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fix(mcp-macros): use JSON serialization instead of Debug format in tool returns
Fixes #62 Changes: - Modified utils.rs generate_error_handling() to use serde_json::to_string() instead of format!("{:?}") for tool return values - Now populates structured_content field per MCP 2025-06-18 specification - Graceful fallback to Debug format for types without Serialize trait - Added comprehensive test suite (8 tests) in json_serialization_test.rs Breaking Change: - Tool return types should implement Serialize trait for optimal JSON output - Types without Serialize will fallback to Debug format Before: "SearchResult { items: [...] }" After: {"items": [...]} Version bumped to 0.13.0 due to breaking change requirement.
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‎.claude/settings.local.json‎

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"Bash(gh release list:*)",
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"Bash(gh release view:*)",
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"Bash(gh pr list:*)",
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"Bash(pre-commit:*)"
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"Bash(pre-commit:*)",
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"WebFetch(domain:spec.modelcontextprotocol.io)"
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],
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"deny": []
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}

‎CHANGELOG.md‎

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@@ -5,6 +5,76 @@ All notable changes to the PulseEngine MCP Framework will be documented in this
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The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
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and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
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## [0.13.0] - 2025-01-11
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### Fixed
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#### mcp-macros
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- **[BREAKING] Fixed `#[mcp_tools]` macro to use JSON serialization instead of Rust Debug format** ([#62](https://github.com/pulseengine/mcp/issues/62))
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- Tool return values are now properly serialized as JSON using `serde_json::to_string()`
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- Populates `structured_content` field in `CallToolResult` per MCP 2025-06-18 specification
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- Graceful fallback to Debug format for types that don't implement `Serialize`
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- **Breaking Change**: Tool return types should implement `Serialize` trait for optimal JSON output
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- Previously returned Rust Debug format like `SearchResult { items: [...] }` which broke JSON parsing
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- Now returns proper JSON: `{"items": [...]}`
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### Added
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#### Testing
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- **Comprehensive JSON serialization test suite** (`mcp-macros/tests/json_serialization_test.rs`)
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- Tests for structured return types (nested structs, vectors, enums)
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- Tests for Result<T, E> return types
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- Tests for simple types (string, number, bool, vector)
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- Verification that `structured_content` field is populated
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- Verification that Debug format markers are not present in output
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- 8 comprehensive test cases covering all scenarios
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### Changed
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- **Version bumped to 0.13.0** (breaking change due to Serialize requirement)
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- Tool responses now comply with MCP 2025-06-18 specification for structured content
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### Migration Guide
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If you have tools that return structured types:
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```rust
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// Add Serialize to your return types
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#[derive(Debug, Serialize)] // Add Serialize
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struct MyResult {
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data: Vec<String>,
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}
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#[mcp_tools]
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impl MyServer {
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pub fn my_tool(&self) -> MyResult {
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MyResult { data: vec!["item1".to_string()] }
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}
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}
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```
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For types that can't implement Serialize, the macro will gracefully fall back to Debug format.
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## [0.12.0] - 2025-01-11
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### Added
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- **MCP 2025-06-18 protocol support**
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- `NumberOrString` type for request IDs
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- Optional `_meta` fields across protocol types
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### Fixed
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- **Fixed flaky tests** with `serial_test` crate
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- All environment variable tests now run serially to prevent race conditions
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- Added `#[serial_test::serial]` to tests in mcp-security-middleware, mcp-cli-derive, and mcp-cli
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### Changed
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- CI now validates all changes with pre-commit hooks
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## [0.4.1] - 2024-07-06
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### Added

‎Cargo.lock‎

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‎Cargo.toml‎

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resolver = "2"
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[workspace.package]
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version = "0.12.0"
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version = "0.13.0"
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rust-version = "1.88"
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edition = "2024"
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license = "MIT OR Apache-2.0"
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serde_yaml = "0.9"
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# Framework internal dependencies (published versions)
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pulseengine-mcp-protocol = { version = "0.12.0", path = "mcp-protocol" }
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pulseengine-mcp-logging = { version = "0.12.0", path = "mcp-logging" }
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pulseengine-mcp-auth = { version = "0.12.0", path = "mcp-auth" }
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pulseengine-mcp-security = { version = "0.12.0", path = "mcp-security" }
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pulseengine-mcp-security-middleware = { version = "0.12.0", path = "mcp-security-middleware" }
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pulseengine-mcp-monitoring = { version = "0.12.0", path = "mcp-monitoring" }
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pulseengine-mcp-transport = { version = "0.12.0", path = "mcp-transport" }
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pulseengine-mcp-cli = { version = "0.12.0", path = "mcp-cli" }
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pulseengine-mcp-cli-derive = { version = "0.12.0", path = "mcp-cli-derive" }
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pulseengine-mcp-server = { version = "0.12.0", path = "mcp-server" }
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pulseengine-mcp-macros = { version = "0.12.0", path = "mcp-macros" }
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pulseengine-mcp-external-validation = { version = "0.12.0", path = "mcp-external-validation" }
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pulseengine-mcp-protocol = { version = "0.13.0", path = "mcp-protocol" }
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pulseengine-mcp-logging = { version = "0.13.0", path = "mcp-logging" }
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pulseengine-mcp-auth = { version = "0.13.0", path = "mcp-auth" }
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pulseengine-mcp-security = { version = "0.13.0", path = "mcp-security" }
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pulseengine-mcp-security-middleware = { version = "0.13.0", path = "mcp-security-middleware" }
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pulseengine-mcp-monitoring = { version = "0.13.0", path = "mcp-monitoring" }
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pulseengine-mcp-transport = { version = "0.13.0", path = "mcp-transport" }
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pulseengine-mcp-cli = { version = "0.13.0", path = "mcp-cli" }
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pulseengine-mcp-cli-derive = { version = "0.13.0", path = "mcp-cli-derive" }
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pulseengine-mcp-server = { version = "0.13.0", path = "mcp-server" }
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pulseengine-mcp-macros = { version = "0.13.0", path = "mcp-macros" }
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pulseengine-mcp-external-validation = { version = "0.13.0", path = "mcp-external-validation" }
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[profile.release]
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opt-level = "s"

‎mcp-macros/src/utils.rs‎

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@@ -158,29 +158,62 @@ pub fn generate_error_handling(return_type: &syn::ReturnType) -> TokenStream {
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if let Some(segment) = type_path.path.segments.last() {
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if segment.ident == "Result" {
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// It's already a Result, wrap it properly for the dispatch context
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// Use JSON serialization for structured data, with fallback
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return quote! {
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match result {
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Ok(value) => Ok(pulseengine_mcp_protocol::CallToolResult {
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content: vec![pulseengine_mcp_protocol::Content::text(format!("{:?}", value))],
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is_error: Some(false),
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structured_content: None,
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_meta: None,
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}),
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Ok(value) => {
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// Try JSON serialization first (for structured data)
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let (text_content, structured) = match serde_json::to_value(&value) {
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Ok(json_value) => {
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// Serialize as JSON string for text content
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let text = serde_json::to_string(&value)
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.unwrap_or_else(|_| format!("{:?}", value));
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(text, Some(json_value))
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}
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Err(_) => {
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// Fallback to Debug if not serializable
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(format!("{:?}", value), None)
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}
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};
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Ok(pulseengine_mcp_protocol::CallToolResult {
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content: vec![pulseengine_mcp_protocol::Content::text(text_content)],
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is_error: Some(false),
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structured_content: structured,
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_meta: None,
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})
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}
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Err(e) => Err(pulseengine_mcp_protocol::Error::internal_error(e.to_string())),
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}
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};
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}
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}
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}
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// Not a Result, wrap it with simple Display formatting
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// Not a Result, wrap it with JSON serialization (preferred) or Display formatting
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quote! {
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Ok(pulseengine_mcp_protocol::CallToolResult {
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content: vec![pulseengine_mcp_protocol::Content::text(result.to_string())],
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is_error: Some(false),
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structured_content: None,
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_meta: None,
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})
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{
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// Try JSON serialization first (for structured data)
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let (text_content, structured) = match serde_json::to_value(&result) {
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Ok(json_value) => {
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// Serialize as JSON string for text content
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let text = serde_json::to_string(&result)
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.unwrap_or_else(|_| format!("{:?}", result));
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(text, Some(json_value))
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}
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Err(_) => {
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// Fallback to Debug if not serializable
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(format!("{:?}", result), None)
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}
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};
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Ok(pulseengine_mcp_protocol::CallToolResult {
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content: vec![pulseengine_mcp_protocol::Content::text(text_content)],
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is_error: Some(false),
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structured_content: structured,
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_meta: None,
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})
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}
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}
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}
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}

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