Did God harden Pharaoh's heart, or did Pharaoh harden his own?

The honest answer is: both. And if you look at the actual Hebrew text, verse by verse, it is not a tie or a compromise — it is two things the narrative holds together without flinching.

Before a single plague begins, God says twice, in the first person, that he will harden Pharaoh's heart. At the burning bush: "I will make his heart firm" (Exodus 4:21) — the Hebrew is a Piel of chazaq (חָזַק, H2388), an agent-explicit causative with God as subject. Then again at the start of Exodus 7: "I will harden Pharaoh's heart" (Exodus 7:3), a Hiphil of qashah (קָשָׁה, H7185). Two forecasts, God as the stated agent, before Pharaoh has refused even once. The hardening is not God's reaction to Egyptian stubbornness — it is declared upfront as strategy, and the stated purpose is given immediately: "that I may multiply my signs and wonders" so that Egypt will know who YHWH is (Exodus 7:3–5).

Then the plagues begin, and through the first five of them the grammar shifts. Every hardening in plagues one through five is either what Hebrew calls a Qal intransitive — "Pharaoh's heart became firm" (7:13, 7:22, 8:19, 9:7), where no agent is named and the heart itself is the subject — or a Hiphil with Pharaoh as the explicit subject: "Pharaoh made his heart heavy" (8:15), "Pharaoh made heavy his heart" (8:32). Not once in those five plagues is YHWH named as the one doing the hardening.

He did not set his heart even to this. (Exodus 7:23)

The hinge comes at the sixth plague, the boils. For the first time in the narrative, the sentence reads: "YHWH made firm the heart of Pharaoh" (Exodus 9:12) — Piel of chazaq, with YHWH as the explicit grammatical subject. A Dead Sea Scroll fragment and the ancient Greek both confirm this reading. From that point forward, through plagues eight, nine, and ten, and into the pursuit at the sea, YHWH is the consistent stated agent. At the sea, God goes further still: "I am hardening the heart of Egypt" (Exodus 14:17) — an active participle, the most intensive form, God as the present ongoing agent.

But here is what the text refuses to do: it refuses to let divine hardening erase Pharaoh's own responsibility. After God has begun hardening Pharaoh explicitly, Pharaoh still hardens himself (9:34). And when Moses gives the first Passover speech — after the ten plagues, after all the divine hardening — he still says, in a Hiphil with Pharaoh as subject: "Pharaoh made his heart stubborn when he refused to let us go" (Exodus 13:15). The same Pharaoh whose heart God was hardening is the same Pharaoh who was hardening himself, and both are true at the same time.

The Apostle Paul reads it exactly this way. He quotes Exodus 9:16 — "for this very purpose I raised you up, that I might show my power in you" — and concludes: "he hardens whom he wills" (Romans 9:17–18). He is not importing a foreign theology; he is reading what the Hebrew narrative showed from the opening forecast. God's sovereignty is total, and Pharaoh's guilt is real, and Exodus never tries to resolve the tension because the text does not think it is a contradiction. Two things are both true: Pharaoh weighs his own heart down, and that very weight becomes the stage on which God displays his glory.