feat(mcp): Phase 3 — auth preflight

src/mcp/mod.rs

@@ -440,13 +440,16 @@ fn handle_tools_call(
             "tools/call params.arguments must be an object when present",
         );
     }
-    if tool_by_name(name).is_none() {
+    let Some(tool) = tool_by_name(name) else {
         return json_rpc_error(
             id,
             -32602,
             "Invalid params",
             format!("unknown MCP tool '{name}'"),
         );
+    };
+    if let Some(auth_error) = validate_tool_auth(tool, ctx) {
+        return json_rpc_result(id, mcp_content_envelope(auth_error, true));
     }
 
     let arguments = params
@@ -472,6 +475,36 @@ fn handle_tools_call(
     }
 }
 
+fn validate_tool_auth(tool: &ToolDef, ctx: &McpContext) -> Option<serde_json::Value> {
+    if tool.requires_github && ctx.github_token.is_none() {
+        return Some(serde_json::json!({
+            "error": {
+                "code": "AUTH_REQUIRED",
+                "message": format!("Tool '{}' requires a delegated GitHub token.", tool.name),
+                "detail": format!(
+                    "Pass a session token with {} access.",
+                    github_scope_detail(tool.github_scopes)
+                ),
+                "tool": tool.name,
+            }
+        }));
+    }
+
+    None
+}
+
+fn github_scope_detail(scopes: &[GithubScope]) -> String {
+    if scopes.is_empty() {
+        return "GitHub".to_string();
+    }
+
+    scopes
+        .iter()
+        .map(|scope| scope.as_str())
+        .collect::<Vec<_>>()
+        .join(", ")
+}
+
 fn mcp_content_envelope(payload: serde_json::Value, is_error: bool) -> serde_json::Value {
     serde_json::json!({
         "content": [{
@@ -759,6 +792,57 @@ mod tests {
             .is_some_and(|text| text.contains("\"tool\":\"worktree_status\"")));
     }
 
+    #[test]
+    fn tools_call_rejects_required_github_tool_without_token() {
+        let request = serde_json::json!({
+            "jsonrpc": "2.0",
+            "id": "auth",
+            "method": "tools/call",
+            "params": {
+                "name": "ci_status",
+                "arguments": {"ticket": "ABC-123"}
+            }
+        });
+
+        let response = dispatch_json_rpc(request).expect("tools/call should produce a response");
+        let text = response["result"]["content"][0]["text"]
+            .as_str()
+            .expect("MCP error text should be present");
+
+        assert_eq!(response["id"], "auth");
+        assert_eq!(response["result"]["isError"], true);
+        assert!(text.contains("\"code\":\"AUTH_REQUIRED\""));
+        assert!(text.contains("\"tool\":\"ci_status\""));
+        assert!(text.contains("checks:read"));
+    }
+
+    #[test]
+    fn tools_call_allows_required_github_tool_with_token() {
+        let ctx = McpContext::from_cwd(false)
+            .expect("should build context")
+            .with_github_token("ghp_test");
+        let request = serde_json::json!({
+            "jsonrpc": "2.0",
+            "id": "auth-ok",
+            "method": "tools/call",
+            "params": {
+                "name": "ci_status",
+                "arguments": {"ticket": "ABC-123"}
+            }
+        });
+
+        let response = dispatch_json_rpc_with_context(request, &ctx)
+            .expect("tools/call should produce a response");
+        let text = response["result"]["content"][0]["text"]
+            .as_str()
+            .expect("MCP error text should be present");
+
+        assert_eq!(response["id"], "auth-ok");
+        assert_eq!(response["result"]["isError"], true);
+        assert!(!text.contains("AUTH_REQUIRED"));
+        assert!(text.contains("\"tool\":\"ci_status\""));
+    }
+
     #[test]
     fn notifications_do_not_produce_responses() {
         let request = serde_json::json!({

tests/mcp/fixtures/stdio_smoke.jsonl

@@ -3,6 +3,7 @@
 {"name":"initialized-notification","request":{"jsonrpc":"2.0","method":"notifications/initialized","params":{}},"no_response":true}
 {"name":"tools-list","request":{"jsonrpc":"2.0","id":"tools","method":"tools/list"},"assertions":[{"pointer":"/jsonrpc","equals":"2.0"},{"pointer":"/id","equals":"tools"},{"pointer":"/result/tools","min_len":10},{"pointer":"/result/tools","contains_tool":"worktree_list"},{"pointer":"/result/tools","contains_tool":"worktree_ship"},{"pointer":"/result/tools","contains_tool":"smartlog"},{"pointer":"/result/tools","contains_tool":"ci_status"},{"pointer":"/result/tools","contains_tool":"reviews"}]}
 {"name":"tools-call-unwired","request":{"jsonrpc":"2.0","id":"call","method":"tools/call","params":{"name":"worktree_status","arguments":{"ticket":"ABC-123"}}},"assertions":[{"pointer":"/jsonrpc","equals":"2.0"},{"pointer":"/id","equals":"call"},{"pointer":"/result/isError","equals":true},{"pointer":"/result/content","min_len":1},{"pointer":"/result/content/0/type","equals":"text"},{"pointer":"/result/content/0/text","contains_text":"worktree_status"}]}
+{"name":"tools-call-github-auth-required","request":{"jsonrpc":"2.0","id":"auth","method":"tools/call","params":{"name":"ci_status","arguments":{"ticket":"ABC-123"}}},"assertions":[{"pointer":"/jsonrpc","equals":"2.0"},{"pointer":"/id","equals":"auth"},{"pointer":"/result/isError","equals":true},{"pointer":"/result/content","min_len":1},{"pointer":"/result/content/0/type","equals":"text"},{"pointer":"/result/content/0/text","contains_text":"AUTH_REQUIRED"},{"pointer":"/result/content/0/text","contains_text":"checks:read"}]}
 {"name":"tools-call-unknown-tool","request":{"jsonrpc":"2.0","id":"bad-tool","method":"tools/call","params":{"name":"nope","arguments":{}}},"assertions":[{"pointer":"/jsonrpc","equals":"2.0"},{"pointer":"/id","equals":"bad-tool"},{"pointer":"/error/code","equals":-32602},{"pointer":"/error/message","equals":"Invalid params"}]}
 {"name":"tools-call-missing-name","request":{"jsonrpc":"2.0","id":"missing-name","method":"tools/call","params":{"arguments":{}}},"assertions":[{"pointer":"/jsonrpc","equals":"2.0"},{"pointer":"/id","equals":"missing-name"},{"pointer":"/error/code","equals":-32602},{"pointer":"/error/message","equals":"Invalid params"},{"pointer":"/error/data","contains_text":"params.name"}]}
 {"name":"tools-call-non-object-arguments","request":{"jsonrpc":"2.0","id":"bad-arguments","method":"tools/call","params":{"name":"worktree_status","arguments":"ticket=ABC-123"}},"assertions":[{"pointer":"/jsonrpc","equals":"2.0"},{"pointer":"/id","equals":"bad-arguments"},{"pointer":"/error/code","equals":-32602},{"pointer":"/error/message","equals":"Invalid params"},{"pointer":"/error/data","contains_text":"arguments"}]}