod terminal;
 mod tool_call;
 
@@ -41,6 +43,8 @@ pub use nes::*;
 pub use plan::*;
 pub use protocol_level::*;
 pub use serde_json::value::RawValue;
+#[cfg(feature = "unstable_session_inject")]
+pub use session_inject::*;
 pub use terminal::*;
 pub use tool_call::*;