diff --git a/Cargo.toml b/Cargo.toml
index a94707f..bc966dd 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -1,3 +1,3 @@
 [workspace]
-members = ["sunbeam-sdk", "sunbeam"]
+members = ["sunbeam-sdk", "sunbeam", "sunbeam-proto"]
 resolver = "3"
diff --git a/sunbeam-proto/Cargo.toml b/sunbeam-proto/Cargo.toml
new file mode 100644
index 0000000..68f03d7
--- /dev/null
+++ b/sunbeam-proto/Cargo.toml
@@ -0,0 +1,12 @@
+[package]
+name = "sunbeam-proto"
+version = "0.1.0"
+edition = "2024"
+description = "Shared protobuf definitions for Sunbeam gRPC services"
+
+[dependencies]
+tonic = "0.13"
+prost = "0.13"
+
+[build-dependencies]
+tonic-build = "0.13"
diff --git a/sunbeam-proto/build.rs b/sunbeam-proto/build.rs
new file mode 100644
index 0000000..457a429
--- /dev/null
+++ b/sunbeam-proto/build.rs
@@ -0,0 +1,4 @@
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    tonic_build::compile_protos("proto/code.proto")?;
+    Ok(())
+}
diff --git a/sunbeam-proto/proto/code.proto b/sunbeam-proto/proto/code.proto
new file mode 100644
index 0000000..a15cfd2
--- /dev/null
+++ b/sunbeam-proto/proto/code.proto
@@ -0,0 +1,121 @@
+syntax = "proto3";
+package sunbeam.code.v1;
+
+// Sol's coding agent service. Bidirectional streaming between
+// the `sunbeam code` TUI client and Sol's server-side agent loop.
+service CodeAgent {
+  rpc Session(stream ClientMessage) returns (stream ServerMessage);
+}
+
+// ── Client → Sol ───────────────────────────────────────────────
+
+message ClientMessage {
+  oneof payload {
+    StartSession start = 1;
+    UserInput input = 2;
+    ToolResult tool_result = 3;
+    ToolApproval approval = 4;
+    EndSession end = 5;
+  }
+}
+
+message StartSession {
+  string project_path = 1;
+  string prompt_md = 2;
+  string config_toml = 3;
+  string git_branch = 4;
+  string git_status = 5;
+  repeated string file_tree = 6;
+  string model = 7;
+  repeated ToolDef client_tools = 8;
+}
+
+message UserInput {
+  string text = 1;
+}
+
+message ToolResult {
+  string call_id = 1;
+  string result = 2;
+  bool is_error = 3;
+}
+
+message ToolApproval {
+  string call_id = 1;
+  bool approved = 2;
+}
+
+message EndSession {}
+
+// ── Sol → Client ───────────────────────────────────────────────
+
+message ServerMessage {
+  oneof payload {
+    SessionReady ready = 1;
+    TextDelta delta = 2;
+    TextDone done = 3;
+    ToolCall tool_call = 4;
+    ApprovalNeeded approval = 5;
+    Status status = 6;
+    SessionEnd end = 7;
+    Error error = 8;
+  }
+}
+
+message SessionReady {
+  string session_id = 1;
+  string room_id = 2;
+  string model = 3;
+}
+
+message TextDelta {
+  string text = 1;
+}
+
+message TextDone {
+  string full_text = 1;
+  uint32 input_tokens = 2;
+  uint32 output_tokens = 3;
+}
+
+message ToolCall {
+  string call_id = 1;
+  string name = 2;
+  string args_json = 3;
+  bool is_local = 4;
+  bool needs_approval = 5;
+}
+
+message ApprovalNeeded {
+  string call_id = 1;
+  string name = 2;
+  string args_json = 3;
+  string summary = 4;
+}
+
+message Status {
+  string message = 1;
+  StatusKind kind = 2;
+}
+
+enum StatusKind {
+  INFO = 0;
+  TOOL_RUNNING = 1;
+  TOOL_DONE = 2;
+  THINKING = 3;
+}
+
+message SessionEnd {
+  string summary = 1;
+}
+
+message Error {
+  string message = 1;
+  bool fatal = 2;
+}
+
+message ToolDef {
+  string name = 1;
+  string description = 2;
+  string schema_json = 3;
+}
diff --git a/sunbeam-proto/src/lib.rs b/sunbeam-proto/src/lib.rs
new file mode 100644
index 0000000..a5f9e99
--- /dev/null
+++ b/sunbeam-proto/src/lib.rs
@@ -0,0 +1,3 @@
+pub mod sunbeam_code_v1 {
+    tonic::include_proto!("sunbeam.code.v1");
+}