diff --git a/.opencode/plugins/ecc-hooks.ts b/.opencode/plugins/ecc-hooks.ts
index 472f80f5a..d496e61a5 100644
--- a/.opencode/plugins/ecc-hooks.ts
+++ b/.opencode/plugins/ecc-hooks.ts
@@ -523,6 +523,7 @@ export const ECCHooksPlugin: ECCHooksPluginFn = async ({
         env.PRIMARY_LANGUAGE = detected[0]
       }
 
+      // OpenCode reads the supplied output object and ignores callback return values.
       output.env = { ...output.env, ...env }
     },
 
@@ -561,11 +562,17 @@ export const ECCHooksPlugin: ECCHooksPluginFn = async ({
         contextBlock.push("")
       }
 
-      output.context = [
-        ...output.context,
+      const eccContext = [
         contextBlock.join("\n"),
         "Focus on preserving: 1) Current task status and progress, 2) Key decisions made, 3) Files created/modified, 4) Remaining work items, 5) Any security concerns flagged. Discard: verbose tool outputs, intermediate exploration, redundant file listings.",
       ]
+
+      // OpenCode requires output assignment and skips context when a prompt is set.
+      if (output.prompt !== undefined) {
+        output.prompt = [output.prompt, ...eccContext].join("\n\n")
+      } else {
+        output.context = [...output.context, ...eccContext]
+      }
     },
 
     /**
diff --git a/tests/opencode-plugin-hooks.test.js b/tests/opencode-plugin-hooks.test.js
index 4a6511c53..a6c3ed2ae 100644
--- a/tests/opencode-plugin-hooks.test.js
+++ b/tests/opencode-plugin-hooks.test.js
@@ -179,12 +179,14 @@ async function main() {
           const $ = createFailingShell()
           const hooks = await ECCHooksPlugin({ client, $, directory: projectDir })
 
-          const output = { env: { EXISTING_ENV: "preserved" } }
+          const existingEnv = Object.freeze({ EXISTING_ENV: "preserved" })
+          const output = { env: existingEnv }
           await hooks["shell.env"]({ cwd: projectDir }, output)
           const { env } = output
 
           assert.deepStrictEqual($.calls, [], `Unexpected shell probes: ${$.calls.join(", ")}`)
           assert.strictEqual(env.EXISTING_ENV, "preserved")
+          assert.notStrictEqual(env, existingEnv)
           assert.strictEqual(env.PROJECT_ROOT, projectDir)
           assert.strictEqual(env.PACKAGE_MANAGER, "pnpm")
           assert.strictEqual(env.DETECTED_LANGUAGES, "typescript,python")
@@ -266,18 +268,38 @@ async function main() {
         const client = createClient()
         const $ = createFailingShell()
         const hooks = await ECCHooksPlugin({ client, $, directory: projectDir })
-        const output = { context: ["Existing plugin context"] }
+        const existingContext = Object.freeze(["Existing plugin context"])
+        const output = { context: existingContext }
 
         await hooks["experimental.session.compacting"]({ sessionID: "session-1" }, output)
 
         assert.strictEqual(output.context[0], "Existing plugin context")
+        assert.notStrictEqual(output.context, existingContext)
         const prompt = output.prompt ?? ["Default compaction prompt", ...output.context].join("\n\n")
         assert.ok(prompt.includes("Default compaction prompt"))
         assert.ok(prompt.includes("# ECC Context"))
         assert.ok(prompt.includes("Current task status and progress"))
-        const customOutput = { context: [], prompt: "Another plugin's custom prompt" }
-        await hooks["experimental.session.compacting"]({ sessionID: "session-1" }, customOutput)
-        assert.strictEqual(customOutput.prompt, "Another plugin's custom prompt")
+        assert.deepStrictEqual($.calls, [])
+      }),
+    ],
+    [
+      "compacting appends ECC guidance to custom prompts, including an empty prompt",
+      async () => withTempProject([], async (projectDir) => {
+        const client = createClient()
+        const $ = createFailingShell()
+        const hooks = await ECCHooksPlugin({ client, $, directory: projectDir })
+
+        for (const customPrompt of ["Another plugin's custom prompt", ""]) {
+          const existingContext = Object.freeze(["Existing plugin context"])
+          const output = { context: existingContext, prompt: customPrompt }
+          await hooks["experimental.session.compacting"]({ sessionID: "session-1" }, output)
+
+          const prompt = output.prompt ?? ["Default compaction prompt", ...output.context].join("\n\n")
+          assert.ok(prompt.startsWith(`${customPrompt}\n\n`))
+          assert.ok(prompt.includes("# ECC Context"))
+          assert.ok(prompt.includes("Current task status and progress"))
+          assert.strictEqual(output.context, existingContext)
+        }
         assert.deepStrictEqual($.calls, [])
       }),
     ],