Is there any evidence that this character in Harry Potter was a double agent all along, prior to the revelation?

Question

Warning: MASSIVE spoilers ahead. If you haven't read the last Harry Potter book, but plan to do so, turn back now

---

Throughout the entire Harry Potter series, there was one character who was the comic 'bad guy' - designed to be disliked. This of course is Snape.

We think that he is a Death Eater, working for Voldemort, with a personal disliking of Harry.

However, in the final book in chapter 33 'The Prince's (Snape's) Tale', that all changes.

Our entire view of Snape changes, and we learn he isn't who we thought he was. He wasn't working against Dumbledore and for Voldemort, but actually the other way round:

"Hide them all, then," he croaked. "Keep her—them—safe. Please."
"And what will you give me in return, Severus?"
"In—in return?" Snape gaped at Dumbledore, and Harry expected him to protest, but after a long moment he said, "Anything."
...

"Snape raised his eyebrows and his tone was sardonic as he asked,
"Are you intending to let him kill you?"
[Dumbledore]: "Certainly not. You must kill me."
...

"Severus Snape wasn’t yours," said Harry. "Snape was Dumbledore’s, Dumbledore’s from the moment you started hunting down my mother."

We learn all this when Harry sees the memories he gave him just before he died.

This is possibly the biggest twist in the entire Harry Potter series. It shocked many, and seems completely unexpected.

But maybe it shouldn't have been so unexpected...

Was there any evidence prior to the revelation that Snape is actually a 'double-agent' and not who we thought he was?

Answer 1

The strongest evidence (in my opinion) is Dumbledore's unwavering trust. When other characters doubted him, Dumbledore always affirmed his confidence, but never gave others a reason to share it. This clearly suggests that he possesses some secret knowledge.

For example, when Harry and Lupin discuss Snape (Half Blood Prince) we have the following exchange

"But," said Harry, "just say — just say Dumbledore's wrong about Snape —"

"People have said it, many times. It comes down to whether or not you trust Dumbledore’s judgement. I do; therefore, I trust Severus."

Lupin, who is at the heart of the Order of the Phoenix, is able to offer no better proof of Snape's fidelity than this, suggesting that Dumbledore must have some good reason that he is unwilling to share with even his closest confidants.

Answer 2

It depends on what you mean by "all along." Snape was actually working for Voldemort, and then after he realized that Voldemort intended to kill the Potters he became loyal to Dumbledore and (apparently) remained so for the rest of his life. (His actions in the sixth book were entirely taken in an effort to re-establish his cover with the Death Eaters, not out of disloyalty to Dumbledore).

After Snape accidentally referred to Lily as a mudblood and attempted to apologize, Lily said

‘It’s too late. I’ve made excuses for you for years. None of my friends can understand why I even talk to you. You and your precious little Death Eater friends – you see, you don’t even deny it! You don’t even deny that’s what you’re all aiming to be! You can’t wait to join You-Know-Who, can you?’

He opened his mouth, but closed it without speaking.

This suggests that Snape actually joined Voldemort (he didn't even bother to deny that that was his intent), so he clearly wasn't a double agent at this point.

See also this question about whether Snape was actually prejudiced.

If he had been a double agent prior to agreeing to do whatever Dumbledore asked, it would make no sense for Dumbledore to ask what he'd do in return for protecting the Potters. He had also given Voldemort accurate (albeit unintentionally missing critical information) information about what he heard about the prophecy. This suggests that Snape was actually working for Voldemort up until the point that he found out that Voldemort intended to kill the Potters.

Evidence suggests that he remained working for Dumbledore all along (e.g. the fact that he saved Harry in the first book and the fact that he went to great lengths to protect the Sorcerer's Stone).

In the sixth book, you see him working with Dumbledore to protect Draco Malfoy and working to re-establish his cover with the Death Eaters by meeting with Bellatrix and taking the Unbreakable Vow to cooperate with Draco's plan.

There are multiple lines of evidence that he was lying to Bellatrix at their meeting, such as the fact that he apparently got the information on Draco's mission by using Legilimency on Draco (rather than directly from the Dark Lord like he claimed), the fact that Bellatrix believed him to be lying until he agreed to take the Unbreakable Vow, and the fact that Dumbledore apparently had to persuade Snape to agree to kill him. Even at the end, Dumbledore apparently had to persuade Snape to go through with killing him. Thus, he appeared to continue to be working with Dumbledore at this point.

Clearly, he was also secretly carrying out Dumbledore's intentions throughout the seventh book as well, up until the point that he was killed by Voldemort.

Answer 3

One piece of evidence (I'm sure that there's more):

In an early Quidditch match, Harry's broom starts trying to buck him off. Hermione sees that Snape is mouthing an incantation so sneaks over to try and put him off by setting fire to his robe.

It transpires that Quirrell is performing the incantation and Snape is muttering a counter-curse to keep Harry safe.

Setting Snape's robe on fire disrupts the entire teachers' seating area and, as a result, Quirrell breaks his magic.