@@ -2,6 +2,7 @@
 members = [
     "crates/framework_bridge",
     "plugins/rust/python-package/encoded_exfil_detection",
+    "plugins/rust/python-package/output_length_guard",
     "plugins/rust/python-package/pii_filter",
     "plugins/rust/python-package/rate_limiter",
     "plugins/rust/python-package/retry_with_backoff",
 
@@ -0,0 +1,35 @@
+[package]
+name = "output_length_guard"
+version = "0.1.0"
+edition.workspace = true
+authors.workspace = true
+license.workspace = true
+repository.workspace = true
+description = "Rust-backed output length guard plugin for MCP Gateway"
+
+[lib]
+name = "output_length_guard_rust"
+crate-type = ["cdylib", "rlib"]
+
+[[bin]]
+name = "stub_gen"
+path = "src/bin/stub_gen.rs"
+required-features = ["stub-gen"]
+
+[features]
+default = []
+stub-gen = ["dep:pyo3-stub-gen"]
+
+[dependencies]
+cpex_framework_bridge = { workspace = true }
+log = { workspace = true }
+mutants = { workspace = true }
+pyo3 = { workspace = true }
+pyo3-log = { workspace = true }
+pyo3-stub-gen = { workspace = true, optional = true }
+serde = { workspace = true }
+serde_json = { workspace = true }
+thiserror = { workspace = true }
+
+[dev-dependencies]
+criterion = { workspace = true }
@@ -0,0 +1,132 @@
+.PHONY: help
+help:
+	@grep '^# help\:' $(firstword $(MAKEFILE_LIST)) | sed 's/^# help\: //'
+
+PACKAGE_NAME := cpex-output-length-guard
+WHEEL_PREFIX := cpex_output_length_guard
+CARGO := cargo
+CARGO_PACKAGE := output_length_guard
+NEXTEST_PROFILE ?= default
+STUB_FILES := cpex_output_length_guard/__init__.pyi
+WHEEL_DIR := ../../../../target/wheels
+
+GREEN := \033[0;32m
+YELLOW := \033[0;33m
+NC := \033[0m
+
+# help: fmt                   - Format Rust code with rustfmt
+# help: fmt-check             - Check Rust code formatting (CI)
+# help: clippy                - Run clippy lints
+.PHONY: fmt fmt-check clippy
+
+fmt:
+	$(CARGO) fmt
+
+fmt-check:
+	$(CARGO) fmt -- --check
+
+clippy:
+	$(CARGO) clippy -- -D warnings
+
+# help: sync                  - Install plugin development dependencies
+# help: test                  - Run Rust unit tests and Python integration tests
+# help: test-unit             - Run Rust unit tests
+# help: test-verbose          - Run Rust tests with verbose output
+# help: test-integration      - Run repo-level integration tests for output_length_guard
+# help: test-all              - Alias for test
+.PHONY: sync test test-unit test-verbose test-python test-integration test-all verify-stubs
+
+sync:
+	uv sync --dev
+
+test-unit:
+	@echo "$(GREEN)Running output_length_guard Rust tests...$(NC)"
+	$(CARGO) nextest run --profile $(NEXTEST_PROFILE) -p $(CARGO_PACKAGE)
+
+test: test-unit test-integration
+
+test-verbose:
+	@echo "$(GREEN)Running output_length_guard Rust tests (verbose)...$(NC)"
+	$(CARGO) nextest run --profile $(NEXTEST_PROFILE) -p $(CARGO_PACKAGE) --no-capture
+
+test-python:
+	$(MAKE) test-integration
+
+test-integration:
+	@echo "$(GREEN)Running Python tests...$(NC)"
+	CPEX_TEST_PLUGIN_HOOKS=1 uv run pytest ../../../tests/output_length_guard/test_integration.py -v -rs
+
+test-all: test
+
+verify-stubs:
+	@test -f cpex_output_length_guard/__init__.pyi
+
+# help: stub-gen              - Generate Python type stubs (.pyi files)
+# help: build                 - Build release wheel (no install)
+# help: install               - Build and install editable extension into project venv
+# help: install-wheel         - Install the previously built wheel into project venv
+.PHONY: stub-gen build install install-wheel uninstall
+
+stub-gen:
+	@echo "$(GREEN)Generating Python type stubs...$(NC)"
+	$(CARGO) run --features stub-gen --bin stub_gen
+	@echo "$(GREEN)Stubs generated$(NC)"
+
+build:
+	@echo "$(GREEN)Building $(PACKAGE_NAME)...$(NC)"
+	uv run maturin build --release
+	@echo "$(GREEN)Build complete$(NC)"
+
+install:
+	@echo "$(GREEN)Installing $(PACKAGE_NAME)...$(NC)"
+	uv run maturin develop --release
+	@echo "$(GREEN)Installation complete$(NC)"
+
+install-wheel: build
+	@echo "$(GREEN)Installing built wheel for $(PACKAGE_NAME)...$(NC)"
+	python3 ../../../../tools/install_built_wheel.py --wheel-dir "$(WHEEL_DIR)" --wheel-prefix "$(WHEEL_PREFIX)" --package-name "$(PACKAGE_NAME)" --venv-dir .venv
+	@echo "$(GREEN)Wheel installation complete$(NC)"
+
+uninstall:
+	@echo "$(YELLOW)Uninstalling $(PACKAGE_NAME)...$(NC)"
+	@uv pip uninstall -y $(PACKAGE_NAME) 2>/dev/null || true
+
+.PHONY: clean clean-all
+
+clean:
+	$(CARGO) clean
+	rm -rf target/ coverage/
+	find . -name "*.whl" -delete
+
+clean-all: clean
+
+# help: doc                   - Generate Rust documentation
+# help: doc-open              - Generate and open documentation
+.PHONY: doc doc-open
+
+doc:
+	$(CARGO) doc --no-deps --document-private-items
+
+doc-open: doc
+	$(CARGO) doc --no-deps --document-private-items --open
+
+# help: verify                - Verify plugin installation
+# help: check-all             - Run fmt-check + clippy + Rust tests
+# help: ci-build              - Run CI build/static verification without integration tests
+# help: ci                    - Run the full CI-equivalent plugin verification flow
+.PHONY: verify check-all ci-build ci pre-commit
+
+verify:
+	@uv run python -c "from cpex_output_length_guard import output_length_guard_rust; print('output_length_guard_rust available')" || echo "output_length_guard_rust not installed — run: make install"
+
+check-all: fmt-check clippy test-unit
+	@echo "$(GREEN)All checks passed$(NC)"
+
+ci-build: check-all verify-stubs build install-wheel
+
+ci: ci-build test-integration
+	@echo "$(GREEN)CI verification passed$(NC)"
+
+pre-commit: check-all
+
+.DEFAULT_GOAL := help
@@ -0,0 +1,79 @@
+# cpex-output-length-guard
+
+Rust-backed output length guard plugin for MCP Gateway. Guards tool outputs by enforcing configurable minimum/maximum character or token limits, with either truncation or blocking strategies.
+
+## Features
+
+- **Character mode** (`limit_mode: "character"`): enforce min/max character counts
+- **Token mode** (`limit_mode: "token"`): enforce min/max estimated token counts (using configurable `chars_per_token` ratio)
+- **Truncate strategy**: shorten over-limit output, optionally at word boundaries, with configurable ellipsis
+- **Block strategy**: return a `PluginViolation` to halt processing when limits are exceeded
+- **Supported input shapes**:
+  - Plain `str`
+  - `dict` with a `text` field
+  - `list[str]`
+  - MCP content array: `[{"type": "text", "text": "..."}]`
+  - MCP `CallToolResult` dict with `content` list (and optional `structuredContent`)
+- **Numeric string preservation**: numeric values (integers, floats, scientific notation) pass through without modification
+- **Security limits**: `max_text_length`, `max_structure_size`, `max_recursion_depth` prevent DoS from oversized inputs
+
+## Configuration
+
+```yaml
+kind: "cpex_output_length_guard.output_length_guard.OutputLengthGuardPlugin"
+available_hooks:
+  - "tool_post_invoke"
+config:
+  min_chars: 0           # Minimum characters (0 = disabled)
+  max_chars: 15000       # Maximum characters (null = disabled)
+  min_tokens: 0          # Minimum estimated tokens (0 = disabled)
+  max_tokens: null       # Maximum estimated tokens (null = disabled)
+  chars_per_token: 4     # Characters per token estimate (1–10)
+  limit_mode: "character"  # "character" or "token"
+  strategy: "truncate"    # "truncate" or "block"
+  ellipsis: "…"          # Appended on truncation (empty = none)
+  word_boundary: false   # Truncate at word boundary
+  max_text_length: 1000000    # Security: max bytes to process (1KB–10MB)
+  max_structure_size: 10000   # Security: max items in list/dict (1–100K; Rust intentionally supports tighter limits than Python)
+  max_recursion_depth: 100    # Security: max nesting depth (10–1000)
+```
+
+## Observability
+
+When an OpenTelemetry trace is active (via `extensions.request.trace_id`), the plugin emits metrics to `result.metadata["output_length_guard"]`:
+
+```python
+result.metadata["output_length_guard"] = {
+    "chars_seen": 42000,       # characters in the oversized content
+    "truncated_count": 1,      # number of items truncated
+    "blocked": False,          # True if blocked, False if truncated
+    "limit_mode": "character", # enforcement mode used
+    "strategy": "truncate",    # strategy applied
+    "stage": "tool_post_invoke",
+}
+```
+
+Metrics never contain raw output content — only counts, labels, and status indicators.
+
+## Violation Codes
+
+| Code | Description |
+|------|-------------|
+| `OUTPUT_LENGTH_VIOLATION` | String length outside configured bounds |
+| `OUTPUT_TOKEN_VIOLATION`  | Estimated token count outside configured bounds |
+| `STRUCTURE_SIZE_VIOLATION` | List/dict too large (security limit) |
+| `STRUCTURE_DEPTH_VIOLATION` | Nesting too deep (security limit) |
+
+## Development
+
+```bash
+uv sync --dev
+make install          # Build Rust extension and install
+make test-all         # Run Rust + Python tests
+make test-integration # Run plugin-framework integration tests
+make check-all        # fmt-check + clippy + Rust tests
+```
+
+## License
+
+Apache-2.0
@@ -0,0 +1,15 @@
+# -*- coding: utf-8 -*-
+"""Output length guard plugin package."""
+
+from __future__ import annotations
+
+
+def __getattr__(name: str):
+    if name == "OutputLengthGuardPlugin":
+        from cpex_output_length_guard.output_length_guard import OutputLengthGuardPlugin
+
+        return OutputLengthGuardPlugin
+    raise AttributeError(f"module {__name__!r} has no attribute {name!r}")
+
+
+__all__ = ["OutputLengthGuardPlugin"]
@@ -0,0 +1,8 @@
+# This file is automatically generated by pyo3_stub_gen
+# ruff: noqa: E501, F401, F403, F405
+
+from .output_length_guard import OutputLengthGuardPlugin
+
+__all__ = [
+    "OutputLengthGuardPlugin",
+]
@@ -0,0 +1,23 @@
+# -*- coding: utf-8 -*-
+# Copyright 2025
+# SPDX-License-Identifier: Apache-2.0
+"""Thin compatibility shim for the Rust-owned output length guard plugin."""
+
+from __future__ import annotations
+
+from cpex.framework import Plugin
+from cpex_output_length_guard.output_length_guard_rust import OutputLengthGuardPluginCore
+
+
+class OutputLengthGuardPlugin(Plugin):
+    """Gateway-facing Plugin subclass that delegates behavior to Rust."""
+
+    def __init__(self, config) -> None:
+        super().__init__(config)
+        self._core = OutputLengthGuardPluginCore(config.config or {})
+
+    async def tool_post_invoke(self, payload, context, extensions=None):
+        return self._core.tool_post_invoke(payload, context, extensions)
+
+
+__all__ = ["OutputLengthGuardPlugin"]
@@ -0,0 +1,18 @@
+description: "Rust-backed output length guard for tool outputs — truncates or blocks responses that exceed configurable character or token limits"
+author: "ContextForge Contributors"
+version: "0.1.0"
+kind: "cpex_output_length_guard.output_length_guard.OutputLengthGuardPlugin"
+available_hooks:
+  - "tool_post_invoke"
+default_configs:
+  min_chars: 0
+  max_chars: 15000
+  min_tokens: 0
+  chars_per_token: 4
+  limit_mode: "character"
+  strategy: "truncate"
+  ellipsis: "…"
+  word_boundary: false
+  max_text_length: 1000000
+  max_structure_size: 10000
+  max_recursion_depth: 100
@@ -0,0 +1,38 @@
+[build-system]
+requires = ["maturin>=1.13.3,<2.0"]
+build-backend = "maturin"
+
+[project]
+name = "cpex-output-length-guard"
+dynamic = ["version"]
+description = "Rust-backed output length guard plugin for MCP Gateway"
+authors = [{ name = "ContextForge Contributors" }]
+license = { text = "Apache-2.0" }
+readme = "README.md"
+requires-python = ">=3.11"
+dependencies = [
+    "cpex>=0.1.3,<0.2",
+    "mcp<2",
+]
+classifiers = [
+    "Programming Language :: Rust",
+    "Programming Language :: Python :: Implementation :: CPython",
+    "Programming Language :: Python :: 3.11",
+    "Programming Language :: Python :: 3.12",
+    "Programming Language :: Python :: 3.13",
+]
+
+[project.entry-points."cpex.plugins"]
+output_length_guard = "cpex_output_length_guard.output_length_guard:OutputLengthGuardPlugin"
+
+[tool.maturin]
+module-name = "cpex_output_length_guard.output_length_guard_rust"
+python-source = "."
+features = ["pyo3/extension-module"]
+
+[dependency-groups]
+dev = [
+    "maturin>=1.13.3",
+    "pytest>=9.1.1",
+    "pytest-asyncio>=1.3.0",
+]