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Local MCP Development with COBOL, and Gemini CLI

#gnucobol #gemini #agents #mcps

Leveraging Gemini CLI and the underlying Gemini LLM to build Model Context Protocol (MCP) AI applications in COBOL with a local development environment.

Why not just use Python?

Python has traditionally been the main coding language for ML and AI tools. One of the strengths of the MCP protocol is that the actual implementation details are independent of the development language. The reality is that not every project is coded in Python- and MCP allows you to use the latest AI approaches with other coding languages.

COBOL? For AI? What century are you from, Time Traveler?

The goal of this article is to provide a minimal viable basic working MCP stdio server in COBOL that can be run locally without any unneeded extra code or extensions.

So how does this Sorcery Work?

It is theoretically possible to implement the stack needed with only valid COBOL language constructs. The key to this solution is that modern compiled COBOL supports importing bindings from C during the compilation process.

A fully functional C MCP library is here:

GitHub - micl2e2/mcpc: Cross-platform C SDK for Model Context Protocol (MCP), in modern🚀 C23.

What Is COBOL?

COBOL (COmmon Business-Oriented Language) is a high-level, English-like programming language developed in the 1950s, designed for business, finance, and administrative systems, especially on mainframe computers, handling large-scale data processing. Despite its age, it remains crucial for critical infrastructure like banking and government systems, powering applications that process massive transaction volumes, due to its reliability and self-documenting syntax.

There are some legacy sites at IBM, but for the modern compiler version- here is the GNU COBOL site:

GnuCOBOL

Installing COBOL

The step by step instructions vary by platform- for a basic Debian system here are the steps:

sudo apt update
sudo apt install build-essential
sudo apt install gcc
sudo apt install gnucobol4

cobc -V

xbill@penguin:~$ cobc -V
cobc (GnuCOBOL) 4.0-early-dev.0
Copyright (C) 2020 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <https://gnu.org/licenses/gpl.html>
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
Written by Keisuke Nishida, Roger While, Ron Norman, Simon Sobisch, Edward Hart
Built Sep 19 2022 04:28:17
Packaged Jun 06 2020 20:56:36 UTC
C version "12.2.0"
xbill@penguin:~$
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Gemini CLI

If not pre-installed you can download the Gemini CLI to interact with the source files and provide real-time assistance:

npm install -g @google/gemini-cli
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Testing the Gemini CLI Environment

Once you have all the tools and the correct Node.js version in place- you can test the startup of Gemini CLI. You will need to authenticate with a Key or your Google Account:

gemini
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Node Version Management

Gemini CLI needs a consistent, up to date version of Node. The nvm command can be used to get a standard Node environment:

GitHub - nvm-sh/nvm: Node Version Manager - POSIX-compliant bash script to manage multiple active node.js versions

C MCP Documentation

The official MCP C page provides samples and documentation for getting started:

GitHub - micl2e2/mcpc: Cross-platform C SDK for Model Context Protocol (MCP), in modern🚀 C23.

This C library is built and the low level bindings can be used from COBOL code.

Where do I start?

The strategy for starting MCP development with COBOL is a incremental step by step approach.

First, the basic development environment is setup with the required system variables, and a working Gemini CLI configuration.

Then, a minimal Hello World Style COBOL MCP Server is built with stdio transport using the C library bindings. This server is validated with Gemini CLI in the local environment.

This setup validates the connection from Gemini CLI to the local process via MCP. The MCP client (Gemini CLI) and the MCP server both run in the same local environment.

Next- the basic MCP server is extended with Gemini CLI to add several new tools in standard code.

Setup the Basic Environment

At this point you should have a working COBOL and C environment and a working Gemini CLI installation. The next step is to clone the GitHub samples repository with support scripts:

cd ~
git clone https://github.com/xbill9/gemini-cli-codeassist
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Then run init.sh from the cloned directory.

The script will attempt to determine your shell environment and set the correct variables:

cd gemini-cli-codeassist
source init.sh
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If your session times out or you need to re-authenticate- you can run the set_env.sh script to reset your environment variables:

cd gemini-cli-codeassist
source set_env.sh
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Variables like PROJECT_ID need to be setup for use in the various build scripts- so the set_env script can be used to reset the environment if you time-out.

Hello World with STDIO Transport

One of the key features that the standard MCP libraries provide is abstracting various transport methods.

The high level MCP tool implementation is the same no matter what low level transport channel/method that the MCP Client uses to connect to a MCP Server.

The simplest transport that the SDK supports is the stdio (stdio/stdout) transport — which connects a locally running process. Both the MCP client and MCP Server must be running in the same environment.

The connection over stdio will look similar to this:

           CALL "mcpc_server_new_iostrm" USING BY VALUE stdin-ptr 
                                                BY VALUE stdout-ptr 
                                       RETURNING server-ptr.
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Package Information

The code depends on several standard libraries for MCP and logging:

       >>SOURCE FORMAT FREE
       IDENTIFICATION DIVISION.
       PROGRAM-ID. server.
       ENVIRONMENT DIVISION.
       CONFIGURATION SECTION.
       REPOSITORY.
           FUNCTION ALL INTRINSIC.
       DATA DIVISION.
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Installing and Running the Code

Run the install make release target on the local system:

xbill@penguin:~/gemini-cli-codeassist/mcp-stdio-cobol$ make
cc -std=c17 -Wall -Wextra -Imcpc/src -Imcpc -Dis_unix -DMCPC_C23PTCH_KW1 -DMCPC_C23PTCH_CKD1 -DMCPC_C23PTCH_UCHAR1 -DMCPC_C23GIVUP_FIXENUM -O2 -c c_helpers.c
cc -std=c17 -Wall -Wextra -Imcpc/src -Imcpc -Dis_unix -DMCPC_C23PTCH_KW1 -DMCPC_C23PTCH_CKD1 -DMCPC_C23PTCH_UCHAR1 -DMCPC_C23GIVUP_FIXENUM -O2 -c cob_helpers.c
make -C mcpc/src 
make[1]: Entering directory '/home/xbill/gemini-cli-codeassist/mcp-stdio-cobol/mcpc/src'
cc -Dis_unix -std=c17 -DMCPC_C23PTCH_KW1 -DMCPC_C23PTCH_CKD1 -DMCPC_C23PTCH_UCHAR1 -DMCPC_C23GIVUP_FIXENUM -Wall -Wextra -Werror -Wno-unused-function -Wno-unused-parameter -Wno-unused-label -Wno-error=unused-variable -Wno-error=unused-but-set-variable -O2 -Os -I.. -fPIC alloc.c log.c errcode.c anydata.c tool.c rsc.c prmpt.c server.c retbuf.c ucbr.c complt.c serlz.c mjson.c -c 
ar rcs libmcpc.a alloc.o log.o errcode.o anydata.o tool.o rsc.o prmpt.o server.o retbuf.o ucbr.o complt.o serlz.o mjson.o 
cc -s -o libmcpc.so alloc.o log.o errcode.o anydata.o tool.o rsc.o prmpt.o server.o retbuf.o ucbr.o complt.o serlz.o mjson.o -shared ../src/libmcpc.a 
make[1]: Leaving directory '/home/xbill/gemini-cli-codeassist/mcp-stdio-cobol/mcpc/src'
cobc -x -free -o server-cobol server.cob c_helpers.o cob_helpers.o mcpc/src/libmcpc.a -lm
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To lint the code:

xbill@penguin:~/gemini-cli-codeassist/mcp-stdio-cobol$ make lint
cc -std=c17 -Wall -Wextra -Imcpc/src -Imcpc -Dis_unix -DMCPC_C23PTCH_KW1 -DMCPC_C23PTCH_CKD1 -DMCPC_C23PTCH_UCHAR1 -DMCPC_C23GIVUP_FIXENUM -Wpedantic -Wshadow -Wpointer-arith -Wcast-qual -Wstrict-prototypes -Wmissing-prototypes -fsyntax-only c_helpers.c
cc -std=c17 -Wall -Wextra -Imcpc/src -Imcpc -Dis_unix -DMCPC_C23PTCH_KW1 -DMCPC_C23PTCH_CKD1 -DMCPC_C23PTCH_UCHAR1 -DMCPC_C23GIVUP_FIXENUM -Wpedantic -Wshadow -Wpointer-arith -Wcast-qual -Wstrict-prototypes -Wmissing-prototypes -fsyntax-only cob_helpers.c
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To test the code:

xbill@penguin:~/gemini-cli-codeassist/mcp-stdio-cobol$ make test
python3 test_server_cobol.py
Initialize response received
Greet response received: {
  "jsonrpc": "2.0",
  "id": 2,
  "result": {
    "content": [
      {
        "type": "text",
        "text": "Hello, World!"
      }
    ]
  }
}
Test PASSED
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Gemini CLI settings.json

In this example — the source code uses a compiled binary that can be called directly from Gemini CLI.

The default Gemini CLI settings.json has an entry for the source:

 {
    "mcpServers": {
      "mcp-stdio-cobol": {
        "command": "$HOME/gemini-cli-codeassist/mcp-stdio-cobol/server-cobol"
        }
      }
    }
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Validation with Gemini CLI

Finally- Gemini CLI is restarted and the MCP connection over stdio to the COBOL Code is validated. The full Gemini CLI Session will start:

> /mcp schema

Configured MCP servers:

🟢 mcp-stdio-cobol - Ready (2 tools)
  Tools:
  - greet
    Get a greeting from a local stdio server.
      Parameters:
      {
        "type": "object",
        "properties": {
          "param": {
            "type": "string",
            "title": "param",
            "description": "Greeting parameter"
          }
        }
      }

> /mcp list

Configured MCP servers:

🟢 mcp-stdio-cobol - Ready (2 tools)
  Tools:
  - greet
  - mcpc-info

> greet FLOW-MATIC

✦ I will call the greet tool with the parameter FLOW-MATIC to get a greeting.
╭──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────╮
│ ✓ greet (mcp-stdio-cobol MCP Server) {"param":"FLOW-MATIC"} │
│ │
│ Hello, FLOW-MATIC! │
╰──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────╯
✦ Hello, FLOW-MATIC!
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Extending the MCP Server with Gemini CLI

Now that the basic MCP functionality has been tested and verified with Gemini CLI- the server code can be extended with new MCP tools:

> with @server.cob add a new tool exposed by 
MCP that generates a sample paycheck
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Gemini CLI will get right to work:

│ 292 + │
│ 293 + *> Calculations │
│ 294 + COMPUTE gross-pay = rate-val * hours-val. │
│ 295 + COMPUTE tax-amount = gross-pay * 0.20. │
│ 296 + COMPUTE net-pay = gross-pay - tax-amount. │
│ 297 + │
│ 298 + MOVE gross-pay TO formatted-gross. │
│ 299 + MOVE tax-amount TO formatted-tax. │
│ 300 + MOVE net-pay TO formatted-net. │
│ 301 + MOVE rate-val TO formatted-rate. │
│ 302 + │ │
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Once completed — the changes will be summarized:

✦ I have successfully added the generate_paycheck tool to the COBOL-based MCP server. This involved:
   - Implementing the tool logic in server.cob (calculating gross pay, 20% taxes, and net pay).
   - Registering the tool and its properties (employee_name, hourly_rate, hours_worked) in the MCP server setup.
   - Adding a C wrapper in cob_helpers.c to bridge the callback to COBOL.
   - Verifying the implementation with a new test script, test_paycheck.py.
   - Updating GEMINI.md documentation.

  The tool can now be called via MCP to generate a formatted paycheck summary.
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Restart Gemini CLI and check for the new MCP tool:

xbill@penguin:~/gemini-cli-codeassist/mcp-stdio-cobol$ make
cc -std=c17 -Wall -Wextra -Imcpc/src -Imcpc -Dis_unix -DMCPC_C23PTCH_KW1 -DMCPC_C23PTCH_CKD1 -DMCPC_C23PTCH_UCHAR1 -DMCPC_C23GIVUP_FIXENUM -O2 -c c_helpers.c
cc -std=c17 -Wall -Wextra -Imcpc/src -Imcpc -Dis_unix -DMCPC_C23PTCH_KW1 -DMCPC_C23PTCH_CKD1 -DMCPC_C23PTCH_UCHAR1 -DMCPC_C23GIVUP_FIXENUM -O2 -c cob_helpers.c
make -C mcpc/src 
make[1]: Entering directory '/home/xbill/gemini-cli-codeassist/mcp-stdio-cobol/mcpc/src'
cc -Dis_unix -std=c17 -DMCPC_C23PTCH_KW1 -DMCPC_C23PTCH_CKD1 -DMCPC_C23PTCH_UCHAR1 -DMCPC_C23GIVUP_FIXENUM -Wall -Wextra -Werror -Wno-unused-function -Wno-unused-parameter -Wno-unused-label -Wno-error=unused-variable -Wno-error=unused-but-set-variable -O2 -Os -I.. -fPIC alloc.c log.c errcode.c anydata.c tool.c rsc.c prmpt.c server.c retbuf.c ucbr.c complt.c serlz.c mjson.c -c 
ar rcs libmcpc.a alloc.o log.o errcode.o anydata.o tool.o rsc.o prmpt.o server.o retbuf.o ucbr.o complt.o serlz.o mjson.o 
cc -s -o libmcpc.so alloc.o log.o errcode.o anydata.o tool.o rsc.o prmpt.o server.o retbuf.o ucbr.o complt.o serlz.o mjson.o -shared ../src/libmcpc.a 
make[1]: Leaving directory '/home/xbill/gemini-cli-codeassist/mcp-stdio-cobol/mcpc/src'
cobc -x -free -o server-cobol server.cob c_helpers.o cob_helpers.o mcpc/src/libmcpc.a -lm
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And run the new MCP tool:

> /mcp list

Configured MCP servers:

🟢 mcp-stdio-cobol - Ready (3 tools)
  Tools:
  - generate_paycheck
  - greet
  - mcpc-info

> generate_paycheck

✦ I will read test_paycheck.py to see how the generate_paycheck tool is expected to be used and what sample data it employs.
╭──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────╮
│ ✓ ReadFile test_paycheck.py │
│ │
╰──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────╯
✦ I will generate a sample paycheck for John Doe with an hourly rate of 50 and 40 hours worked.
╭──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────╮
│ ✓ generate_paycheck (mcp-stdio-cobol MCP Server) {"employee_name":"John Doe","hours_worked":40,"hourly_rate":50} │
│ │
│ Paycheck for: John Doe │
│ Rate: $ 50.00 / Hours: 40 │
│ Gross Pay: $ 2,000.00 │
│ Taxes (20%): $ 400.00 │
│ Net Pay: $ 1,600.00 │
╰──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────╯
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This information can then be used inside the Gemini CLI context:

✦ The generate_paycheck tool produced the following result:

   1 Paycheck for: John Doe
   2 Rate: $ 25.00 / Hours: 40
   3 Gross Pay: $ 1,000.00
   4 Taxes (20%): $ 200.00
   5 Net Pay: $ 800.00

  Would you like to see the COBOL implementation of this tool in server.cob?

> did John Doe have any overtime?

✦ I will check the COBOL implementation in server.cob to see how it calculates pay and if it handles overtime.

╭──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────╮
│ ✓ ReadFile server.cob │
│ │
╰──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────╯
✦ The current COBOL implementation for generate_paycheck (specifically the generate_paycheck_cb_impl program) does not handle overtime. It
  performs a simple calculation:

   1 *> Calculations
   2 COMPUTE gross-pay = rate-val * hours-val.
   3 COMPUTE tax-amount = gross-pay * 0.20.
   4 COMPUTE net-pay = gross-pay - tax-amount.

  As you can see, it simply multiplies the rate-val by the hours-val to get the gross-pay, regardless of how many hours were worked.

  Would you like me to update the implementation to include overtime pay (e.g., time-and-a-half for hours over 40)?

> how many hours did John Doe work?

✦ John Doe worked 40 hours.

  Since the current implementation doesn't have an overtime threshold, all 40 hours were paid at the regular rate. If I were to implement
  overtime (e.g., for hours over 40), his pay would stay the same in this specific case, but it would change for any hours worked beyond 40.
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Summary

The strategy for using COBOL with MCP development with Gemini CLI was validated with an incremental step by step approach.

A minimal stdio transport MCP Server was started from COBOL source code and validated with Gemini CLI running as a MCP client in the same local environment.

Gemini CLI was then used to extend the sample code with a new MCP tool and use the tool inside the context for the underlying LLM.

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