diff --git a/java/src/test/java/com/github/copilot/e2e/ErgonomicTestTools$$CopilotToolMeta.java b/java/src/test/java/com/github/copilot/e2e/ErgonomicTestTools$$CopilotToolMeta.java
index 703a6b010..3e6291984 100644
--- a/java/src/test/java/com/github/copilot/e2e/ErgonomicTestTools$$CopilotToolMeta.java
+++ b/java/src/test/java/com/github/copilot/e2e/ErgonomicTestTools$$CopilotToolMeta.java
@@ -44,6 +44,25 @@ public List<ToolDefinition> definitions(ErgonomicTestTools instance, ObjectMappe
                             Map<String, Object> args = invocation.getArguments();
                             String keyword = (String) args.get("keyword");
                             return CompletableFuture.completedFuture(instance.searchItems(keyword));
+                        }, null, null, null),
+                new ToolDefinition("get_status", "Returns the current status",
+                        Map.of("type", "object", "properties", Map.of(), "required", List.of()), invocation -> {
+                            return CompletableFuture.completedFuture(instance.getStatus());
+                        }, null, null, null),
+                new ToolDefinition("combine_values", "Combines two values into a single string", Map.of(
+                        "type", "object", "properties", Map
+                                .ofEntries(
+                                        Map.entry("value1",
+                                                (Map<String, Object>) (Map) withMeta(Map.of("type", "string"),
+                                                        "First value", null)),
+                                        Map.entry("value2",
+                                                (Map<String, Object>) (Map) withMeta(Map.of("type", "string"),
+                                                        "Second value", null))),
+                        "required", List.of("value1", "value2")), invocation -> {
+                            Map<String, Object> args = invocation.getArguments();
+                            String value1 = (String) args.get("value1");
+                            String value2 = (String) args.get("value2");
+                            return CompletableFuture.completedFuture(instance.combineValues(value1, value2));
                         }, null, null, null));
     }
 }
diff --git a/java/src/test/java/com/github/copilot/e2e/ErgonomicTestTools.java b/java/src/test/java/com/github/copilot/e2e/ErgonomicTestTools.java
index e70e9b4dc..15b2c087a 100644
--- a/java/src/test/java/com/github/copilot/e2e/ErgonomicTestTools.java
+++ b/java/src/test/java/com/github/copilot/e2e/ErgonomicTestTools.java
@@ -29,4 +29,15 @@ public String setCurrentPhase(@CopilotToolParam("The phase to transition to") St
     public String searchItems(@CopilotToolParam("Search keyword") String keyword) {
         return "Found: " + keyword + " -> item_alpha, item_beta";
     }
+
+    @CopilotTool("Returns the current status")
+    public String getStatus() {
+        return "Status: OK";
+    }
+
+    @CopilotTool("Combines two values into a single string")
+    public String combineValues(@CopilotToolParam("First value") String value1,
+            @CopilotToolParam("Second value") String value2) {
+        return "combined: " + value1 + " + " + value2;
+    }
 }
diff --git a/java/src/test/java/com/github/copilot/e2e/ErgonomicToolDefinitionIT.java b/java/src/test/java/com/github/copilot/e2e/ErgonomicToolDefinitionIT.java
index df031f354..412acd4c4 100644
--- a/java/src/test/java/com/github/copilot/e2e/ErgonomicToolDefinitionIT.java
+++ b/java/src/test/java/com/github/copilot/e2e/ErgonomicToolDefinitionIT.java
@@ -84,6 +84,120 @@ void ergonomicToolDefinition() throws Exception {
         }
     }
 
+    @Test
+    void ergonomicToolArity0() throws Exception {
+        ctx.configureForTest("tools", "ergonomic_tool_arity0");
+
+        ErgonomicTestTools tools = new ErgonomicTestTools();
+        List<ToolDefinition> toolDefs = ToolDefinition.fromObject(tools);
+
+        try (CopilotClient client = ctx.createClient()) {
+            CopilotSession session = client
+                    .createSession(new SessionConfig().setOnPermissionRequest(PermissionHandler.APPROVE_ALL)
+                            .setAvailableTools(new ToolSet().addCustom("*")).setTools(toolDefs))
+                    .get(30, TimeUnit.SECONDS);
+
+            try {
+                AssistantMessageEvent response = session
+                        .sendAndWait(new MessageOptions().setPrompt("Call get_status and tell me the result."), 60_000)
+                        .get(90, TimeUnit.SECONDS);
+
+                assertNotNull(response, "Expected a response from the assistant");
+                String content = response.getData().content().toLowerCase();
+                assertTrue(content.contains("ok"),
+                        "Response should mention the status: " + response.getData().content());
+            } finally {
+                session.close();
+            }
+        }
+    }
+
+    @Test
+    void ergonomicToolArity2() throws Exception {
+        ctx.configureForTest("tools", "ergonomic_tool_arity2");
+
+        ErgonomicTestTools tools = new ErgonomicTestTools();
+        List<ToolDefinition> toolDefs = ToolDefinition.fromObject(tools);
+
+        try (CopilotClient client = ctx.createClient()) {
+            CopilotSession session = client
+                    .createSession(new SessionConfig().setOnPermissionRequest(PermissionHandler.APPROVE_ALL)
+                            .setAvailableTools(new ToolSet().addCustom("*")).setTools(toolDefs))
+                    .get(30, TimeUnit.SECONDS);
+
+            try {
+                AssistantMessageEvent response = session.sendAndWait(
+                        new MessageOptions().setPrompt(
+                                "Call combine_values with 'alpha' and 'beta', then report the combined result."),
+                        60_000).get(90, TimeUnit.SECONDS);
+
+                assertNotNull(response, "Expected a response from the assistant");
+                String content = response.getData().content().toLowerCase();
+                assertTrue(content.contains("alpha") && content.contains("beta"),
+                        "Response should contain the combined values: " + response.getData().content());
+            } finally {
+                session.close();
+            }
+        }
+    }
+
+    @Test
+    void lambdaToolArity0() throws Exception {
+        ctx.configureForTest("tools", "ergonomic_tool_arity0");
+
+        ToolDefinition getStatus = ToolDefinition.from("get_status", "Returns the current status", () -> "Status: OK");
+
+        try (CopilotClient client = ctx.createClient()) {
+            CopilotSession session = client
+                    .createSession(new SessionConfig().setOnPermissionRequest(PermissionHandler.APPROVE_ALL)
+                            .setAvailableTools(new ToolSet().addCustom("*")).setTools(List.of(getStatus)))
+                    .get(30, TimeUnit.SECONDS);
+
+            try {
+                AssistantMessageEvent response = session
+                        .sendAndWait(new MessageOptions().setPrompt("Call get_status and tell me the result."), 60_000)
+                        .get(90, TimeUnit.SECONDS);
+
+                assertNotNull(response, "Expected a response from the assistant");
+                String content = response.getData().content().toLowerCase();
+                assertTrue(content.contains("ok"),
+                        "Response should mention the status: " + response.getData().content());
+            } finally {
+                session.close();
+            }
+        }
+    }
+
+    @Test
+    void lambdaToolArity2() throws Exception {
+        ctx.configureForTest("tools", "ergonomic_tool_arity2");
+
+        ToolDefinition combineValues = ToolDefinition.from("combine_values", "Combines two values into a single string",
+                Param.of(String.class, "value1", "First value"), Param.of(String.class, "value2", "Second value"),
+                (v1, v2) -> "combined: " + v1 + " + " + v2);
+
+        try (CopilotClient client = ctx.createClient()) {
+            CopilotSession session = client
+                    .createSession(new SessionConfig().setOnPermissionRequest(PermissionHandler.APPROVE_ALL)
+                            .setAvailableTools(new ToolSet().addCustom("*")).setTools(List.of(combineValues)))
+                    .get(30, TimeUnit.SECONDS);
+
+            try {
+                AssistantMessageEvent response = session.sendAndWait(
+                        new MessageOptions().setPrompt(
+                                "Call combine_values with 'alpha' and 'beta', then report the combined result."),
+                        60_000).get(90, TimeUnit.SECONDS);
+
+                assertNotNull(response, "Expected a response from the assistant");
+                String content = response.getData().content().toLowerCase();
+                assertTrue(content.contains("alpha") && content.contains("beta"),
+                        "Response should contain the combined values: " + response.getData().content());
+            } finally {
+                session.close();
+            }
+        }
+    }
+
     @Test
     void lambdaToolDefinition() throws Exception {
         ctx.configureForTest("tools", "ergonomic_tool_definition");
diff --git a/test/snapshots/tools/ergonomic_tool_arity0.yaml b/test/snapshots/tools/ergonomic_tool_arity0.yaml
new file mode 100644
index 000000000..a55f48681
--- /dev/null
+++ b/test/snapshots/tools/ergonomic_tool_arity0.yaml
@@ -0,0 +1,21 @@
+models:
+  - claude-sonnet-4.5
+conversations:
+  - messages:
+      - role: system
+        content: ${system}
+      - role: user
+        content: Call get_status and tell me the result.
+      - role: assistant
+        content: I'll call get_status now.
+        tool_calls:
+          - id: toolcall_0
+            type: function
+            function:
+              name: get_status
+              arguments: '{}'
+      - role: tool
+        tool_call_id: toolcall_0
+        content: "Status: OK"
+      - role: assistant
+        content: "The status is: OK"
diff --git a/test/snapshots/tools/ergonomic_tool_arity2.yaml b/test/snapshots/tools/ergonomic_tool_arity2.yaml
new file mode 100644
index 000000000..e34c695bd
--- /dev/null
+++ b/test/snapshots/tools/ergonomic_tool_arity2.yaml
@@ -0,0 +1,21 @@
+models:
+  - claude-sonnet-4.5
+conversations:
+  - messages:
+      - role: system
+        content: ${system}
+      - role: user
+        content: Call combine_values with 'alpha' and 'beta', then report the combined result.
+      - role: assistant
+        content: I'll call combine_values with those arguments.
+        tool_calls:
+          - id: toolcall_0
+            type: function
+            function:
+              name: combine_values
+              arguments: '{"value1":"alpha","value2":"beta"}'
+      - role: tool
+        tool_call_id: toolcall_0
+        content: "combined: alpha + beta"
+      - role: assistant
+        content: "The combined result is: alpha + beta"