How can I tell whether a Git merge will fast-forward or create a merge commit?

The key question is whether the current branch tip is an ancestor of topic’s tip. If it is, Git can fast-forward: it advances the current branch pointer to topic without creating an extra commit. If both branches have commits the other does not, their histories have diverged; a true merge is needed and is recorded in a merge commit with both tips as parents. [S1]

First, check which branch you are on and inspect the commit ranges:

git branch --show-current

git log --oneline HEAD..topic

git log --oneline topic..HEAD

The first range lists commits reachable from topic but not from your current HEAD. The second lists commits reachable from your current branch but not from topic. [S1] These commands describe the relationship between the tips; they do not promise that a merge will be conflict-free. [S1]

Assume your current branch is main and you plan to merge topic. These are illustrative histories, not output from a real repository.

In a fast-forwardable history, main points to an earlier commit on the path to topic:

A---B---C---D
    ^       ^
   main    topic

Here, main is an ancestor of topic. Git can move main to D without creating a merge commit. [S1] git log --oneline HEAD..topic shows commits ahead on topic, while git log --oneline topic..HEAD shows no commits unique to main.

A diverged history has commits on both sides of a shared ancestor:

      C---D  topic
     /
A---B
     \
      E---F  main

Each range has commits: HEAD..topic shows C and D, while topic..HEAD shows E and F. A true merge is needed, recorded in a merge commit with both branch tips as parents. [S1]

git merge-base HEAD topic helps you inspect the common ancestor of the two tips. Compare the result with the tips: if the current tip is already on the history leading to topic, the relationship permits a fast-forward. If each tip has commits beyond the shared history, the branches have diverged. The Git manual identifies git merge-base as a way to inspect ancestry. [S1]

If you want to suppress a fast-forward and record a merge commit even when one is possible, Git documents --no-ff for that purpose. [S1] That option changes how the merge is recorded; it does not tell you whether the histories have diverged. Inspect the ranges first to understand the relationship.

For a local example, replace topic with the branch you plan to merge, then run:

git branch --show-current
git log --oneline HEAD..topic
git log --oneline topic..HEAD
git merge-base HEAD topic

Read both log ranges, not just the common ancestor. If HEAD..topic has commits and topic..HEAD has none, the current branch is behind topic in a fast-forwardable relationship. If both ranges have commits, the histories have diverged and a true merge is needed. If neither has commits, there are no commits reachable from either tip that are absent from the other. [S1]

These checks describe commit relationships, not whether the eventual merge will be conflict-free. [S1] If you want a merge commit even when fast-forwarding is possible, Git documents --no-ff for that purpose. [S1]