diff --git a/Cargo.lock b/Cargo.lock
index 443dbf8..d641a42 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -384,6 +384,12 @@ dependencies = [
  "wasip2",
 ]
 
+[[package]]
+name = "glob"
+version = "0.3.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0cc23270f6e1808e30a928bdc84dea0b9b4136a8bc82338574f23baf47bbd280"
+
 [[package]]
 name = "h2"
 version = "0.4.13"
@@ -726,6 +732,7 @@ dependencies = [
  "tokio-util",
  "tracing",
  "tracing-subscriber",
+ "trybuild",
 ]
 
 [[package]]
@@ -1178,6 +1185,15 @@ dependencies = [
  "zmij",
 ]
 
+[[package]]
+name = "serde_spanned"
+version = "1.0.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f8bbf91e5a4d6315eee45e704372590b30e260ee83af6639d64557f51b067776"
+dependencies = [
+ "serde_core",
+]
+
 [[package]]
 name = "sharded-slab"
 version = "0.1.7"
@@ -1264,6 +1280,12 @@ dependencies = [
  "syn",
 ]
 
+[[package]]
+name = "target-triple"
+version = "1.0.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "591ef38edfb78ca4771ee32cf494cb8771944bee237a9b91fc9c1424ac4b777b"
+
 [[package]]
 name = "tempfile"
 version = "3.24.0"
@@ -1277,6 +1299,15 @@ dependencies = [
  "windows-sys 0.61.2",
 ]
 
+[[package]]
+name = "termcolor"
+version = "1.4.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "06794f8f6c5c898b3275aebefa6b8a1cb24cd2c6c79397ab15774837a0bc5755"
+dependencies = [
+ "winapi-util",
+]
+
 [[package]]
 name = "thiserror"
 version = "2.0.17"
@@ -1375,6 +1406,45 @@ dependencies = [
  "tokio",
 ]
 
+[[package]]
+name = "toml"
+version = "1.0.3+spec-1.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c7614eaf19ad818347db24addfa201729cf2a9b6fdfd9eb0ab870fcacc606c0c"
+dependencies = [
+ "indexmap",
+ "serde_core",
+ "serde_spanned",
+ "toml_datetime",
+ "toml_parser",
+ "toml_writer",
+ "winnow",
+]
+
+[[package]]
+name = "toml_datetime"
+version = "1.0.0+spec-1.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "32c2555c699578a4f59f0cc68e5116c8d7cabbd45e1409b989d4be085b53f13e"
+dependencies = [
+ "serde_core",
+]
+
+[[package]]
+name = "toml_parser"
+version = "1.0.9+spec-1.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "702d4415e08923e7e1ef96cd5727c0dfed80b4d2fa25db9647fe5eb6f7c5a4c4"
+dependencies = [
+ "winnow",
+]
+
+[[package]]
+name = "toml_writer"
+version = "1.0.6+spec-1.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ab16f14aed21ee8bfd8ec22513f7287cd4a91aa92e44edfe2c17ddd004e92607"
+
 [[package]]
 name = "tower"
 version = "0.5.2"
@@ -1487,6 +1557,21 @@ version = "0.2.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "e421abadd41a4225275504ea4d6566923418b7f05506fbc9c0fe86ba7396114b"
 
+[[package]]
+name = "trybuild"
+version = "1.0.116"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "47c635f0191bd3a2941013e5062667100969f8c4e9cd787c14f977265d73616e"
+dependencies = [
+ "glob",
+ "serde",
+ "serde_derive",
+ "serde_json",
+ "target-triple",
+ "termcolor",
+ "toml",
+]
+
 [[package]]
 name = "unicode-ident"
 version = "1.0.22"
@@ -1640,6 +1725,15 @@ dependencies = [
  "wasm-bindgen",
 ]
 
+[[package]]
+name = "winapi-util"
+version = "0.1.11"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c2a7b1c03c876122aa43f3020e6c3c3ee5c05081c9a00739faf7503aeba10d22"
+dependencies = [
+ "windows-sys 0.61.2",
+]
+
 [[package]]
 name = "windows-link"
 version = "0.2.1"
@@ -1802,6 +1896,12 @@ version = "0.53.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "d6bbff5f0aada427a1e5a6da5f1f98158182f26556f345ac9e04d36d0ebed650"
 
+[[package]]
+name = "winnow"
+version = "0.7.14"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5a5364e9d77fcdeeaa6062ced926ee3381faa2ee02d3eb83a5c27a8825540829"
+
 [[package]]
 name = "wit-bindgen"
 version = "0.46.0"
diff --git a/llm-worker/Cargo.toml b/llm-worker/Cargo.toml
index 727ed92..e268175 100644
--- a/llm-worker/Cargo.toml
+++ b/llm-worker/Cargo.toml
@@ -26,3 +26,4 @@ schemars = "1.2"
 tempfile = "3.24"
 dotenv = "0.15"
 tracing-subscriber = { version = "0.3", features = ["env-filter"] }
+trybuild = "1.0.116"
diff --git a/llm-worker/tests/compile_fail.rs b/llm-worker/tests/compile_fail.rs
new file mode 100644
index 0000000..fa876c2
--- /dev/null
+++ b/llm-worker/tests/compile_fail.rs
@@ -0,0 +1,6 @@
+#[test]
+fn compile_fail_state_constraints() {
+    let t = trybuild::TestCases::new();
+    t.compile_fail("tests/ui/cache_locked_register_tool.rs");
+    t.compile_fail("tests/ui/tool_server_handle_register_tool.rs");
+}
diff --git a/llm-worker/tests/ui/cache_locked_register_tool.rs b/llm-worker/tests/ui/cache_locked_register_tool.rs
new file mode 100644
index 0000000..4f8be01
--- /dev/null
+++ b/llm-worker/tests/ui/cache_locked_register_tool.rs
@@ -0,0 +1,11 @@
+use llm_worker::Worker;
+use llm_worker::llm_client::providers::ollama::OllamaClient;
+use std::sync::Arc;
+
+fn main() {
+    let client = OllamaClient::new("dummy-model");
+    let worker = Worker::new(client);
+    let mut locked = worker.lock();
+    let def: llm_worker::tool::ToolDefinition = Arc::new(|| panic!("unused"));
+    let _ = locked.register_tool(def);
+}
diff --git a/llm-worker/tests/ui/cache_locked_register_tool.stderr b/llm-worker/tests/ui/cache_locked_register_tool.stderr
new file mode 100644
index 0000000..0c3b097
--- /dev/null
+++ b/llm-worker/tests/ui/cache_locked_register_tool.stderr
@@ -0,0 +1,8 @@
+error[E0599]: no method named `register_tool` found for struct `Worker<OllamaClient, CacheLocked>` in the current scope
+  --> tests/ui/cache_locked_register_tool.rs:10:20
+   |
+10 |     let _ = locked.register_tool(def);
+   |                    ^^^^^^^^^^^^^ method not found in `Worker<OllamaClient, CacheLocked>`
+   |
+   = note: the method was found for
+           - `Worker<C>`
diff --git a/llm-worker/tests/ui/tool_server_handle_register_tool.rs b/llm-worker/tests/ui/tool_server_handle_register_tool.rs
new file mode 100644
index 0000000..ff75d0c
--- /dev/null
+++ b/llm-worker/tests/ui/tool_server_handle_register_tool.rs
@@ -0,0 +1,11 @@
+use llm_worker::Worker;
+use llm_worker::llm_client::providers::ollama::OllamaClient;
+use std::sync::Arc;
+
+fn main() {
+    let client = OllamaClient::new("dummy-model");
+    let worker = Worker::new(client);
+    let handle = worker.tool_server_handle();
+    let def: llm_worker::tool::ToolDefinition = Arc::new(|| panic!("unused"));
+    let _ = handle.register_tool(def);
+}
diff --git a/llm-worker/tests/ui/tool_server_handle_register_tool.stderr b/llm-worker/tests/ui/tool_server_handle_register_tool.stderr
new file mode 100644
index 0000000..57d7139
--- /dev/null
+++ b/llm-worker/tests/ui/tool_server_handle_register_tool.stderr
@@ -0,0 +1,13 @@
+error[E0624]: method `register_tool` is private
+  --> tests/ui/tool_server_handle_register_tool.rs:10:20
+   |
+10 |       let _ = handle.register_tool(def);
+   |                      ^^^^^^^^^^^^^ private method
+   |
+  ::: src/tool_server.rs
+   |
+   | /     pub(crate) fn register_tool(
+   | |         &self,
+   | |         factory: WorkerToolDefinition,
+   | |     ) -> Result<(), ToolServerError> {
+   | |____________________________________- private method defined here
diff --git a/llm-worker/tests/worker_state_test.rs b/llm-worker/tests/worker_state_test.rs
index 496f6da..c4f92a0 100644
--- a/llm-worker/tests/worker_state_test.rs
+++ b/llm-worker/tests/worker_state_test.rs
@@ -5,9 +5,14 @@
 
 mod common;
 
+use std::sync::Arc;
+use std::sync::atomic::{AtomicUsize, Ordering};
+
+use async_trait::async_trait;
 use common::MockLlmClient;
 use llm_worker::Worker;
 use llm_worker::llm_client::event::{Event, ResponseStatus, StatusEvent};
+use llm_worker::tool::{Tool, ToolDefinition, ToolError, ToolMeta};
 use llm_worker::Item;
 
 // =============================================================================
@@ -91,6 +96,56 @@ fn test_mutable_extend_history() {
     assert_eq!(worker.history().len(), 4);
 }
 
+#[derive(Clone)]
+struct CountingTool {
+    name: String,
+    calls: Arc<AtomicUsize>,
+}
+
+impl CountingTool {
+    fn new(name: impl Into<String>) -> Self {
+        Self {
+            name: name.into(),
+            calls: Arc::new(AtomicUsize::new(0)),
+        }
+    }
+
+    fn definition(&self) -> ToolDefinition {
+        let tool = self.clone();
+        Arc::new(move || {
+            (
+                ToolMeta::new(&tool.name)
+                    .description("Counting tool")
+                    .input_schema(serde_json::json!({"type":"object","properties":{}})),
+                Arc::new(tool.clone()) as Arc<dyn Tool>,
+            )
+        })
+    }
+
+    fn call_count(&self) -> usize {
+        self.calls.load(Ordering::SeqCst)
+    }
+}
+
+#[async_trait]
+impl Tool for CountingTool {
+    async fn execute(&self, _input_json: &str) -> Result<String, ToolError> {
+        self.calls.fetch_add(1, Ordering::SeqCst);
+        Ok(format!("{}-ok", self.name))
+    }
+}
+
+/// Verify that tools can be registered in Mutable state.
+#[test]
+fn test_mutable_can_register_tool() {
+    let client = MockLlmClient::new(vec![]);
+    let mut worker = Worker::new(client);
+    let tool = CountingTool::new("count_tool");
+
+    let result = worker.register_tool(tool.definition());
+    assert!(result.is_ok(), "Mutable should allow tool registration");
+}
+
 // =============================================================================
 // State Transition Tests
 // =============================================================================
@@ -330,6 +385,67 @@ async fn test_unlock_edit_relock() {
     assert_eq!(relocked.locked_prefix_len(), 1);
 }
 
+/// Verify that tools registered before lock and after unlock remain effective.
+#[tokio::test]
+async fn test_lock_unlock_relock_tools_remain_effective() {
+    let client = MockLlmClient::with_responses(vec![
+        vec![
+            Event::tool_use_start(0, "call_1", "tool_a"),
+            Event::tool_input_delta(0, r#"{}"#),
+            Event::tool_use_stop(0),
+            Event::Status(StatusEvent {
+                status: ResponseStatus::Completed,
+            }),
+        ],
+        vec![
+            Event::text_block_start(0),
+            Event::text_delta(0, "done-a"),
+            Event::text_block_stop(0, None),
+            Event::Status(StatusEvent {
+                status: ResponseStatus::Completed,
+            }),
+        ],
+        vec![
+            Event::tool_use_start(0, "call_2", "tool_b"),
+            Event::tool_input_delta(0, r#"{}"#),
+            Event::tool_use_stop(0),
+            Event::Status(StatusEvent {
+                status: ResponseStatus::Completed,
+            }),
+        ],
+        vec![
+            Event::text_block_start(0),
+            Event::text_delta(0, "done-b"),
+            Event::text_block_stop(0, None),
+            Event::Status(StatusEvent {
+                status: ResponseStatus::Completed,
+            }),
+        ],
+    ]);
+
+    let mut worker = Worker::new(client);
+    let tool_a = CountingTool::new("tool_a");
+    worker
+        .register_tool(tool_a.definition())
+        .expect("register tool_a should succeed");
+
+    let mut locked = worker.lock();
+    locked.run("first").await.expect("first run");
+    assert_eq!(tool_a.call_count(), 1, "tool_a should be called once");
+
+    let mut unlocked = locked.unlock();
+    let tool_b = CountingTool::new("tool_b");
+    unlocked
+        .register_tool(tool_b.definition())
+        .expect("register tool_b after unlock should succeed");
+
+    let mut relocked = unlocked.lock();
+    relocked.run("second").await.expect("second run");
+
+    assert_eq!(tool_a.call_count(), 1, "tool_a should not be called again");
+    assert_eq!(tool_b.call_count(), 1, "tool_b should be called once");
+}
+
 // =============================================================================
 // System Prompt Preservation Tests
 // =============================================================================