fix(core): allow environment variable expansion and explicit overrides for MCP servers by galz10 · Pull Request #18837 · google-gemini/gemini-cli

galz10 · 2026-02-11T19:26:22Z

Summary

Fixes a regression where environment variable expansion was missing in MCP server configurations, and sanitization was redacting explicitly provided user credentials (introduced in #17311). This caused MCP servers to fail with 'Connection closed' errors when configured with dynamic secrets.

Details

PR #17311 introduced environment sanitization that merged all environment variables into a single pool before shredding them. This prevented the use of dynamic variables like $VAR in the configuration and accidentally redacted intentional secrets provided by the user.

This PR:

Introduces a robust expandEnvVars utility in packages/core/src/utils/envExpansion.ts that supports $VAR, ${VAR}, and %VAR%.
Updates createTransport in packages/core/src/tools/mcp-client.ts to expand variables in the env config using the host process environment.
Refines the security model to sanitize the base process environment while trusting and expanding user-provided environment variables and extension settings.
Updates docs/tools/mcp-server.md to document expansion and clarify security behavior for explicit variables.

Related Issues

Fixes unintended credential exposure regression from #17311.

How to Validate

Configure an MCP server in settings.json with an environment variable reference: "env": { "API_KEY": "$MY_SECRET" }.
Set MY_SECRET=1234 in the shell environment.
Launch the CLI and verify the MCP server connects and receives the expanded value (API_KEY=1234).
Configure an MCP server with a "secret-looking" key: "env": { "SECRET_KEY": "my-secret" }.
Verify the key is NOT redacted by the sanitizer in the debug logs (--debug).

Pre-Merge Checklist

…s for MCP servers

gemini-code-assist · 2026-02-11T19:26:40Z

Summary of Changes

Hello @galz10, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses issues where MCP servers failed to connect due to unexpanded environment variables and over-zealous sanitization of user-provided credentials. It introduces a robust environment variable expansion utility and refines the security model for passing environment variables to MCP servers. The changes ensure that user-defined variables are correctly expanded and explicitly trusted, while the base environment remains sanitized, enhancing both functionality and security.

Highlights

Environment Variable Expansion: Enabled environment variable expansion in MCP server configurations, supporting POSIX ($VAR, ${VAR}) and Windows (%VAR%) syntaxes.
Refined Security Model: Implemented a refined security model that sanitizes the base process.env but explicitly trusts and expands user-defined environment variables in mcpServerConfig.env and extension settings, preventing unintended redaction of explicit credentials.
Core Logic Update: Updated the createTransport function to incorporate the new environment variable expansion logic and the refined sanitization process.
Comprehensive Testing: Added comprehensive unit and integration tests for the new expansion utility and the mcp-client's handling of environment variables.
Documentation Updates: Updated documentation to clearly explain environment variable expansion and the security model for MCP server configurations.

Changelog

docs/tools/mcp-server.md
- Updated documentation to include support for Windows-style %VAR% environment variable expansion.
- Added a new section detailing environment variable expansion capabilities and providing examples.
- Introduced a new section on security and environment sanitization, explaining automatic redaction and explicit overrides.
- Revised the security considerations section to reference the new environment sanitization documentation.
packages/core/src/tools/mcp-client.test.ts
- Added a new test case to verify that environment variables defined in mcpServerConfig.env are correctly expanded and are not redacted.
packages/core/src/tools/mcp-client.ts
- Imported the new expandEnvVars utility.
- Refactored the createTransport function to first sanitize the base process.env, then expand and merge environment variables from extensions and mcpServerConfig.env into the final environment passed to the transport.
packages/core/src/utils/envExpansion.test.ts
- Added a new test file containing comprehensive unit tests for the expandEnvVars utility, covering POSIX, Windows, mixed formats, missing variables, and edge cases.
packages/core/src/utils/envExpansion.ts
- Added a new utility function expandEnvVars that expands environment variables in a string, supporting $VAR, ${VAR}, and %VAR% syntaxes.

Activity

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Code Review

This pull request introduces a valuable feature for expanding environment variables in MCP server configurations and refines the environment sanitization logic. A security review found no vulnerabilities in the audited files. However, there are a couple of high-severity issues that need attention: unit tests for Windows-style paths are flawed due to improper handling of backslash characters, and the documentation includes a misleading example for default value expansion (e.g., ${VAR:-default}) which is not supported by the new expandEnvVars function.

github-actions · 2026-02-11T19:42:58Z

Size Change: +4.89 kB (+0.02%)

Total Size: 25.7 MB

Filename	Size	Change
`./bundle/gemini.js`	25.2 MB	+4.89 kB (+0.02%)

ℹ️ View Unchanged

Filename	Size
`./bundle/node_modules/@google/gemini-cli-devtools/dist/client/main.js`	221 kB
`./bundle/node_modules/@google/gemini-cli-devtools/dist/src/_client-assets.js`	227 kB
`./bundle/node_modules/@google/gemini-cli-devtools/dist/src/index.js`	11.5 kB
`./bundle/node_modules/@google/gemini-cli-devtools/dist/src/types.js`	132 B
`./bundle/sandbox-macos-permissive-open.sb`	890 B
`./bundle/sandbox-macos-permissive-proxied.sb`	1.31 kB
`./bundle/sandbox-macos-restrictive-open.sb`	3.36 kB
`./bundle/sandbox-macos-restrictive-proxied.sb`	3.56 kB
`./bundle/sandbox-macos-strict-open.sb`	4.82 kB
`./bundle/sandbox-macos-strict-proxied.sb`	5.02 kB

_{compressed-size-action}

…CP client

jacob314

Approved once the expandEnvVars logic is hardened.
If we must keep it we should go through at least a couple iterations with Gemini 3.0 pro to harden it.

jacob314

g-samroberts

Docs LGTM

…s for MCP servers (#18837)

…s for MCP servers (google-gemini#18837)

fix(core): allow environment variable expansion and explicit override…

1f18797

…s for MCP servers

galz10 requested review from a team as code owners February 11, 2026 19:26

gemini-code-assist Bot reviewed Feb 11, 2026

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Comment thread docs/tools/mcp-server.md Outdated

Comment thread packages/core/src/utils/envExpansion.test.ts Outdated

galz10 added 2 commits February 11, 2026 11:35

test(core): fix backslash escaping in env expansion tests

1b3335a

docs(core): correct mcp server env expansion example

96f4021

gemini-cli Bot added the priority/p1 Important and should be addressed in the near term. label Feb 11, 2026

jacob314 reviewed Feb 11, 2026

View reviewed changes

Comment thread packages/core/src/utils/envExpansion.test.ts

galz10 added 3 commits February 11, 2026 12:13

fix(core): improve environment variable handling and type safety in M…

4cfdf82

…CP client

test(core): parameterize envExpansion tests

8779818

chore: fix lint error

c12fa73

jacob314 reviewed Feb 11, 2026

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Comment thread packages/core/src/utils/envExpansion.ts

jacob314 reviewed Feb 11, 2026

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galz10 added 2 commits February 12, 2026 11:26

chore: use dotenv-expand to expand vars

31195c0

Merge branch 'main' into galzahavi/fix/mcp-error

f356d4f

galz10 requested a review from a team as a code owner February 17, 2026 18:27

galz10 and others added 9 commits February 17, 2026 10:27

Merge branch 'main' into galzahavi/fix/mcp-error

2bdf7bf

chore:fix lint error

5e0f060

chore: fix docs

cf763a1

Merge branch 'main' into galzahavi/fix/mcp-error

2a86c23

chore: fix lint error

6a41081

Merge branch 'main' into galzahavi/fix/mcp-error

45ad2a6

revert strict-development-rules.md change

90cd774

Merge branch 'main' into galzahavi/fix/mcp-error

3b1a3ef

Revert sidebar.json changes.

da635bc

jacob314 reviewed Feb 20, 2026

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Comment thread packages/core/src/utils/envExpansion.ts

jacob314 approved these changes Feb 20, 2026

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chore: address pr comments

81fbe71

galz10 and others added 2 commits February 23, 2026 11:41

Merge branch 'main' into galzahavi/fix/mcp-error

d4236ef

chore: fix lint error

41258bd

g-samroberts approved these changes Feb 23, 2026

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galz10 added this pull request to the merge queue Feb 23, 2026

Merged via the queue into main with commit 0bc2d3a Feb 23, 2026
27 checks passed

galz10 deleted the galzahavi/fix/mcp-error branch February 23, 2026 21:50

sripasg pushed a commit that referenced this pull request Feb 27, 2026

fix(core): allow environment variable expansion and explicit override…

aed0f21

…s for MCP servers (#18837)

BryanBradfo pushed a commit to BryanBradfo/gemini-cli that referenced this pull request Mar 5, 2026

fix(core): allow environment variable expansion and explicit override…

7046a28

…s for MCP servers (google-gemini#18837)

liamhelmer pushed a commit to badal-io/gemini-cli that referenced this pull request Mar 12, 2026

fix(core): allow environment variable expansion and explicit override…

bdebbb8

…s for MCP servers (google-gemini#18837)

warrenzhu25 pushed a commit to warrenzhu25/gemini-cli that referenced this pull request Apr 9, 2026

fix(core): allow environment variable expansion and explicit override…

6366fcb

…s for MCP servers (google-gemini#18837)

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fix(core): allow environment variable expansion and explicit overrides for MCP servers#18837
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