Support context injection via SessionStart hook. #15746

gundermanc · 2025-12-31T00:24:41Z

Summary

Finishes wiring up systemMessage and additionalContext fields from the SessionStart hook.

Now you can inject context and info messages into the history using a SessionStart hook.

Details

Related Issues

Fixes #15413.

How to Validate

Pre-Merge Checklist

gemini-code-assist · 2025-12-31T00:25:04Z

Summary of Changes

Hello @gundermanc, 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 introduces a significant enhancement by enabling the SessionStart hook to inject contextual information directly into the model's input and display system messages. This functionality allows for greater extensibility, letting external hooks dynamically influence the initial conversation state and user experience across both the command-line interface and the interactive UI.

Highlights

Context Injection via SessionStart Hook: The SessionStart hook can now return additionalContext and systemMessage as part of its output, allowing external logic to influence the initial state of a session.
CLI Integration: In the command-line interface, any additionalContext returned by the SessionStart hook is prepended to the model's input, and systemMessage is displayed in stderr.
UI Integration: For the interactive UI, additionalContext is added to the chat history as a user message, and systemMessage is shown as an informational message.
Improved Hook Output Handling: The fireSessionStartHook function now returns a structured output object, enabling calling components to react to the hook's decisions and any injected data.
Test Coverage: A new integration test has been added to validate the end-to-end functionality of context injection and system message display via the SessionStart hook.

🧠 New Feature in Public Preview: You can now enable Memory to help Gemini Code Assist learn from your team's feedback. This makes future code reviews more consistent and personalized to your project's style. Click here to enable Memory in your admin console.

Using Gemini Code Assist

The full guide for Gemini Code Assist can be found on our documentation page, here are some quick tips.

Invoking Gemini

You can request assistance from Gemini at any point by creating a comment using either /gemini <command> or @gemini-code-assist <command>. Below is a summary of the supported commands on the current page.

Feature	Command	Description
Code Review	`/gemini review`	Performs a code review for the current pull request in its current state.
Pull Request Summary	`/gemini summary`	Provides a summary of the current pull request in its current state.
Comment	@gemini-code-assist	Responds in comments when explicitly tagged, both in pull request comments and review comments.
Help	`/gemini help`	Displays a list of available commands.

Customization

To customize Gemini Code Assist for GitHub experience, repository maintainers can create a configuration file and/or provide a custom code review style guide (such as PEP-8 for Python) by creating and adding files to a .gemini/ folder in the base of the repository. Detailed instructions can be found here.

Limitations & Feedback

Gemini Code Assist may make mistakes. Please leave feedback on any instances where its feedback is incorrect or counter productive. You can react with 👍 and 👎 on @gemini-code-assist comments. If you're interested in giving your feedback about your experience with Gemini Code Assist for Github and other Google products, sign up here.

You can also get AI-powered code generation, chat, as well as code reviews directly in the IDE at no cost with the Gemini Code Assist IDE Extension.

Review the Privacy Notices, Generative AI Prohibited Use Policy, Terms of Service, and learn how to configure Gemini Code Assist in GitHub here. Gemini can make mistakes, so double check it and use code with caution. ↩

github-actions · 2025-12-31T00:28:02Z

Size Change: +1.5 kB (+0.01%)

Total Size: 22.2 MB

Filename	Size	Change
`./bundle/gemini.js`	22.2 MB	+1.5 kB (+0.01%)

ℹ️ View Unchanged

Filename	Size
`./bundle/sandbox-macos-permissive-closed.sb`	1.03 kB
`./bundle/sandbox-macos-permissive-open.sb`	890 B
`./bundle/sandbox-macos-permissive-proxied.sb`	1.31 kB
`./bundle/sandbox-macos-restrictive-closed.sb`	3.29 kB
`./bundle/sandbox-macos-restrictive-open.sb`	3.36 kB
`./bundle/sandbox-macos-restrictive-proxied.sb`	3.56 kB

_{compressed-size-action}

gemini-code-assist

Code Review

This pull request successfully implements context injection via the SessionStart hook. The changes are well-structured, covering both non-interactive and interactive modes. In non-interactive mode, context is correctly prepended to the input, and in interactive mode, it's added to the conversation history. The addition of a comprehensive integration test validates the new functionality effectively. A notable improvement is the change to wrap the FakeContentGenerator with LoggingContentGenerator, which enhances testability by allowing API request inspection even with fake responses. Overall, the implementation is robust and the code quality is high.

…s quoting issues

gundermanc · 2026-01-02T20:13:17Z

packages/cli/src/ui/AppContainer.tsx

+    // item, including the ones from the start session hook will cause a
+    // re-render and an error when we try to reload config.
+    //
+    // eslint-disable-next-line react-hooks/exhaustive-deps


One thing I want to call out is that our use of useEffect() for first-time initialization is pretty brittle and possible bad practice, though I'm not a react expert.

We load the config inside of the useEffect()

Config expects to only get loaded once, so if the use effect runs multiple times, it causes an error.

Adding the new item to history with historyManager trips a linter error that wants us to include historyManager in the dependency array.

Doing so causes us to react to the addItem() call, causing the useEffect() to run again immediately after the first time.

For now I'm just excluding the historyManager from the dependencies because I think we're really just trying to cause a side effect here, not cause a redundant repaint, but I think we should rethink this in the future.

cc @jacob314, @scidomino do either of you have any insights?

packages/cli/src/ui/AppContainer.tsx

NTaylorMullen · 2026-01-03T00:35:59Z

packages/cli/src/ui/commands/clearCommand.ts

+      if (result?.systemMessage) {
+        context.ui.addItem(
+          {
+            type: MessageType.INFO,


I wonder if we should have a new message type for information like this. Right now we utilize this for "updates pending" or "warning xyz", it's a bit of everything.

Don't block on this comment (i.e. can always do in another PR) but food for thought. It's worth putting out a parallel feeler to UX folks to see their take.

NTaylorMullen · 2026-01-03T00:40:43Z

packages/cli/src/ui/commands/clearCommand.ts

+          Date.now(),
+        );
+      }
    }


Should this also be adding a message to history post-clear?

Ah, great catch!

Support context injection via SessionStart hook.

5022393

gemini-code-assist bot reviewed Dec 31, 2025

View reviewed changes

gundermanc added 9 commits December 30, 2025 16:40

Integration test.

47fb8e2

Fix SessionStart hook race conditions and CI timeout issues

5f4b44d

Fix build.

c0ce42c

Fix test

daf0361

Merge remote-tracking branch 'origin' into gundermanc/session-start-hook

ddea4f1

Fix build.

e784201

Fix test.

cfdc2ea

fix: use script files for SessionStart hooks in tests to avoid Window…

36532ae

…s quoting issues

Merge remote-tracking branch 'origin' into gundermanc/session-start-hook

935dccd

gundermanc force-pushed the gundermanc/session-start-hook branch 2 times, most recently from 5ab9c61 to 935dccd Compare January 2, 2026 17:15

Revert unneeded changes.

1af770f

gundermanc force-pushed the gundermanc/session-start-hook branch from fcd35b0 to 1af770f Compare January 2, 2026 17:36

Fix duplicate render issue.

8b8b5b2

gundermanc marked this pull request as ready for review January 2, 2026 20:07

gundermanc requested a review from a team as a code owner January 2, 2026 20:07

Delete accidentally added files.

4aca401

gundermanc commented Jan 2, 2026

View reviewed changes

Merge remote-tracking branch 'origin' into gundermanc/session-start-hook

28408df

NTaylorMullen reviewed Jan 3, 2026

View reviewed changes

Post hook message on clear.

8747611

NTaylorMullen approved these changes Jan 5, 2026

View reviewed changes

Fix formatting.

8b5d95c

gundermanc added this pull request to the merge queue Jan 5, 2026

Merged via the queue into main with commit 6d1e276 Jan 5, 2026
20 checks passed

gundermanc deleted the gundermanc/session-start-hook branch January 5, 2026 21:43

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Uh oh!

Support context injection via SessionStart hook. #15746

Support context injection via SessionStart hook. #15746

gundermanc commented Dec 31, 2025 •

edited

Loading

Uh oh!

gemini-code-assist bot commented Dec 31, 2025

Uh oh!

github-actions bot commented Dec 31, 2025 •

edited

Loading

Uh oh!

gemini-code-assist bot left a comment

Uh oh!

gundermanc Jan 2, 2026

Uh oh!

gundermanc Jan 5, 2026

Uh oh!

Uh oh!

NTaylorMullen Jan 3, 2026

Uh oh!

NTaylorMullen Jan 3, 2026

Uh oh!

gundermanc Jan 5, 2026

Uh oh!

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

2 participants

Support context injection via SessionStart hook. #15746

Support context injection via SessionStart hook. #15746

Conversation

gundermanc commented Dec 31, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Summary

Details

Related Issues

How to Validate

Pre-Merge Checklist

Uh oh!

gemini-code-assist bot commented Dec 31, 2025

Summary of Changes

Highlights

Footnotes

Uh oh!

github-actions bot commented Dec 31, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

gemini-code-assist bot left a comment

Choose a reason for hiding this comment

Code Review

Uh oh!

gundermanc Jan 2, 2026

Choose a reason for hiding this comment

Uh oh!

gundermanc Jan 5, 2026

Choose a reason for hiding this comment

Uh oh!

Uh oh!

NTaylorMullen Jan 3, 2026

Choose a reason for hiding this comment

Uh oh!

NTaylorMullen Jan 3, 2026

Choose a reason for hiding this comment

Uh oh!

gundermanc Jan 5, 2026

Choose a reason for hiding this comment

Uh oh!

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

2 participants

gundermanc commented Dec 31, 2025 •

edited

Loading

github-actions bot commented Dec 31, 2025 •

edited

Loading