How grep-mcp Manages and Responds to Different HTTP Status Codes from API Calls

grep-mcp wraps calls to the Grep.app search endpoint inside the asynchronous grep_query tool, inspecting HTTP response status with aiohttp and mapping specific codes to custom exceptions or structured JSON outputs to ensure consistent error handling for MCP agents.

The grep-mcp repository provides a Model Context Protocol (MCP) server that interfaces with the Grep.app API. When managing and responding to different HTTP status codes from API calls, the server implements a robust error-handling strategy in src/grep_mcp/server.py that converts technical HTTP responses into user-friendly messages while preserving type safety for downstream consumers.

HTTP Status Code Mapping in grep_query

The core logic resides in the grep_query function within src/grep_mcp/server.py. After dispatching requests to https://grep.app/api/search using aiohttp, the tool evaluates the response status code and branches accordingly.

Handling 429 Too Many Requests (Rate Limiting)

When the API returns HTTP 429, grep-mcp raises a dedicated GrepAPIRateLimitError. This exception is caught upstream and transformed into a readable error string:

# Result returned to MCP agent

"❌ Error: Rate limit exceeded. Please wait before making another request."

This approach prevents raw HTTP errors from propagating to the LLM context while signaling the specific rate-limit condition.

Handling 404 Not Found (Empty Results)

A HTTP 404 response does not trigger an exception. Instead, the tool returns a minimal JSON payload indicating zero results:

{
  "total_results": 0,
  "results": []
}

This preserves the expected return type of the tool (a JSON string) and allows downstream logic to handle empty result sets gracefully without branching into error-handling code paths.

Handling 200 OK (Success)

For HTTP 200 responses, the tool parses the JSON body using await response.json() and passes the data to _format_grep_response. This helper structures the raw API output into a consistent format suitable for MCP tool responses, extracting relevant fields like repository names, file paths, and code snippets.

Handling 5xx and Other Client Errors

Any unexpected status code (e.g., 500, 403, 502) triggers a generic GrepAPIError containing the specific status code and response text. The outer exception handler converts these into prefixed error messages:

"❌ Error: Unexpected error occurred: <status_code> - <details>"

This ensures that even unanticipated API behaviors result in deterministic, user-friendly output rather than stack traces.

Network-Level Error Management

Beyond HTTP status codes, grep-mcp normalizes network-level failures using a hierarchy of custom exceptions defined in src/grep_mcp/server.py.

Timeouts and Connection Failures

When a request exceeds the configured timeout (default 30 seconds), asyncio.TimeoutError is caught and re-raised as GrepAPITimeoutError. Similarly, aiohttp client errors (DNS failures, connection resets, SSL issues) are wrapped in GrepAPIError. Both result in consistent error prefixes:

"❌ Error: Request timed out. The grep.app API may be experiencing issues."
"❌ Error: Network error while contacting grep.app API: <details>"

Catch-All Exception Handling

A final except Exception block captures any unanticipated errors (parsing failures, unexpected None values, etc.) and returns:

"❌ Error: Unexpected error occurred: <exception_message>"

This defensive programming ensures the MCP server never crashes or returns raw Python exceptions to the client, maintaining protocol stability.

Implementation Details in server.py

The error-handling architecture centers on src/grep_mcp/server.py, which contains:

• grep_query: The main tool function that orchestrates HTTP requests and dispatches to handlers based on status codes.
• Custom Exception Classes: GrepAPIError, GrepAPIRateLimitError, and GrepAPITimeoutError provide semantic meaning to failure modes.
• _format_grep_response: Transforms successful API payloads into MCP-compatible structures.

All network operations use aiohttp.ClientSession with explicit timeout configuration, ensuring that asynchronous context managers properly clean up connections regardless of success or failure paths.

Practical Code Examples

The following examples demonstrate how grep-mcp handles various HTTP scenarios:

# Example 1 – Normal successful query

result_json = await grep_query("asyncio.run", language="python")
print(result_json)          # → JSON string with matches

# Example 2 – Rate‑limit hit (429)

# Simulate many rapid calls or use a test mock that returns 429

result = await grep_query("foo")

# result == "❌ Error: Rate limit exceeded. Please wait before making another request."

print(result)

# Example 3 – Repository not found (404)

result = await grep_query("nonexistentfunction", repo="unknown/unknown")

# Returns a minimal JSON payload with total_results = 0

print(result)

# Example 4 – Network timeout

# If the API does not respond within 30 seconds

result = await grep_query("slowquery")

# result == "❌ Error: Request timed out. The grep.app API may be experiencing issues."

print(result)

Summary

• grep-mcp maps specific HTTP status codes to custom exceptions in src/grep_mcp/server.py, ensuring semantic error handling.
• HTTP 429 triggers GrepAPIRateLimitError and returns a user-friendly rate-limit message.
• HTTP 404 returns a valid JSON payload with zero results rather than raising an exception.
• HTTP 200 responses are parsed and formatted via _format_grep_response for consistent MCP output.
• Network failures (timeouts, connection errors) are normalized through GrepAPITimeoutError and GrepAPIError.
• All error paths converge on deterministic string outputs prefixed with ❌ Error:, preventing raw exceptions from reaching MCP clients.

Frequently Asked Questions

How does grep-mcp handle rate limiting from the Grep.app API?

When the API returns HTTP 429 Too Many Requests, grep-mcp raises a GrepAPIRateLimitError that gets converted into the message "❌ Error: Rate limit exceeded. Please wait before making another request." This prevents the MCP agent from crashing and signals the user to retry after a delay.

What happens when the Grep.app API returns a 404 status code?

Rather than treating HTTP 404 as an error condition, grep-mcp returns a minimal JSON payload with "total_results": 0 and an empty results array. This preserves the tool's expected return type and allows downstream logic to handle empty result sets gracefully without branching into exception handling.

How are network timeouts and connection errors managed?

Network-level failures such as asyncio.TimeoutError or aiohttp client errors are caught and wrapped in GrepAPITimeoutError or GrepAPIError respectively. These are then formatted into user-friendly strings like "❌ Error: Request timed out. The grep.app API may be experiencing issues." ensuring the MCP server remains stable and informative during connectivity issues.

Where is the HTTP status code handling logic implemented in the codebase?

All HTTP status code inspection and error mapping logic resides in src/grep_mcp/server.py, specifically within the grep_query asynchronous function. This file defines the custom exception hierarchy (GrepAPIError, GrepAPIRateLimitError, GrepAPITimeoutError) and the _format_grep_response helper used for successful response processing.

{
  "mcpServers": {
    "instagit": {
      "command": "npx",
      "args": ["-y", "instagit@latest"]
    }
  }
}