diff --git a/packages/opencode/src/provider/provider.ts b/packages/opencode/src/provider/provider.ts
index cf6917a2c962..d11734dfd33f 100644
--- a/packages/opencode/src/provider/provider.ts
+++ b/packages/opencode/src/provider/provider.ts
@@ -1177,7 +1177,10 @@ function cost(c: ModelsDev.Model["cost"]): Model["cost"] {
       tier: item.tier,
     }))
   }
-  if (c?.context_over_200k) {
+  // NOTE (Gabe, 2026-09-02): models.dev spells a model's single long-context tier as
+  // context_over_200k even when its tiers put the threshold elsewhere (272K for gpt-5.x), and
+  // Session.getUsage falls back to this field below the tier size, so tiers own the threshold.
+  if (c?.context_over_200k && !result.tiers?.length) {
     result.experimentalOver200K = {
       cache: {
         read: c.context_over_200k.cache_read ?? 0,
@@ -1422,6 +1425,9 @@ const layer = Layer.effect(
               if (model.id && model.id !== modelID) return modelID
               return existingModel?.name ?? modelID
             })
+            // NOTE (Gabe, 2026-09-02): a config context_over_200k prices everything above 200K, but
+            // Session.getUsage consults tiers first, so it rides as a 200K tier above the catalog's lower ones.
+            const over200k = model.cost?.context_over_200k ? cost(model.cost).experimentalOver200K : undefined
             const parsedModel: Model = {
               id: ModelV2.ID.make(modelID),
               api: {
@@ -1468,6 +1474,14 @@ const layer = Layer.effect(
                   read: model?.cost?.cache_read ?? existingModel?.cost?.cache.read ?? 0,
                   write: model?.cost?.cache_write ?? existingModel?.cost?.cache.write ?? 0,
                 },
+                // NOTE (Gabe, 2026-09-02): config cannot express tiers, so an entry that only names a model keeps the catalog's.
+                tiers: over200k
+                  ? [
+                      ...(existingModel?.cost?.tiers ?? []).filter((item) => item.tier.size < 200_000),
+                      { ...over200k, tier: { type: "context", size: 200_000 } },
+                    ]
+                  : existingModel?.cost?.tiers,
+                experimentalOver200K: over200k ?? existingModel?.cost?.experimentalOver200K,
               },
               options: mergeDeep(existingModel?.options ?? {}, model.options ?? {}),
               limit: {
diff --git a/packages/opencode/test/provider/provider.test.ts b/packages/opencode/test/provider/provider.test.ts
index 18ec8f9fbeeb..b9930f1b27dc 100644
--- a/packages/opencode/test/provider/provider.test.ts
+++ b/packages/opencode/test/provider/provider.test.ts
@@ -15,6 +15,8 @@ import { Config } from "@/config/config"
 import { Env } from "../../src/env"
 import { Plugin } from "../../src/plugin/index"
 import { Provider } from "@/provider/provider"
+import { Session } from "@/session/session"
+import { Usage } from "@opencode-ai/llm"
 
 import { RuntimeFlags } from "@/effect/runtime-flags"
 import { Filesystem } from "@/util/filesystem"
@@ -935,6 +937,95 @@ it.instance(
   },
 )
 
+it.instance(
+  "config entry that only names a model keeps the database over-200k pricing",
+  Effect.gen(function* () {
+    yield* set("OPENROUTER_API_KEY", "test-api-key")
+    const providers = yield* list
+    const model = providers[ProviderV2.ID.make("openrouter")].models["openai/gpt-5.4"]
+    expect(model.cost.input).toBe(2.5)
+    expect(model.cost.tiers).toEqual([
+      { input: 5, output: 22.5, cache: { read: 0.5, write: 0 }, tier: { type: "context", size: 272_000 } },
+    ])
+    // NOTE (Gabe, 2026-09-02): OpenRouter's override for gpt-5.x starts at 272K, so the legacy
+    // over-200k field must not price the 200K-272K band at the tier rate.
+    expect(model.cost.experimentalOver200K).toBeUndefined()
+    const price = (inputTokens: number) =>
+      Session.getUsage({ model, usage: new Usage({ inputTokens, outputTokens: 0, totalTokens: inputTokens }) }).cost
+    expect(price(250_000)).toBeCloseTo(0.625, 6)
+    expect(price(300_000)).toBeCloseTo(1.5, 6)
+  }),
+  {
+    config: {
+      provider: {
+        openrouter: {
+          models: { "openai/gpt-5.4": { options: {} } },
+        },
+      },
+    },
+  },
+)
+
+it.instance(
+  "config over-200k cost overrides the database over-200k pricing",
+  Effect.gen(function* () {
+    yield* set("OPENROUTER_API_KEY", "test-api-key")
+    const providers = yield* list
+    const model = providers[ProviderV2.ID.make("openrouter")].models["openai/gpt-5.4"]
+    expect(model.cost.experimentalOver200K).toEqual({
+      input: 7,
+      output: 30,
+      cache: { read: 0.7, write: 0 },
+    })
+    // NOTE (Gabe, 2026-09-02): the catalog's 272K tier would otherwise outrank the configured
+    // over-200k price in Session.getUsage for every context above 272K.
+    expect(model.cost.tiers).toEqual([
+      { input: 7, output: 30, cache: { read: 0.7, write: 0 }, tier: { type: "context", size: 200_000 } },
+    ])
+    const priced = Session.getUsage({
+      model,
+      usage: new Usage({ inputTokens: 300_000, outputTokens: 0, totalTokens: 300_000 }),
+    })
+    expect(priced.cost).toBeCloseTo(2.1, 6)
+  }),
+  {
+    config: {
+      provider: {
+        openrouter: {
+          models: { "openai/gpt-5.4": { cost: { input: 3.5, output: 15, context_over_200k: { input: 7, output: 30, cache_read: 0.7 } } } },
+        },
+      },
+    },
+  },
+)
+
+it.instance(
+  "config over-200k cost keeps the database tiers below 200k",
+  Effect.gen(function* () {
+    yield* set("OPENROUTER_API_KEY", "test-api-key")
+    const providers = yield* list
+    const model = providers[ProviderV2.ID.make("openrouter")].models["x-ai/grok-4"]
+    expect(model.cost.tiers).toEqual([
+      { input: 6, output: 30, cache: { read: 0, write: 0 }, tier: { type: "context", size: 128_000 } },
+      { input: 9, output: 45, cache: { read: 0, write: 0 }, tier: { type: "context", size: 200_000 } },
+    ])
+    const price = (inputTokens: number) =>
+      Session.getUsage({ model, usage: new Usage({ inputTokens, outputTokens: 0, totalTokens: inputTokens }) }).cost
+    expect(price(100_000)).toBeCloseTo(0.3, 6)
+    expect(price(150_000)).toBeCloseTo(0.9, 6)
+    expect(price(300_000)).toBeCloseTo(2.7, 6)
+  }),
+  {
+    config: {
+      provider: {
+        openrouter: {
+          models: { "x-ai/grok-4": { cost: { input: 3, output: 15, context_over_200k: { input: 9, output: 45 } } } },
+        },
+      },
+    },
+  },
+)
+
 it.instance(
   "completely new provider not in database can be configured",
   Effect.gen(function* () {