The Stacks project

Proof. Say $X = \mathop{\mathrm{Spec}}(A)$. Recall that a perfect complex is pseudo-coherent, see Cohomology, Lemma 20.49.5. By Lemma 36.13.3 we can find a bounded above complex $\mathcal{F}^\bullet $ of finite free $A$-modules such that $E$ is isomorphic to $\mathcal{F}^\bullet |_ U$ in $D(\mathcal{O}_ U)$. By Cohomology, Lemma 20.49.5 and since $U$ is quasi-compact, we see that $E$ has finite tor dimension, say $E$ has tor amplitude in $[a, b]$. Pick $r < a$ and set

Since $E$ has tor amplitude in $[a, b]$ we see that $\mathcal{F} = \mathcal{K}|_ U$ is flat (Cohomology, Lemma 20.48.2). Hence $\mathcal{F}$ is flat and of finite presentation, thus finite locally free (Properties, Lemma 28.20.2). It follows that

is a strictly perfect complex on $U$ representing $E$. On the other hand, the complex $P = (\mathcal{F}^ r \to \mathcal{F}^{r + 1} \to \ldots )$ is a perfect complex on $X$. Using stupid truncations we obtain a distinguished triangle

If the map $E \to \mathcal{F}[-r - 1]$ is zero in $D(\mathcal{O}_ U)$, then $P|_ U = \mathcal{F}[-r - 2] \oplus E$, see Derived Categories, Lemma 13.4.11. This will be true for $r \ll 0$ for example by Lemma 36.13.5. $\square$