diff --git a/packages/opencode/src/session/llm.ts b/packages/opencode/src/session/llm.ts
index adacfc431549..2b86fea241a7 100644
--- a/packages/opencode/src/session/llm.ts
+++ b/packages/opencode/src/session/llm.ts
@@ -294,11 +294,11 @@ const live: Layer.Layer<
           // Copilot returns the authoritative billed amount only in provider-specific response fields.
           includeRawChunks: input.model.providerID.includes("github-copilot"),
           async experimental_repairToolCall(failed) {
-            const lower = failed.toolCall.toolName.toLowerCase()
-            if (lower !== failed.toolCall.toolName && prepared.tools[lower]) {
+            const repaired = repairToolName(prepared.tools, failed.toolCall.toolName)
+            if (repaired) {
               return {
                 ...failed.toolCall,
-                toolName: lower,
+                toolName: repaired,
               }
             }
             return {
@@ -401,6 +401,16 @@ export const defaultLayer = Layer.suspend(() =>
 
 export const hasToolCalls = LLMRequestPrep.hasToolCalls
 
+// Match case/hyphen variants back to the registered key; ambiguous collisions resolve to undefined.
+const normalizeToolName = (name: string) => name.toLowerCase().replaceAll("-", "_")
+
+export function repairToolName(tools: Record<string, unknown>, name: string) {
+  if (tools[name]) return name
+  const target = normalizeToolName(name)
+  const matches = Object.keys(tools).filter((key) => normalizeToolName(key) === target)
+  return matches.length === 1 ? matches[0] : undefined
+}
+
 export const node = LayerNode.make(layer, [
   Auth.node,
   Config.node,
diff --git a/packages/opencode/test/session/llm.test.ts b/packages/opencode/test/session/llm.test.ts
index 0c5dadaf17d6..973897025377 100644
--- a/packages/opencode/test/session/llm.test.ts
+++ b/packages/opencode/test/session/llm.test.ts
@@ -173,6 +173,35 @@ describe("session.llm.hasToolCalls", () => {
   })
 })
 
+describe("session.llm.repairToolName", () => {
+  const stub = tool({
+    description: "",
+    inputSchema: z.object({}),
+    execute: async () => "",
+  })
+  const tools = (...names: string[]) => Object.fromEntries(names.map((name) => [name, stub]))
+
+  test("matches a lowercased capitalized-prefix MCP tool (#31506)", () => {
+    expect(LLM.repairToolName(tools("Tool_get_prop"), "tool_get_prop")).toBe("Tool_get_prop")
+  })
+
+  test("matches an all-underscore variant of a hyphenated MCP tool (#27396)", () => {
+    expect(LLM.repairToolName(tools("server_get-by-id"), "server_get_by_id")).toBe("server_get-by-id")
+  })
+
+  test("returns the exact name when it already matches", () => {
+    expect(LLM.repairToolName(tools("Tool_get_prop"), "Tool_get_prop")).toBe("Tool_get_prop")
+  })
+
+  test("returns undefined for an unknown tool", () => {
+    expect(LLM.repairToolName(tools("Tool_get_prop"), "totally_unknown")).toBeUndefined()
+  })
+
+  test("returns undefined when the normalized name is ambiguous", () => {
+    expect(LLM.repairToolName(tools("Tool_get_prop", "tool_get_prop"), "TOOL_GET_PROP")).toBeUndefined()
+  })
+})
+
 describe("session.llm.ai-sdk adapter", () => {
   type AISDKAdapterEvent = Parameters<typeof LLMAISDK.toLLMEvents>[1]