---
title: "Mock a gRPC dependency | Speedscale Docs"
description: "Answer your service's calls to a gRPC dependency from a proxymock recording, so tests and AI coding assistants run with no dependency, and fix calls the mock does not match."
canonical: "https://docs.speedscale.com/proxymock/grpc/mock-a-grpc-dependency/"
---
[Documentation index](https://docs.speedscale.com/proxymock/llms.txt) | [Markdown](https://docs.speedscale.com/proxymock/grpc/mock-a-grpc-dependency/index.md) | [Canonical page](https://docs.speedscale.com/proxymock/grpc/mock-a-grpc-dependency/)
# Mock a gRPC dependency
`proxymock mock` answers your service's gRPC calls from recorded or generated calls, so the service runs with no dependency: nothing to deploy, no test account to share, and the same answers every time.
## Run the mock
Start the mock with the same mapping you recorded with, and point your service at the mapped port:
```bash
proxymock mock --in ./proxymock/recorded-fraud --map 18580=localhost:18551 -- ./your-service
```
Calls that match a recorded call get its response. Calls that do not match go on to the real dependency at `localhost:18551`, and the mock's output lists each call as a match or a pass-through. Add `--no-passthrough` to answer a call that matches nothing with an HTTP 404 instead, so a test can never reach the real dependency. A gRPC client reports that 404 as `Unimplemented`:
```text
ERROR:
Code: Unimplemented
Message: unexpected HTTP status code received from server: 404 (Not Found); malformed header: missing HTTP content-type
```
A service that sets `grpc_proxy` instead of using a mapped port works the same way: point `grpc_proxy` at the mock's outbound port (4140 by default, `--proxy-out-port` to change it).
You can call the mock yourself to check it, with the dependency stopped:
```bash
grpcurl -plaintext -proto fraud.proto -d '{"account_id":"acct-1001","user_id":"user-7","amount":12.5,"transaction_type":"DEPOSIT","merchant_category":"5411"}' localhost:18580 fraud.v1.FraudChecker/CheckTransaction
```
## What a call has to match
A gRPC mock matches on the method path, the host and the whole request message. A call that asks for a different account than the recording did is a different call, so it does not match. That keeps answers exact, and it means a mock answers only the requests that were recorded or written into it.
When a call does not match, the mock's output shows the closest recorded call and the fields that differ:
```text
Almost matched signature #0: ...
< http:requestBodyJSON: {"1:LEN":"acct-2","2:LEN":"user-7",...}
> http:requestBodyJSON: {"1:LEN":"acct-1001","2:LEN":"user-7",...}
```
Three ways to make it match:
* **Record the call.** Run the scenario once more with `proxymock record`, so the recording holds the request your service really sends.
* **Edit the recorded request.** Change the values in the mock file's request body to the ones your service sends. The mock matches on the edited message the next time it starts.
* **Ignore the volatile field.** When a field changes on every call, such as a request id or a pod name, a transform removes it from the match. [Improve Mock Match Rate with AI](https://docs.speedscale.com/proxymock/guides/mock-match-rate/) has an AI assistant find the calls that missed and write the fix, and [Using Transforms for better Mocks](https://docs.speedscale.com/guides/replay/mocks/edit-sig/) walks through the same fix for a protobuf field by hand.
## Mock a dependency you have never recorded
When there is no recording yet, generate the mock from the dependency's `.proto`:
```bash
proxymock generate --direction outbound --out ./proxymock/mocks-fraud ./fraud.proto
```
A generated mock answers only the request it was generated with, whose strings are `example_value`. Edit its request body to the values your service sends, as above, and treat its response as made up until you replace it with a recorded one. [Generate tests and mocks from a .proto](https://docs.speedscale.com/proxymock/grpc/generate-from-a-proto/) explains what the generated messages contain.