cpex-retry-with-backoff 0.3.6

High-performance retry and backoff policy engine for MCP Gateway

Retry With Backoff (Rust)

High-performance retry and backoff policy engine for ContextForge and MCP Gateway.

Features

• Rust-backed retry state tracking for tool invocations
• Exponential backoff with optional jitter
• Per-tool policy overrides without duplicating whole plugin configs
• Retry decisions based on isError, structured status_code, or optional parsed text payloads
• Automatic state eviction for stale request entries
• Gateway ceiling enforcement for max_retries
• Retry policy metadata returned on tool and resource hooks

Build

make install

Runtime Requirements

This plugin depends on cpex>=0.1.0,<0.2 and imports hook models from cpex.framework. The compiled Rust extension is mandatory; there is no Python fallback implementation.

Usage

The plugin runs on tool_post_invoke and resource_post_fetch.

Typical uses:

• Retry transient upstream failures such as 429, 500, 502, 503, and 504
• Clamp aggressive plugin settings to the gateway-wide retry ceiling
• Apply stricter retry budgets to fragile or expensive tools

Configuration

Core settings

• max_retries: maximum retry attempts before giving up
• backoff_base_ms: base delay for exponential backoff
• max_backoff_ms: upper bound for computed retry delays
• retry_on_status: HTTP or structured status codes treated as retriable
• jitter: randomize delay within the current exponential ceiling
• check_text_content: inspect text content for JSON-encoded error payloads when structured content is absent

Per-tool overrides

Use tool_overrides to change retry behavior for a specific tool:

• max_retries
• backoff_base_ms
• max_backoff_ms
• retry_on_status
• jitter

Behavior Notes

• Successful responses clear retry state for the (tool, request_id) pair.
• Retry state expires after a short TTL so abandoned request state does not accumulate indefinitely.
• If check_text_content is disabled, the hot path uses the Rust state manager directly.
• If check_text_content is enabled, Python-side payload inspection supplements the Rust state manager before applying retry policy.

Returned Metadata

tool_post_invoke — OpenTelemetry metrics

tool_post_invoke accepts an optional extensions parameter carrying OpenTelemetry trace context. When a trace context is present (via extensions.request.trace_id), the plugin emits operational metrics on result.metadata["retry_with_backoff"] with the following schema:

result.metadata["retry_with_backoff"] = {
    "retry_count": 1,      # int — consecutive_failures after this call's outcome is recorded; 0 on success
    "retry_delay_ms": 100, # int — the per-attempt delay computed for this call; 0 on success or once exhausted
}

Every call (success, within-budget retry, or exhausted) has a meaningful outcome to report, including the all-zero success case — there is deliberately no total_backoff_ms cumulative counter, only these two per-call fields.

Gating: Metrics are only emitted when a valid trace_id is present in the trace context (extensions.request.trace_id). No trace context means no result.metadata write at all, regardless of any config flag.

resource_post_fetch — unchanged config echo

resource_post_fetch is out of scope for the OTel metrics contract above — it never receives extensions and continues to unconditionally emit the plugin's active retry policy configuration (not per-call outcome data) on result.metadata:

• max_retries
• backoff_base_ms
• max_backoff_ms
• retry_on_status

Migration Note

Version 0.3.6 is a breaking change for tool_post_invoke consumers only (resource_post_fetch is unaffected):

• tool_post_invoke no longer emits the flat, unconditional retry_policy config echo (max_retries, backoff_base_ms, max_backoff_ms, retry_on_status). That echo is replaced by the namespaced, trace-gated result.metadata["retry_with_backoff"] schema above (retry_count, retry_delay_ms).
• tool_post_invoke now accepts a new optional extensions parameter carrying OpenTelemetry trace context. Emission is gated solely on extensions.request.trace_id being present and valid — if no trace context is supplied, no metrics are written at all.
• Consumers that previously read the config echo from tool_post_invoke unconditionally must migrate to reading result.metadata["retry_with_backoff"] and must pass a trace_id via extensions to receive metrics.
• resource_post_fetch keeps its pre-existing, un-namespaced config echo byte-for-byte unchanged — it is a different contract, not covered by this migration.

Testing

# Full plugin CI
make ci

Performance

The retry state manager is implemented in Rust so the common retry decision path avoids Python bookkeeping overhead for normal structured tool results.